Closing in on drought proof cereals by Bernie Reppel

Above: Norman Borlaug Center for Southern Crop Improvement (Texas A&M University). Left: Dr Andrew Borrell (DPI Queensland, left), Professor Henry Nguyen (University of Missouri, centre), and Professor Darrell Rosenow (Texas A&M University, right) at Texas Tech University.

SCIENTISTS HAVE moved closer to unlocking genetic codes allowing cereals to cope better with drought, with the formal signing of agreements that link the relatively small Australian sorghum research effort with three major American universities.

The agreements - between Queensland's Department of Primary Industries and the Texas Tech, Texas A&M and Missouri Universities - centre on a characteristic called Stay-Green discovered in some sorghum plants.

Stay-Green - delayed leaf ageing and death - is a trait that helps the sorghum plant continue filling its grain in water-limited situations. It increases resistance to lodging and improves stubble quality, ratooning ability and ground cover. Research trials have shown hybrid sorghums incorporating Stay-Green can out yield those without it by up to 25 per cent under water-limited conditions.

Can it happen with other cereals?

What excites scientists is the possibility of incorporating Stay-Green genes - once they're isolated and cloned - into other major cereals like wheat, barley, maize and even rice.

The QDPI sorghum-breeding program - led by Bob Henzell at the Hermitage Research Station, outside Warwick - has been working for more than two decades on the development of the Stay-Green characteristic for incorporation into commercial sorghum varieties, here their use is now common.

The breeding program is supported by growers and the Federal Government through the GRDC, as is a physiological study of Stay-Green by Hermitage's Andrew Borrell.

Dr Borrell explained how Stay-Green works:

"Grain sorghum plants with Stay-Green are able to take up more nitrogen under water-limited conditions, keeping their leaves green for longer during postanthesis drought (hence they can continue to photosynthesise and fill grain).

"We hypothesise that increased nitrogen uptake by Stay-Green hybrids is a result of greater biomass accumulation during grain filling in response to increased sink demand (higher grain numbers) which, in turn, is the result of increased radiation-use efficiency and transpiration efficiency due to higher specific leaf nitrogen (SLN).

"Delayed leaf senescence resulting from higher SLN should, in turn, allow more carbon and nitrogen to be allocated to the roots of Stay-Green hybrids during grain filling, thereby maintaining a greater capacity to extract N from the soil compared with senescent hybrids."

He said determining the genetic base for Stay-Green in other cereals is possible because of the general kinship of grasses.

"Take wheat, for example. It is a major food crop in the world, yet not as droughtresistant as sorghum. Because of the high level of homology between grasses, it is likely that the genes conferring drought-resistance in sorghum also exist in wheat.

"However, the fact that wheat does not display the Stay-Green phenotype to the same extent as sorghum suggests that the key alleles in wheat may not be nearly as effective. One approach would be to search for high expression of Stay-Green alleles in wild wheat popUlations, then introduce these alleles into modern wheat cultivars.

"This strategy would not involve genetic engineering. Another approach would be to transfer the key alleles from sorghum into wheat via transformation. The advantage of this genetic engineering strategy is that the alleles are already known to confer Stay-Green in sorghum, and would be expected to function similarly in wheat.

"Alternatively, the ineffective alleles in wheat could be modified to enhance Stay-Green expression. However, this approach would involve considerable understanding of the biochemical pathways associated with the trait, and would therefore be a longer-term solution. So there are a number of routes, some involving genetic engineering and some not, to develop a Stay-Green wheat plant.

Overseas collaboration

"The Americans have supplied us with four different isogenic sorghum lines, each containing key chromosomal regions associated with Stay-Green;' Dr Borrell said. "We will be growing those lines in the field - sometimes under a rain-out shelter - to determine the function of each of these chromosomal regions. We may discover genes associated with increased water-use efficiency, reduced rate of leaf senescence, higher leaf nitrogen status or increased nitrogen uptake under drought conditions."

Program 2 Contact: Dr Andrew Borrell 07 4661 2944

Region North