PULLMAN, Wash. — Maurice Ku, a Washington State University botanist,
announced at a conference last month in Manila that he and Japanese
colleagues have developed a strain of rice with increased yields of up to 35
percent. By inserting a gene from maize, the scientists allowed the rice to
overcome the natural inefficiency of rice photosynthesis.

Photosynthesis is the process by which most green plants use sunlight to
manufacture sugars from carbon dioxide and water. Most crop species,
including rice, belong to a category of plants called C3, whose photosynthesis
is limited by the current level of atmospheric carbon dioxide (CO2). Their
photosynthesis machinery evolved at a time when the atmosphere was much
richer in carbon dioxide, when the leakage of carbon dioxide through
photorespiration was not a hindrance to productivity.

Approximately 5 percent of the flowering plants (angiosperms) have developed
a more efficient means of photosynthesis whereby they concentrate CO2 with
a sort of biochemical pump or turbocharger. Through a complex process,
carbon storage molecules in C4 plants acquire an extra carbon atom. Thus the
C4 designation.

C4 plants evolved much more recently, adapting to increasingly lower CO2
concentration in the atmosphere. The current level of CO2 was reached about
20 million years ago.

C4 plants are also more tolerant of drought and heat. Not surprisingly, many
weedy species are C4, as are a few crop plants such as maize (corn) and
sugarcane. Recreating C4 photosynthesis in other crop plants is something of
the holy grail of crop genetic engineering.

Ku, with Makoto Matsuoka of Nagoya University and Mitsue Miyao of the
National Institute of Agrobiological Resources in Japan, had set out to
genetically modify rice to mimic the C4 plants using three genes from maize,
through which they hoped to reconstruct a C4 biochemical pathway. Ku
expected the process, which they are continuing, to take 10-20 years, as the
development requires structural as well as biochemical change.

The process entailed inserting the genes individually in different lines of rice,
screening for a homozygous, or genetically stable, plant — and then using
conventional hybridization techniques to integrate the traits expressed by the
individual lines. Since Ku and his colleagues began, technology has
developed through which more than one gene can be added to a plant
simultaneously.

To the researchers’ surprise and pleasure, the increased maize enzyme activity
by the individual genes alone increased the rice’s photosynthetic activity. Still,
says Ku, in retrospect the development is not that surprising, given the
metabolic function of the enzymes.

Two of the enzymes, PEPC and PPDK, appear to increase the stomatal
conductance of the rice plant. Stomata are the pores on the leaf surface
through which CO2 is drawn in — and through which it normally leaks back out
in C3 plants. This increased conductance provides more CO2 for use by the
plant.

Increased efficiency by C4 plants is due mainly to the C4 biochemical pathway,
which concentrates CO2 in the leaf. The concentration of CO2 in C4 plants is
10-20 times the atmospheric concentration, whereas because of diffusion the
concentration in C3 plants is actually less than atmospheric level.

Ku believes that increased production of organic solutes by the enzymes in
the stomatal cells might be responsible for the transgenic rice’s increased
stomatal conductance. This is because stomates open by pumping up their
levels of solutes.

Preliminary field trials in China and Korea show 10-30 percent increases in
grain yield for the PEPC transgenic rice and 30-35 percent increases for the
PPDK rice. These rice strains have grown through six generations, and the
traits seem stable.

Ku believes the technology can be applied to other C3 crops, such as wheat.
Although the researchers have not tested whether the modified rice requires
more nutrient input, Ku believes it does not. However, it does require more
water.

Nearly half the world’s population relies on rice as a staple. Despite shrinking
agricultural land and water supplies, farmers must produce an additional 6.7
million tons of rice every year to meet the needs of a rapidly growing world
population. This represents more than a 10 percent increase annually.

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