Regulation of Plant Senescence by Peptide Hormones

Plant hormones have long been characterized as small, low-molecular-weight compounds and are divided into several classes, such as auxins, cytokinins, and brassinosteroids. In addition to these nonpeptide hormones, plants also have peptide hormones, which have gone unnoticed until the discovery of systemin. Following the identification of systemin, several peptide hormones were detected in plants. Plant peptide hormones contain hundreds of amino acids and undergo extensive posttranslational modifications, e.g. proteolytic processing. Although they can be secreted or nonsecreted, plant peptide hormones are involved in numerous processes, including stress responses, development, and senescence. Plant-specific senescence is a programmed cell death pathway mediated by sugar and hormone signaling in contrast to animal-specific senescence, which is caused by mutations and telomere shortening. Senescence in plants can occur as organ senescence, which is the transport of nutrients from dying organs to growing parts, or reproductive senescence or monocarpic senescence, which indicates the final stage of development. Both pathways are coordinated and their regulation contributes to yield and seed content. Senescence induced by unfavorable conditions such as drought can confer survival, yet there are contrasting findings. Novel studies have reported the contribution of peptide hormones such as CLE14 and CLE42, in addition to classical plant hormones, to senescence. In this chapter, after a brief introduction of peptide hormones, plant senescence and its role in stress response are covered. Then, we discuss recent findings reporting the role of peptide hormones in senescence.