INDUCING THE SYRB GENE IN PSEUDOMONAS-SYRINGAE PV SYRINGAE IN TWIG EXTRACTS FROM CHERRY GENOTYPES

The syrB gene required by Pseudomonas syringae pv. syringae van Hall t o produce the phytotoxin syringomycin is activated by plant signal mol ecules. Extracts from twigs of 12 cherry (Prunus) genotypes were teste d for their ability to induce syrB::lacZ fusion in P. syringae pv. syr ingae strain B3AR132 to determine whether signal activity is correlate d with susceptibility to bacterial canker. One-year-old twigs of 'Napo leon', 'Corum', and 12 cherry rootstocks (F12/1, 'Colt', MxM2, MxM39, MxM60, Gi 148-1, Gi 148-9, Gi 154-2, Gi 154-5, Gi 169-15, Gi 172-9, an d Gi 173-9) were tested at concentrations of 0.2, 1.0, and 2.0 mg twig dry weight/ml solution for their ability to induce syrB::lacZ fusion, as measured by beta-galactosidase activity. Extracts from all cherry genotypes induced syrB::lacZ fusion, but to varying degrees. The highe st beta-galactosidase activity was observed in 'Napoleon' and 'Corum'- the most susceptible genotypes; activities were two to four times high er than that of F12/1, a disease-resistant genotype. Activities higher than that of F12/1 were induced at the lowest extract concentration b y rootstocks MxM60, Gi 148-1, Gi 148-9, and Gi 154-5, whereas rootstoc ks MxM2, MxM39, Gi 154-2, Gi 172-9, Gi 173-9, and 'Colt' were not sign ificantly different from F12/1. At the two highest extract concentrati ons tested, only 'Napoleon' and 'Corum' consistently had higher induct ion activity than F12/1. At high extract concentrations, interfering s ubstances seemed to suppress or antagonize the induction of syrB::lacZ fusion. These results suggest that susceptible genotypes contain high er signal activities than resistant genotypes.