ROLE OF CUTINASE IN THE INVASION OF PLANTS

Cutin as the polymeric component of plant cuticles has to be breached during the direct penetration of plant surfaces by invading fungal pathogens. Several lines of evidence had suggested that cutinases produced under saprophytic growth conditions with cutin as sole carbon source were also involved in the penetration of host surfaces during plant infection. However, retained pathogenicity of cutinase gene disrupted mutants of Magnaporthe grisea and Nectria haematococca was not in support of the model. The controversial involvement of cutinase in plant infection was investigated with Alternaria brassicicola. The fungus produced two cutinase isozymes during saprophytic growth on cutin. The disruption of the single gene CUTAB1 abolished the expression of both isozymes and gene-disrupted transformants lost their trait of utilizing cutin as saprophytic carbon source. However, small amounts of two serine esterases with cutinolytic activities and molecular masses of 31 and 19 kDa continued to be secreted into the extracellular fluid. These two esterases were also expressed on leaves of cabbage during host invasion. In contrast, the two cutinase isozymes expressed by the wild-type strain under saprophytic conditions were not detected on host surfaces. The results indicate that different cutinase isozymes are expressed during saprophytic and parasitic stages of the fungus.