The United States is the largest grower of commercial crops that have been genetically engineered in the world, but not without domestic and international opposition.
In
this case, the plaintiff argued both for mandatory labeling on the
basis of consumer demand, and that GMO foods should undergo the same
testing requirements as food additives because they are "materially
changed" and have potentially unidentified health risks. The plaintiff
also alleged that the FDA did not follow the Administrative Procedures Act
in formulating and disseminating its policy on GMO's. The federal
district court rejected all of those arguments and found that the FDA's
determination that GMO's are Generally Recognized as Safe
was neither arbitrary nor capricious. The court gave deference to the
FDA's process on all issues, leaving future plaintiffs little legal
recourse to challenge the FDA's policy on GMO's. Alliance for Bio-Integrity v Shalala, 116 F.Supp.2d 166 (D.D.C. 2000).
Diamond v. Chakrabarty, 447 U.S. 303 (1980), was a United States Supreme Court case dealing with whether genetically modified organisms can be patented.
The Court held that a living, man-made micro-organism is patentable
subject matter as a "manufacture" or "composition of matter" within the
meaning of the Patent Act of 1952. The fact that the organism sought to be patented is alive is no bar to patentability.
Numerous
organizations based in the U.S. oppose or have concerns about genetic
engineering for various reasons. Groups such as the Center for Food Safety, the nonprofit science advocacy group Union of Concerned Scientists, Greenpeace and the World Wildlife Fund have expressed concerns about the FDA's lack of a requirement for additional testing for GMO's, lack of required labeling and the presumption that GMO's are "Generally Recognized as Safe" (GRAS). Some of these groups have questioned whether the FDA is too close to companies that seek approval for their products.
Health concerns
Although
there have been no recorded instances of harm to human health due to
the consumption of genetically engineered foods, there is concern over
their impact on health. One of the largest food recalls in US history,
was the Taco Bell GMO recall, where a Bt corn plant
not approved for human consumption due its risk as an allergen, had
contaminated food products like the tacos at Taco Bell, and a huge
percentage of US's seed supply. No health problems were linked to
Starlink corn, and subsequent evaluations of the Bt trait determined that there is medium risk to human health.
The USA is the largest commercial grower of genetically modified crops in the world. United States regulatory policy is governed by the Coordinated Framework for Regulation of Biotechnology. The United States is not a signatory to the Cartagena Protocol on Biosafety.
For a genetically modified organism to be approved for release it is
assessed by the USDA, the FDA and the EPA. USDA evaluates the plant's
potential to become weeds, the FDA reviews plants that could enter or
alter the food supply and the EPA regulates the genetically modified
plants with pesticide properties. Most developed genetically modified
plants are reviewed by at least two of the agencies, with many subject
to all three. Final approval can still be denied by individual counties within each state. In 2004, Mendocino County,
California became the first and only county to impose a ban on the
"Propagation, Cultivation, Raising, and Growing of Genetically Modified
Organisms", the measure passing with a 57% majority. (See Mendocino County GMO Ban)
U.S. Department of Agriculture
The Biotechnology Regulatory Services program of the Animal and Plant Health Inspection Service (APHIS) agency within the USDA is concerned with protecting agriculture and the environment from potential pests under the Plant Protection Act of 2000 (part of the Agriculture Risk Protection Act) and the National Environmental Policy Act (NEPA). Each transgenic event is regulated separately as the transgene insertion locus varies even when using identical constructs and host genotypes. This could result in different expression patterns or could affect the function of other endogenous genes within the host. The USDA is responsible for approving field trials of GM plants under either the notification or permit procedures.
The notification procedure is a streamlined process for the simplest or
most familiar genetically engineered plants that meet six criteria (is
not a noxious weed,
the function of the genetic material is known and characterized, stable
integration, no significant risk of creating new viruses and that no
animal or human pathogen sequences are present). Most field trials are approved under the notification procedure.
The permit procedure is much more elaborate and is required for all
genetically engineered organisms that do not meet the notification
requirements or any plant-made pharmaceuticals or plant-made industrial products.
APHIS officials are responsible for inspecting the field trials.
At least one inspection is carried out for each state listed on a
permit, while inspection of field trials authorized by notification is
conducted based on the relative risk of each trial.
For field trials of organisms that contain pharmaceutical or industrial
compounds, inspections are carried out more frequently (five times
during establishment and twice yearly after that). If the inspectors are
satisfied that there are no regulatory concerns they issue a Notice of
Compliance. If the regulations are not being adhered to the inspectors
will issue a Notice of Non-Compliance requesting that the deviations be
fixed, or for more serious breaches a warning letter requiring a written
response and corrective action to be taken within a given time frame.
Formal investigations are carried out on developers who may not be
adhering to regulations, permit conditions, or other requirements, which
can result in civil penalties or criminal charges.
In 1993, the USDA proposal to remove regulatory oversight from GM organisms deemed environmentally benign was approved and four GM plants (Flavr Savr tomato, virus-resistant squash, bromoxynil-tolerant cotton and glyphosate-tolerant soybean) obtained non-regulatory status that year.
Non-regulated status means that permits and notifications are no longer
required for introductions of this organism. Applicants can petition
APHIS for non-regulated status if the GM organism poses no more of a
plant pest risk than an equivalent non-GM organism. APHIS will prepare at least two documents (an Environmental Assessment and a determination of non-regulatory status) under the NEPA while considering the application.
Four federal district courtsuits have been brought against APHIS challenging their regulation of GM plants. Two involved field trials (herbicide-tolerant turfgrass in Oregon; pharmaceutical-producing corn and sugar in Hawaii) and the other two were the deregulation of GM alfalfa and GM Sugar Beet.
APHIS initially lost all four cases, with the judges ruling they failed
to diligently follow the NEPA guidelines. However, the Supreme Court
overturned the nationwide ban on GM alfalfa and an appeal court allowed the partial deregulation of GM sugar beet crops. After APHIS prepared Environmental Impact Statements for both crops they were deregulated again.
