Since the beginning of agriculture eight to ten thousand years ago,
farmers have been altering the genetic makeup of the crops they grow
Since the beginning of agriculture eight to ten thousand years ago,
farmers have been altering the genetic makeup of the crops they grow.
Early farmers selected the best looking plants and seeds and saved
them to plant for the next year. The selection for features such as
faster growth, higher yields, pest and disease resistance, larger seeds,
or sweeter fruits has dramatically changed domesticated plant species
compared to their wild relatives. Plant breeding came into being when
man learned that crop plants could be artificially mated or crosspollinated to be able to improve the characters of the plant. Desirable
characteristics from different parent plants could be combined in the
offspring. When the science of plant breeding was further developed
in the 20th century, plant breeders understood better how to select
superior plants and breed them to create new and improved varieties
of different crops. This has dramatically increased the productivity and
quality of the plants we grow for food, feed and fiber.
34
Conventional plant breeding (Figure 1) has been the method used to develop
new varieties of crops for hundreds of years. However, conventional plant
breeding can no longer sustain the global demand with the increasing
population, decline in agricultural resources such as land and water, and the
apparent plateauing of the yield curve of the staple crops. Thus, new crop
improvement technologies should be developed and utilized.
Figure 1. Conventional breeding entails sexual hybridization followed by
careful selection
Source: Alfonso, A. 2007
Mutation breeding
The art of recognizing desirable traits and incorporating them into future
generations is very important in plant breeding. Breeders inspect their fields
and travel long distances in search of individual plants that exhibit desirable
traits. A few of these traits occasionally arise spontaneously through a process
called mutation, but the natural rate of mutation is very slow and unreliable to
produce plants that breeders would like to see.
In the late 1920s, researchers discovered that they could greatly increase the
number of these variations or mutations by exposing plants to X-rays and
mutation-inducing chemicals. “Mutation breeding” accelerated after World
War II, when the techniques of the nuclear age became widely available. Plants
were exposed to gamma rays, protons, neutrons, alpha particles, and beta
particles to see if these would induce useful mutations. Chemicals such as
sodium azide and ethyl methanesulphonate, were also used to cause mutations.
Mutation breeding efforts continue around the world today. Of the 2,252
officially released mutation-derived varieties, 1,019 or almost half have been 5
released during the last 15 years. Some varieties of wheat, barley, rice, potatoes,
soybeans, onions and others were produced via mutation breeding with
agronomically-desirable characteristics.
Pure line and hybrid seed technology
The end result of plant breeding is either an open-pollinated (OP for corn) or
inbred (for rice) varieties or an F1 (first filial generation) hybrid variety. OP and
inbred varieties, when maintained and properly selected and produced, retain
the same characteristics when multiplied.
Hybrid seeds are an improvement over OP and inbred seeds in terms of yield,
resistance to pests and diseases, and time to maturity.
Hybrid seeds are developed by the hybridization or crossing of diverselyrelated parent lines. Pure lines are offsprings of several cycles of repeated selfpollination that “breed true” or produce sexual offspring that closely resemble
their parents.
Pure line development involves firstly, the selection of lines in the existing
germplasm which express the desired characteristics such as resistance to pest
and diseases, early maturity, yield, and others. These traits may not be present
in only one line, thus selected lines are bred together by hand. In self-pollinated
plants, flowers are emasculated by removing the anthers or the male part of
the flower by hand, and are pollinated by pollen from another line. The female
parent is usually the line that possesses the desired agronomic trait while the
male parent is the donor of the new trait. F1 (first filial generation) offsprings
are planted and selfed, as well as the F2 generation. Breeders then select
in the F3 and F4 generation the lines which exhibit their desired agronomic
characteristics and the added trait. Testing for resistances to pests and abiotic
stresses are conducted also at this time. Lines with desired traits and are rated
intermediate to resistant/tolerant to the pests and abiotic stresses are selected
and selfed in two to three more generations. Lines which do not lose the new
traits and are stable are termed pure lines and are stable.
In hybrid seed technology, two pure lines with complementing traits and are
derived from diversely related parents are bred together by hand. F1 hybrids
are tested for hybrid vigor in all agronomic and yield parameters and compared
to both parents. The resulting offsprings will usually perform more vigorously
than either parents.
Since the technology has been developed, it has brought tremendous impact
in major crops including rice, corn, wheat, cotton, and other crops including
many vegetables. In the USA, the widespread use of corn hybrids, coupled with
improved cultural practices by farmers, has more than tripled corn grain yields
over the past 50 years from an average of 35 bushels per acre in the 1930s to
115 bushels per acre in the 1990s. No other major crop anywhere in the world
even comes close to equaling that sort of success story.6
Pure Stable
Hybridization
Hybrid rice technology helped China to increase its rice production from 140
million tons in 1978 to 188 million tons in 1990. Research at the International
Rice Research Institute (IRRI) and in other countries indicates that hybrid rice
technology offers opportunities for increasing rice yields by 15-20% beyond
those achievable with improved, semi-dwarf, inbred varieties.
With the proven impact of hybrid seed technology, new tools for hybrid
breeding were discovered and utilized for self-pollinating crops including
cytoplasmic male sterility (cms). Cytoplasmic male sterility is a condition where
the plant is unable to produce functional pollen and would rely on other
pollen source to produce seeds. This greatly facilitates large scale hybrid seed
production, by-passing hand pollination.
Current hybrid seed technology uses three lines in order to produce the hybrid
seed: a) the A line which contains a defective mitochondrial genome in the
cytoplasm and a suppressed restorer gene, b) the B line which is genetically
similar to the A line but contains a normal cytoplasm and a suppressed restorer
gene, and c) the restorer line, a distinctly unrelated line which contains normal
cytoplasm and an active restorer gene (dominant).
The two line hybrid system, another hybrid seed technology relies on
temperature and geographic location affecting the nuclear genome of the
plant, manifested as male sterile. Hybrid seed technology assures hybrid vigor
in the progenies but discovery and development of cms lines requires a lot of
work and time.
Figure 2. Pure line (inbred line) development
Source: Alfonso, A. 20077
Conventional plant breeding resulting in open pollinated varieties or hybrid
varieties has had a tremendous impact on agricultural productivity over the last
decades. While an extremely important tool, conventional plant breeding also
has its limitations. First, breeding can only be done between two plants that
can sexually mate with each other. This limits the new traits that can be added
to those that already exist in that species. Second, when plants are crossed,
many traits are transferred along with the trait of interest including traits with
undesirable effects on yield potential. Agricultural biotechnology is an option
for breeders to overcome these problems.
Sources:
Alfonso, A. 2007. Rice Biotechnology. Presentation during PhilRice R&D. March 13-15,
2007.
Eckart N. A. 2006. Cytoplasmic male sterility and fertility restoration, The Plant Cell 18
(515-517)
Food and Agriculture Organization. 2002. Crop Biotechnology: A working paper for
administrators and policy makers in sub-Saharan Africa.
History of Plant Breeding-
TransgenicCrops/history.html
Hybrid varieties and saving seed
vegetables/seed.html)
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