Out, damned spot: wheat disease faces genetic nemesis

Wheat stem rust, an inexorable pathogen of Australia's wheat belt since 1925, is destined to be run over by the genetic equivalent of a four-decker bus.

A CSIRO Plant Industry research team, led by Dr Rohit Mago, has identified DNA markers tightly linked to four major stem rust resistance genes imported into the wheat genome from two primitive wheat varieties, and two of wheat's relatives: wheatgrass (Agropyron) and rye (Secale).

They are now working to create a single breeding line, homozygous for all four resistance genes.

It will be game over for stem rust (Puccinia graminis) -- any new cultivar bred from the quadruply protected line will be impervious to infection by any current or future stem rust strain.

The four resistance loci -- Sr24, Sr26, Sr31 and SrR -- individually provide strong resistance to stem rust. Mago said Sr24 is already widely used in Australian wheat cultivars, which typically carry four to six resistance loci.

When Sr26 was imported, it came with a bunch of deleterious genes that reduced yield in resistant varieties.

Collaborating research groups, at the University of Adelaide's Waite Institute, and in the US, used repeated crosses and recombination events to 'prune' the imported chromosome segments to a bare minimum, while retaining the resistance genes.

Mago's team then developed new DNA markers for the pruned chromosome segments, so tightly linked to the Sr26, Sr31 and SrR loci that they virtually guarantee if a DNA marker is present, and so is the resistance gene it marks.

Each new resistance gene imposes intense selection pressure on the rust. In the pattern of the past eight decades, cultivars protected by Sr24 would eventually succumb to a 'breakthrough' mutant, forcing breeders to import a new resistance gene.

According to Mago, the rust will eventually win every battle in this style of serial, gene-for-gene combat. Any mutation that deletes or modifies the component of the rust that induces the resistance response leaves the plant vulnerable.

With another resistance gene like Sr26 to back up Sr24, the odds against two 'breakthrough' mutations occurring simultaneously in two genes in the same rust fungus increase enormously. With three genes the chances of breakthrough are effectively zero; a fourth resistance gene is overkill.

Mago said his team already developed crosses featuring Sr24 paired with Sr31 or Sr26, and will now cross these lines, with the aim of having its first quadruple-whammy cultivar ready for field testing in four years.

The CSIRO team is already moving to employ a similar strategy with the other two major rust pathogens of wheat: stripe rust (Puccinia striiformis), and leaf rust (P. triticina). The triumvirate of rusts cause total production losses averaging AUD$300 million annually in the Australian wheat industry.

Mago said the resistance genes -- more accurately, genetic loci tightly linked to as-yet anonymous genes conferring high-level resistance to rust -- have been identified over the past few decades by Canadian, Australian and European geneticists.

He said the precision of the new DNA markers will halve the time required to coalesce all four genes in a single cultivar. The simple presence of a DNA marker for each resistance gene will confirm the breeding line is resistant, without the need to expose it to rust infection.

The task of coalescing the full range of resistance genes for stem, strip and leaf rust could be completed even more rapidly with recombinant DNA technology, but Mago said public concerns with gene technology would preclude this approach for the time being.