Food and Drug Administration
The FDA
is responsible for the safety and security of human and animal food and
drugs, including any that are genetically modified. The FDA was
responsible for approving the first commercialized GMO, Genetech's
genetically modified human insulin (Humulin) in 1982 and the first commercialized GM whole food, Calgene's Flavr Savr tomato in 1994. When evaluating new GM foods or feed the FDA looks for the presence of any new or altered allergens and toxicants and examines changes in the levels of nutritional and anti-nutritional substances. Food and feed that is identical or nearly identical in composition to current products is deemed to be substantially equivalent and is not required to undergo review by the FDA.
The FDA has been criticized for using substantial equivalence, with a
major accusation being that FDA review is essentially voluntary as
almost all GM products are substantially equivalent. However, all GM food and feed currently on the US market (as of 2008) have undergone a FDA consultation, where the developer submits the compositional data and FDA scientist compare it to regular food and feed.
The FDA consultation focuses on whether the new food or feed
contains any new allergens or toxic substances and whether the
nutritional components of the food or feed have increased or decreased.
The developer submits documentation to the FDA describing the food or
feed and a FDA assigned caseworker can then request additional
information on expected dietary exposure, in particular if any risk
groups (children, elderly etc.) might be exposed. As of 2007, the FDA
has not identified any genetically modified foods with unexpected
changes in the nutrient composition or levels of allergens or toxic
substances. However, allergic proteins have been detected when some GM products have undergone testing. Pioneer Hi-Bred inserted a gene from the Brazil nut into transgenic soybean resulting in soy with an enhanced nutritional profile. The inserted gene did not translate into a known allergen at the time, but when tested with serum from people who are allergic to Brazil nut the allergenic nature of the protein was discovered.
The development of the transgenic soybean expressing a Brazil nut
allergen was stopped after these tests. The FDA consultation process is
relatively (when compared to the other agencies regulating GM) informal
and they do not approve new GM products. Instead they issue a memo stating whether the new food is the same as or different from the non-modified variety.
The Center for Veterinary Medicine of the FDA regulates genetically modified animals in consultation with Centers at the FDA responsible for regulating pharmaceuticals or other medical products derived from biopharm animals. The FDA also has extra guidelines that apply to genetically modified animals that will be used in the manufacturing and testing of therapeutic products and xenotransplantation.
The FDA guidance documents do not establish legally binding laws and
are viewed as recommendations, unless specific regulatory or statutory
requirements are cited. Any relevant federal, State, or local laws and
regulations must also be adhered to.
Environmental Protection Agency
The EPA regulates substances with pesticide characteristics, looking at potential threats to human health or the environment.
They claim not to regulate the genetically modified plants, but the
pesticides produced by the plants or properties that change the usage of
applied pesticides . This includes; plants engineered to produce
resistance to herbicides (e.g. Roundup Ready), plants that produce their own pesticides (e.g. BT)
and virus resistant plants. Authority to regulate the pesticide
properties in genetically modified organisms was granted in the Federal Insecticide, Fungicide, and Rodenticide Act (FIFRA) and the Federal Food, Drug, and Cosmetic Act
(FFDCA). The EPA published regulations in 1994 and begun acting on them
in 1995. In 1994 they proposed the exemption of three categories of
genetically modified plants under their regulation. These were plants
where the genetic material originated in sexually compatible plants (cisgenic), plants that used physical barriers to prevent the target pest from attaching itself, and plants expressing viral coat proteins to protect against virus infection.
In 2001, rules regarding exemption of cisgenic plants had been
finalised. The other two proposed exemptions were still under review in
2010.
The EPA evaluated each submission on a case-by-case basis. The
EPA assesses data concerning the characterisation of the end-product of
the engineered organism (presently all plants evaluated produce
proteins), as well as data on mammalian toxicity, effects on non-target
organisms and environmental metabolism.
For Bt products the producer must also supply an insect resistance
management program. For herbicide resistant plants the EPA co-ordinates
with the USDA and FDA, but does not regulate the plant itself. Instead
it regulates the herbicide and its use on the new cultivar. The EPA examines the construct
used to transform the plant and the biology of recipient plant. The
sequence of the resulting protein must be described, expression pattern
and intencity verified and any modifications to the protein reported.
The EPA considers the potential allergenicity of the product, issues
surrounding gene flow into wild species, possible effects on non-target
organisms, likelihood of it persisting in the environment and the
potential for insect resistance developing when assessing submissions.
The Yecoro wheat (right) cultivar
is sensitive to salinity, plants resulting from a hybrid cross with
cultivar W4910 (left) show greater tolerance to high salinity
Plant breeding is the science of changing the traits of plants in order to produce desired characteristics. It has been used to improve the quality of nutrition in products for humans and animals.
The goals of plant breeding are to produce crop varieties that boast
unique and superior traits for a variety of applications. The most
frequently addressed agricultural traits are those related to biotic and
abiotic stress tolerance, grain or biomass yield, end-use quality
characteristics such as taste or the concentrations of specific
biological molecules (proteins, sugars, lipids, vitamins, fibers) and
ease of processing (harvesting, milling, baking, malting, blending,
etc.).
Plant breeding can be performed through many different techniques
ranging from simply selecting plants with desirable characteristics for
propagation, to methods that make use of knowledge of genetics and
chromosomes, to more complex molecular techniques. Genes in a plant are
what determine what type of qualitative or quantitative traits it will
have. Plant breeders strive to create a specific outcome of plants and
potentially new plant varieties, and in the course of doing so, narrow down the genetic diversity of that variety to a specific few biotypes.
It is practiced worldwide by individuals such as gardeners and
farmers, and by professional plant breeders employed by organizations
such as government
institutions, universities, crop-specific industry associations or
research centers. International development agencies believe that
breeding new crops is important for ensuring food security
by developing new varieties that are higher yielding, disease
resistant, drought tolerant or regionally adapted to different
environments and growing conditions.
A recent study shows that without plant breeding, Europe
would have produced 20% fewer arable crops over the last 20 years,
consuming an additional 21.6 million hectares (53 million acres) of land
and emitting 4 billion tonnes (3.9×109 long tons; 4.4×109 short tons) of carbon. Wheat species created for Morocco
are currently being crossed with plants to create new varieties for
northern France. Soy beans, which were previously grown predominantly in
the south of France, are now grown in southern Germany.
