The role of the proteinase VCP1 produced by the nematophagous Verticillium chlamydosporium in the infection process of nematode eggs

Abstract

The infection process of the nematophagous fungus Verticillium chlamydosporium (isolate Vc1O) was studied in two nematode species that differed in susceptibility according to standardized in vitro assays. After 14 d, V. chlamydosporium infected 9 and 80% of Globodera rostochiensis and Meloidogyne incognita eggs, respectively. The purified proteinase VCPl, from V. chlamydosporium, removed the protein coat from eggs of M. incognita but not of G. rostochiensis. VCPl was more effective at hydrolysing proteins from M. incognita eggs than the related enzymes Prl and proteinase K, whereas the latter two were slightly more active against G. rostochiensis eggs than VCPl. Pretreatment of eggs with VCPl increased susceptibility to V. chlamydosporium from 80–100% in M. incognita eggs and from 9 to 29% in G. rostochiensis eggs. These observations suggest that VCPl is a host specificity-determining factor and also confirm it to be a pathogenicity determinant which works best in concert with other enzymes secreted by the actively growing fungus. Fungal growth on the surface of eggs of both species was commensurate with the infection frequencies observed; growth was limited and fewer appressoria were observed on eggs of G. rostochiensis compared with those of M. incognita. Eggs of M. incognita denatured by heat or killed with sodium azide, like eggs treated with VCPl, were more susceptible than untreated, healthy eggs. In contrast heat and azide treatment of G. rostochiensis eggs did not increase the number of infections. Appressoria were observed on the eggs of both nematode species in all treatments, but the numbers and types varied greatly. On M. incognita, the dominant type was swollen and branched laterally from hyphae, while on G. rostochiensis lateral inconspicuous appressoria prevailed. Differentiation of appressoria on live and dead nematode eggs showed that their production was not limited to the parasitic state of the fungus.