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A high-yielding rice plant which does not fall over in bad weather has been
created by a team of researchers. Their approach could help plant breeders
develop more productive cultivars of rice ? the crop that provides nearly a
quarter of the world?s calories - without the need to use genetic
modification technology, June 2005 by Anna Gosline.

Breeding short, sturdy and high yield cereal crops ? a hallmark of the
?Green Revolution? in the 1960s ? has often been credited with saving the
world from starvation. And in recent years, plant biologists have begun to
unravel the genetics behind these salvation cultivars, with hope to improve
them further - and faster.

?Generally speaking, it takes over 10 years to produce a new variety by
conventional selective breeding. However, if we can use molecular markers
linked with the gene controlling the trait, we can dramatically reduce time
and laborious human work,? says Makoto Matsuoka at Nagoya University, Japan,
one of the team.

The genes uncovered to date have been those referred to as ?dwarf? genes,
which are linked to growth hormone pathways. Stubbier plants are less likely
to topple over in bad weather and often devote their remaining energy into
grain production.

Matsuoka?s team, lead by Motoyuki Ashikari and Hitoshi Sakakibara, wanted to
look directly at the genes for increased yield. They used two varieties of
rice: Japan?s favourite rice, called Koshihikari and a shorter, more
productive variant called Habataki.

Seed growth hormone

They began by investigating a stretch of the rice genome, called a
quantitative trait loci (QTL), which previous breeding experiments had shown
to control the number of grains produced on a rice branch. They found that
the QTL consisted of two genes, Gn1aand Gn1b. Cross referencing the
sequences to the published rice genome, revealed that Gn1a matched the gene
for cytokinin oxidase OsCKX2 ? an enzyme that breaks down the seed growth
hormone cytokinin.

The higher-yield Habataki cultivar seemed to have less of the enzyme which
breaks down cytokinin, likely leading to its 306 grains per branch, compared
with the Koshihikari?s 164. But when they cloned the full Gn1 Habataki
region and inserted into the Koshihikari genome, its grain number jumped to
237 per branch ? a 45% increase.

And to strengthen the now top-heavy Koshihikari plant, they also inserted a
gene from a dwarf variety which reduced height and beefed up stalk strength.
This combination saw a smaller rise of 26% in grain number. However, the
plant?s increased sturdiness makes it more practical as a crop.

Harsh environments

Plant breeders the world over are likely to immediately search for similar
genes in wheat or corn, says Peter Hedden, a plant geneticist at the
Rothamsted Research in Hertfordshire, UK. ?I think that now in cereals, we
are coming to a limit using the Green Revolution genes. New approaches are
need. And that?s what this research really has.?

He cautions, however, that rice has a very simple genome and other species
such as wheat, which has six sets of chromosomes, may prove trickier. But
the efforts are well worth it, he says because searching for natural gene
variations means more productive plants can be bred without having to
genetically modify them.

But while Matsuoka and his colleagues support the use of GM, the team plans
to next use wild varieties of rice, which often have much greater tolerance
to harsh environments, in order to cross- breed hardier rice. ?The
long-range target is combining all these useful traits into one cultivar to
produce the ideal rice,? he says.

[www.newscientist.com]

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