CONTRIBUTION OF THE REGULATORY GENE LEMA TO FIELD FITNESS OF PSEUDOMONAS-SYRINGAE PV. SYRINGAE

In Pseudomonas syringae pv. syringae, lemA is required for brown spot lesion formation on snap bean and for production of syringomycin and e xtracellular proteases (E. M. Hrabak and D. K. Willis, J, Bacteriol. 1 74: 3011-3022, 1992; E. M. Hrabak and D. K. Willis, Mol. Plant-Microbe Interact. 6:368-375, 1993; D. K. Willis, E. M. Hrabak, J. J. Rich, T. M. Barta, S. E. Lindow, and N. J. Panopoulos, Mol. Plant-Microbe Inte ract. 3:149-156, 1990). The lemA mutant NPS3136 (lemA1::Tn5) was previ ously found to be indistinguishable from its pathogenic parent B728a i n its ability to grow when infiltrated into bean leaves of plants main tained under controlled environmental conditions (Willis et al., Mol. Plant-Microbe Interact, 3:139-156, 1990), We compared population sizes of NPS3136 and B728aN (a Nal(r) clone of wild-type B728a) in two fiel d experiments to determine the effect of inactivation of lemA on the f itness of P. syringae pv. syringae. in one experiment, the bacterial s trains were spray inoculated onto the foliage of 25-day-old bean plant s. In the other, seeds were inoculated at the time of planting. In bot h experiments, the strains were inoculated individually and coinoculat ed in a 1:1 ratio. NPS3136 and B728aN achieved similar large populatio n sizes on germinating seeds. However, in association with leaves, pop ulation sizes of NPS3136 were diminished relative to those of B728aN i n both experiments. Thus, lemA contributed significantly to the fitnes s of P. syringae pv, syringae in association with bean leaves but not on germinating seeds under held conditions. When NPS3136 was coinocula ted with B728aN, the mutant behaved as it did when inoculated alone. H owever, population sizes of B728aN in the coinoculation treatment were much lower than those when it was inoculated alone, Inactivation of t he lemA gene appeared to have rendered the mutant suppressive to B728a N.