KLAUS PALME AND JEFF SCHELL PLANT SIGNALLING Auxin receptors take shape An end to the long search for phytohormone receptors is firmly in sight. A better understanding of how plants respond to environmental changes should follow. Unlike most eukaryotic organisms, plants must meet the specific demands of a sessile, autotrophic habit. They ad- just to a wide range of changes in their local environment by plastic growth responses. OveraLl, they increase their mass and volume throughout their lifetime by continuous growth. Most of the details regulating these processes are not yet known, but it has been well documented that they rely heavily on phytohormones, relatively sim- ple chemical factors that are used by plants as growth and differentiation signals. Animal hormones are produced mainly in particular glands, from which they are secreted to produce a re- sponse distant from their point of origin. In plants, how- ever, it is often very difhcult to discriminate clearly be- tween the sites of synthesis and action of phytohor- mones. They elicit a wide range of diRerent responses (Table 1) and it is conceivable that their mode of action is different from that of animal hormones. For example, coleoptile cells respond to auxin (indole-3-acetic acid) by undergoing cell enlargement whereas cambium cells respond by dividing mitotically. Our present knowledge of plant signalling networks is still quite rudimentary but recent results indicate some ways in which both plant- specilic and general eukaryotic mechanisms play a part. One intriguing parallel with other eukaryotes emerged with the discovery of a plant gene that encodes a pre- sumptive cell surface receptor with similarity to some re- ceptors of other eukaryotes that have protein kinase activ- ity. This gene, Zmpkl, isolated from Zeu WZUJIS, encodes a protein with a catalytic domain sharing structural mo- tifs with the &family of serine-threomne kinases [l]. The extracellular domain, which is probably involved in recognition of extracellular signals and is separated from the kinase domain by a single transmembrane domain, has a sequence that is closely related (greater than 50% similarity) to a glycoprotein that is involved in the control of self-incompatibility. Although the precise tunction of the Zm.pkZ protein is not yet known, its structure suggests an involvement in ce&cell recognition. One is provoked to speculate that such receptor kinases could be involved in recognition of oligosaccharide signals derived from the plant cell wall. Why? Because oligogalacturonate fragments, for exam- ple, are among the earliest signals that are released af- ter pathogen attack, are possibly involved in the activa- tion of defence genes, and strongly enhance the in vitro phosphorylation of a 34kD plasma membrane protein in several plant species [2,3]. The possibility exists that 228 phosphorylation of a set of proteins by such plasma- membrane-associated receptor kinases would initiate cas- cades of responses necessary for regulating plant gene transcription and morphogenesis. Knowing that the G proteins, which regulate various in- formation processing circuits, in most eukaryotes were perfected early in evolution, it has been satisfying rather than surprising to find plants have them too. As in other eukaryotes, plant G proteins occur in two large families, the small G proteins, typically found as single polypep- tides of about 200 ammo acids, and the heterotrimeric G proteins, with 01,p and y subunits, that are probably associated with the cytoplasmic side of the plasma mem- brane and involved in transmission of signals from trans- membrane receptors to effector proteins. Among the hrst G proteins to be found in plants are those encoded by the maize genes yptml and yptm.2, which are related to rnembers of the ras family such as yptl from Sac&~ romyces cerevkkze or rdbl from rat [4]. These proteins are assumed to be involved in the control of secretion and intracellular vesicle interactions. A gene, GPAl , that encodes a protein with significant sim- ilarity to G protein c1 subunits has recently been iden- tified in Arabidopsh t&&&a [51. The 383 ammo acid GPoll protein shares 36% of its amino acid residues with other G proteins, and a further 37% of its amino acid residues are conservative changes when compared with mammalian G protein ~1 subunits and transducins. Al- tlhough there is still no evidence for a functional role of GPul in An&i&$x& its similarity to other eukaryotic G protein CI subunits suggests it may also be similar in in- teracting with subsets of eifectors, including phospholi- pase C, phosphodiesterases and ion channels, or plasma- membrane-spanning G-coupled receptors. Perhaps GPal will lead to the identification in higher plants of G-cou- pled receptors that could be involved in sensing a broad array of environmental and cellular signals. The continous growth of plants is largely confined to primary or secondary meristematic zones - embryonic tissues that retain the ability to divide and differentiate throughout the life of a plant - and it will be impor- tant to understand the molecular mechanisms that reg- ulate these cellular divisions, in particular with regard to. the role played by phytohormones. In this respect, it is interesting that a variety of plant species have now been shown to have a protein kinase related to the pro- tedns encoded by the c&2 gene of S&.zosa&romyces pombe and &28 of S. cerev&tie that are involved in the @ 1991 Current Biology