In recent years, next-generation sequencing of rRNA amplicons has made its way into the toolbox of phyllosphere microbiologists, providing in-depth descriptions of the bacterial and fungal community composition associated with leaves of oak ( Jumpponen and Jones, 2009 Jumpponen and Jones, 2010), soybean, clover and mouse-ear cress ( Delmotte et al., 2009), various tree species ( Redford et al., 2010), salt cedar ( Finkel et al., 2011), spinach ( Lopez-Velasco et al., 2011) and grape ( Leveau and Tech, 2011). Culture-independent methods based on the analysis of ribosomal RNA (rRNA) genes have revealed that phyllosphere microbiota are complex and more diverse than the results from culture-dependent approaches would suggest ( Yang et al., 2001 Handschur et al., 2005 Jackson et al., 2006 Lambais et al., 2006 Redford and Fierer, 2009 Yashiro et al., 2011). The phyllosphere or leaf surface ( Ruinen, 1956) represents a biome that is inhabited by a variety of bacteria, fungi, archaea and other microorganisms ( Lindow and Leveau, 2002 Lindow and Brandl, 2003 Leveau, 2006). As one of the largest surveys of leaf surface microbiology, this study offers new insights into the extent and underlying causes of variability in bacterial community composition on plant leaves as a function of time, space and environment. The distance between fields or the timing of a dust storm, but not Romaine cultivar, explained differences in bacterial community composition between several of the fields sampled. Summer samples showed an overrepresentation of Enterobacteriaceae sequences and culturable coliforms compared with winter samples. vitians, which is the causal agent of bacterial leaf spot of lettuce, correlated positively with the relative representation of bacteria from the genus Alkanindiges, but negatively with Bacillus, Erwinia and Pantoea. The foliar presence of Xanthomonas campestris pv. At the genus level, Pseudomonas, Bacillus, Massilia, Arthrobacter and Pantoea were the most consistently found across samples, suggesting that they form the bacterial ‘core' phyllosphere microbiota on lettuce. Pyrosequencing of 16S rRNA gene amplicons from 88 samples revealed that Proteobacteria, Firmicutes, Bacteroidetes and Actinobacteria were the most abundantly represented phyla. Total bacterial populations averaged between 10 5 and 10 6 per gram of tissue, whereas counts of culturable bacteria were on average one (summer season) or two (winter season) orders of magnitude lower. We collected 106 samples of field-grown Romaine lettuce from commercial production regions in California and Arizona during the 2009–2010 crop cycle. However, information is scarce regarding their composition and how it changes along geographical and seasonal scales. The presence, size and importance of bacterial communities on plant leaf surfaces are widely appreciated.
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