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Plants are exposed continuously to a variety to adversely changing environmental factors such as heat, cold, light, drought, acidity, alkalinity, oxidative damage and metal damage which affect their distribution, growth, development and productivity. These stressful conditions are associated with the losses in the productivity of many of the agriculturally important crops and therefore, also affect the economic returns of the country. Thus efforts are concerted worldwide to understand the mechanism of the stresses and studies performed to combat the problems before the damage occurs. There are several natural ways of self defense in the plants to cope with these stressful conditions: they can induce several functional or regulatory genes (Bartels and Sunkar 2005) or can undergo different physiological or biochemical changes. The accumulation of some functional substances, such as compatible solute and protective proteins, is an important element of the physiological and biochemical response to the stressful conditions (Liu et al., 2007). In addition to these response by the plants, molecules known as 'polyamines', have also been known to be an integral part of plant stress response (Bouchereau et al. 1999; Walters 2003; Alcázar et al. 2006b).
Polyamines, putrescine, spermine and spermidne and cadaverine, are one of the widely and distributed N containing organic molecules which were discovered more than 100 years ago and hold their significance from the minutest bacteria to multicellular pants, animals and mammals. In addition to their stabilizing effects, which they confer by binding to the intracellular anions (DNA, RNA, Chromatin and proteins), they are also known to possess several regulatory functions as well (Igarasahi et al., 2000: Alcazar et al., 2006b; Kusano et al., 2008; Alcazar et al., 2010). In plants, they have been associated with regulating many physiological processes, such as organogenesis, embryogenesis, floral initiation and development, leaf senescence, fruit development and ripening, and abiotic and biotic plant stress responses (Galston and Kaur-Sawhney 1990; Kumar et al. 1997; Walden et al. 1997; Malmberg et al. 1998; Bouchereau et al. 1999; Bagni and Tassoni 2001; Alcázar et al. 2006b; Kusano et al. 2008; Alcazar et al, 2010).
Several changes in the concentrations of the polyamines in the plant cells take place while responding to the stressful conditions (Bouchereau et al. 1999; Alcázar et al. 2006b; Groppa and Benavides 2008; Alcazar et al., 2010). The importance of this process can be exemplified by the fact that the levels of Put may accounted for 1.2% of the dry matter, representing at least 20% of the nitrogen (Galston 1991) in the stressful conditions. Though the exact mechanism of the involvement of polyamines during stressful conditions is not fully understood, studies are ongoing to study the molecular aspects (Liu et al., 2007; Alcazar et al., 2010). Evaluating the complete genome sequence of the Arabidopsis has facilitated the use of global 'omic' approaches in the identification of target genes in polyamine biosynthesis and signalling pathways (Alcazar et al., 2010). The advantages of the progress made in these directions has made possible the generation of Arabidopsis transgenic plants which are resistant to various stresses (Alcazar et al., 2010). Efforts can be made towards the development of such varieties for the agriculturally important crops as well. Such studies add to the economic potential from the agricultural sector touched by the biotechnological advances and hence further research in these directions is noteworthy.
Role of Polyamines in abiotic stress
Richards and Coleman in 1952 observed the presence of a predominant unknown ninhydrin positive spot that accumulated in barley plants when exposed to potassium starvation. This compound was identified as Put. Later on it was shown that K+-defcient shoots fed with L-14C-arginine produced labelled Put in a more rapid way compared to feeding with labelled ornithine (Alcazar et al., 2010). These results suggested that decarboxylation of arginine was the main way of accumulation of Put under K+ defciency (Smith and Richards 1964). The relevance of the ADC pathway in plant responses to abiotic stress was later on established by Galston et al. at Yale University (Flores and Galston 1982). It has been observed that polyamines accumulate during various stressful conditions (see Bouchereau et al. 1999; Alcázar et al. 2006b; Groppa and Benavides 2008; Alcazar et al., 2010). These all support the fact that polyamines do play a protective role durng the streeful conditions. Several examples have been quoted by Alcazar et al (2010) in which genetic modification of the genes involved in the biosynthetic pathway have proven useful in discerning the function of the polyamines in plant responses to the abiotic stress. The different stress factors have been briefly discussed below:
Mineral deficiency: This is one of the most common stress related factors affecting plants almost everywhere. However, studies related to this type of stress are often performed on leaves and/or seedlings, as the external symptons of deficiency become acute. The accumulation of Put in leaves of K+ deficient barley plants was first reported by Richards and Coleman and subsequent studies by others have established that specific role of Put in maintaining a cation- anion balance in plant tissues. As a result of K+ starvation, this diamine accumulation (via ADC activation), is widespread among mono- and dicotyledonous species and may well be a universal response (Bouchereau et al., 1999). The exact reason behind the increase in Put is unclear. The induced high levels of Put might be the cause of the stress injury. Put might also be beneficial for the plants. Alternatively, high levels of Put could be one of the many physiological changes induced by mineral nutrient deficiency without any special significance (Bouchereau et al., 1999). There are several other examples listing the changes in the polyamines content while responding to the mineral deficiencies (Geny et al., 197; Geny et al., 1998). However the changes differed according to the tissue and the stage of development.
