Inducing the syrB Gene in Pseudomonas syringae pv. syringae in Twig Extracts from Cherry Genotypes

Abstract

The syrB gene required by Pseudomonas syringae pv. syringae van Hall to produce the phytotoxin syringomycin is activated by plant signal molecules. Extracts from twigs of 12 cherry (Prunus) genotypes were tested for their ability to induce syrB::lacZ fusion in P. syringae pv. syringae strain B3AR132 to determine whether signal activity is correlated with susceptibility to bacterial canker. One-year-old twigs of `Napoleon', `Corum', and 12 cherry rootstocks (F12/1, `Colt', M×M2, M×M39, M×M60, Gi 148-1, Gi 148-9, Gi 154-2, Gi 154-5, Gi 169-15, Gi 172-9, and 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 β -galactosidase activity. Extracts from all cherry genotypes induced syrB::lacZ fusion, but to varying degrees. The highest β -galactosidase activity was observed in `Napoleon' and `Corum'-the most susceptible genotypes; activities were two to four times higher than that of F12/1, a disease-resistant genotype. Activities higher than that of F12/1 were induced at the lowest extract concentration by rootstocks M×M60, Gi 148-1, Gi 148-9, and Gi 154-5, whereas rootstocks M×M2, M×M39, Gi 154-2, Gi 172-9, Gi 173-9, and `Colt' were not significantly different from F12/1. At the two highest extract concentrations tested, only `Napoleon' and `Corum' consistently had higher induct&m activity than F12/1. At high extract concentrations, interfering substances seemed to suppress or antagonize the induction of syrB::lacZ fusion. These results suggest that susceptible genotypes contain higher signal activities than resistant genotypes.