Drought tolerance gene identified in wheat

In the nick of time, climatically speaking, an Australian National University research team has cloned a plant gene that could markedly increase water-use efficiency in major food crops, including cereals.

Dr Josette Masle, Scott Gilmore and Prof Graham Farquhar identified the role of the ERECTA gene by screening Arabidopsis inbred lines and mutant plants.

They reported their discovery in an advance on-line publication in the international journal Nature this week.

Masle said her team has confirmed ERECTA is a major contributor to water use efficiency, and detected ERECTA orthologs in both dicot and monocot crop species, including cereals.

CSIRO Plant Industry wheat breeder Dr Richard Richards said the ERECTA gene is a strong candidate to explain at least some of the exceptional drought tolerance of the CSIRO-developed semi-dwarf wheat cultivar 'Drysdale', released in 2002, which provides a 10 per cent yield advantage over conventional cultivars in dry years.

Carbon discrimination

'Drysdale' emerged from a seminal discovery by Farquhar and Richards two decades ago that wheat lines differ significantly in their 'preference' for the two major carbon isotopes -- C12 and C13 -- assimilated in atmospheric carbon dioxide during photosynthesis.

Using mass spectrometry to detect the vanishingly small mass differences between the two carbon isotopes, Farquhar discovered that 'non-discriminators', which assimilate the two carbon isotopes in the ratios found naturally in the atmosphere, have superior drought tolerance to cultivars that 'prefer' the lighter C12 isotope.

Masle and colleagues at the ANU were screening recombinant inbred Arabidopsis lines for quantitiative trait loci (QTLs) associated with the carbon discrimination trait, and found a QTL on chromosome 2. That QTL mapped to a region that incorporated the ERECTA gene. The ANU team already had mutant plants which, Masle had earlier noticed, exhibited premature wilting.

"I put two and two together," Masle said. "I did direct measurements of water vapour and CO<->2</-> fluxes in these lines, and showed that where the gene was knocked out or knocked down, there was an increase in transpiration relative to photosynthesis."

Masle confirmed the association with a complementation assay, in which she introduced a wild-type ERECTA gene from more water use efficient Arabidopsis lines into the wilting mutants.

It not only restored their normal water use efficiency, it re-established the normal phenotype of drought-tolerant Arabidopsis: a compact habit, shorter stems and stubby siliques or 'pods.'

Understanding drought tolerance

The ERECTA gene codes for a protein kinase receptor, one of a very large and complex family of signalling molecules called protein kinases, and their receptors, which mediate plants' responses to disease, predation and stress.

Masle says ERECTA has marked effects on leaf organogenesis. It reduces the density of stomata on the leaf undersurface -- plants lose water through their open stomata, while absorbing carbon dioxide. Fewer stomata means reduced transpiration, and improved water economy.

In addition to the drought-tolerant plants' compact, robust habit, their leaves have more numerous and closely packed mesophyll cells, which form the spongy inner layer housing the leaf's photosynthetic organelles, the chloroplasts.

The gene is expressed in apical meristem tissues, at the tips of the plants' new shoots.

"We want to understand, at the molecular level, how this gene is working, and identify its partners and the pathways it regulates," Masle said.

"We now plan to do experiments in silencing and over-expressing ERECTA, or looking for allelic variation in its sequence, to see if it performs the same function in key crop species, including monocots like wheat, rice and maize."

Adding value in dry conditions and favourable conditions

CSIRO's Richards describes the cloning of the gene as "very exciting", and is looking forward to checking for an overactive allele in 'Drysdale', whose drought tolerance in dry years is winning over growers in NSW.

CSIRO has recently released a sister variety, Rees, bred for northern NSW and southern Queensland, and has others ready for release in WA and south-eastern Australia.

"The nice thing about this discovery is that if we increase its expression it could be very valuable under dry conditions as global warming increases evapotranspiration and pressure on water resources.

"But if we move it in the opposite direction, we could greatly increase yields under favourable conditions where there is no water stress."

Richards said gene technology could be used to move selected alleles of the gene into wheat and other crop species, but the wide range of drought tolerance already exhibited by wheat and other major crops suggests similar results could be achieved by using conventional breeding to exploit existing variability in the ERECTA gene.

He said it is very likely that perennial plants in the arid zone have highly efficient versions of ERECTA, that could be imported into crop species via gene technology.