PRODUCTION OF ERWINIA-CHRYSANTHEMI PECTINASES IN POTATO-TUBERS SHOWING HIGH-LEVEL OR LOW-LEVEL OF RESISTANCE TO SOFT-ROT

Erwinia chrysanthemi was used for inoculation of two lines of potato t ubers with different degrees of resistance to soft-rot: cultivar Katah din, a very sensitive material, and the somatic hybrid of Solanum brev idens and Solanum tuberosum as a resistant plant. Infection was perfor med with the E. chlysanthemi wild-type strain 3937 and various mutants containing a transcriptional fusion of one pectinase gene (pem, pelA or pelf), together with the reporter gene gus. Compared with the susce ptible tubers, the accumulation of pectate lyase in the resistant tube rs is limited, This limitation of the pectate lyase production seems t o take place at the transcriptional level, with a striking decrease of the expression of the pelA::gus fusion and a decrease in the first st eps of infection of the pelE::gus fusion expression. The pectinase gen es appeared to be differentially regulated in the two potato lines. Th is study also revealed the major role of pectin methyl esterase in the infection process which occurs in potato tubers resistant to soft-rot . Indeed, the maceration of somatic hybrid tubers by the pem mutant wa s strongly reduced in comparison with the wild-type. We suggest that t he resistant tubers have the advantage of a high methylated pectin whi ch is difficult to degrade by the E. chlysanthemi pectinases.