Genome-wide identification ofPseudomonas syringaegenes required for competitive fitness during colonization of the leaf surface and apoplast

The foliar plant pathogen Pseudomonas syringae can establish large epiphytic populations on leaf surfaces before infection. However, the bacterial genes that contribute to these lifestyles have not been completely defined. The fitness contributions of most genes in P. syringae pv. syringae 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 Phaseolus vulgaris . Genes within the functional categories of amino acid and polysaccharide (including alginate) biosynthesis contributed most to fitness both on the leaf surface (epiphytic) or in the leaf interior (apoplast), while genes in the type III secretion system and syringomycin synthesis were primarily important in the apoplast. Numerous other genes that had not been previously associated with in planta growth were also required for maximum epiphytic or apoplastic fitness. Many 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 in planta and either the magnitude of their expression in planta or degree of induction in planta 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. Significance Statement Many plant pathogenic bacteria can extensively colonize leaf surfaces before entry and multiplication within the leaf to cause disease. While these habitats presumably require distinct adaptations, the genes required in these habitats and how they would differ was unknown. Using a genome-wide library of barcoded insertional mutants in the plant pathogen Pseudomonas syringae , we ascertained the common and unique genes required to colonize these habitats. A lack of association between gene expression and contribution to fitness suggests that many genes that are highly expressed or induced in planta are dispensable or redundant. As a model bacterium for plant pathogenesis and colonization, our comprehensive genetic dataset allows us to better understand the traits needed for association with leaves.