Hunting for wheat rust resistance amid relentless fungal mutations

By Kerstin Hoppenhaus & Sibylle Grunze

The video below is the third part in a six-part series examining the scourge of Ug99, a type of fungus that causes disease in wheat crops — one that scientists worry could threaten global food supplies. 

In combatting older forms of wheat rust, researchers and plant breeders had mostly relied on introducing single resistance genes to new wheat varieties. These so-called “major genes” are fairly easy to handle for breeders and they provide strong protection — though only while they last. When the pathogen evolves and defeats the resistance gene, the plant becomes susceptible again — and sometimes all it takes is a single mutation. Given the large number of spores that the fungus produces in every reproductive cycle, it is only a matter of time until a major gene becomes ineffective.

This cat-and-mouse game between breeders and pathogens leads to recurring cycles of boom and bust. When a new resistance gene is available, more and more farmers will plant the new resistant wheat variety. But in the background the fungus keeps evolving until, one day, the crucial mutation appears, and the pathogen becomes immune to the resistance gene. This new strain now finds large areas with unprotected plants.

The time between the release of a new wheat variety and its breakdown is currently about three to five years. For this reason, industrialized countries with their established large-scale agricultural systems keep up permanent breeding pipelines for new varieties. Small farmers in Africa and elsewhere, however, can’t do that, which is why many experts argue that better resistance mechanisms would protect wheat over much longer periods of time, and be effective against several newly emerging types of rust disease.

The only way to achieve that, they say, is a complete change of strategy. Instead of single major resistance genes that can provide total protection but are easily overcome, scientists increasingly count on so-called minor genes. In isolation, any one of these can provide some, but not total protection. But when combined, they can form a powerful shield. Instead of one resistance gene, the fungus has to overcome several genes at the same time in order to take hold. This is not impossible, but it does take time. Such a combination of minor genes can often extend the period of protection for the plant from three to five years, to 10 to 15.

Of course, this sort of strategy is much more difficult for breeders to maintain. But if they succeed, they will be able to keep the overall fungal load on a much lower level, and slow down the evolution of new types of pathogens.