Effect of solar UV-B radiation on a phyllosphere bacterial community

The effect of solar UV-B radiation on the population dynamics and compositi on of the culturable bacterial community from peanut (Arachis hypogeae L.) was examined in field studies using plants grown under UV-B -transmitting ( UV-B+) or UV-B-excluding (UV-B-) plastic filters. Our data demonstrate that solar UV-B selection alters phyllosphere bacterial community composition a nd that UV tolerance is a prevalent phenotype late in the season. The total bacterial population size was not affected by either UV-B treatment. Howev er, isolates from the UV-B+ plots (n = 368) were significantly more UV tole rant than those from the UV-B- (n = 363) plots. UV sensitivity was determin ed as the minimal inhibitory dose of UV that resulted in an inhibition of g rowth compared to the growth of a nonirradiated control. The difference in minimal inhibitory doses among bacterial isolates from UV-B+ and UV-B- trea tments was mainly partitioned among nonpigmented isolates, with pigmented i solates as a group being characterized as UV tolerant. A large increase in UV tolerance was observed within isolate groups collected late (89 and 96 d ays after planting) in the season. Identification of 200 late-season isolat es indicated that the predominant UV-tolerant members of this group were Ba cillus coagulans, Clavibacter michiganensis, and Curtobacterium flaccumflac iens. We selected C. michiganensis as a model UV-tolerant epiphyte to study if cell survival on UV-irradiated peanut leaves was increased relative to UV survival in vitro. The results showed an enhancement in the survival of C. michiganensis G7.1, especially following high UV-C doses (300 and 375 J m(-2)), that was evident between 24 and 96 h after inoculation. A dramatic increase in the in planta/in vitro survival ratio was observed over the ent ire 96-h experiment period for C. michiganensis T5.1.