Against friend and foe: type 6 effectors in plant-associated bacteria

Bacterial secretion systems play critical roles in communication with neighboring bacteria and in the modulation of host immune responses via the secretion of small proteins called effectors. Several secretion systems have been identified and these are denoted types I-VII. Of these, the type VI secretion system (T6SS) and its effectors were only recently elucidated. Most studies on the role and significance of the T6SS and its effectors have focused on human pathogens. In this review, type 6 effectors from plant-associated beneficial and pathogenic bacteria are discussed, including effectors from Agrobacterium tumefaciens, Dickeya dadanti, Rhizobium leguminosarum, Pectobacterium atroseptium, Ralstonia solanacearum, Pseudomonas syringae, Pseudomonas fluorescens, and Pseudomonas protegens. Type 6 effectors act in symbiosis, biofilm formation, virulence, and interbacterial competition. Understanding the impact of type 6 effectors on pathogenesis will contribute to the management of bacterial pathogens in crop plants by allowing the manipulation of intra and inter-specific interactions.