Peptide signaling plays a significant role in various aspects of plant growth and development and specific receptors for various peptides have been identified as being membrane-localized receptor kinases, the largest family of receptor-like molecules in plants. Signaling peptides include members of the following protein families. Systemin — is a small polypeptide functioning as a long-distance signal to activate chemical defenses against herbivores. It was the first plant hormone proven to be a peptide. Systemin induces the production of protein defense compound called protease inhibitors. Systemin was first identified in tomato leaves. It was found to be an 18-amino acid peptide processed from the C-terminus of a 200-amino acid precursor, which is called prosystemin. CLV3/ESR-related ('CLE') peptide family — CLV3 encodes a small secreted peptide that functions as a short range ligand to the membrane-bound CLV1 receptor like kinase that together with CLV2 (a receptor-like protein) function to maintain stem cell homeostasis in Arabidopsis shoot apical meristems. Although the maize embryo-surrounding region protein (ESR). and CLV3 are very different, they are both members of the CLE peptide family given that they share a short conserved 14-amino acid sequence at the carboxy terminal region. To date, more than 150 CLE signaling peptides are identified. This proteolytically processed bioactive region is important for both promoting and inhibiting cellular differentiation in both apical and cambial meristems. Recently it was found that CLE25 can act as a long distance signal to communicate water stress from the roots to leaves. ENOD40 — is an early nodulin gene, hence ENOD, that putatively encodes two small peptides, one of 12 and the other of 18 amino acid residues. Controversy exists on whether the mRNA or peptides themselves are responsible for bioactivity. Both peptides have been shown "in vivo" to bind to the 93 kDa subunit of sucrose synthase, an essential component in sucrose metabolism. Sucrose degradation is a key step in nitrogen fixation, and is a pre-requisite for normal nodule development. Phytosulfokine (PSK) — was first identified as a "conditioning factor" in asparagus and carrot cell cultures. The bioactive five amino acid peptide (PSK) is proteolytically processed from an ~80 amino acid precursor secreted peptide. PSK has been demonstrated to promote cellular proliferation and transdifferentiation. It has been demonstrated that PSK binds to a membrane bound LRR receptor like kinase (PSKR). POLARIS (PLS) — The PLS peptide has a predicted length of 36 amino acids however possesses no