© Jupiterimages Corporation.You eat mutants! Everybody does, every day. Genomes
are constantly undergoing variations in DNA
code—mutating—with each generation. Every item
in the grocery store has been touched by genetic
changes, ranging from the minor to the profound.
Today's mammoth tomatoes, for example, can weigh
as much as 1,000 times the weight of their wild ancestors.
Everything about our food, from size to
texture, color, taste, and nutritional content—plus
much, much more—is influenced by plant genes.
For many centuries, humans have deliberately
cultivated plants with desirable
mutations through selective breeding—
choosing plants with the best traits and
breeding them together. Kiwi fruits, originally
hard, unpalatable (but edible) berries
from China, were dramatically altered
through selective breeding in New Zealand
to become the fruit we know today.
Scientists can now delve into plants' genomes
to find the genes behind particular
traits—a much more targeted approach
than selective breeding. Identifying key
genes enables them to influence changes
in plant characteristics more rapidly than
ever before.
Genetic mutations—in plants, animals,
or any living thing—can be good, bad, or
irrelevant for the fitness of the organism
© Jupiterimages Corporation.itself, or for its human uses. Sometimes,
mutations that are beneficial in one way
can be debilitating in another. For example,
celery naturally produces psoralens,
which are irritant chemicals that deter
insects from feeding on the plant. Celery
plants with elevated levels of psoralens
suffer less damage from disease and insects
and appeal more to consumers, and
therefore, were selectively bred in the
1980s. Unfortunately, workers harvesting
or packaging high psoralen-producing
celery developed severe skin rashes as a
result, and the celery strain was subsequently
removed from the market.
Corn: Improving a Critical
Staple Crop
You'd be sorely disappointed to find an
ear of corn's predomestication ancestor,
known as teosinte, on your plate. The
edible portions of that plant consisted of
just 5 to 12 tiny kernels, each encased
in a rock-hard shell. Thousands of years
of selective breeding have produced a
vastly more productive corn—offering
500 or more delectable kernels on each
cob—that has become a staple food in
many cultures.
Image courtesy Nicolle Rager Fuller, National Science Foundation.
Interbreeding modern corn with its
ancestor teosinte has shed some light on
the genetic changes behind corn's domestication,
and even offered clues on when
(6,000-10,000 years ago) and where (in
southern Mexico) domestication occurred.
Scientists have already identified
several genes that are responsible for the
main changes that differentiate corn from
teosinte; these genes have a huge effect
on plant growth, as well as on fruit and
seed formation. This research has advanced
understanding not just of corn,
but of how new physical characteristics
evolve in other plant species.
Armed with advanced molecular research
tools, scientists and farmers are
working to improve corn even more. For
example, researchers recently identified
the genes responsible for corn's vitamin
A content. Inexpensive molecular markers,
used to identify which plants are expressing
a particular gene, are being used
to help farmers selectively breed for
strains that produce increased amounts
of vitamin A. In developing countries,
vitamin A deficiency causes eye disease
in millions of children each year, as well
as many other health effects. Producing
corn with a higher concentration of this
critical nutrient will be a major boon to
public health, particularly in cultures with
a corn-based diet.
Cultivated Mutants:
Romanesco Cauliflower
Image courtesy of Jon Sullivan.
Romanesco cauliflower attracts attention for its
branching, spiraling pattern of growth. Scientists
have traced the origin of Romanesco and related
cauliflowers to selective breeding by 15th-century
Italian farmers. The vegetable's striking shape
highlights the incredible amount of variation that
can occur within a single plant species; studying
such natural variation can provide clues as to
how traits develop in plants.
A head of cauliflower or broccoli is actually
a collection of immature flower buds. Studying
Romanesco and its relatives has provided
clues to how flowers are formed. Researchers
identified a gene that allowed them to induce
Arabidopsis, a weed not closely related to cauliflower,
to produce cauliflower-like structures
instead of flowers. They dubbed the gene responsible
"CAULIFLOWER."
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Manipulation of
Plant Genes
Many people are aware of the debates
about the safety and long-term effects of
genetically modifying food crops. Most are
not aware, however, of the broad range
of approaches genetic modification can
include.
Altering a plant's genes does not
necessarily mean transplanting genes from
another species into the plant's genome.
