Genome-wide identification of genes required for fitness during colonization of the leaf surface and apoplast.

The foliar plant pathogen can establish large epiphytic populations on leaf surfaces before apoplastic colonization. However, the bacterial genes that contribute to these lifestyles have not been completely defined. The fitness contributions of 4,296 genes in pv. B728a were determined by genome-wide fitness profiling with a randomly barcoded transposon mutant library that was grown on the leaf surface and in the apoplast of the susceptible plant Genes within the functional categories of amino acid and polysaccharide (including alginate) biosynthesis contributed most to fitness both on the leaf surface (epiphytic) and in the leaf interior (apoplast), while genes involved in type III secretion system and syringomycin synthesis were primarily important in the apoplast. Numerous other genes that had not been previously associated with growth were also required for maximum epiphytic or apoplastic fitness. Fourteen hypothetical proteins and uncategorized glycosyltransferases were also required for maximum competitive fitness in and on leaves. For most genes, no relationship was seen between fitness and either the magnitude of their expression or degree of induction compared to in vitro conditions measured in other studies. A lack of association of gene expression and fitness has important implications for the interpretation of transcriptional information and our broad understanding of plant-microbe interactions.