Until recently, various physiological and biochemical processes of plants were thought be regulated by the classical five phytohormones i.e. auxins, cytokinins, gibberellins, abscisic acid, and ethylene. But this view is changing. Recently jasmonic acid, brassinosteroids, polyamines, salicylic acid and strigolactones have been added to the group of plant hormones. There is a new entrant in the group Peptide hormone. There are compelling evidences that suggest that some peptides function as signaling molecule in the cell to cell interactions to coordinate and integrate cellular functions such as cell division, cell differentiation, reproduction, nodulation and plant defence, environmental responses, anitcarcinogenic, anitoncogenic properties and many others. Further these peptides are effective at very low concentrations.
On the basis of their production peptide hormones can be classified into two class, peptides produced by selective action of peptidases on larger precursor proteins and peptides that are the result of protein turnover. Some peptides also need post transcriptional modifications for their activity such as hydroxyprolination, tyrosine sulphation and arabinosylation for bioactivity. A number of peptide hormones have been identified with diverse functions. Some of them are involved in defence and others in non defence functions. Systemin was the first plant peptide confirmed to be a plant hormone. It was isolated from tomato leaves.
Systemin act as a signal to induce protease inhibitor and are involved in wound response in solanaceae family. It is 18 amino acid peptide processed from a precursor prosystemin. Phytosulfokine-alpha (PSK-alpha), a sulfated growth factor ubiquitously found in monocotyledons and dicotyledons, very active in promoting proliferation of plant cells in culture. ROTUNDIFOLIA4/DEVIL1 (ROT4/DVL1) was involved in regulation of polar cell proliferation. POLARIS (PLS), peptide predicted to be 36 amino acid peptide, was involved in vascularization, longitudinal cell expansion and increased radial expansion. SCR/SP11are small peptides produced by the anther cells and is involved in self-incompatibility of Brassica species. Inflorescence deficient in abscission (IDA) peptide family has important role in petal abscission. Cyclotides are a family of peptide hormones with a unique head to tail cyclized backbone. This cyclization provides unique feature to cyclotides such as thermostability and resistance to enzyme hydrolysis. AtPep1 is a 23 amino acid peptide found in Arabidopsis thaliana. This peptide acts as an elicitor resulting in the activation of defence genes and turns on the immune response in plants.
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