Long-range signalling in plant reproductive development.

Animals and plants produce regulatory signals at specific places of their bodies, in order to regulate developmental events which take place at a distance. Plants use this mechanism to adjust their development to the changing environment. Flowering and tuber formation are controlled by signals generated in the leaves that travel throughout the plant to reach their target tissues: the shoot apical meristem for flowering and the underground stolons for tuberization. Although the existence of these long-distance plant messengers was postulated almost seventy years ago, their chemical nature is still not clear. These leaf-derived signals are graft-transmissible and move through the plant vascular system. Presumably they are very similar or even identical for flowering and tuberization and common to most plant species. It is generally accepted that their composition is complex and includes positive and negative regulators. Many different substances, including classical plant hormones and metabolites have been postulated to be components of these mobile signals, but conclusive evidence of this is still lacking. Recent work has positioned these signals within the genetic network that regulates flowering time and suggests roles for specific genes in the generation, transport or response to the signalling molecules. Current knowledge of long-range signalling mechanisms in other physiological and developmental events, together with the finding of common regulators involved in flowering, tuberization and other processes like pathogen and wound responses, should help to establish the biochemical composition of these elusive messenger signals.