SURVIVAL, GROWTH, AND LOCALIZATION OF EPIPHYTIC FITNESS MUTANTS OF PSEUDOMONAS-SYRINGAE ON LEAVES

Among 82 epiphytic fitness mutants of a Pseudomonas syringae pv. syrin gae strain that were characterized in a previous study, 4 mutants were particularly intolerant of the stresses associated with dry leaf surf aces. These four mutants each exhibited distinctive behaviors when ino culated onto and into plant leaves. For example, while none showed mea surable growth on dry potato leaf surfaces, they grew to different pop ulation sizes in the intercellular spaces of bean leaves and on dry be an leaf surfaces, and one mutant appeared incapable of growth in both environments although it grew well on moist bean leaves. The presence of the parental strain did not influence the survival of the mutants i mmediately following exposure of leaves to dry, high-light incubation conditions, suggesting that the reduced survival of the mutants did no t result from an inability to produce extracellular factors in planta. On moist bean leaves that were colonized by either a mutant or the wi ld type, the proportion of the total epiphytic population that was loc ated in sites protected from a surface sterilant was smaller for the m utants than for the wild type, indicating that the mutants were reduce d in their ability to locate, multiply in, and/or survive in such prot ected sites. This reduced ability was only one of possibly several fac tors contributing to the reduced epiphytic fitness of each mutant, The ir reduced fitness,vas not specific to the host plant bean, since they also exhibited reduced fitness on the nonhost plant potato; the funct ions altered in these strains are thus of interest for their contribut ion to the general fitness of bacterial epiphytes.