Effect of heat and cycloheximide treatment of tobacco on the ability of Pseudomonas syringae pv. syringae 61 hrp/hrmA mutants to cause HR

In this study the effects of heat treatment (50 degrees C, 13 s) and protein synthesis inhibition by cycloheximide on the defence reactions of tobacco leaves were investigated. As a consequence of both treatments confluent necrosis was caused by the hrpK and hrmA mutants in all treated leaves. Normally hrp mutants cause no confluent necrosis and hrmA mutant causes only delayed and/or weakened necrosis. In the treated plant tissue the hrpK mutant strain needed a longer induction period (3-5 h) to induce HR than the wild type (1-1.5 h). The hrmA mutant had at most a 1 h longer induction period than the wild type (2-2.5 h). Incubation of the hrpK mutant in hrp-inducing minimal medium prior to inoculation increased the probability of induction of the HR and shortened the induction period by 1-2 h. The two treatments enhanced both the multiplication and the hrpK gene activity (measured with beta-glucuronidase reporter gene) of the hrpK mutant in plant tissue. The present study shows that the appearance of the HR and the behaviour of mutant bacteria can be altered either on the plant side by influencing plant metabolism with cycloheximide or heat-treatment, or on the bacterial side by incubation of bacteria in hrp-inducing medium. The results suggest that the length of the induction period may be critical in determining whether or not HR occurs in certain circumstances. (C) 1999 Academic Press.