Novel breeding strategy for plant resistance

Disabling certain plant genes instead of adding resistance genes is a promising strategy for giving crops long-term resistance to diseases. Researcher Yuling Bai and professors Evert Jacobsen and Richard Visser from Wageningen UR explain the new breeding strategy in the last issue of Molecular Breeding.

Photo: Left plant, carrying the Mlo S-gene, shows powdery mildew infection on leaves. Right plant, loss-of-function mutant of the Mlo S-gene, shows no powdery mildew infection on leaves. Photograph was taken 17 days post inoculation with tomato powdery mildew, Oidium neolycopersici.

Switching off genes, better known as gene silencing, has been used for many years to improve crop quality, but has not been used to increase resistance of crops to pathogens in order to mimic recessive mutations. The dominant strategy in resistance breeding is to add dominant resistance genes (R genes) into a crop.

Over the past few years, scientists have obtained a better understanding of how pathogens cause diseases in plants. Pathogens exploit effector molecules to interfere with specific genes in the plants. Some of these plant genes play a negative role in plant defense and these genes are so called susceptibility genes or S genes. 'By using these S genes, pathogens reprogram the plant cell', says Bai. 'S genes give pathogens an entrance to the plant. If you switch these genes off, you block the entrance of the pathogen. As a result, the plant becomes resistant.'

The first example of this was found in barley. Researchers found a susceptibility gene (the Mlo gene) for powdery mildew disease. They discovered that this S gene is not functional in barley varieties resistant to powdery mildew. A remarkable aspect was that these varieties had been resistant to powdery mildew for more than thirty years. 'The resistant mutant must have been the result of spontaneous classical breeding, without a good understanding among the breeders of what they were doing', says Jacobsen. 'S genes usually have other functions in plants. Mutation of S genes gives recessive resistance, which is more difficult to use in plant breeding'.

When scientists switched this Mlo S gene off in Arabidopsis, the plant model for genetic research, this plant also became resistant to powdery mildew. Subsequently, Bai found in 2007 that tomato plants become resistant to powdery mildew too, if you silence this susceptibility gene. She expects that this method can be used for many other crops to achieve this type of resistance to powdery mildews.

So far, only four S gene families have been used for many years in resistance breeding. One other example is an S gene found in several crops which helps the spreading of viruses in plants. When this S gene is switched off, the virus cannot spread in the plant anymore. As a result, plants are resistant to the virus. Bai has a fast growing list of potential S genes for different diseases, mainly found in Arabidopsis.

Because the S genes have a function in plant growth or reproduction, silencing or mutation of the genes may have side effects on the plants performance. 'But other plant genes can compensate for these side effects', says Jacobsen. 'That's the art of plant breeding.'

R genes and S genes are the two sides of the same coin of plant disease resistance. R genes combat pathogens by playing positive roles in plant defense mechanisms. In such a battle, R genes often loose their resistance within five years of introduction because of mutations in the pathogen. S genes play a negative role in plant defense. Examples have shown that loss of functions in such S genes caused sustainable resistance to the pathogen, Bai explains.

Jacobsen now wants to investigate whether potatoes have S genes which are involved in the susceptibility to late blight. He hopes to find a combination of S and stacked R genes to develop a more lasting resistance to this tough disease.

The new breeding strategy is still controversial among plant scientists and breeders. 'We have already been discussing this strategy for two and a half years', says Jacobsen. 'Not everybody is convinced of its potential. People say: gene silencing is old, we need resistance genes. But you have to investigate new techniques and strategies - that's the task of a university.'  / Albert Sikkema