Bumper rice plant created by novel approach
Anna Gosline
New Scientist, 23 June 2005
http://www.newscientist.com/article.ns?id=dn7572

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.

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, Gn1a and 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.

Journal reference: Science (DOI: 10.1126/science.1113373)

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