Adult plant resistance genes can give cereal crops protection from rust infection.

Adult plant resistance (APR) genes in cereal crops such as wheat, barley and oats can provide effective, lasting protection from rust infection under field conditions - and often also cumulative protection in combination with either seedling or other APR genes.

For wheat, APR genes Lr34/Yr18/Sr57/Pm38, Lr46/Yr29/Sr58 and Lr67/Yr46/Sr55 are widely effective across a range of environments and also confer resistance to multiple diseases (pleiotropic).

However, in some cases, APR (Lr12 and Lr22b) has broken down due to shifts in the virulence of prevailing rust pathogen populations.

In barley, relatively few APR genes have been genetically characterised.

Research initiated by Professor Robert Park and his team at the University of Sydney discovered numerous barley accessions that carried no effective seedling resistance, yet they were highly resistant in the field over numerous seasons.

Adult plant resistance (APR) genes in cereal crops such as wheat, barley and oats can provide effective, lasting protection from rust infection under field conditions - and often also cumulative protection in combination with either seedling or other APR genes.

Researchers have since developed doubled haploid (DH) barley populations, segregating for APR, which led to the discovery and genetic characterisation of three APR genes in barley (Rph20, Rph23 and Rph24) during the past decade.

When present on their own in a rust-susceptible genetic background, the three APR genes confer high (Rph20), moderate (Rph24) and minor (Rph23) levels of resistance to leaf rust respectively.

When two or more of these APRs are present in combination, they provide cumulative resistance, even under high disease pressure.

Fast-tracked breeding

The development and use of molecular marker technologies helps fast-track breeding for rust resistance in cereals.

Marker assisted selection (MAS) is a highly effective method, used in combination with traditional phenotypic selection, to combine resistance alleles in high-yielding elite cultivars.

For the APR gene - Rph20 - a predictive dominant marker was developed as part of collaborative research with the University of Queensland.

When the targeted locus or section of DNA was formally catalogued, however, researchers found the marker was not suitable for MAS in terms of segregating early generation material.

But more recently, University of Sydney researcher Dr Peter Dracatos has developed and validated (co-dominant) predictive markers for all three APR genes, which are suitable for MAS application.

The development and use of molecular marker technologies helps fast-track breeding for rust resistance in cereals.

Using these predictive markers, both Rph20 and Rph24 APR genes were found in combination, at high frequencies, across diverse germplasm, especially within European and Australian cultivars.

However, Rph20 was not present in African and Eastern European accessions and Rph23 was identified in only a few accessions.

Two German barley cultivars (Lenka and Volla), which are immune to leaf rust in the field, tested positive for all three APR markers, confirming the importance of these genes in providing leaf rust control, especially in combination.

Australian barley cultivars that carry both Rph20 and Rph24 APR genes include: Cosmic, Corvette, Dove, Henley, Quickstar, Starmalt and Shepherd (PBR).

Although many Australian barley cultivars have high levels of APR, diversification and characterisation of new sources of leaf rust resistance are still needed.

These new genetic tools are being used at the University of Sydney as part of international barley germplasm screening to identify and prioritise new sources of APR.

Despite the high levels of resistance achieved, particularly when all three APR genes are present, this cumulative effect is often enhanced by the presence of effective seedling resistance genes.

The overall aim of our research is to develop a toolbox of diverse, effective resistance genes, including diagnostic, high-throughput markers to help breeders more efficiently incorporate enduring rust resistance into barley cultivars.

GRDC Research Code UOS1707-003RTX

More information: Robert Park, 02 9351 8806, Robert.park@sydney.edu.au; Peter Dracatos, peter.dracatos@sydney.edu.au; Davinder Singh, davinder.singh@sydney.edu.au