Inhibitory effects of polypeptides secreted by the grapevine pathogens Phaeomoniella chlamydospora and Phaeoacremonium aleophilum on plant cell activities

Esca is a devastating disease affecting grapevines all around the world induced by a complex of xylem-inhabiting fungi. In order to elucidate the mechanisms of virulence of the esca-associated fungi Phaeomoniella chlamydospora (Pch) and Phaeoacremonium aleophilum (Pal), the phytotoxic activities of the polypeptides secreted in their culture medium were investigated. Both polypeptide fractions (PFs), which are composed of proteins with an apparent molecular mass ranging from 6 to 250 kDa, triggered the death of grapevine 41BT cells in culture and induced transitory H+ flux modifications and membrane depolarization of cells. Complementary assays with plasma membrane vesicles showed that the H+-ATPase is targeted by these polypeptides, as shown by the inhibition of the enzyme catalytic activity and the decrease of proton conductance of plasma membrane. Due to this impairment, an inhibition of uptake of assimilates occurred. Furthermore, PFs also induced the activation of the plant secondary metabolism as indicated by induction of the anthocyanin synthesis. In addition, PFs acted on key enzyme reactions known to participate to the elicitation process, namely NADPH oxidase and phenylalanine ammonia-lyase (PAL). Considering the differential effect of the PF secreted respectively by Pch and Pal on the latter enzymes, it could be hypothesized that the toxic polypeptides of the two fungi modify the plant cell metabolism by different pathways, hence indicating that the proteinaceous compounds secreted by both esca-associated fungi Pch and Pal may act as virulence factors.