Involvement of auxin pathways in modulating root architecture during beneficial plant-microorganism interactions

The community of researchers studying molecular plant-microbe interactions under the banners of fundamental plant science, biofuel-bioenergy, and crop productivity and sustainability research is expanding rapidly. The review summarizes multiple, separate lines of evidences linking auxin transport, signaling, and synthesis pathways to beneficial plant-microbe interactions and modulations in host root architecture. Compelling physiology and functional genomics-based evidence was found in support of a delicate and precise orchestration of distinct root phenotypic effects achieved via a shared auxin biosynthesis and signaling machinery involving signaling crosstalk. A hypothetical and simplified model on role of auxin in beneficial plant-microbe interactions is presented, and outstanding research challenges and potential future directions are discussed. A wide variety of microorganisms known to produce auxin and auxin precursors form beneficial relationships with plants and alter host root development. Moreover, other signals produced by microorganisms affect auxin pathways in host plants. However, the precise role of auxin and auxin-signalling pathways in modulating plant-microbe interactions is unknown. Dissecting out the auxin synthesis, transport and signalling pathways resulting in the characteristic molecular, physiological and developmental response in plants will further illuminate upon how these intriguing inter-species interactions of environmental, ecological and economic significance occur. The present review seeks to survey and summarize the scattered evidence in support of known host root modifications brought about by beneficial microorganisms and implicate the role of auxin synthesis, transport and signal transduction in modulating beneficial effects in plants. Finally, through a synthesis of the current body of work, we present outstanding challenges and potential future research directions on studies related to auxin signalling in plant-microbe interactions.