Polypeptide hormones

Abstract

The polypeptide hormones that have now been identified in plants are presented in Table I. The various scenarios in plants for synthesis, storage, processing, and release of polypeptide hormones are not yet known, and from the limited data available it is apparent that no consistent patterns can yet be deduced. Prosystemin and ENOD40 lack signal sequences that would target them through the secretory system, and they may be synthesized in the cytoplasm. PSKs, CLAVATA3, and SCR pre-proteins all exhibit signal peptides, as does RALF, and are likely synthesized through the secretory system. Signal peptidase sites, where known, appear to have similar specificity requirements as the proteinases that cleave animal, yeast, and bacterial pre-proteins. Only systemin and phytosulfokines are known to be internally processed to produce smaller signaling polypeptides, although others may be. No pro-protein processing enzymes or their possible processing sites have yet been identified for any of the plant polypeptide hormones. In addition to the polypeptide receptor proteins and genes mentioned above, several other receptor genes have been identified through mutagenesis and cloning, including Erecta (1), Crinkly4 (14), PRK1 (7), SLRK (12), and BRl1 (4). All exhibit Leu repeat domains that are typical of extracellular LRR polypeptide-binding motifs. LRRs are associated with protein-protein interactions in animals and plants, which are often found associated with polypeptide hormone receptors. Over 100 LRR-containing genes have been identified in Arabidopsis alone and it is possible that many of these proteins are receptors for polypeptide hormones. The signaling pathway for systemin is a complex cascade that bears striking similarities to the inflammatory response of animals (10) that has raised interesting questions concerning the ancestral origins of both signaling systems. If the two pathways are found to share a common ancestral origin, then it must be established whether other polypeptide signals and signaling pathways in plants also track back to ancestral origins common to plants and animals. It will be important not only to investigate the occurrence of polypeptide hormones, their receptors, and their signaling pathways throughout the plant kingdom, but also to begin to understand how various stimuli cause the release of these signals and orchestrate their activities within the overall environmental and developmental status of the plants. The understanding of the scope and roles of polypeptide hormones in plants and their relationships with other plant hormones and signals should provide new insights into many of the complex signaling networks that orchestrate plant growth and development, and the responses of plants to biotic and abiotic stresses.