Plant breeding started with sedentary agriculture and particularly the domestication of the first agricultural plants, a practice which is estimated to date back 9,000 to 11,000 years. Initially early farmers simply selected food plants with particular desirable characteristics, and employed these as progenitors for subsequent generations, resulting in an accumulation of valuable traits over time.
Grafting technology had been practiced in China before 2000 BCE.
By 500 BCE grafting was well established and practiced.
Gregor Mendel (1822–84) is considered the "father of genetics". His experiments with plant hybridization led to his establishing laws of inheritance. Genetics stimulated research to improve crop production through plant breeding.
Selective breeding enlarged desired traits of the wild cabbage plant (Brassica oleracea) over hundreds of years, resulting in dozens of today's agricultural crops. Cabbage, kale, broccoli, and cauliflower are all cultivars of this plant.
One major technique of plant breeding is selection,
the process of selectively propagating plants with desirable
characteristics and eliminating or "culling" those with less desirable
characteristics.
Another technique is the deliberate interbreeding (crossing) of
closely or distantly related individuals to produce new crop varieties
or lines with desirable properties. Plants are crossbred to introduce traits/genes from one variety or line into a new genetic background. For example, a mildew-resistant pea
may be crossed with a high-yielding but susceptible pea, the goal of
the cross being to introduce mildew resistance without losing the
high-yield characteristics. Progeny from the cross would then be crossed
with the high-yielding parent to ensure that the progeny were most like
the high-yielding parent, (backcrossing).
The progeny from that cross would then be tested for yield (selection,
as described above) and mildew resistance and high-yielding resistant
plants would be further developed. Plants may also be crossed with
themselves to produce inbred varieties for breeding. Pollinators may be excluded through the use of pollination bags.
Successful commercial plant breeding concerns were founded from the late 19th century. Gartons Agricultural Plant Breeders in England
was established in the 1890s by John Garton, who was one of the first
to commercialize new varieties of agricultural crops created through
cross-pollination. The firm's first introduction was the Abundance Oat, an oat variety. It is one of the first agricultural grain varieties bred from a controlled cross, introduced to commerce in 1892.
In the early 20th century, plant breeders realized that Gregor Mendel's findings on the non-random nature of inheritance could be applied to seedling populations produced through deliberate pollinations to predict the frequencies of different types. Wheat hybrids were bred to increase the crop production of Italy during the so-called "Battle for Grain" (1925–1940). Heterosis was explained by George Harrison Shull.
It describes the tendency of the progeny of a specific cross to
outperform both parents. The detection of the usefulness of heterosis
for plant breeding has led to the development of inbred lines that
reveal a heterotic yield advantage when they are crossed. Maize was the first species where heterosis was widely used to produce hybrids.
Statistical
methods were also developed to analyze gene action and distinguish
heritable variation from variation caused by environment. In 1933
another important breeding technique, cytoplasmic male sterility (CMS), developed in maize, was described by Marcus Morton Rhoades. CMS is a maternally inherited trait that makes the plant produce sterile pollen. This enables the production of hybrids without the need for labor-intensive detasseling.
These early breeding techniques resulted in large yield increase in the United States in the early 20th century. Similar yield increases were not produced elsewhere until after World War II, the Green Revolution increased crop production in the developing world in the 1960s.
Following World War II
a number of techniques were developed that allowed plant breeders to
hybridize distantly related species, and artificially induce genetic
diversity.
When distantly related species are crossed, plant breeders make use of a number of plant tissue culture
techniques to produce progeny from otherwise fruitless mating.
Interspecific and intergeneric hybrids are produced from a cross of
related species or genera that do not normally sexually reproduce with each other. These crosses are referred to as Wide crosses. For example, the cerealtriticale is a wheat and rye
hybrid. The cells in the plants derived from the first generation
created from the cross contained an uneven number of chromosomes and as a
result was sterile. The cell division inhibitor colchicine was used to double the number of chromosomes in the cell and thus allow the production of a fertile line.
Failure to produce a hybrid may be due to pre- or post-fertilization incompatibility. If fertilization is possible between two species or genera, the hybrid embryo
may abort before maturation. If this does occur the embryo resulting
from an interspecific or intergeneric cross can sometimes be rescued and
cultured to produce a whole plant. Such a method is referred to as embryo rescue. This technique has been used to produce new rice for Africa, an interspecific cross of Asian rice Oryza sativa and African riceO. glaberrima.
Hybrids may also be produced by a technique called protoplast fusion. In this case protoplasts are fused, usually in an electric field. Viable recombinants can be regenerated in culture.
When a desirable trait has been bred into a species, a number of
crosses to the favored parent are made to make the new plant as similar
to the favored parent as possible. Returning to the example of the
mildew resistant pea being crossed with a high-yielding
but susceptible pea, to make the mildew resistant progeny of the cross
most like the high-yielding parent, the progeny will be crossed back to
that parent for several generations (See backcrossing).
This process removes most of the genetic contribution of the mildew
resistant parent. Classical breeding is therefore a cyclical process.
With classical breeding techniques, the breeder does not know
exactly what genes have been introduced to the new cultivars. Some scientists therefore argue that plants produced by classical breeding methods should undergo the same safety testing regime as genetically modified
plants. There have been instances where plants bred using classical
techniques have been unsuitable for human consumption, for example the poisonsolanine was unintentionally increased to unacceptable levels in certain varieties of potato through plant breeding. New potato varieties are often screened for solanine levels before reaching the marketplace.
Modern plant breeding may use techniques of molecular biology to
select, or in the case of genetic modification, to insert, desirable
traits into plants. Application of biotechnology or molecular biology is
also known as molecular breeding.
Modern facilities in molecular biology are now used in plant breeding.
