CUTINASE OF NECTRIA-HAEMATOCOCCA (FUSARIUM-SOLANI F SP PISI) IS NOT REQUIRED FOR FUNGAL VIRULENCE OR ORGAN SPECIFICITY ON PEA

A detailed quantitative and microscopic examination of the infection and colonization process of a wild-type strain of Nectria haematococca and cutinase-deficient mutants constructed by transformation-mediated gene disruption was performed. The assays were done under natural conditions. Quantitative analyses were performed by evaluating disease symptoms, measuring plant biomass development and ergosterol content of infected and non-infected plants at different times during the infection process. Qualitative histological observations showed that the wild-type as well as the cutinase-deficient mutants invaded the pea foot by direct penetration of the outer cell layers of epicotyl, hypocotyl, and taproot. After systemic colonization of the root and stem cortex by intra- and intercellular growth wild-type and mutants invaded the vascular system. The infection progressed vertically into the stem by fungal growth into the xylem. The infection of aerial plant parts was restricted to the xylem. This was shown with beta-glucuronidase expressing transformants which were stained selectively in the host tissue. Infection of intact or wounded leaves by wild-type or cutinase-deficient mutants was inhibited by a hypersensitive reaction of leaf cells. After having shown recently that cutinase is not required for pathogenicity on pea (Stahl and Schafer, Plant Cell 4:621-629, 1992), we have now found no evidence that cutinase is a virulence factor of Nectria haematococca.