Science - May 29, 2010

Better armed to fight potato blight

Stocks of genetic material for potatoes with sustainable resistance against phytophthora are now available, says plant breeder Evert Jacobsen. He is, however, not allowed to conduct any field trials with these recently acquired resistant genes.

Six dissertations on the potato and its number one adversary will appear shortly. Four of these, from the Laboratory of Plant Breeding, concern potato resistance against phytophthora, a fungus which causes potato blight. Mathieu Pel graduated last week, while Anoma Lokossou, Nicolas Champouret and Estelle Verzaux will graduate in June. Co-promoter Evert Jacobsen is contented with the results. 'We now know much more about the possibilities to ward off phytophthora than four years ago.'

R-genes
Firstly, the researchers have found many resistance genes (R-genes) in wild potatoes and cultivars which can protect the potato against the pathogen. 'In total, we have now isolated 24 R-genes', says Jacobsen. 'Among these are genes of different potato species which look different, but are in fact based on the same resistance gene. If this duplication is removed, you will end up with eight to nine different clusters with resistance genes.'

Secondly, the researchers have found several related avirulence genes (Avr genes) in Phytophthora infestans. These Avr genes produce substances which are recognized by the R-genes in the potato, and this sets off an immune reaction in the potato, which prevents the pathogen from attacking. The PhD students have found Avr genes which can get hold of different defence mechanisms in potatoes. Therefore, if genetic modification can incorporate more of the resistance genes in potatoes to pick out the different avirulence genes, it seems possible, theoretically, to arrive at sustainable resistance.

Field trials

Jacobsen suspects that three to five resistance genes are needed for sustainable resistance. He wants to test this by using field trials to find out which R-genes are penetrated quickest by phytophthora, which are slow to be penetrated, and which are impenetrable. 'We can then shift the R-genes tactically to find the right combination for establishing resistance. What we have in mind is dynamic species development, where a different set of R-genes is used for every generation. We will still have to keep on spraying because phytophthora is a very adaptive and aggressive pathogen.'
Adaptive
Pythopathologist Francine Govers also thinks so. This expert in potato blight will be the promoter in the coming weeks at two graduation ceremonies. 'Klaas Bouwmeester, who will be defending his thesis on Monday, has proven that one of the avirulence genes in Phytophthora infestans which seemed impenetrable has in fact been broken through now', she says. 'P. Infestans is very adaptive. It changes its attack strategy according to the circumstances. You therefore have to assemble your R-genes in potatoes very tactically. Moreover, different types of Phytophthora can cross-breed with one another, as shown by the PhD research conducted by Laurens Kroon. Natural hybrids pose more and more threat for agriculture and the nature.'

Govers is a staunch supporter of field trials and monitoring Phytophthora in the field. 'You should in fact be constantly measuring if Phytophthora possesses abilities to break through resistance.'

Block
Those who oppose genetic modification have however been able to block the field trials by getting the Council of State to suspend the permit application for the trials. 'It takes more effort to carry out field trials nowadays than ten years ago', says Jacobsen. Govers: 'The arsenal with R-genes to fight Phytophthora is growing, but I don't foresee a GMO potato in the Dutch market in the very near future.'

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