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The extracellular region of the receptors contains an LRR, which forms a superhelix structure providing the structural backbone to form an interaction surface for a corresponding peptide ligand. Co-crystal structures of several peptide–receptor pairs have been solved recently ( Song et al. Binding of the peptide hormones to their receptors is thought to recruit additional co-receptors for the activation of downstream signaling events by transphosphorylation between kinase domains in proximity ( Fig. 2017) are in the subclass XI of the LRR-RK family, while the PSK receptor PSKR is in the subclass X ( Matsubayashi et al. The receptors for CLE/TDIF ( Hirakawa et al. LRR-RKs are single transmembrane domain kinases containing extracellular LRRs which can participate in versatile molecular recognition.
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The major receptor class is the LRR-RKs ( Shiu and Bleecker 2001). Receptors for these peptides have been identified genetically and biochemically ( Butenko et al. 2013) such as tyrosine sulfation, proline hydroxylation and hydroxyproline arabinosylation. 2016) and modifications of specific residues by modifying enzymes ( Hieta and Myllyharju 2002, Tiainen et al. They are encoded in the genome as precursor proteins and mature into active forms via post-translational processing including proteolytic cleavage by proteases ( Tamaki et al. 1991, Matsubayashi and Sakagami 1996, Pearce et al. Small peptide signals of plants include systemin, PSK (phytosulfokine), HypSys (hydroxyproline-rich glycopeptide systemin), Pep1, CLE (CLAVATA3/EMBRYO SURROUNDING REGION-related)/TDIF (tracheary element differentiation inhibitory factor), PSY (plant peptide containing sulfated tyrosine), CEP (C-terminally encoded peptide), RGF/CLEL/GLV (root meristem growth factor/CLE-like/GOLVEN), PIP (PAMP-INDUCED PEPTIDE), IDA (INFLORESCENCE DEFICIENT IN ABSCISSION) and CIF (Casparian strip integrity factor) subclasses ( Pearce et al. Made in Plants Peptide classes, receptors and structural insights In addition to the naturally occurring mechanisms shaping these peptide hormones, we will introduce synthetic approaches to design novel bioactive peptides in the last section. 2016, Zipfel and Oldroyd 2017, Stegmann et al. Since this review focuses on molecular structures of peptides, especially in dicots, please refer to other literature for the latest information and discussion on biological functions of peptide signals in diverse plant species including monocots ( Grienenberger and Fletcher 2015, Higashiyama and Takeuchi 2015, Je et al. In the first and second sections, we will describe variations found in plants and parasitic phytopathogens, respectively. Interestingly, such peptide signals are also made by phytopathogens to hijack functions of host plants. In this mini-review, we focus on the short peptides, all of which are perceived by leucine-rich repeat receptor kinases (LRR-RKs). The first two are both approximately 5–20 amino acids in length and do not undergo intramolecular disulfide bonding while the CRPs consist of 50–100 amino acids and have a relatively fixed structure due to intracellular disulfide bridges ( Ohki et al. Based on (i) mature peptide structures and (ii) modes of trafficking into the extracellular space, these peptide signals, or plant peptide hormones, are classified into three groups: secreted small peptides, non-secreted small peptides and secreted cysteine-rich peptides (CRPs) ( Matsubayashi 2014). They act extracellularly through recognition by their receptors on the plasma membrane of target cells. 1991), a number of different classes of peptide signals have been identified. Since the discovery of tomato systemin ( Pearce et al. In this mini-review, we aim to discuss how molecular structures of plant peptide hormones have been shaped and how one can design artificial peptide hormones with novel biological functions.