Polyamines (Plant Hormone)
Polyamines (Plant Hormone)
Polyamines are ubiquitous low molecular weight amino group containing organic molecules with cationic property and appear to be necessary for plant growth and development. Polyamines and their biological activities in animals and bacteria have been known for a very long time. But, perhaps the first plant physiological publication on polyamines is that of F.J. Richards and R.G. Coleman, published in 1952 in “Nature”, where they demonstrated that the deficiency of potassium in barley caused a dramatic accumulation of polyamine, putrescine. It is only during the last two decades (70’s and 80’s) that the importance of polyamines as growth regulators has been recognised. The pioneering work in this direction was done by N. Bagni and D. Serafini-Fracassini (1974), who demonstrated that polyamines stimulated growth in sunflower explants (cultured tissues). Evidences are now available to indicate the role of polyamines in growth, differentiation, stress and senescence. They have also been considered to act as secondary messengers in cytokine action and perhaps in some other regulatory responses.
Polyamines are organic compounds with more than one amino group. Some important polyamines and their structure formula are as follows:
- Diamino propane
Stimulation of growth and morphogenesis-
Exogenously supplied polyamines are known to stimulate growth of intact plants and of cultured explants in many species. In fact in many tissue culture protocols, the addition of a polyamine is essential to obtain callus formation and for organogenesis. The endogenous level of polyamines also seems to very well correlated with the embryogenesis in cultured callus and also with the growth of the plants. The increase in growth or embryogenesis is considered to be due to the stimulatory effect of polyamines on mitotic cell division.
Promotion of seed germination and seedling growth-
Increased rate of seed germination by 0.1 or 1mM polyamines has been reported in chickpea, maize and a few others species. The polyamines also counteract the inhibitory effect of high temperature and of abscisic acid on seed germination in chickpea. The endogenous polyamines are believed to play an important role in seed germination and seedling formation. Treatment of Picea abies seeds with the inhibitors of polyamine biosynthesis reduces seed germination.
The effect of polyamines on seedling growth is rather less clear cut, although in a few species the seedling growth is stimulated. The stimulation is more pronounced on shoot growth than on root growth.
Delaying of senescence-
Perhaps the most significant effect of polyamines is on senescence. They are known to retard dark induced senescence by inhibiting chlorophyll degradation in detached leaves of barley, radish, soyabean, rice, mustard and Hydrilla. However, they usually accelerate chlorophyll degradation in light conditions. In oat leaves, dark induced senescence is retarded by ImM polyamines diaminopropane, spermidine or spermine apparently by stabilizing the thylakoid structure and hence regulating the loss of chlorophyll. Stabilization of membranes of cell and cell organelles is considered to be the most important factor in the retardation of senescence by polyamines.
In greening tissues, the polyamines seem to have either no effect or inhibitory on chlorophyll biosynthesis. However in light grown maize seedlings they seem to promote anthocyanin synthesis.
Polyamines are nitrogenous molecules and hence they are expected to influence basic nitrogen metabolism in plants. In cultured cells and tissues, they are able to fulfill the nitrogen requirement. In nitrate supplied cultures and seedlings, their endogenous level is either unaffected or is decreased. However, under these conditions, the exogenously supplied polyamines usually stimulate nitrate assimilation. In ammonium supplied cultures, there is an increase in endogenous polyamine level. It is believed that polyamines act as metabolic buffers under the conditions of excess ammonium.
Mechanism of Action
Induction of gene expression-
Because of their polycationic charge, polyamines are able to bind with the negatively charged DNA and thereby regulating transcription and translation processes. In the embryonic axis of chickpea, spermine supply enhances the formation of some m-RNA species. The induction or repression of some enzymes in response to polyamines are also known.
Free radical scavenging-
As mentioned earlier, the most significant effect of polyamines is on senescence, which is usually linked with free radical generation and their damaging effects on cellular components. The free radical of oxygen (superoxide anion) and of hydroxyl are known to be produced during senescence which cause membrane disintegration and celluar damage. It is believed that polyamines inhibit the production of these free radicals during senescence.
Action through ethylene-
Ethylene and polyamines have opposite effects on fruit ripening and senescence; while ethylene accelerates these processes < polyamines retard them. Both are however, synthesized from a common precursor, S-adenosylmethionine, and in some cases there might be a competition for this substrate, between the two growth regulators. It is likely therefore, that in those cases polyamines affected plant growth through reduced ethylene biosynthesis. In fact, polyamines have been reported to inhibit ethylene production in a number of plant tissues including fruits, leaves, petals and hypocotyls.
Polyamines re-synthesized from the amino-acid ornithine. Perhaps the first polyamines to be synthesized is putrescine; and spermine and spermidine are derived from it. One of the important intermediates in poylamine biosynthesis is S-adenosylmethionine (SAM) which is also a precursor for ethylene biosynthesis.
These can be oxidised by diamine oxidases and polyamine oxidases.
Diamine oxidases are of widespread occurrence in plants and they are particularly active in leguminous species. Polyamine oxidases have been reported from barley roots and oat leaves.
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