USE OF GUS FUSION TO STUDY THE EXPRESSION OF ERWINIA-CHRYSANTHEMI PECTINASE GENES DURING INFECTION OF POTATO-TUBERS

Erwinia chrysanthemi mutants, containing transcriptional fusion of one of the major pectinase genes (pem, pelA, pelB, pelC, pelD, pelE) with the reporter gene encoding beta-glucuronidase activity, were studied for their ability to synthesize pectinases and to cause disease symptoms after inoculation of potato tubers and of in vitro plants. There were no differences in growth of all the tested mutants, and all caused soft rot. The pelB::uidA, pelC.:uidA, and pelE::uidA mutants exhibited lower pectate lyase activity than the wild type. Pectinase genes could be separated into two groups according to their expression level, i.e., 1) the moderately expressed genes: pelA, pelB, pelC, and pem; 2) the highly expressed genes: pelD and pelE. Expression of some pel::uidA fusions were different in inoculated tubers and in bacterial culture. The pelA::uidA fusion showed a 23-fold higher GUS activity in tubers compared to synthetic medium. Conversely, the pelE and pem expression is favored after inoculation of in vitro plants. The comparison of the pectinase activity and of the expression of the fusions in bacterial culture and in planta suggests that one or more plant factor(s) modulates the expression of these genes.