Oligopeptide signalling and the action of systemin.

Abstract

In the 19th century, the German botanist Julius von Sachs postulated the existence of chemical messengers that are responsible for the regulation of plant growth and development. The first such messenger was identified in the 1930s as indole-3-acetic acid, an auxin. In the following years, four additional classes of phytohormones were discovered: cytokinins, gibberellins, abscisic acid and ethylene. For decades, these classical phytohormones have been and still are at the centre of research in this area and it was generally believed that plant hormone action is fundamentally different from that in animals. This view is changing, however, since over recent years brassinosteroids and jasmonates, resembling animal steroids and prostaglandins, respectively, have been added to the group of chemical messengers in plants. Furthermore, there is now compelling evidence for the existence of plant peptide hormones. Indirect evidence is derived from the observation that plants possess putative receptor proteins for (poly)peptide ligands. Over 40 receptor-like kinases (RLKs) have been identified in sequence databases (Xiaorong, 1998). These RLKs comprise three functional domains, an extracellular domain, the transmembrane domain and an intracellular catalytic kinase domain. They appear to be involved in processes as diverse as embryogenesis, morphogenesis, selfincompatibility, light and hormone signal transduction, and pathogen resistance. The extracellular domains of RLKs contain regions of leucine-rich repeats (LRRs) which are known to form amphipathic peptide surfaces involved in protein-protein interactions. Based on the sequences of the LRRs and their structural similarity to animal LRR motifs it seems likely that plant RLKs bind peptide ligands which, however, remain to be identified. LRRs are also typically found in resistance gene-encoded proteins that are hypothesized to be the receptors of race-specific polypeptide elicitors produced by phytopathogenic bacteria, fungi, and viruses (Baker et al., 1997). On the other hand, three classes of endogenous oligopeptides have been discovered clearly having signalling function in plants. These include ENOD40, phytosulfokines and systemin and will be the subject of this review article. In a first section, the most salient features of these peptides will be summarized and compared. Most of these data have been reviewed already and for references I refer the reader to recent reviews on systemin (Ryan and Pearce, 1998; Bowles, 1998) and enod40 (Barciszewski and Legocki, 1997; Franssen, 1998) and to the most recent paper on phytosulfokines (Matsubayashi et al., 1999). After this introductory section, the main focus of the article will be on very recent observations that shed light on the molecular mechanisms involved in systemin perception and in the subsequent activation of cellular responses.