RE S EARCH Phytophthora always finds a way to break do F or many years, prof. Francine Govers has been involved in research into the pathology and biology of Phytophthora infestans at Wageningen University. A month ago, she held her inaugural address about her recent findings regarding this pathogen. This was on the occasion of her acceptance of the post of professor at the Laboratory of Phytopathology. This appointment includes the post of doctoral thesis supervisor of trainee research assistants that work in her research group. If you want to annoy the professor, just call Phytophthora a fungus. She will tell you in great detail that Phytophthora is most definitely not a fungus but an oomycete and that is something completely different! Hence the following seven questions about the most feared oomycete in potato country. The Blb2 resistance gene was recently broken down. What does this resistance breakdown mean? We still have a long way to go as far as resistance is concerned. We have made so much progress in our research in Wageningen, that we are able to identify those virulent phytophthora isolates. If we had all the right tools now, we could also do this in the field. The most ideal situation would be if, with the right detection tools, we were able to scan which isolates are around in the Netherlands and on which variety those isolates are virulent, despite crossed resistance. We would then know what to do. Depending on which isolates are found, the grower can adjust his spraying schedule on a particular resistant variety. If no virulent isolates are found, spraying is, of course, not necessary. In practice, only a few growers use resistant potato varieties. This means that most growers have no choice but to spray their crops. The biggest mistake we can make is to start campaigns in which we state that modern methods such as cisgeneses are the solution for late blight. How is it possible that, time and again, Phytophthora succeeds in staying clear of crossed resistances? Until recently, we thought that ´if only we would cross resistances from wild varieties into our Bintje, or introduce genetic modification into them, we can keep Phytophthora at bay’. Regrettably, this idea was mistaken. What we see is that, within a short period of time, the crossed resistances are broken down. This certainly applies to our mono-culture crop cultivation that is highly vulnerable where infectious diseases are concerned. Fifteen years ago, we knew nothing about the molecular basis of resistance genes and how pathogens can break down resistance. Thanks to the uncovering of the phytophthora genome, we now know that Phytophthora has an unlimited capacity to overcome resistances. From every potato resistance gene that we want to utilise, we first have to know how the pathogen is going to react, before starting to use the resistance gene on a large scale. You need to know which gene in the pathogen corresponds with a particular resistance gene and in which way this gene can adapt in Phytophthora. We now have 10 to 15 available resistance genes. Resistance genes which we thought were strong, such as the Blb1 and Blb2, have now been broken down. We also now know the complete protein structure of the accompanying RXLR effectors in Phytophthora, which make the isolates virulent. These Phytophthora RXLR effectors or effector genes are an important subject in your research. Can you explain what they are and how they behave? RXLR effectors are proteins that are made by Phytophthora and which have an effect on the host, the potato plant. Phytophthora has over 560 of these RXLR effector genes. I can tell you that that is quite an army of attack troops and they are very varied into the bargain. What these effectors have in common is an RXLR motif that acts as a kind of postal code. This motif makes sure that the effectors reach their final destination 4 Potato World 2009 • number 4 Pagina 3
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