Genetic defence by numbers

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Discovering and characterising new rust resistance genes is a tedious process. Once a gene has been fully characterised, shown to be new and genetically independent of other resistance genes, it is given a number according to internationally accepted rules, for example, Sr2, Lr13, and Yr27, for stem rust, leaf rust and stripe rust resistance respectively, in wheat.

Professor Robert McIntosh from the University of Sydney has coordinated the allocation of these numbers for wheat rust resistance genes since the early 1970s.

While the process of gene discovery has become faster with the advent of DNA marker technology, the efficiency of finding and characterising new rust resistance genes still relies on accurate ‘phenotyping’, or screening host germplasm with a range of characterised rust isolates under controlled greenhouse and field conditions.

Wheat

For common wheat, the pool of diverse sources of resistance includes wild/cultivated relatives and cultivars from other geographic regions and landraces. For example, 55 genes conferring resistance to stem rust have so far been catalogued (from Sr2 to Sr58). While many of these were discovered in common wheat (Triticum aestivum), quite a few were bred into common wheat from grasses such as Thinopyrum ponticum (for example, Sr24 and Sr26), Aegilops ventricosa (Sr38) and from T. timopheevii, a distant relative of durum wheat (for example, Sr36).

While most of the work conducted globally concentrates on understanding the genetic basis of rust resistance in modern wheat cultivars, University of Sydney staff and students have also focused on identifying resistance genes from winter wheats and landraces.

Over the past 10 years they have identified or helped to characterise at least 20 new genes that confer resistance to stem rust, leaf rust or stripe rust of wheat, and five that confer resistance to leaf rust in barley.

Barley

As reported in the July–August 2013 issue of Ground Cover, work by Australian Cereal Rust Control Program (ACRCP) staff on rust resistance in barley has focused principally on leaf rust and on the exotic barley stripe rust.

Work on leaf rust over the past 25 years has targeted understanding the genetic basis of leaf rust resistance in Australian barley cultivars and on characterising new sources of resistance.

This work has led to the discovery of seven genes for leaf rust resistance, including Rph20 and Rph23 that confer adult plant resistance (APR) and were sourced from European and Australian barley germplasm, respectively.

Work is underway to characterise new sources of APR in more than 100 diverse international barleys, some of which are already being used by Australian barley breeders.

While typical barley stripe rust does not occur in Australia, a form known as barley grass stripe rust (BGYR) does have the ability to infect some barley genotypes, including MaritimePBR logo.

The primary focus of work on stripe rust in barley has been to help breeders eliminate lines with vulnerability to BGYR and also to true barley stripe rust by offshore testing in Mexico.

Oats

Attempts to develop crown rust resistant oat cultivars over the past 25 years have met with limited success, with new pathotypes often being detected soon after the release of resistant cultivars. The most recent example was the detection of a new pathotype of crown rust that overcomes the resistance of DroverPBR logo oats (see the May–June 2013 issue of Ground Cover).

Teasing apart the genetic basis of rust resistance in oats is more difficult than in wheat and barley, simply because the genetic tools available for this crop are not as good.

The ACRCP has nonetheless focused attention on a set of more than 200 oats that it has found carry APR to crown rust.

Genetic studies on nine of these have suggested the presence of between one and three genes conferring the APR. Further work is underway to confirm this.

A never-ending story

Plants have evolved genes that can recognise the presence of a pathogen and then switch on defence mechanisms to stop or slow pathogen growth.

These ‘resistance genes’ have been used to protect Australian cereal crops against a range of biotic stresses, especially the rusts. However, like all living organisms, rust pathogens randomly mutate and occasionally a mutant can develop that overcomes a resistance gene.

A successful program of breeding for disease resistance relies on the availability of well-characterised sources of resistance, so that breeders can keep one step ahead of mutating pathogens.

Scientists at the University of Sydney have been discovering and characterising rust resistance genes in cereals since the 1920s. This gene discovery work is the beginning of a pipeline that has provided and continues to deliver many important rust resistance genes to Australian cereal growers.

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GRDC Project Code US00063, US00064, US00067

Region South, North