Cross-talk between osmosensing and cAMP signalling in Botrytis cinerea

Fungi rapidly adapt to their environment involving to signalling pathways like those of mitogen activated protein kinases (MAPKs). The osmotic signal transduction (ST) cascade in the grey mold agent Botrytis cinerea is controlled by the sensor histidine-kinase Bos1 and the Hog1-like MAPK Sak1. It is involved in the adaptation to various stresses (osmotic, oxidative, cell-wall) including some fungicides, but also in fungal development and pathogenicity. Since previous studies highlighted links of the osmotic signalling pathway to other ST cascades we undertook a comparative proteomics study between the bos1, sak1 mutants and the parental wild-type grown under in vitro conditions.

Applying shot-gun proteomics to mycelial extracts of the three strains, we detected 2425 proteins out of which 638 showed significant differences in abundance among the strains. While 275 proteins were clearly co-regulated by Bos1 and Sak1, 150 were under control of Bos1 solely and 213 under Sak1 control. Among this last category of proteins we saw enrichment in ST proteins. Particularly, proteins of the G-protein, cAMP and calcium ST pathways were affected by Sak1.

Additional data from partial phosphoproteomics after fungicide exposure corroborate these interactions through the identification of a differentially phosphorylated phosducin-like protein and the calcineurin responsive transcription factor Crz1.

Phosducin A (PhnA) is involved in G-protein signalling and hence cAMP pathway. Functional analysis of phnA revealed its involvement in development, pathogenicity and adaptation to cell wall stress, some phenotypes common to ∆sak1. We did not observe any influence of PhnA on Sak1 phosphorylation, but increased cAMP-levels in the ∆sak1 mutant. This result is in agreement with our proteomic data.

In conclusion, our study showed unexpected interaction in B. cinerea between the osmotic ST MAPK and the cAMP pathway that may explain several phenotypes of the ∆sak1 mutant. How this interaction operates remains to be established.