A NONPATHOGENIC MUTANT OF COLLETOTRICHUM-MAGNA IS DEFICIENT IN EXTRACELLULAR SECRETION OF PECTATE LYASE

A nonpathogenic mutant strain (path-1) of Colletotrichum magna (teleom orph Glomerella magna) was isolated and grew endophytically in cucurbi t seedlings. Decay development on the pericarp of avocado fruits was s ignificantly delayed in the wild-type (wt) isolate of C. magna compare d with C. gloeosporioides isolate Cg-14, but both isolates colonized m esocarp tissue equally well. However, path-1 did not cause disease sym ptoms in either the pericarp or the mesocarp, Electrofocusing of cultu re filtrates of Colletotrichum strains revealed the presence of pectat e lyase (PL) and polygalacturonase (PG) at pIs of 7.9 and 4.8, respect ively, but no significant activity from path-1 was detected. Western b lots detected PL in avocado fruit tissue infected with Cg-14 and wild- type C. magna. DNA of the three Colletotrichum isolates hybridized to a pel probe from C. gloeosoporioides indicating that at the DNA level no deletion in the pel gene was evident in path-1. Similar levels of p el mRNA expression were also found for wild-type and path-1 of C. magn a, and Cg-14 of C. gloeosporioides. However, when PL antibodies were c rossreacted with cell lysate of Colletotrichum strains, PL accumulated in the hyphae of path-1 of C. magna, but was not detected in Cg-14 ce ll lysate. This suggests that the malfunction in extracellular secreti on of PL proteins results in differential pathogenicity of path-1 isol ate on avocado.