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

Hybrid breeding has also begun with IRRI’s
new plant type and could give yields of up to 15
t/ha. Golden Rice hybrids are another possibility
for the future. An International Hybrid Rice
Network extends across Asia, involving national
research institutions in Bangladesh, China, India,
Indonesia, the Philippines, Sri Lanka, and
Vietnam.
Researchers in China have used NERICA
lines (see section on “Interspecific rice” below)
as fertility restorers for cytoplasmic male-sterile
lines. Crosses proved that restoration was
controlled by a single dominant gene from the
NERICA lines.
Interspecific rice (“New Rice for
Africa,” or NERICA)
The potential of the cultivated rice species in
Africa (Oryza glaberrima) has been “unlocked”
through the successful crossing of this species
with O. sativa and the development of truebreeding
“interspecific hybrid progenies” (the
NERICA varieties). The yield potential of these
varieties is enhanced by the combination of the
African species’ adaptation to the West African
environment with yield attributes from O. sativa.
In addition, secondary branching on the panicles
(from O. sativa) combined with transgressive
segregation (a form of heterosis) gives NERICAs
more than 400 grains per panicle, compared with
about 250 in O. sativa. These new upland
varieties also combine noninput dependence with
input responsiveness—yielding more grain as
farmers earn more to invest in their crop.
Functional genomics
The rice plant has 12 chromosomes, the tiny
strands of DNA within each cell that hold its
genetic information. Along the chromosomes,
about 50,000 genes make up the genome.
Scientists have been working for several years on
rice gene sequencing: pinpointing each gene and
deciphering DNA sequence structure, variation,
and function. The study is called genomics. The
entire sequence of genes along the rice
chromosomes is being elucidated by various
groups. Syngenta, a multinational agribusiness
corporation, and the Beijing Genomics Institute
published their sequencing of the rice genome in
April 2002. The International Rice Genome
Sequencing Project led by Japan expects to
complete its task by the end of 2002. The
international project, largely supported by governments,
is committed to providing all sequence
information to the public.
The rice genome represents an enormous
pool of information for rice improvement
through marker-aided selection or genetic
transformation. However, a full application of
this wealth of information will not be possible
until the biological functions encoded by the
sequenced DNA are understood. Functional
genomics is the aspect of discovering what the
genes do: how they function, how their functions
combine with those of other genes, and for what
purpose. Thus, functional genomics is expected
to become the engine that drives discovery of
traits and helps solve presently intractable
problems in crop production.
IRRI is in a unique position to contribute to
this study, backed by the vast collection of rice
germplasm that it holds in trust.
One approach to the task is to delete a
particular gene from the plant using chemicals or
irradiation, then examine the plant for missing
characteristics as it grows. IRRI already has a
collection of more than 18,000 of these “deletion
mutants” and the number is growing rapidly. The
Institute is also developing a large collection of
“introgression lines,” plants that carry a wide
range of chromosome segments implanted from
commercially used varieties and wild rice. These
will be used in the discovery of the functional
diversity of the genes, and to understand the
overall genetic, biochemical, and physiological
systems in the plant. The mutants and
introgression lines can be supplied to other
institutions to assist them in the challenging
work of assigning functions to the rice genes.
So far, the functional genomics team at IRRI
has identified several genes giving the plants
enhanced resistance to various types of
pathogens that cause diseases. The team has also
produced plants containing small chromosome
segments from wild rice that confer resistance to
several diseases and pests. In fact, more than 100
genes that can help the plants defend themselves
against pathogens have been found and are
already being used to select better diseaseresistant
varieties. The scientists have also found
introgression lines and mutants that exhibit
variations in growth and yield under water stress.
Such genetic variation is the prerequisite for
selecting better performing germplasm in soil
with too much or too little water. Scientists also
36 Rice almanac