The ecological genetics ofPseudomonas syringaefrom kiwifruit leaves

Abstract

SUMMARYInteractions between commensal microbes and invading pathogens are understudied, despite their likely effects on pathogen population structure and infection processes. We describe the population structure and genetic diversity of a broad range of co-occurringPseudomonas syringaeisolated from infected and uninfected kiwifruit during an outbreak of bleeding canker disease caused byP. syringaepv.actinidiae(Psa) in New Zealand. Overall population structure was clonal and affected by ecological factors including infection status and cultivar. Most isolates are members of a new clade in phylogroup 3 (PG3a), also present on kiwifruit leaves in China and Japan. Stability of the polymorphism between pathogenicPsaand commensalP. syringaePG3a isolated from the same leaf was tested using reciprocal invasion from rare assaysin vitroand in planta.P. syringaeG33C (PG3a) inhibitedPsaNZ54, while the presence ofPsaNZ54 enhanced the growth ofP. syringaeG33C. This effect could not be attributed to virulence activity encoded by the Type 3 secretion system ofPsa. Together our data contribute toward the development of an ecological perspective on the genetic structure of pathogen populations.ORIGINALITY-SIGNIFICANT STATEMENTBacterial pathogen populations are often studied with little consideration of co-occurring microbes and yet interactions between pathogens and commensals can affect both population structure and disease progression. A fine-scale sampling of commensals present on kiwifruit leaves during an outbreak of bleeding canker disease caused byP. syringaepv.actinidiaereveals a clonal population structure. A new clade of non-pathogenicP. syringae(PG3a) appears to be associated with kiwifruit on a global scale. The presence of PG3a on kiwifruit has significant effects on the outcome of infection byP. syringaepv.actinidiae. This emphasises the value of studying the effect of co-occurring bacteria on pathogen-plant interactions.