Sunday, June 26, 2011

Understanding of involvement of polyamines in plants

Polyamines (PAs) are naturally occurring polycationic aliphatic amines, which due to their ubiquity and versatility are involved in the regulation of various cellular and molecular processes. They are positively charged compounds with their charge distributed along the molecule. The common PAs, spermidine (SPD) and spermine (SPM) and their diamine precursor putrescine (PUT) play a critical role in the normal functioning of all cells. They are involved in the cellular functioning both at the molecular and physiological levels due to their association with various Plant macromolecules (DNA, RNA and proteins) and membranes as well as their high concentration in the cytosol thus behaving as osmolytes. The role of PAs is much better studied in animal systems than plants, though they have been suggested to have a role as new plant growth regulators either by mediating the plant hormone effects or independently signalling other responses.

PAs exist in three forms in the cell, viz. as free cations, covalently bound to low molecular weight phenolic compounds like hydroxycinnamic acids (conjugated form of PAs) and bound to marcomolecules or membranes (bound form of PAs). Though the major form is the free cationic form of PAs, there are instances when the amounts of conjugated form exceed the free form and these are known to be critical in certain physiological processes including seed germination, flower development, defence responses and stress reactions. Besides PUT, SPD and SPM, there are certain unusual PAs found in nature, e.g. thermo SPM which have been detected in bacteria residing in hot springs and they seem to be important in protecting the enzymes from heat denaturation and aminobutylhomo-SPD found in fast growing cells of root nodule bacteria Rhizobium. NorSPD and norSPM are found in thermophilic red algae, brown algae,and Chlamydomonas, Nitella and Chlorella. Similarly, some unusual PAs have been reported in plants, homo-SPD was first detected in sandalwood and also in mosses and ferns. In leguminous plants, other unusual PAs like canavalmine, homoagmatine, aminopropylcanavalmine and aminobutylcanavalmine have been detected. NorSPD and NorSPM have been detected in alfalfa grown under drought conditions and have been postulated to play a protective role under stress conditions. As a matter of fact it has been suggested that PA distribution, especially of SPM, may serve as a phylogenetic marker.

PAs have been demonstrated to be associated with regulation of somatic embryogenesis, root and shoot formation, flower and fruit development , stress responses and senescence. In fact, PAs may serve as ‘biomarkers’ for in vitro morphogenetic potential including plant regeneration via somatic embryogenesis. The multifaceted functions of PAs as well as the variations in their levels in response to changes in the physiological state, point towards their role as possible second messengers, though their high titres do not support the view. Various studies have been conducted to investigate the involvement of PAs in cell functioning, using mutants of PA biosynthetic genes and specific substrate-based inhibitors of PAs. Though much information could be generated regarding the involvement and possible mechanisms of action, no clear picture of their functioning emerged. Hence, transgenic plants expressing PA biosynthetic genes in constitutive and regulated manner were generated, with an aim to answer some of the queries regarding the functioning and role of PAs.

Tags: Bio Technology, Bio Genetics, Polyamines in Plants