Pep1 has a role in attenuating plant defences
Pathogenic bacteria and fungi secrete an array of effector proteins to subvert cellular processes in their hosts. In U. maydis, several gene clusters encoding putative effector proteins have been identified, although the function of most of these effectors remains unknown. It had previously been shown that U. maydis mutants in which the pep1 gene is deleted (Δpep1) can form normal penetration structures, but the subsequent establishment of a biotrophic interaction is prevented, suggesting that the effector Pep1 has a role in attenuating plant defences, albeit by an unknown mechanism. Reactive oxygen species (ROS), such as H2O2, are important molecules for plant defence and can either exert direct toxicity at the site of infection or act as second messengers, making the enzymes that produce ROS good targets for inhibition by invading pathogens. Consistent with these observations, the authors found that high levels of H2O2 were present in the penetrated epidermal cells of maize leaves infected with a Δpep1 strain of U. maydis but not in leaves infected with a wild-type strain. To test whether Pep1 could inhibit H2O2 production directly, the authors induced an oxidative burst in maize leaf discs by adding the fungal elicitor chitosan in the presence or absence of purified recombinant Pep1, finding that Pep1 addition almost completely blocked the burst. Next the authors treated maize leaves with ascorbate to scavenge any ROS produced and found that this allowed the Δpep1 strain of U. maydis to enter the leaf tissue and establish an infection.
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