Maclean et al. - 2002 - Rice almanac source book for the most important e

precursors of vitamin A—beta-carotene and
other carotenoids—in its seeds. Rice plants
already have the pathway for producing betacarotene
in their vegetative tissues; by genetic
engineering, genes for beta-carotene driven by
endosperm-specific promoters express betacarotene
(provitamin A) in rice seeds.
National institutes and IRRI are working
together in finding suitable, well-known local
varieties in which to transfer the new pathway.
The resulting new lines will be released to
national agricultural research centers, which will
undertake their own research on the safety and
effectiveness of these vitamin-enriched varieties
in combating vitamin-A deficiency. Each country
will make its own evaluation.
The interspecific varieties (NERICAs)
recently developed by WARDA for West African
upland ecologies have a higher protein content
than either of their parent varieties. They
therefore have the potential to improve the
nutritional status of the subsistence-oriented
farming families who are their target
beneficiaries.
Another new rice variety, one that is
particularly rich in iron and zinc, is awaiting a
large-scale nutrition trial in the Philippines. This
variety is not transgenic and was developed by
exploiting natural variation in rice germplasm.
Iron and zinc are usually deficient in people
eating a rice-heavy diet. Lack of iron causes
widespread anemia in some countries. Zinc
enhances the body’s capacity to absorb iron and
combats diarrhea and cholesterol accumulation
in blood vessels. The new variety, developed by
IRRI, was tested by a group of 27 religious
sisters in Manila, who ate the rice exclusively for
six months, resulting in higher iron levels in their
blood. However, a larger sample is needed to
verify the results and a new trial involving 300
persons is beginning, under the supervision of
two U.S. universities. The trials are organized by
the International Food Policy Research Institute
in Washington, D.C.
Potentially, these two new rice varieties can
help prevent some of the most widespread
micronutrient deficiencies in Asia.
Increasing yield potential
Since the late 1980s, scientists from IRRI and
other institutions have been developing a new
rice plant architecture, one that would increase
yield potential in the tropics from 10 to 12 t/ha
per crop: a fast-growing plant with a deep root
system, dark green erect leaves, and 200 to 250
grains per panicle. It has been a massive task; for
example, a grain-filling problem took three years
of breeding effort to solve. When the number of
tillers had to be increased to deliver a bigger
harvest, the program adjusted to the new goal.
Work on the plant is nearing fruition and in
temperate China it is showing yields of 13 t/ha.
Lines have been developed with resistance to
various pests and diseases. Breeders in different
countries are selecting lines best suited for local
conditions and multiplying seeds for distribution.
It may be four more years before they are
available to farmers.
Meanwhile, global warming is creating new
problems for rice farming and plants are needed
that not only are higher yielding but also will
thrive in higher temperatures, with more carbon
dioxide and pollutants in the air, and where
extremes of weather are commonplace. The
plants will have to use less water and use nitrogenous
nutrients very efficiently. This is an even
more challenging task. There are biophysical
limits to how much grain rice plants can produce,
imposed by the photosynthetic pathway they use
to convert sunlight into organic matter. Only by
changing that pathway can the limit be
overcome.
C 4
rice
In an attempt to improve yields, scientists from
several research centers in both developed and
developing countries are beginning work to
modify the photosynthetic pathway of rice plants
from a C 3
to a C 4
system such as is found in
maize, a more efficient user of sunlight. Most, if
not all, of the genes required for a C 4
system
actually exist in the rice plant, but they are
expressed differently. Importantly, C 4
-type plants
operate well at high temperatures, are extremely
water-efficient, and require less nitrogen, and it
might be these characteristics that will be most
important in a world of climate change. It is
something of an enigma that rice does not
already operate on a C 4
system, given the
advantages of that system. As carbon dioxide
levels increase in the future, the yield gap between
a C 3
plant and a C 4
plant will close, but the
difference in water-use efficiencies will widen.
Nonetheless, this is unlikely to happen before the
end of the 21st century. Improving rice
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