Diverse functions of plant peptides: entering a new phase.

In animals, a variety of extracellular molecules, including peptides and steroids, mediate cell–cell communication. Similarly, in plants, mobile phytohormones, such as auxins, cytokinins and brassinosteroids, have been recognized as major intercellular signals responsible for cell–cell communication. However, the recent discovery of peptides with diverse functions and of their elaborating signaling pathways in plants demonstrates the importance of peptide ligands in plant development (De Smet et al. 2009, Higashiyama 2010, Hirakawa et al. 2010b). The first functional plant peptide to be identified was tomato systemin, an 18 amino acid polypeptide which acts in the rapid expression of defense-responsive genes via cellular communication (Pearce et al. 1991). In the 1990s, Matsubayashi and Sakagami isolated a disulfated pentapeptide, named phytosulfokine [PSK; Tyr(SO3H)-Ile-Tyr(SO3H)Thr-Gln], as a potent mitogenic factor from conditioned medium derived from cultures of asparagus mesophyll cells (Matsubayashi and Sakagami 1996). Intensive studies over the past decade have highlighted diverse plant peptides as key players in the cell–cell communication governing plant development. In most cases, plant peptides are secreted out of cells and act at neighboring cells in a non-cell-autonomous manner. Secreted peptides can be categorized into three groups: (i) peptides that are subjected to proteolytic processing, and often undergo complex post-translational modification such as hydroxylation, glycosylation and sulfation; (ii) peptides that form intramolecular disulfide bonds followed by proteolytic processing; and (iii) peptides that form multiple intramolecular disulfide bonds without proteolytic processing (Matsubayashi 2011). The first group contains: PSK (five amino acid residues; Matsubayashi and Sakagami 1996); CLE [CLAVATA3 (CLV3)/ ESR-related] peptides (12–13 amino acid residues) including CLV3 peptide (Kondo et al. 2006, Ohyama et al. 2009), TDIF (TRACHEARY ELEMENT DIFFERENTIATION INHIBITORY FACTOR; Ito et al. 2006), CEP1 (C-TERMONALLY ENCODED PEPTIDE 1; 15 amino acid residues; Ohyama et al. 2008), and RGF1 (ROOT MERISTEM GROWTH FACTOR 1; 13 amino acid residues; Matsuzaki et al. 2010). There are also many putative peptides in this group (Matsubayashi 2011). The second group contains: RALF (RAPID ALKALINIZATION FACTOR; 49 amino acid residues; Pearce et al. 2001, Covey et al. 2010); and the EPF (EPIDERMAL PATTERNING FACTOR) family (45–75 amino acid residues), to which stomagen (Kondo et al. 2010, Sugano et al. 2010), EPF1 (Hara et al. 2007) and EPF2 (Hara et al. 2009, Hunt and Gray 2009) belong. The third group contains: defensins (approximately 50 amino acid residues; Carvalho and Gomes 2009); SCR/SP11 (S-LOCUS CYSTEINE-RICH PROTEIN/S-LOCUS PROTEIN 11; approximately 50 amino acid residues; Takayama 2001); TPD1 (TAPETUM DETERMINANT 1; approximately 150 amino acid residues; Yang et al. 2003); and LUREs (approximately 60 amino acid residues; Okuda et al. 2009, Higashiyama 2010). Many of these peptides function as ligands in local cell–cell communication governing the organization of various tissues and organs. For example, CLV3, TDIF, RGF1 and EPF1/EPF2/ stomagen play essential roles in the maintenance of shoot apical meristem (SAM) size (Ohyama et al. 2009), vascular stem cell maintenance (Hirakawa et al. 2010a, Hirakawa et al. 2010b), root meristem maintenance (Matsuzaki et al. 2010) and regulation of stomatal development (Hara et al. 2007, Hara et al. 2009, Hunt and Gray 2009, Kondo et al. 2010, Sugano et al. 2010), respectively. Defensin-like LUREs act as attractants, guiding pollen tubes to the embryo sac (Higashiyama 2010). Self-incompatibility in Brassica is controlled by a set of closely linked genes at the S locus, called the S haplotype. SCR/SP11, with a defensin-like structure, is involved in the inhibition of self-pollen germination and pollen tube growth (Takayama et al. 2001). The Arabidopsis thaliana genome contains >600 receptorlike kinase (RLK) genes (Shiu and Bleecker 2001). CLV3 and TDIF, which belong to different subgroups of CLE peptides, can be perceived by similar RLKs, CLV1 (Ogawa et al. 2008) and TDR (Fisher and Turner 2007, Hirakawa et al. 2008), respectively, which are members of the LRR-RLK (LEUCINERICH REPEAT RLK) class XI. PSK is also recognized by an LRR-RLK (Matsubayashi et al. 2002). In addition to these short peptides, relatively long peptides, such as members of the EPF family, may be perceived by LRR proteins, TMM (TOO MANY MOUTH) and/or LRR-RLKs, including Erecta, Erecta-like 1 and Erecta-like 2 (Shpak et al. 2005, Hara et al. 2007, Hara et al. 2009, Kondo et al. 2010, Sugano et al. 2010). However, our knowledge of the authentic receptors for many different types of peptide ligands is still very limited.