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 a nd 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 cond itions. Quantitative analyses were performed by evaluating disease sym ptoms, measuring plant biomass development and ergosterol content of i nfected and non-infected plants at different times during the infectio n 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, a nd taproot. After systemic colonization of the root and stem cortex by intra- and intercellular growth wild-type and mutants invaded the vas cular system. The infection progressed vertically into the stem by fun gal growth into the xylem. The infection of aerial plant parts was res tricted to the xylem. This was shown with beta-glucuronidase expressin g transformants which were stained selectively in the host tissue. Inf ection of intact or wounded leaves by wild-type or cutinase-deficient mutants was inhibited by a hypersensitive reaction of leaf cells. Afte r having shown recently that cutinase is not required for pathogenicit y 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 haema tococca.