By Tan Ee Lyn

HONG KONG, Aug 10 (Reuters) Scientists have identified a gene that enables rice to survive for up to two weeks in water, a breakthrough that could herald the development of rice varieties that withstand flooding.

Rice, like most other crops, dies if submerged in water for more than a few days and flooding causes annual losses of over 1 billion dollars, particularly in parts of South and Southeast Asia.

Researchers have been trying to develop rice with ''submergence tolerance'' since the 1970s, and advances in the field of genetics in the 1990s enabled scientists to determine that chromosome 9 in rice somehow responsible for the trait.

Scientists at the International Rice Research Insitute (IRRI) in the Philippines and the University of California have since gone further to identify the gene -- Sub 1A-1 -- in chromosome 9 that confers submergence tolerance in certain rice varieties.

Their findings are in the Aug. 10 issue of the journal Nature; www.nature.com/nature.

''A lot of our work was to delineate the region (in chromosome 9) carefully and we were able to get it down to a region where there were a fewer number of genes,'' said IRRI's David Mackill.

''We found one that looked very promising, Sub 1A-1 gene ...

we were able get a clone of this gene (from the Indian rice variety FR 13A) and then introduced (injected) it directly into an intolerant variety (Liaogeng variety in China), and this has conferred tolerance to that variety,'' Mackill said.

Mackill and his colleagues also managed to introduce the Sub 1A-1 gene into two other rice varieties using a normal breeding method, called marker-assisted selection or hybridisation.

''The resulting plants that expressed that gene became tolerant ... that allowed us to determine that this was the main gene that conferred tolerance,'' Mackill said.

Mackill hopes to see more flood resistant rice varieties in the world in three to five years because of this finding. He and his colleagues have developed submergence tolerant varieties using natural breeding techniques.

''They need to be evaluated in the various regions and then gradually they have to be released to farmers,'' he said.

''If everything goes to plan, we will see them grown in three to five years because they will have to go through two years of testing by governments in national programmes before approval.'' Mackill is not keen on promoting submergence tolerant rice varieties that are created using DNA introduction, or genetic engineering, where the gene is directly injected.

''When you do that, the plant is subject to many kinds of regulation which conventionally bred varieties are not subject to.

Then it takes extra time to have it approved by national programs and released,'' IRRI's Mackill said.

There are strong concerns among environmentalists that genetically modified crops can harm conventional crops.

IRRI is credited for helping the world feed itself by developing high-yielding rice during the so-called Green Revolution of the 1960s, and is helping with work on genetically modified Vitamin A enriched rice, or ''golden rice''.

REUTERS MS BS1006