Sometimes many different genes can influence a desirable trait in plant breeding. The use of tools such as molecular markers or DNA fingerprinting
can map thousands of genes. This allows plant breeders to screen large
populations of plants for those that possess the trait of interest. The
screening is based on the presence or absence of a certain gene as
determined by laboratory procedures, rather than on the visual
identification of the expressed trait in the plant. The purpose of
marker assisted selection, or plant genome analysis, is to identify the
location and function (phenotype) of various genes within the genome. If all of the genes are identified it leads to genome sequence.All plants have varying sizes and lengths of genomes with genes that
code for different proteins, but many are also the same. If a gene's
location and function is identified in one plant species, a very similar
gene likely can also be found in a similar location in another related
species genome.
Homozygous plants with desirable traits can be produced from heterozygous starting plants, if a haploid cell with the alleles for those traits can be produced, and then used to make a doubled haploid.
The doubled haploid will be homozygous for the desired traits.
Furthermore, two different homozygous plants created in that way can be
used to produce a generation of F1 hybrid
plants which have the advantages of heterozygosity and a greater range
of possible traits. Thus, an individual heterozygous plant chosen for
its desirable characteristics can be converted into a heterozygous
variety (F1 hybrid) without the necessity of vegetative reproduction but as the result of the cross of two homozygous/doubled haploid lines derived from the originally selected plant.
Plant tissue culturing can produce haploid or double haploid plant
lines and generations. This cuts down the genetic diversity taken from
that plant species in order to select for desirable traits that will
increase the fitness of the individuals. Using this method decreases the
need for breeding multiple generations of plants to get a generation
that is homogeneous for the desired traits, thereby saving much time
over the natural version of the same process. There are many plant
tissue culturing techniques that can be used to achieve haploid plants,
but microspore culturing is currently the most promising for producing
the largest numbers of them.
Genetic modification of plants is achieved by adding a specific gene or genes to a plant, or by knocking down a gene with RNAi, to produce a desirable phenotype. The plants resulting from adding a gene are often referred to as transgenic plants.
If for genetic modification genes of the species or of a crossable
plant are used under control of their native promoter, then they are
called cisgenic plants.
Sometimes genetic modification can produce a plant with the desired
trait or traits faster than classical breeding because the majority of
the plant's genome is not altered.
To genetically modify a plant, a genetic construct must be
designed so that the gene to be added or removed will be expressed by
the plant. To do this, a promoter to drive transcription
and a termination sequence to stop transcription of the new gene, and
the gene or genes of interest must be introduced to the plant. A marker
for the selection of transformed plants is also included. In the laboratory, antibiotic resistance
is a commonly used marker: Plants that have been successfully
transformed will grow on media containing antibiotics; plants that have
not been transformed will die. In some instances markers for selection
are removed by backcrossing with the parent plant prior to commercial release.
The construct can be inserted in the plant genome by genetic recombination using the bacteria Agrobacterium tumefaciens or A. rhizogenes, or by direct methods like the gene gun or microinjection. Using plant viruses
to insert genetic constructs into plants is also a possibility, but the
technique is limited by the host range of the virus. For example, Cauliflower mosaic virus (CaMV) only infects cauliflower
and related species. Another limitation of viral vectors is that the
virus is not usually passed on to the progeny, so every plant has to be
inoculated.
The majority of commercially released transgenic plants are currently limited to plants that have introduced resistance to insect pests and herbicides. Insect resistance is achieved through incorporation of a gene from Bacillus thuringiensis (Bt) that encodes a protein that is toxic to some insects. For example, the cotton bollworm, a common cotton pest, feeds on Bt cotton it will ingest the toxin and die. Herbicides usually work by binding to certain plant enzymes and inhibiting their action. The enzymes that the herbicide inhibits are known as the herbicide's "target site".
Herbicide resistance can be engineered into crops by expressing a
version of target site protein that is not inhibited by the herbicide.
This is the method used to produce glyphosate resistant ("Roundup Ready") crop plants.
Genetic modification can further increase yields by increasing
stress tolerance to a given environment. Stresses such as temperature
variation, are signalled to the plant via a cascade of signalling
molecules which will activate a transcription factor to regulate gene expression.
Overexpression of particular genes involved in cold acclimation has
been shown to produce more resistance to freezing, which is one common
cause of yield loss
Genetic modification of plants that can produce pharmaceuticals (and industrial chemicals), sometimes called pharming, is a rather radical new area of plant breeding.
The debate surrounding genetically modified food during the 1990s peaked in 1999 in terms of media coverage and risk perception, and continues today – for example, "Germany has thrown its weight behind a growing European mutiny over genetically modified crops by banning the planting of a widely grown pest-resistant corn variety." The debate encompasses the ecological impact of genetically modified plants, the safety of genetically modified food and concepts used for safety evaluation like substantial equivalence.
Such concerns are not new to plant breeding. Most countries have
regulatory processes in place to help ensure that new crop varieties
entering the marketplace are both safe and meet farmers' needs. Examples
include variety registration, seed schemes, regulatory authorizations
for GM plants, etc.
Breeding and the microbiome
Industrial breeding of plants has unintentionally altered how agricultural cultivars associate with their microbiome.
In maize, for example, breeding has altered the nitrogen cycling taxa
required to the rhizosphere, with more modern lines recruiting less nitrogen fixing taxa and more nitrifiers and denitrifiers.
Microbiomes of breeding lines showed that hybrid plants share much of
their bacterial community with their parents, such as Cucurbita seeds
and apple shoot endophytes.In addition, the proportional contribution of the microbiome from
parents to offspring corresponds to the amount of genetic material
contributed by each parent during breeding and domestication.
Phenotyping and artificial intelligence
As of 2020 machine learning – and especially deep machine learning – has recently become more commonly used in phenotyping. Computer vision
using ML has made great strides and is now being applied to leaf
phenotyping and other phenotyping jobs typically performed by human
eyes. Pound et al. 2017 and Singh et al. 2016 are especially salient
examples of early successful application and demonstration of the
general usability of the process across multiple target plant species. These methods will work even better with large, publicly available open data sets.
Speed breeding
Speed breeding is introduced by Watson et al. 2018. Classical (human performed) phenotyping during speed breeding is also possible, using a procedure developed by Richard et al. 2015. As of 2020 it is highly anticipated that SB and automated phenotyping will, combined, produce greatly improved outcomes – see § Phenotyping and artificial intelligence above.
