The ups and downs of signalling between root and shoot

Models of plant development should account for the concurrent regulation of many processes. For example, nodulation, branching and root-shoot partitioning all involve interactions of signalling components from the shoot and root (or rootstock) (Fig. 2). Perhaps the simplest explanation for the simultaneous regulation of different or overlapping root-shoot signalling systems, and the lack of strong evidence for independent roles of known plant hormones in some of these processes, is that many unknown signals are lurking in the shadows. Additional plant hormone-like signals would not necessarily add complexity to control systems, as they could easily account for the coordination of multiple developmental and physiological events simultaneously. Furthermore, roles for novel signals may not be revealed while the focus of research rests on elucidating roles of known signals. The value of studies with mutants such as DL (Hannah et al.) is that they have been selected on the basis of phenotype alone without a premise that known plant hormones are involved. Mutants can therefore demonstrate genetic regulation of signals, known or unknown. Apart from DL (Hannah et al.), few mutants have been utilized to investigate root-shoot partitioning. Genotypes that enable a route to gene cloning (e.g. Bouchez and Hofte, 1998) will allow progress to unequivocal determination of the role and identity of long-distance signals involved in the control of plant development.