Peptide Hormones in Root Growth and Development

Multicellular organisms require cell-to-cell communication to coordinate growth and development. Plants mainly rely on hormones, such as auxins, abscisic acid, cytokinins, brassinosteroids (BRs), and gibberellins, for intercellular communication. In the past few decades, small peptides have been recognized as essential phytohormones that participate in numerous developmental processes in plants. In 1991, the first functional peptide, SYSTEMIN with the sequence AVQSKPPSKRDPPKMQTD, was isolated from tomato leaves and was shown to be involved in the wounding response (Pearce et al. 1991). Since then, many peptides that play versatile roles during plant growth and development have been identified. Some peptides act locally in response to endogenous stimuli, whereas others are involved in long-distance signaling in order to transmit environmental cues. However, the actual number of peptide hormones in plants may be underestimated due to their small size, low native concentrations, and difficulties in identification. The small peptides are generally considered to be either secreted or non-secreted, and the former can be further categorized into post-translationally modified (PTM) peptides and cysteine-rich peptides based on their structure (Matsubayashi 2014; Figure 18.1). The PTM peptides are produced by a series of proteolytic processing and modifications such as tyrosine sulfation, proline hydroxylation, or hydroxyproline arabinosylation to form the functional mature peptides (usually less than 20 amino acids long) (Matsubayashi 2014). For instance, CLAVATA3 (CLV3)/EMBRYO SURROUNDING REGION peptide (CLE), PHYTOSULFOKINE (PSK), PLANT PEPTIDE CONTAINING SULFATED TYROSINE (PSY), ROOT MERISTEM GROWTH FACTOR (herein called RGF, also known as CLE-like or GOLVEN peptide) (Meng et al. 2012; Whitford et al. 2012), INFLORESCENCE DEFICIENT IN ABSCISSION (IDA), IDA-LIKE (IDL), and CASPARIAN STRIP INTEGRITY FACTOR (CIF) are all classified in the PTM group (Matsubayashi et al. 2002; Ito et al. 2006; Amano et al. 2007; Matsuzaki et al. 2010; Aalen et al. 2013; Haruta et al. 2014; Doblas et al. 2017). In contrast to the PTM peptides, the cysteine-rich peptides are defined by the presence of an even number of cysteine residues (typically six or eight) that can form intramolecular disulfide bonds (Matsubayashi 2014), such as RAPID ALKALINIZATION FACTOR (RALF) (Pearce et al. 2001). Figure 18.1 Structure and maturation of secreted peptides. Most prepropeptides possess an N-terminal secretion signal peptide (yellow) and are cleaved by distinct proteases to release propeptides. Propeptides undergo various post-translational modifications (PTM), such as tyrosine sulfation, proline hydroxylation, and hydroxyproline arabinosylation, followed by proteolytic processing to form mature peptides and are grouped as PTM peptides. In contrast, those that undergo intramolecular disulfide bond formation are defined as cysteine-rich peptides. Abbreviations: A, arabinosylation; C or Cys, cysteine; H, hydroxylation; P, proline; S, sulfation; SP, signal peptide; Y, tyrosine. A flow diagram shows that secreted peptide gene undergoes transcription to form prepropeptide. Signal peptide cleavage of prepropeptide forms propeptide. P T M processing of propeptide forms mature P T M peptide. Processing disulfide bond formation gives rise to cysteine-rich peptide. https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9781003324942/6097e3ab-8b11-490d-b388-39ebf751fcc7/content/fig18_1_C.tif" xmlns:xlink="https://www.w3.org/1999/xlink"/>