Metabolite signaling in plants: So many putative signals, so few definitive signaling components.

A myriad of metabolite signals have been proposed to operate in plants. By contrast to the situation in microbial and mammalian systems, however, except for those characterized in phytohormone research, there is a massive gap in information concerning the perception mechanism underlying non-hormonal metabolite signaling in plants. Here we critically evaluate current evidence for the role of sugars, nitrogen, amino acids, organic acids, polyamines and vitamins as signal metabolites. We discuss emerging concepts suggesting that plants may frequently rely on sensors, transceptors, and integrators rather than dedicated receptors, and we propose experimental approaches to identify and characterize the perception systems underlying non-hormonal plant metabolite signaling.