EFFECTORS TH E WOR LD OF I f you consider the enormous number of diseases like viruses, bacteria and fungi that are able to infect the potato crop one wonders why there is something left for us to eat. Especially if you add many creatures that do not infect them but feed on them such as eating insects – Colorado beetle – or suck their leaves – aphids – or their roots like eelworms or nematodes When a potato is infected with whatever disease, pest or weed one wonders why the potato still exists and why its predators have not looked for some other plant to attack. Alas for the disease but fortunately for the potato crop. There is specificity or a “gene for gene” relationship. We have heard about it but what is it and how does it work? For us working in the potato supply chain we especially wonder if it is of any use. For potato and Phytophthora it means that both organisms have a pair of matching genes: the potato has a resistance gene (R-gene) and the matching Phytophthora gene is called the avirulence (Avr) gene. Upon entering the plant tissue Phytophthora produces a substance, a protein called an effector, that if it is recognized by the potato plant because it has the corresponding R-gene reacts by killing its own cells as soon as infected by Phtyphthora. This is a so called hypersensitivity reaction as is shown in the photo. The oomycete needs living tissue to draw its food from so it dies inside the dead potato cells. The potato only sacrifices a few cells but lives whereas the causal organism of the disease dies. Professor Francine Govers of Wageningen University this year delivered her inaugural address on this relationship between potato and its most devastating disease. Why in nature have not all potatoes disappeared, consumed by Phytophthora and subsequently has the disease not disappeared with it? That is because there is always an “arms race” between the two organisms. Scientists at Wageningen and at the Scottish Crop Research Institute – SCRI – have discovered over seven hundred effector genes in Phytophthora of which three hundred effectively produce effectors that are either or not recognized by potato plants. In order to find out if individual effectors of the disease matches with an individual resistance gene of a potato plant (variety or wild species) the genes responsible for effectors are cloned individually and transferred to agrobacterium. The same is done with resistance genes of the potato. Then the two genes are infiltrated at the same time in a tobacPOTATO R E S EARCH co leaf. If in the tobacco plant a hypersensitivity reaction occurs the potato gene has recognized the Phytophthora gene meaning the potato is resistant. In research where two or more resistance genes are stacked it is important that all three genes act individually to make sure that there is a maximum buffer against the disease. The method described above perfectly shows whether all resistance genes work. Moreover, the dose (concentration) of the effector gene before it shows hypersensitivity reaction tells us something about the level of resistance: the higher the dose the earlier late blight will develop in the field. Presently scientist, if they want to know what kind of blight (pathotype) is attacking their potato, take an isolate of the disease and infect detached leaves of a set of different varieties and wild species (a so called differential set) and from the pattern of infection deduct the pathotype. In future more highthroughput strategies on monitoring resistance genes in potato plants likely will be based on a system using DNA of the effector gene and that of its potato counterpart. ● Dr. Ir. Anton Haverkort anton.haverkort@wur.nl Potato World 2009 • number 3 25 Pagina 24
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