Role of Cyclic di-GMP in the Bacterial Virulence and Evasion of the Plant Immunity

Plant pathogenic bacteria are responsible for the loss of hundreds of millions of dollars each year, impacting a wide range of economically relevant agricultural crops. The plant immune system detects conserved bacterial molecules and deploys an arsenal of effective defence measures at different levels; however, during compatible interactions, some pathogenic bacteria suppress and manipulate the host immunity and colonize and infect the plant host. Different bacteria employ similar strategies to circumvent plant innate immunity, while other tactics are specific to certain bacterial species. Recent studies have highlighted the secondary messenger c-di-GMP as a key molecule in the transmission of environmental cues in an intracellular regulatory network that controls virulence traits in many plant pathogenic bacteria. In this review, we focus on the recent knowledge of the molecular basis of c-di-GMP signalling mechanisms that promote or prevent the evasion of bacterial phytopathogens from the plant immune system. This review will highlight the considerable diversity of mechanisms evolved in plant-associated bacteria to elude plant immunity.