Often, it can mean using the
variability of the DNA sequences
in a plant's own genes (generated
by the mix of DNA from each of
its parents) to trigger or suppress
certain processes.
© Jupiterimages Corporation.For example,
scientists are searching for ways to
change the time when rice plants
flower. If they are able to trigger
flowering earlier, it could allow
farmers to harvest more crops
within a single year in some areas.
Other scientists are investigating
genes that can make rice plants
produce as much as twice the number
of rice grains per plant.
In other cases, scientists suppress
certain genes to improve
a crop's performance. For example,
leafy greens such as spinach and lettuce
are more productive if flowering
is suppressed. This increase is because
flowering diverts energy from leaf production
to flower and seed production,
slowing the growth of the harvested
parts of the plant.
Another type of manipulation includes
turning a plant's genes on or off to change
its height or shape, which can make crops
sturdier and more resistant to wind and
weather damage.
Genetic Resources
Wild plants that humans don't eat can still
be used as a genetic resource. A dozen
varieties of apples or four types of potato
at the supermarket may seem like a large
selection, but these represent a miniscule
fraction of the thousands of varieties of
apple and potato that exist currently or
have existed historically somewhere on
Earth. Studying the DNA sequence variation
in the genomes of the relatives and
ancestors of today's crops can yield insights
into basic plant processes and improve
crop strains and farming methods.
© Jupiterimages Corporation.For example, humans have cultivated
an enormous variety of tomatoes
through selective breeding, and, more
recently, genetic manipulation. These efforts
have been targeted toward producing
a more marketable and easily transported
fruit. However, some of these
beneficial changes to the tomato's size,
shape, color, sensitivity to bruising, and
rate of spoilage have been gained at the
expense of flavor, fueling an increased interest
in heirloom and other tomatoes
known for their superior taste. Genome
sciences provide valuable tools for scientists
in search of the optimal good-tasting,
but robust, tomato.
Another reason the wild ancestors
of crops are of value is their potential for
holding the cure to future plant diseases
or pests. When a new plant foe emerges,
the more places scientists have to
look for genes that can boost crop
defenses, the higher their chances of
finding the right cure for the disease.
The Plant-Pathogen Arms Race
Image courtesy Yue Jin, USDA.
Between 1950 and 1954, a rust-colored fungus
known as wheat stem rust wiped out as much as
20 percent of the wheat crop across a large swath
of midwestern states.
Following the outbreak, scientists created a
stem rust-resistant wheat strain that quickly became
a favored breed worldwide. But in 1999,
a new strain of the fungus was discovered on
what had been rust-resistant strains in Uganda.
The new stem rust, named Ug99, has many
people worrying about the possibility of a global
wheat catastrophe.
Wheat provides a substantial proportion of
food calories around the globe. It also serves as a
feedstock for cows and other livestock. Given the
current climate of increasing food prices, combined
with the fragile structure of food systems
in developing countries, now would be a particularly
bad time for a new strain of stem rust to
strike. Since its discovery, Ug99 has spread from
Africa to the Middle East and appears to be on a
track headed toward Asia and North America.
Scientists are racing to develop Ug99-resistant
wheat strains. But breeding or engineering
a stem rust-resistant wheat strain is a tiny—and
moving—target. Wheat and stem rust have coevolved
in an epic arms race in which a change
in a single gene in the wheat or the fungus can
make a plant resistant or susceptible to disease.
Genomics research, on both wheat and the
wheat rust fungus, are crucial to fighting Ug99
in this high-stakes race against time.
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The iPlant:
Tools for the Future of
Plant Genomics
Genomics research generates enormous
amounts of data. One goal
is to integrate all of these data to
create the "iPlant." The iPlant is
conceived as a large family of mathematical
models that generate computable
"plants" that would provide
insight on how plants grow and develop
as a system and could be used
to predict plant behavior under a range
of environmental conditions. Such a tool
would allow researchers to do "virtual"
experiments to identify the most promising
avenues of research before doing experiments
in the field. The National Research
Council report
Achievements of the National
Plant Genome Initiative and New
Horizons in Plant Biology recommends the
development of computational tools that
are sustainable, adaptable, interoperable,
accessible, and evolvable to achieve the
goal of the iPlant.
This web page is based on the National Academies' educational booklet
New Horizons in Plant Sciences.
