Goat grass' gift to civilization was a suite of glutenin and gliadin alleles that give rise - literally - to the remarkable extensibility of bread wheat dough, and modern bread's soft, airy texture.

Munns says the tauschii genome also endowed bread wheat with a gene controlling sodium transport in root cells that selectively admits potassium (K) ions while excluding sodium (Na) ions.

Kna1 is the source of bread wheat's superior salt tolerance, while its absence explains durum wheat's vulnerability to sodium toxicity.

Dante The wasn't looking for salt-tolerance in his "durum" lines, because he was selecting for rust resistance. He could not have known there was a sodium-exclusion gene on the same chromosome segment - nor that there was a second one on a neighbouring chromosome.

Breeders identified the Kna1 locus in the early 1990s, but the gene remained elusive, because it is present in all bread wheat cultivars, and no deletion mutants were available for comparative DNA analysis.

The's salt tolerant einkorn/durum hybrids provided a crucial clue that allowed Munns and her colleagues to pinpoint and clone the gene from bread wheat, by first cloning its homoeologue from the synthetic 'durums'.

The first gene from the synthetic 'durums' came to light four years ago, as a quantitative trait locus (QTL) contributing to salt tolerance. The gene was called Nax1, for Na exclusion. A second gene was discovered on a neighbouring chromosome, and dubbed Nax2.

"We knew that Nax2 from the 'durum' and Kna1 from bread wheat were physiologically equivalent," Munns says. "We got hold of some chromosome deletion lines, and a gene very similar to Nax2 was found on the same chromosome as the gene known as Kna1. It was even on the same segment of chromosome, so Nax2 is the einkorn homoeologue of Kna1 from the D genome in bread wheat.

"We published it eight months ago in Plant Physiology. We now have a set of three genes - two in the durum wheat, and an extra one in bread wheat. "These are three major salt-tolerance genes in wheat, and physiologically, that's the pathway to improving bread wheat's salt tolerance as well."

Roots to shoots

The CSIRO researchers have already inserted extra copies of Nax1 and Nax2 into bread wheat, using advanced conventional breeding techniques, and will conduct field trials this year to determine if there is an additive effect on salt tolerance.

Nax2 and Kna1 are 94 per cent similar at the DNA sequence level, and Munns' team was not expecting that Nax2 would provide any additive benefit in salt tolerance, because the combined effect would merely be equivalent to over-expressing Kna1.

So they were surprised - and delighted - to find that the plants had 25 per cent less sodium in their leaves than normal bread wheat, equivalent to adding a second, independently acting gene to the three sodium-tolerance genes already present in bread wheat.

So what does Nax2 do differently to Kna1? Using a sodium isotope to track the movement of sodium ions from roots to shoots, Munns' team found that both sodium-transporter molecules were selectively removing sodium ions from nutrients absorbed by the roots before the solutions were sent to the shoots.

"Kna1 and its sister Nax2 works in the older tissues of the roots, pulling sodium out of the water moving up the xylem vessel," she says. "Nax1 also operates in the roots, but it works at the level of the shoots as well. Nax1 removes any residual sodium that makes it to the base of the leaf, so virtually no sodium enters the shoot."

Some of the sequence differences between the genes lie in the promoter region, which may explain why Nax1 is expressed both in roots and shoots.

Munns' team is attempting to over-express the genes in experimental, transgenic barley lines to see if they improve the plants' salt tolerance.

"Barley is already highly salt tolerant, but through a different mechanism," Munns says. "It's not a sodium excluder, and has a high rate of sodium transport to the shoots, so it sequesters sodium instead of excluding it.

"We thought it would be interesting to see if a complementary, exclusion mechanism would improve barley's salt tolerance."

Four years ago the discovery of a QTL for salt tolerance allowed Munns to mark the anonymous gene, so breeders could track it through a conventional hybridisation program - a much quicker and more reliable alternative to measuring sodium concentrations in hybrids.

Collaborating breeders have introduced the gene into a number of durum lines, which are being trialled at a number of sites in NSW, under Ray Hare's supervision. Hare says few wheatbelt soils are free of salt. He cites the case of farmer who planted a sorghum crop on the deep black soils of northern NSW, which are at least four metres deep.

The farmer was astonished when the apparently healthy crop ran out of water and died. Even though there was water deeper in the soil profile, it was inaccessible to the crop because of the presence of a saline layer about a metre beneath the surface.

Similar saline layers occur, usually at much shallower depth, in wheatbelt soils across southern Australia - both durum and bread wheats would benefit from increased salt tolerance.

"Even if you could give growers a five per cent yield increase on these soils, by enhancing their ability to access saline groundwater, it would help," Hare says.

"Rana's glasshouse work shows this is quite achievable. It would be a partial solution to the salinity problem. What we really need is enough rain to wash the salt further down in the soil profile."

Hare believes the transfer of the einkorn Nax1 gene to bread wheat, via The's 'durum' hybrids, exemplifies the enormous potential to improve bread wheat through hybridisation with tetraploid varieties.

He is most interested in east Asian tetraploids, from countries like Korea, China and Japan, which have been isolated from bread wheat's centre of origin in Asia Minor for millennia.

"East Asian tetraploids seem to share a mutation that is absent from western varieties, that facilitates gene flow when they are hybridized with bread wheat," he says. "Chinese spring wheats are widely used in cytogenetic research, because they're so easy to cross."

Salt tolerance is one thing, drought tolerance another. Munns says early single-gene experiments with transgenic wheats have shown that drought hardiness is genetically complex, and will be much more difficult to achieve.