Genomic selection (GS)
The
NGS platform has substantially declined the time and cost required for
sequencing and facilitated SNP discovery in model and non-model plants.
This in turn has led to employing large-scale SNP markers in genomic
selection approaches which aim at predicting genomic breeding
values/GEBVs of genotypes in a given population. This method can
increase the selection accuracy and decrease the time of each breeding
cycle. It has been used in different crops such as maize, wheat, etc.
Participatory plant breeding
Participatory
plant breeding (PPB) is when farmers are involved in a crop improvement
programme with opportunities to make decisions and contribute to the
research process at different stages. Participatory approaches to crop improvement can also be applied when
plant biotechnologies are being used for crop improvement.
Local agricultural systems and genetic diversity are strengthened by
participatory programs, and outcomes are enhanced by farmers knowledge
of the quality required and evaluation of the target environment.
A 2019 review of participatory plant breeding indicated that it
had not gained widespread acceptance despite its record of successfully
developing varieties with improved diversity and nutritional quality, as
well as greater likelihood of these improved varieties being adopted by
farmers. This review also found participatory plant breeding to have a
better cost/benefit ratio than non-participatory approaches, and
suggested incorporating participatory plant breeding with evolutionary
plant breeding.
Evolutionary plant breeding describes practices which use mass
populations with diverse genotypes grown under competitive natural
selection. Survival in common crop cultivation environments is the
predominant method of selection, rather than direct selection by growers
and breeders. Individual plants that are favored under prevailing
growing conditions, such as environment and inputs, contribute more seed
to the next generation than less-adapted individuals. Evolutionary plant breeding has been successfully used by the Nepal National Gene Bank to preserve landrace diversity within Jumli Marshi rice while reducing its susceptibility to blast disease. These practices have also been used in Nepal with bean landraces.
In 1929, Harlan and Martini proposed a method of plant breeding
with heterogeneous populations by pooling an equal number of F2 seeds
obtained from 378 crosses among 28 geographically diverse barley
cultivars. In 1938, Harlan and Martini demonstrated evolution by natural
selection in mixed dynamic populations as a few varieties that became
dominant in some locations almost disappeared in others; poorly-adapted
varieties disappeared everywhere.
Evolutionary breeding populations have been used to establish
self-regulating plant–pathogen systems. Examples include barley, where
breeders were able to improve resistance to Rynchosporium secalis scald over 45 generations. An evolutionary breeding project grew F5 hybrid bulk soybean populations on soil infested by the soybean cyst nematode and was able to increase the proportion of resistant plants from 5% to 40%. The International Center for Agricultural Research in the Dry Areas
(ICARDA) evolutionary plant breeding is combined with participatory
plant breeding in order to allow farmers to choose which varieties suit
their needs in their local environment.
An influential 1956 effort by Coit A. Suneson to codify this approach coined the term evolutionary plant breeding
and concluded that 15 generations of natural selection are desirable to
produce results that are competitive with conventional breeding. Evolutionary breeding allows working with much larger plant population sizes than conventional breeding.
It has also been used in tandem with conventional practices in order to
develop both heterogeneous and homogeneous crop lines for low input
agricultural systems that have unpredictable stress conditions.
Evolutionary plant breeding has been delineated into four stages:
Stage 1: Genetic diversity is created, for example by manual
crosses of inbreeding species or mixing of cultivars in outcrossing
species.
Stage 2: Multiplication of seeds
Stage 3: Seeds of each cross are then mixed to produce the first
generation of the Composite Cross Population (CCP). The entire offspring
is sown to grow and set seed. As the number of plants in the population
increases, a proportion of the harvested seed is saved for sowing.
Stage 4: The seed can be used for continued evolutionary plant
breeding or as a starting point for a conventional breeding effort.
Issues and concerns
Breeding and food security
Issues
facing plant breeding in the future include the lack of arable land,
increasingly harsh cropping conditions and the need to maintain food
security, which involves being able to provide the world population with
sufficient nutrition. Crops need to be able to mature in multiple
environments to allow worldwide access, which involves solving problems
including drought tolerance. It has been suggested that global solutions
are achievable through the process of plant breeding, with its ability
to select specific genes allowing crops to perform at a level which
yields the desired results. One issue facing agriculture is the loss of landraces and other local varieties which have diversity that may have useful genes for climate adaptation in the future.
Conventional breeding intentionally limits phenotype plasticity within genotypes and limits variability between genotypes. Uniformity does not allow crops to adapt to climate change and other biotic stresses and abiotic stresses.
Plant breeders rights
Plant breeders' rights
is an important and controversial issue. Production of new varieties is
dominated by commercial plant breeders, who seek to protect their work
and collect royalties through national and international agreements
based in intellectual property rights.
The range of related issues is complex. In the simplest terms, critics
of the increasingly restrictive regulations argue that, through a
combination of technical and economic pressures, commercial breeders are
reducing biodiversity and significantly constraining individuals (such as farmers) from developing and trading seed on a regional level. Efforts to strengthen breeders' rights, for example, by lengthening periods of variety protection, are ongoing.
Intellectual property legislation for plants often uses
definitions that typically include genetic uniformity and unchanging
appearance over generations. These legal definitions of stability
contrast with traditional agronomic usage, which considers stability in
terms of how consistent the yield or quality of a crop remains across
locations and over time.
As of 2020, regulations in Nepal only allow uniform varieties to
be registered or released. Evolutionary plant populations and many
landraces are polymorphic and do not meet these standards.
Environmental stressors
Uniform and genetically stable cultivars can be inadequate for dealing with environmental fluctuations and novel stress factors.
Plant breeders have focused on identifying crops which will ensure
crops perform under these conditions; a way to achieve this is finding
strains of the crop that is resistance to drought conditions with low
nitrogen. It is evident from this that plant breeding is vital for
future agriculture to survive as it enables farmers to produce stress
resistant crops hence improving food security.
