RE S EARCH wn resistance and that is the inside of a plant cell, where they can manipulate the host. It has recently been shown that a few RXLR effectors can suppress the defence in their host to pave the way for an infection. Thanks to this, Phytophthora can penetrate without any problem, withdraw nutrients, sporulate and produce progeny. Each phytophthora species has its own specific set of these dynamic RXLR effectors. This is the scientific answer to the question of why Phytophthora continues to be successful in breaking down the crossed resistances via breeding from wild varieties. Can a potato plant arm itself against these RXLR effectors? When there is a natural balance, the potato plant can block these effectors and that is what it does. The weapons it uses are resistance genes. Each resistance gene produces a resistance protein and each resistance protein has its own characteristic activity. Under normal circumstances, one resistance protein can only block one isolate from one type of virus, fungus or oomycete. The many wild potato varieties, like those that grow in the Andes, have a long series of different resistance proteins that can block Phytophthora, so that dissemination is hindered. As I said before, we have mono-cultures here. Moreover, since the two years that we have known the genome sequence of Phytophthora infestans, we know that almost 75 percent of the genome consists of repetitive DNA. The genes in the genome are thus able to jump from one spot to another, and it is possible that this also changes the function. That is what makes Phytophthora so flexible. This is shown in practice once more with the recently discovered isolates that are virulent in varieties with the Blb2 resistance gene. ‘I hear politicians say again and again that the problem of late blight will now soon be solved by Wageningen University’, is what you mentioned in your inaugural address with some scepticism. How long is it going to take? I don’t think that we are far enough yet to be able to say that we can take some resistance genes, put them into a potato and there we are: a problem solved. It is also not credible if you put promising resistant potato varieties on the market and their resistance is broken down soon afterwards. 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. It is my view that we first have to think carefully about a balanced strategy. Only when we know exactly how the RXLR effectors in Phytophthora infestans work, can we get more of a grip on resistance. That is the message I would like to pass on to the politicians. It may be that DNA markeroriented technology offers a solution for the future, that’s true, but we have to keep on monitoring. How does Phytophthora react to that specific resistance gene you work with in practice? Is the resistance strong or can Phytophthora easily break it down? In order to know that, it is essential to have much more knowledge about the RXLR effectors in Phytophthora. You don’t dare forecast yet when the phytophthora monitoring system you so much want will be operational. Why not? Because, for each promising resistance gene, the accompanying RXLR effector must be traced. Moreover, we need to find out which variants of the effector occur elsewhere in the total Phytophthora infestans population in the world, and which variants are being recognised by the resistance gene and which ones are not. What we would like to have soon is a tailor-made DNA diagnosis chip, with which we can check which RXLR effectors are present in the phytophthora strains that are in the field. If we have that information we can forecast which varieties are going to be susceptible to an epidemic and which ones are not. If we find an isolate that a variety cannot deal with, then the only option that remains is spraying. If a variety has no problems, then the grower can simply leave his sprayer in the shed without any concerns whatsoever. There is, however, one more hurdle ahead: financing. We don’t have guaranteed financial backing yet. However, that financing is of the utmost importance. Without any further knowledge of the effectors in Phytophthora, projects such as DuRPh are also meaningless and we won’t ever be able to achieve a sustainable resistance to late blight. This is a process that can be compared to the construction of the North-South line of the subway in Amsterdam. The Veerman Committee recently concluded that, if Amsterdam stops now, a lot of money would go down the drain, and the town would be further away from its goal than if it continues. That is exactly the same situation as with Phytophthora. Considering all your knowledge about late blight, do you think that there will ever be a potato variety that will be lastingly resistant to Phytophthora infestans? As long as resistance is based on resistance genes, I think that Phytophthora will always be able to find a way to break that down. If there is more than one resistance gene in a potato, it will no doubt take longer to break these down. And it would take even longer if the potato had something that, in its turn, could attack Phytophthora. So, the strategy for the future is to work from different angles at the same time. It may not actually be possible to kill late blight completely, which is why we have to continue looking for new strategies regularly. ● Leo Hanse Potato World 2009 • number 4 5 Pagina 4

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