Onion‐pathogenic Burkholderia species: Role and regulation of characterized virulence determinants

Abstract Members of the bacterial genus Burkholderia are a routine threat to onion production worldwide. In addition to the common onion‐pathogenic species, Burkholderia cepacia , Burkholderia orbicola and Burkholderia gladioli , other Burkholderia species have the potential to cause onion disease. Despite their impacts and long‐known association with onion disease, the virulence mechanisms of onion‐pathogenic Burkholderia are far less well understood than Burkholderia in human and murine infection models. In this review, we will focus on genetically characterized virulence factors in species that contribute to symptom production in onion and other plant hosts. Specifically, we will focus on the variable roles of specialized protein secretion systems (T2SS, T3SS and T4SS) and secreted proteins, thiosulphinate tolerance gene (TTG) clusters and the well‐characterized phytotoxin toxoflavin in virulence. The regulation and roles of LuxI/LuxS quorum‐sensing system and IclR‐type transcriptional regulator, qsmR , as master regulators of secondary metabolite production and virulence factors will also be discussed. The TTG clusters, involved in bacterial tolerance to thiosulphinate defence compounds, exhibit onion tissue‐specific contributions to virulence. This suggests that Burkholderia onion pathogens have tissue‐specific virulence strategies for causing disease.