In countries that experience harsh winters such as Iceland, Germany
and further east in Europe, plant breeders are involved in breeding for
tolerance to frost, continuous snow-cover, frost-drought (desiccation
from wind and solar radiation under frost) and high moisture levels in
soil in winter.
Long-term process
Breeding
is not a quick process, which is especially important when breeding to
ameliorate a disease. The average time from human recognition of a new
fungal disease threat to the release of a resistant crop for that
pathogen is at least twelve years.
Maintaining specific conditions
When
new plant breeds or cultivars are bred, they must be maintained and
propagated. Some plants are propagated by asexual means while others are
propagated by seeds. Seed propagated cultivars require specific control
over seed source and production procedures to maintain the integrity of
the plant breeds results. Isolation is necessary to prevent cross
contamination with related plants or the mixing of seeds after
harvesting. Isolation is normally accomplished by planting distance but
in certain crops, plants are enclosed in greenhouses or cages (most
commonly used when producing F1 hybrids).
Nutritional value
Modern
plant breeding, whether classical or through genetic engineering, comes
with issues of concern, particularly with regard to food crops. The
question of whether breeding can have a negative effect on nutritional
value is central in this respect. Although relatively little direct
research in this area has been done, there are scientific indications
that, by favoring certain aspects of a plant's development, other
aspects may be retarded. A study published in the Journal of the American College of Nutrition in 2004, entitled Changes in USDA Food Composition Data for 43 Garden Crops, 1950 to 1999, compared nutritional analysis of vegetables done in 1950 and in 1999, and found substantial decreases in six of 13 nutrients measured, including 6% of protein and 38% of riboflavin. Reductions in calcium, phosphorus, iron and ascorbic acid were also found. The study, conducted at the Biochemical Institute, University of Texas at Austin, concluded in summary: "We
suggest that any real declines are generally most easily explained by
changes in cultivated varieties between 1950 and 1999, in which there
may be trade-offs between yield and nutrient content."
Plant breeding can contribute to global food security as it is a
cost-effective tool for increasing nutritional value of forage and
crops. Improvements in nutritional value for forage crops from the use
of analytical chemistry and rumen fermentation technology have been
recorded since 1960; this science and technology gave breeders the
ability to screen thousands of samples within a small amount of time,
meaning breeders could identify a high performing hybrid quicker. The
genetic improvement was mainly in vitro dry matter digestibility (IVDMD)
resulting in 0.7-2.5% increase, at just 1% increase in IVDMD a single
Bos Taurus also known as beef cattle reported 3.2% increase in daily
gains. This improvement indicates plant breeding is an essential tool in
gearing future agriculture to perform at a more advanced level.
Yield
With an
increasing population, the production of food needs to increase with it.
It is estimated that a 70% increase in food production is needed by
2050 in order to meet the Declaration of the World Summit on Food
Security. But with the degradation of agricultural land, simply planting
more crops is no longer a viable option. New varieties of plants can in
some cases be developed through plant breeding that generate an
increase of yield without relying on an increase in land area. An
example of this can be seen in Asia, where food production per capita
has increased twofold. This has been achieved through not only the use
of fertilisers, but through the use of better crops that have been
specifically designed for the area.
Role of plant breeding in organic agriculture
Some critics of organic agriculture
claim it is too low-yielding to be a viable alternative to conventional
agriculture in situations when that poor performance may be the result
in part of growing poorly-adapted varieties.
It is estimated that over 95% of organic agriculture is based on
conventionally adapted varieties, even though the production
environments found in organic vs. conventional farming systems are
vastly different due to their distinctive management practices.
Most notably, organic farmers have fewer inputs available than
conventional growers to control their production environments. Breeding
varieties specifically adapted to the unique conditions of organic
agriculture is critical for this sector to realize its full potential.
This requires selection for traits such as:
Currently, few breeding programs are directed at organic agriculture
and until recently those that did address this sector have generally
relied on indirect selection (i.e. selection in conventional
environments for traits considered important for organic agriculture).
However, because the difference between organic and conventional
environments is large, a given genotype may perform very differently in each environment due to an interaction between genes and the environment (see gene–environment interaction).
If this interaction is severe enough, an important trait required for
the organic environment may not be revealed in the conventional
environment, which can result in the selection of poorly adapted
individuals.
To ensure the most adapted varieties are identified, advocates of
organic breeding now promote the use of direct selection (i.e. selection
in the target environment) for many agronomic traits.
There are many classical and modern breeding techniques that can
be utilized for crop improvement in organic agriculture despite the ban
on genetically modified organisms.
For instance, controlled crosses between individuals allow desirable
genetic variation to be recombined and transferred to seed progeny via
natural processes. Marker assisted selection
can also be employed as a diagnostics tool to facilitate selection of
progeny who possess the desired trait(s), greatly speeding up the
breeding process.
This technique has proven particularly useful for the introgression of
resistance genes into new backgrounds, as well as the efficient
selection of many resistance genes pyramided into a single individual. Molecular markers are not currently available for many important traits, especially complex ones controlled by many genes.
Nitrogen assimilation is the formation of organic nitrogen compounds like amino acids from inorganic nitrogen compounds present in the environment. Organisms like plants, fungi and certain bacteria that can fix nitrogen gas (N2) depend on the ability to assimilate nitrate or ammonia for their needs. Other organisms, like animals, depend entirely on organic nitrogen from their food.
Nitrogen assimilation in plants
Plants absorb nitrogen from the soil in the form of nitrate (NO3−) and ammonium (NH4+). In aerobic soils where nitrification can occur, nitrate is usually the predominant form of available nitrogen that is absorbed. However this is not always the case as ammonia can predominate in grasslands and in flooded, anaerobic soils like rice paddies.
Plant roots themselves can affect the abundance of various forms of
nitrogen by changing the pH and secreting organic compounds or oxygen.
This influences microbial activities like the inter-conversion of
various nitrogen species, the release of ammonia from organic matter in
the soil and the fixation of nitrogen by non-nodule-forming bacteria.
Ammonium ions are absorbed by the plant via ammonia transporters. Nitrate is taken up by several nitrate transporters that use a proton gradient to power the transport.
