Joining forces to defeat wheat disease

28th Feb 2012

Rusts never sleep" warned the Nobel Prize winner Norman Borlaug, referring to the crop-destroying fungi that rank among humankind's most formidable agricultural foes.

A prescient warning it remains – Borlaug was father of the 1940s-70s 'green revolution' that saved millions from starvation by developing new varieties of wheat that were resistant to these rust diseases. While always present, the spectre of fungal attack has returned to haunt cereal growers the world over in the form of rusts that can destroy wheat varieties that were until very recently still immune to the major rust pathogens.

To fight back, researchers in the UK and Africa, through the SARID programme, have been collaborating and developing research capabilities in Africa to breed a new crop that is once again a step ahead of the deadly rust diseases. At stake is nothing less than the food security, wellbeing and wealth of millions of people across both the developed and developing world.

New enemies
Stem and stripe rusts are major constraints of wheat production. Because wheat is the third largest yielding and produced crop in the world – as well as covering more land than any other crop – problems with wheat production are massive global issues that affect local trade and wider economies.

"The breakdown of a major rust-resistance gene in China in 1990 resulted in wheat yield losses of 2.65 million metric tonnes," says Dr Lesley Boyd, Project Leader at the John Innes Centre (JIC), which receives strategic funding from the Biotechnology and Biological Sciences Research Council (BBSRC). "While a second stripe rust epidemic in 2002 caused a loss of 1.31 million metric tonnes."

Such attacks can make the difference between a country being an importer or exporter of food. South Africa used to be a net exporter of wheat, but the appearance of stripe rust in 1996 contributed to a decline in wheat outputs and the country subsequently becoming an importer of wheat.

The food-producing world then received a massive scare in 1999 when a new, hyper virulent race (Ug99) of the fungal stem rust pathogen Puccinia graminis appeared in Uganda and rapidly spread to Kenya and Ethiopia.

Ug99 has overcome the rust-resistance gene in wheat (Sr31) which was present in 70% of the world's wheat varieties. Progeny of Ug99 have subsequently overcome other rust-resistance genes (Sr24 and Sr36), thus exposing other wheat varieties to attack. The Ug99 lineage reached Yemen in 2005, Iran in 2008, South Africa in 2010, and will eventually reach all wheat growing regions of the world, including Europe causing millions, if not billions, of pounds worth of damages in reduced yields and lost productivity.

As if that wasn't bad enough, a new and more aggressive race of stripe rust, caused by the fungus P. striiformis, which is able to grow at higher temperatures, was detected in the US in 2000. This race spread to Europe in 2001 and onto Western Australia in 2002, where previously the warmer climate was not conducive to stripe rust infection, thereby becoming the fastest reported spread of a new pathogen race.

In response to this global crisis the wheat international community established the Borlaug Global Rust Initiative on which Boyd and her project collaborator Professor Zakkie Pretorius, University of the Free State, South Africa, are active members.

Battle Plan
Combating the new rusts is a priority. Under the SARID initiative, Boyd and collaborators including Dr Rene Prins in South Africa and Mexico (at the historic CIMMYT centre from where Norman Borlaug worked) set about looking for ways to make wheat resistant to the new rusts.

Breeding resistance into the crop is the best strategy; rusts can be controlled by spraying fungicides but the chemicals are too expensive compared to the relatively small yield returns. "In Kenya 70% of small-scale farmers grow wheat for sale, providing them with a source of cash," says Boyd. "Rust resistant wheat varieties are the only way that small-scale farmers in Africa can achieve a return on a wheat crop where stem and stripe rust are endemic."

Boyd says to stay one step ahead of the pathogen we must find, and genetically and biologically characterise sources of rust resistance which will remain effective over long and large scale usage, unlike race-specific R-genes such as Sr31.

Hence, the Boyd team's strategy has been to identify and biologically characterise new sources of adult plant resistance (APR) for both stripe and stem rust which are effective in Africa (and the UK in the case of stripe rust), and to develop molecular markers for these APR genes for use in marker-assisted breeding programmes.

Opening gambit
Boyd says the work has gone extremely well and they have achieved more than originally proposed.

The first strand of work was to map stripe rust APR genes in fine detail in a South African wheat variety called Kareiga using single changes in the DNA, known as single nucleotide polymorphisms (SNPs), to develop markers that can be used to highlight desirable traits and thus catalyse the selection process.

This represents the introduction of a new marker technology in wheat into South Africa," Boyd explains. "The markers developed provide a valuable resource that South African wheat breeders can use to identify these stripe rust APR genes in their current wheat breeding programmes."

In a second project, researchers completed a genetic mapping and gene identification analysis of stripe rust resistance in the old French wheat variety Cappelle Desprez. Boyd says this has lead to the identification of a number of APR genes which are effective against stripe rust in South Africa, and provided materials that can now be tested to identify which resistance genes from Cappelle Desprez are still effective against stripe (yellow) rust in the UK.

Finally, and perhaps most importantly for the long-term engagement with the enemy, Boyd and collaborators have characterised over 300 wheat genotypes held in the Gene Bank at JIC for stripe and stem (Ug99) resistance in Kenya and South Africa. "These 300 lines were screened in field trials in Kenya over two growing seasons and have undergone initial field trials in South Africa," says Boyd. "From this collection of 300 lines two have initially been selected for further characterisation."

In all of these studies the characterisation involves the development of DNA markers linked to each resistance gene, providing tools whereby each gene can be followed through generations of wheat during breeding programmes. In this way, individual APR genes can be combined together within a single wheat variety, providing effective and superior resistance to stripe and stem rust.

Middle game
The battle against crop pathogens, including the rusts of wheat, is guaranteed to go on as long as these organisms continue to evolve. It makes sense to build domestic research capabilities within developing countries to broaden the wider campaign.

For instance, the studies on stem rust and identification of new sources of durable resistance against Ug99 could not have been undertaken without the field trial facilities at the Kenyan Agricultural Research Institute (KARI) at Njoro, Kenya, funded by the Bill and Melinda Gates Foundation under the Durable Rust Resistance in Wheat (DRRW) I project. This programme has recently received additional funding from DFID to continue its activities.

Furthermore, Boyd says that African researchers and breeders have a tremendous enthusiasm and passion for their work, but are constantly frustrated by the lack of both infrastructural and human resources.

"We now need to support the development of a new generation of self-confident Africans who can provide solutions to problems before they reach a crisis level," says Boyd. To ensure the uptake of new knowledge and technologies it is imperative that the next generation of plant breeders are educated and trained in the value and the implementation of new methods, a philosophy greatly supported by Boyd.

The SARID programme was designed to help strengthen links between UK and Southern research organisations and increase training opportunities for African researchers. Boyd and her colleagues have begun this process, having established the foundations of a knowledge and technology pipeline from the UK science base into South Africa.

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