When plants are infested with parasites a sort of arms race is triggered. The plant tries to produce defense molecules as fast as possible to kill the intruder or at least keep it in check. At the same time the parasitic organisms are trying to overcome the plants defense responses or to turn them off altogether. An important parasitic fungus is Ustilago maydis, the cause of the smut disease on maize. Infected plants can be identified by the characteristic black galls. About 150 proteins are secreted by the fungus and are responsible for its nutrition, cell wall modifications and the penetration of the host plant's tissue. One of those proteins -- chorismate mutase -- is able to redirect the plant's metabolism in a way that it produces less salicylic acid, which is required for the production of defense molecules.
Ustilago maydis is an unusual fungus. Outside of plants, for example in petri dishes in a laboratory, its behavior is undistinguishable of that of normal baker's yeast. It grows and occasionally produces offspring by budding off daughter cells. Only at the surface of a plant two compatible cells of U. maydis can merge and enter the plant tissue. At first the fungus only grows in between individual cells, before it starts to infiltrate them with its mycelia. For quite some time now researchers have addressed the question exactly how U. maydis manages to repress the plant's defense response mechanisms. A repertoire of no less than 386 genes, most of them with an unknown function, came into consideration. A team of researchers of eight different research institutes, including four Max Planck Institutes, managed to identify the enzyme chorismate mutase as an important virulence factor.
The chorismate mutase is part of a metabolic network of the plastids that links chorismate to the phytohormone salicylic acid (SA). SA plays an important role in growth, development, nutrient uptake and transport processes in the plant. At the same time SA is part of a signal cascade that ends in the production of defense proteins to fight against attack. Following a different course of action it is also possible to convert chorismate into the amino acid tryptophan, whereby no salicylic acid is produced.
The fungus U. maydis secretes its very own chorismate mutase called Cmu1, which can enter the maize cells by a so-far unknown mechanism. The concentration of chorismate mutase in the cytoplasm rises and stimulates the transportation of chorismate from the plastids into the cytoplasm. But since the synthesis of salicylic acid can only take place in the plastids that are now lacking chorismate as a precursor molecule, the process comes to a halt and the plant's defense response collapses.
The secreted chorismate mutase is probably not the sole initiator of all detected morphological changes of the plant. It is more likely that Cmu1 is part of a cocktail of effector proteins. But because genes for chorismate mutases have been found in many biotrophic plant pathogens and symbionts it seems to be a widely spread strategy for the manipulation of the host's genome.
Materials provided by Max Planck Institute of Molecular Plant Physiology. Note: Content may be edited for style and length.
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