Type VI secretion systems of plant-pathogenicBurkholderia glumaeBGR1 play a functionally distinct role in interspecies interactions and virulence

In the environment, bacteria show close association, such as interspecies interaction, with other bacteria as well as host organisms. The type VI secretion system (T6SS) in gram-negative bacteria is involved in bacterial competition or virulence. The plant pathogenBurkholderia glumaeBGR1, causing bacterial panicle blight in rice, has four T6SS gene clusters. The presence of at least one T6SS gene cluster in an organism indicates its distinct role, like in the bacterial and eukaryotic cell targeting system. In this study, deletion mutants targeting fourtssDgenes, which encode the main component of T6SS needle formation, were constructed to functionally dissect the four T6SSs inB. glumaeBGR1. We found that both T6SS group_4 and group_5, belonging to the eukaryotic targeting system, act independently as bacterial virulence factors toward host plants. In contrast, T6SS group_1 is involved in bacterial competition by exerting antibacterial effects. The Delta tssD1mutant lost the antibacterial effect of T6SS group_1. The Delta tssD1mutant showed similar virulence as the wild-type BGR1 in rice because the Delta tssD1mutant, like the wild-type BGR1, still has key virulence factors such as toxin production towards rice. However, metagenomic analysis showed different bacterial communities in rice infected with the Delta tssD1mutant compared to wild-type BGR1. In particular, the T6SS group_1 controls endophytic plant-associated bacteria such asLuteibacterandDyellain rice plants and may have an advantage in competing with endophytic plant-associated bacteria for settlement inside rice plants in the environment. Thus,B. glumaeBGR1 causes disease using T6SSs with functionally distinct roles.