Breeding rust-resistant wheat with DNA technology

CSIRO scientists are breeding new varieties of disease-resistant wheat in an effort to improve crop yields and avert a potential food supply crisis.

In recent years the growth in demand for staple food crops such as wheat, rice and corn has outstripped the growth in supply, causing worldwide concerns about food security.

Population growth, climate change and increased production of biofuels are recognised as key drivers behind an emerging food supply crisis.

Compounding this problem are four wheat diseases – stem rust, leaf rust, stripe rust and powdery mildew – which are threatening worldwide crop yields and grain quality.

Australias crops have been mostly well protected for the past 60 years by breeding rust-resistant varieties, which carry sets of genes called rust resistance genes.
If not for current measures, such as rust-resistant wheat varieties, the magnitude of potential loss could be in excess of A$900 million.

Rust pathogens, however, are very adaptable and can rapidly evolve into new strains that can infect previously rust-resistant plants. For example, in 2002 a new virulent strain of the stripe rust pathogen appeared in Australia and has continued to cause serious annual crop losses ever since.

It is a constant battle for wheat breeders to try to develop new cereal varieties with effective and long-lasting rust resistance.

CSIROs rust resistance research team, headed by plant molecular biologist Dr Jeff Ellis, is part of the Australian Cereal Rust Control Program that is always on the lookout for more effective ways to breed new rust-resistant cereal varieties.

It is vital that we continue to develop more effective controls to combat all four rust diseases to minimise their broader economic and social impacts, says Dr Ellis.
Gene-based crop disease and pest control is highly desirable in plants as it is more environmentally friendly and profitable than alternative solutions like spraying pesticides.

The team is also part of an international fight to control the stem rust strain Ug99 that evolved in Africa and is spreading into Asia, posing a significant threat to worldwide wheat production.

DNA markers for breeding

When developing new plant varieties, plant breeders must combine the most desirable sets of genes and associated traits of two parents into a single new plant variety.

Breeding new resistant crop varieties is often hindered by the slow process of identifying and selecting for resistance genes during the breeding process that produces new and improved varieties.

This process can be sped up through the use of a gene technology tool called DNA marker-assisted breeding.

DNA markers clearly and simply tag the presence of important genes and allow breeders to quickly and accurately identify whether rust resistance genes are present in a plants DNA.

This can save time because it reduces the need for breeders to expose plants to rust strains to assess performance. It also allows simultaneous selection to be applied for multiple different resistance genes during the complex breeding process.

Dr Ellis said CSIRO had identified DNA markers for important stem rust and stripe rust resistance genes that will allow more effective breeding for rust resistance, and had also identified genes used by rust fungi for causing disease.

Cloning rust resistance genes

Dr Elliss team is now working towards cloning rust resistance genes and incorporating them directly into wheat as part of the Grains Research and Development Corporations Triple Rust Initiative.

Recently team members Dr Evans Lagudah and Dr Wolfgang Spielmeyer, together with collaborators in Switzerland and Mexico have cloned an important wheat rust resistance gene called Lr34 that provides durable resistance to leaf and stripe rust and powdery mildew.

In addition to conventional breeding with DNA markers, the team also aims to stack multiple cloned resistance genes into wheat as a disease control cassette, producing genetically modified (GM) wheat varieties with a solid resistance to all three rust types.

Stacking genes is beneficial for improving the plants immune response by having more than one line of defence against different strains of rust.

GM technology enables those genes to be stacked together on a single DNA molecule, or cassette, which stays intact in subsequent breeding and are the key to durability of rust resistance in wheat.

The genes will get transmitted through the breeding process together as a unit rather than fall apart as the conventionally bred gene stack does when breeders make new crosses, Dr Ellis says.

It offers a way of making the breeding process simpler and rust resistance more long lasting.

Once several resistance genes are introduced into a wheat breeding line, CSIRO and partners will do further quality and food safety testing.

If this is successful, CSIRO plans to deliver new super-resistant wheat germplasm to Australian wheat breeders for the development of new rust-resistant varieties for farmers.