Effect of Solar UV-B Radiation on a Phyllosphere Bacterial Community

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Abstract
The effect of solar UV-B radiation on the population dynamics and composition 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 and that UV tolerance is a prevalent phenotype late in the season. The total bacterial population size was not affected by either UV-B treatment. However, isolates from the UV-B+ plots ( n = 368) were significantly more UV tolerant than those from the UV-B− ( n = 363) plots. UV sensitivity was determined as the minimal inhibitory dose of UV that resulted in an inhibition of growth compared to the growth of a nonirradiated control. The difference in minimal inhibitory doses among bacterial isolates from UV-B+ and UV-B− treatments was mainly partitioned among nonpigmented isolates, with pigmented isolates as a group being characterized as UV tolerant. A large increase in UV tolerance was observed within isolate groups collected late (89 and 96 days after planting) in the season. Identification of 200 late-season isolates indicated that the predominant UV-tolerant members of this group were Bacillus coagulans , Clavibacter michiganensis , and Curtobacterium flaccumfaciens . 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 entire 96-h experiment period for C . michiganensis T5.1.

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