The MAP kinase BcSak1 of Botrytis cinerea is required for pathogenic development and has broad regulatory functions beyond stress response.

Heller J, Ruhnke N, Espino J, Massaroli M, Collado IG, Tudzynski P

Zusammenfassung

The mitogen-activated protein kinase (MAPK) BcSak1 of Botrytis cinerea is activated upon exposure to H2O2 and hence might be involved in coping with oxidative stress during infection. However, beside osmotic and oxidative stress sensitivity ∆bcsak1 mutants have a pleiotropic phenotype as they do not produce conidia and are unable to penetrate unwounded host tissue. In this study the role of BcSak1was investigated in the stress response and during infection of French beans by Botrytis cinerea. Using a macroarray approach it was shown that BcSak1 is only marginally involved in the specific oxidative stress response. In fact, the induction of several genes after oxidative stress treatment is BcSak1-dependent, but most of these genes are also induced under conditions of osmotic stress. The majority of genes regulated by BcSak1 are not involved in the stress response at all. Using a translational fusion of BcSak1 to GFP, it was shown clearly that the localization of this MAPK depends on the type of stress being applied: it associates rapidly to the nucleus only under osmotic stress. Therefore a model is proposed in which BcSak1 acts in the cytosol by activation of (a) transcription factor(s) under oxidative stress, and at the same time it reacts to osmotic stress by migrating to the nucleus. Interestingly, the MAPK is also involved in the regulation of secondary metabolism, as the major phytotoxins secreted by this fungus are reduced in the ∆bcsak1 deletion mutant. Experiments done in planta underlined the essential role of BcSak1 in the early stages of infection when it translocates to the nucleus and then changes to cytosolic distribution during hyphal growth within the tissue.