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Introduction
The discoveries of peptide hormones in human and animals are one of the greatest inventions in the last century. Many of these peptides such as insulin have been used extensively in pharmacy for saving people’s life and improving life quality. For a long time, peptide hormones have not been considered to be present and important in plants. Most plant hormones identified thus far, such as auxin, cytokinins, gibberellins, abscisic acid and ethylene, are low-molecule weight metabolites, with high sensitivities and critical functions at different stages of plant growth and development. However, since the first peptide hormone, systemin, was discovered in tomato by Ryan’s group in 1991 (Pearce et al. 1991), putative peptide hormones have continuously been identified in different plant species, showing their importance in both short- and long-range signal transductions. Their roles are implicated in processes such as pollination, self-incompatibility, fertilization, embryo and endosperm development, stem cell homeostasis, plant architecture, cell differentiation, organogenesis, dehiscence, senescence, plant-pathogen and plant-insect interactions, and stress responses. This article aims to provide a general overview for the discoveries, functions, chemical natures, transcriptional regulations, and post-translational modifications (PTMs) of plant peptide hormones. Attentions are also given to peptide-sensing receptor kinases and downstream signal transduction pathways.
Identifications and classifications of plant peptide hormones
Identifications of peptide hormones in plants
Peptide hormones are defined as small secreted polypeptide-based intercellular communication signal molecules (Lease and Walker 2006). The lengths of peptide hormones range from a few amino acids to up to 250 amino acids. Traditionally, the identifications of potential peptide hormones relied on various analytical methods such as chemical separations and bioassays, phenotype-based genetic analyses, and chromatographic detections (Cammue et al. 1992; Fletcher et al. 1999; Butenko et al. 2003). However, since peptide hormones are present with extremely low concentrations in plants, posing substantial challenges for liquid-phase separations, purifications, and biochemical analyses (Hou et al. 2014). To address these challenges, high-throughput omics approaches such as transcriptomics, proteomics and peptideomics, combined with detection methods including high-throughput mass spectrometry (MS) and in vitro assays, have been used to identify peptide hormones in plants (Yan et al. 2022). In addition, advances in bioinformatics tools and machine-learning approaches have enabled the identifications of putative peptide hormones across different plant species (Yang et al. 2011; Chen et...