Cyclic di-GMP Regulation in Beneficial Plant-Microbe Interactions

Cyclic diguanilate, c-di-GMP, has emerged as an ubiquitous secondary messenger in bacteria. It influences a wide range of cellular processes, including cell-cell signalling, cell cycle progression and virulence. However, it is best known for controlling the transition from a planktonic/motile lifestyle to a sessile biofilm mode of growth. Plant-interacting bacteria can live as free motile single cells or in association with their plant hosts, which provide them with numerous adaptive advantages. Also, the permanent exchange of multiple signals between plant and bacteria during the establishment of mutualistic interactions need to be integrated by the bacteria in order to modify their lifestyle to the new ecological niche. Most of the studied systems evidenced a temporal and spatial requirement for opposite c-di-GMP-metabolizing processes at different stages, culminating with an effective interaction with the plant host. In this review we highlight the importance of c-di-GMP signaling in beneficial plant-bacteria interactions, gathering the yet limited but significant knowledge acquired from different rhizobial and other plant growth-promoting rhizobacteria (PGPR).