Nitrogen is transported from the root to the shoot via the xylem in the
form of nitrate, dissolved ammonia and amino acids. Usually (but not always)
most of the nitrate reduction is carried out in the shoots while the
roots reduce only a small fraction of the absorbed nitrate to ammonia.
Ammonia (both absorbed and synthesized) is incorporated into amino acids
via the glutamine synthetase-glutamate synthase (GS-GOGAT) pathway. While nearly all
the ammonia in the root is usually incorporated into amino acids at the
root itself, plants may transport significant amounts of ammonium ions
in the xylem to be fixed in the shoots. This may help avoid the transport of organic compounds down to the roots just to carry the nitrogen back as amino acids.
Nitrate reduction is carried out in two steps. Nitrate is first reduced to nitrite (NO2−) in the cytosol by nitrate reductase using NADH or NADPH. Nitrite is then reduced to ammonia in the chloroplasts (plastids in roots) by a ferredoxin dependent nitrite reductase. In photosynthesizing tissues, it uses an isoform of ferredoxin (Fd1) that is reduced by PSI
while in the root it uses a form of ferredoxin (Fd3) that has a less
negative midpoint potential and can be reduced easily by NADPH. In non photosynthesizing tissues, NADPH is generated by glycolysis and the pentose phosphate pathway.
In the chloroplasts, glutamine synthetase incorporates this ammonia as the amide group of glutamine using glutamate as a substrate. Glutamate synthase (Fd-GOGAT and NADH-GOGAT)
transfer the amide group onto a 2-oxoglutarate molecule producing two
glutamates. Further transaminations are carried out make other amino
acids (most commonly asparagine) from glutamine. While the enzyme glutamate dehydrogenase
(GDH) does not play a direct role in the assimilation, it protects the
mitochondrial functions during periods of high nitrogen metabolism and
takes part in nitrogen remobilization.
pH and Ionic balance during nitrogen assimilation
Different plants use different pathways to different levels. Tomatoes take in a lot of K+
and accumulate salts in their vacuoles, castor reduces nitrate in the
roots to a large extent and excretes the resulting alkali. Soy bean
plants moves a large amount of malate to the roots where they convert it
to alkali while the potassium is recirculated.
Every nitrate ion reduced to ammonia produces one OH− ion.
To maintain a pH balance, the plant must either excrete it into the
surrounding medium or neutralize it with organic acids. This results in
the medium around the plants roots becoming alkaline when they take up
nitrate.
To maintain ionic balance, every NO3− taken
into the root must be accompanied by either the uptake of a cation or
the excretion of an anion. Plants like tomatoes take up metal ions like K+, Na+, Ca2+ and Mg2+ to exactly match every nitrate taken up and store these as the salts of organic acids like malate and oxalate. Other plants like the soybean balance most of their NO3− intake with the excretion of OH− or HCO3−.
Plants that reduce nitrates in the shoots and excrete alkali from
their roots need to transport the alkali in an inert form from the
shoots to the roots. To achieve this they synthesize malic acid in the
leaves from neutral precursors like carbohydrates. The potassium ions
brought to the leaves along with the nitrate in the xylem are then sent
along with the malate to the roots via the phloem. In the roots, the
malate is consumed. When malate is converted back to malic acid prior to
use, an OH− is released and excreted. (RCOO− + H2O -> RCOOH +OH−)
The potassium ions are then recirculated up the xylem with fresh
nitrate. Thus the plants avoid having to absorb and store excess salts
and also transport the OH−.
Plants like castor reduce a lot of nitrate in the root itself,
and excrete the resulting base. Some of the base produced in the shoots
is transported to the roots as salts of organic acids while a small
amount of the carboxylates are just stored in the shoot itself.
Nitrogen use efficiency
Nitrogen use efficiency
(NUE) is the proportion of nitrogen present that a plant absorbs and
uses. Improving nitrogen use efficiency and thus fertilizer efficiency
is important to make agriculture more sustainable, by reducing pollution (fertilizer runoff) and production cost and increasing yield. Worldwide, crops generally have less than 50% NUE. Better fertilizers, improved crop management, selective breeding, and genetic engineeringcan increase NUE.
Nitrogen use efficiency can be measured at various levels: the
crop plant, the soil, by fertilizer input, by ecosystem productivity,
etc. At the level of photosynthesis in leaves, it is termed photosynthetic nitrogen use efficiency (PNUE).
Agricultural biotechnology, also known as agritech, is an area of agricultural science involving the use of scientific tools and techniques, including genetic engineering, molecular markers, molecular diagnostics, vaccines, and tissue culture, to modify living organisms: plants, animals, and microorganisms. Crop
biotechnology is one aspect of agricultural biotechnology which has
been greatly developed upon in recent times. Desired trait are exported
from a particular species of Crop to an entirely different species. These transgene
crops possess desirable characteristics in terms of flavor, color of
flowers, growth rate, size of harvested products and resistance to
diseases and pests.
History
Farmers have manipulated plants and animals through selective breeding
for decades of thousands of years in order to create desired traits. In
the 20th century, a surge in technology resulted in an increase in
agricultural biotechnology through the selection of traits like the
increased yield, pest resistance, drought resistance, and herbicide resistance. The first food product produced through biotechnology
was sold in 1990, and by 2003, 7 million farmers were utilizing biotech
crops. More than 85% of these farmers were located in developing
countries.
Crop modification techniques
Traditional breeding
Traditional crossbreeding has been used for centuries to improve crop quality and quantity. Crossbreeding
mates two sexually compatible species to create a new and special
variety with the desired traits of the parents. For example, the honeycrisp apple
exhibits a specific texture and flavor due to the crossbreeding of its
parents. In traditional practices, pollen from one plant is placed on
the female part of another, which leads to a hybrid that contains
genetic information from both parent plants. Plant breeders select the
plants with the traits they're looking to pass on and continue to breed
those plants. Note that crossbreeding can only be utilized within the
same or closely related species.
