Ultraviolet radiation (UVR) sensitivity analysis and UVR survival strategies of a bacterial community from the phyllosphere of field-grown peanut (Arachis hypogeae L.)

The short-term population dynamics of the culturable bacterial community fr om field-grown peanut (Arachis hypogeae L.) was analyzed over three 2-day p eriods. As in other phyllosphere studies, significant numbers of pigmented organisms were detected, suggesting the importance of pigmentation in the c olonization of this habitat. Isolates were grouped according to pigmentatio n (orange, pink, yellow, nonpigmented), and the sensitivity of each isolate in the collection (n = 617) to ultraviolet radiation (UVR) was determined as the minimal inhibitory dose (MIDC) of UVR that resulted in an inhibition of growth compared to an unirradiated control. The majority of isolates re covered (56.1%) had an MIDC equal to or exceeding that of Pseudomonas syrin gae 8B48, a known UV-tolerant strain. Among pigmentation groups, the mean M IDC of pink- and orange-pigmented isolates was significantly greater than t hat of yellow- or nonpigmented isolates at each sampling time of day. Ident ification of 213 of the isolates using fatty acid methyl ester analysis ind icated that a large proportion of the isolates were gram-positive, with Bac illus spp. alone accounting for 35.7% of the total. The genus Curtobacteriu m contained the largest percentage of highly WR-tolerant strains. Nonpigmen ted mutants of four Curtobacterium strains were selected following ethyl me thane sulfonate mutagenesis; these nonpigmented mutants were significantly altered in survival following irradiation with UV-A wavelengths. The strate gy of avoidance of WR through colonization of the abaxial leaf surface was evaluated on three separate occasions by leaf imprint sampling. Only 3 of 1 20 leaves (2.5%) contained larger bacterial populations on the adaxial surf ace, indicating that colonization of the abaxial leaf surface is important to phyllosphere survival. Our results indicate that tolerance to UVR is a c ommon phenotype among phyllosphere bacteria, suggesting that solar radiatio n has a strong influence on the microbial ecology of the phyllosphere.