Mutagenesis
Mutations can occur randomly in the DNA of any organism. In order to create variety within crops, scientists can randomly induce mutations within plants. Mutagenesis uses radioactivity to induce random mutations in the hopes of stumbling upon the desired trait. Scientists can use mutating chemicals such as ethyl methanesulfonate, or radioactivity to create random mutations within the DNA. Atomic gardens
are used to mutate crops. A radioactive core is located in the center
of a circular garden and raised out of the ground to radiate the
surrounding crops, generating mutations within a certain radius.
Mutagenesis through radiation was the process used to produce ruby red grapefruits.
Polyploidy
Polyploidy
can be induced to modify the number of chromosomes in a crop in order
to influence its fertility or size. Usually, organisms have two sets of chromosomes, otherwise known as a diploidy.
However, either naturally or through the use of chemicals, that number
of chromosomes can change, resulting in fertility changes or size
modification within the crop. Seedless watermelons are created in this manner; a 4-set chromosome watermelon is crossed with a 2-set chromosome watermelon to create a sterile (seedless) watermelon with three sets of chromosomes.
Protoplast fusion
Protoplast fusion
is the joining of cells or cell components to transfer traits between
species. For example, the trait of male sterility is transferred from
radishes to red cabbages by protoplast fusion. This male sterility helps plant breeders make hybrid crops.
RNA interference
RNA interference (RNAIi) is the process in which a cell's RNA
to protein mechanism is turned down or off in order to suppress genes.
This method of genetic modification works by interfering with messenger
RNA to stop the synthesis of proteins, effectively silencing a gene.
Transgenics
Transgenics
involves the insertion of one piece of DNA into another organism's DNA
in order to introduce new genes into the original organism. This
addition of genes into an organism's genetic material creates a new
variety with desired traits. The DNA must be prepared and packaged in a
test tube and then inserted into the new organism. New genetic
information can be inserted with gene guns/biolistics.
An example of a gene gun transgenic is the rainbow papaya, which is
modified with a gene that gives it resistance to the papaya ringspot virus.
Genome editing
Genome editing
is the use of an enzyme system to modify the DNA directly within the
cell. Genome editing is used to develop herbicide resistant canola to
help farmers control weeds.
Improved nutritional content
Agricultural
biotechnology has been used to improve the nutritional content of a
variety of crops in an effort to meet the needs of an increasing
population. Genetic engineering can produce crops with a higher
concentration of vitamins. For example, golden rice contains three genes that allow plants to produce compounds that are converted to vitamin A in the human body. This nutritionally improved rice is designed to combat the world's leading cause of blindness—vitamin A deficiency. Similarly, the Banana 21 project has worked to improve the nutrition in bananas to combat micronutrient deficiencies
in Uganda. By genetically modifying bananas to contain vitamin A and
iron, Banana 21 has helped foster a solution to micronutrient
deficiencies through the vessel of a staple food and major starch source
in Africa. Additionally, crops can be engineered to reduce toxicity or
to produce varieties with removed allergens.
Genes and traits of interest for crops
Agronomic traits
Insect resistance
One highly sought after trait is insect resistance.
This trait increases a crop's resistance to pests and allows for a
higher yield. An example of this trait are crops that are genetically
engineered to make insecticidal proteins originally discovered in (Bacillus thuringiensis).
Bacillus thuringiensis is a bacterium that produces insect repelling
proteins that are non-harmful to humans. The genes responsible for this
insect resistance have been isolated and introduced into many crops. Bt corn and cotton are now commonplace, and cowpeas, sunflower, soybeans, tomatoes, tobacco, walnut, sugar cane, and rice are all being studied in relation to Bt.
Herbicide tolerance
Weeds
have proven to be an issue for farmers for thousands of years; they
compete for soil nutrients, water, and sunlight and prove deadly to
crops. Biotechnology has offered a solution in the form of herbicide
tolerance. Chemical herbicides are sprayed directly on plants in order
to kill weeds and therefore competition, and herbicide resistant crops
have to the opportunity to flourish.
Disease resistance
Often, crops are afflicted by disease spread through insects (like aphids).
Spreading disease among crop plants is incredibly difficult to control
and was previously only managed by completely removing the affected
crop. The field of agricultural biotechnology offers a solution through
genetically engineering virus resistance. Developing GE
disease-resistant crops now include cassava, maize, and sweet potato.
Temperature tolerance
Agricultural
biotechnology can also provide a solution for plants in extreme
temperature conditions. In order to maximize yield and prevent crop
death, genes can be engineered that help to regulate cold and heat
tolerance. For example, tobacco plants have been genetically modified to
be more tolerant to hot and cold conditions, with genes originally
found in Carica papaya. Other traits include water use efficiency, nitrogen use efficiency and salt tolerance.
Quality traits
Quality
traits include increased nutritional or dietary value, improved food
processing and storage, or the elimination of toxins and allergens in
crop plants.
Common GMO crops
Currently, only a small number of genetically modified crops
are available for purchase and consumption in the United States. The
USDA has approved soybeans, corn, canola, sugar beets, papaya, squash,
alfalfa, cotton, apples, and potatoes. GMO apples (arctic apples) are non-browning apples and eliminate the need for anti-browning treatments, reduce food waste,
and bring out flavor. The production of Bt cotton has skyrocketed in
India, with 10 million hectares planted for the first time in 2011,
resulting in a 50% insecticide application reduction. In 2014, Indian
and Chinese farmers planted more than 15 million hectares of Bt cotton.
Safety testing and government regulations
Agricultural biotechnology regulation in the US falls under three main government agencies: The Department of Agriculture (USDA), the Environmental Protection Agency (EPA), and the Food and Drug Administration
(FDA). The USDA must approve the release of any new GMOs, EPA controls
the regulation of insecticide, and the FDA evaluates the safety of a
particular crop sent to market. On average, it takes nearly 13 years and
$130 million of research and development for a genetically modified organism to come to market. The regulation process takes up to 8 years in the United States.
The safety of GMOs has become a topic of debate worldwide, but
scientific articles are being conducted to test the safety of consuming
GMOs in addition to the FDA's work. In one such article, it was
concluded that Bt rice did not adversely affect digestion and did not
induce horizontal gene transfer.