Cold stress: The injury due to the cold causes alteration of the membrane structure as proposed by Raison and Lyons (Raison and Lyon, 1970) that the chilling injury involves phase transition in the molecular ordering of membrane lipids. This can cause several deleterious effects like increase in membrane permeability and alteration of the activity of membrane proteins. Cold treatment has been reported to increase the levels of Put and this correlates with the increase in the induction of Arginine Decarboxylase (ADC) genes (ADC1, ADC2 and SAMDC2) (Urano et al. 2003; Cuevas et al. 2008; Cuevas et al. 2009). On the other hand, levels of free Spd and Spm remain constant or even decrease in response to cold treatment (Alcazar et al., 2010). The absence of correlation between enhanced SAMDC2 expression and the decrease of Spm levels may be a result of increased Spm catabolism (Cuevas et al. 2008; Alcazar et al., 2010). Boucereau et al. (1999) reports that in the chilling-tolerant-cultivar, chilling induced an increase of free Absicissic acid (ABA) levels first, then ADC activity and finally free Put levels. Fluridone, an inhibitor of ABA synthesis, inhibited the increase of free ABA levels, ADC activity and free Put levels in chilled seedlings of the chilling-tolerant cultivar. These effects resulted in a lower tolerance to chilling and could be reversed by the pre-chilling treatment with ABA. All these results suggest that Put and ABA are integrated in a positive feedback loop, in which ABA and Put reciprocally promote each other's biosynthesis in response to abiotic stress (Figure 1). This highlights a novel mode of action of polyamines as regulators of ABA biosynthesis (Alcazar et al., 2010).
Thermal stress: When exposed to heat stress, plants have the ability to synthesize uncommon long chain PA's (caldine, thermine). The levels of free and bound PAs, as well as ADC and Polyamine oxidases (PAO) activities, were higher in tolerant than in sensitive cultures (Kuehn et al., 1990; Philipps et al., 1991; Roy and Ghosh 1996; Bouchereau et al., 1999). The increased activities of the transglutaminases indicated the high content of the polamines. This indicates a correlation between heat-stress tolerance, ADC, PAO and transglutaminase activities (Bouchereau et al., 1999).
Dought stress: Certain plants during water scarcity tend to accumulate putrescine (Put) which is supported by the fact that transcript profiling under these conditions induces the expression of certain genes involved in the biosynthetic pathway. The expression of some of these genes is also induced by ABA treatment (Perez-Amador et al. 2002; Urano et al. 2003; Alcazar et al., 2010). This throws light upon the fact that up- regulation of PA-biosynthetic genes and accumulation of Put under water stress are mainly ABA-dependent responses (Alcazar et al., 2010).
Salt stress: Differences in PA (Put, Spd, Spm) response under salt-stress have been reported among and within species. For example, according to Prakash et al. (1988), endogenous levels of PAs (Put, Spd and Spm) decreased in rice seedlings under NaCl stress, whereas Basu et al. (1988) reported that salinity results in accumulation of these compounds in the same material (Bouchereau et al., 1999). Santa-Cruz et al. (1997) reported that the (Spd+Spm):Put ratios increased with salinity in the salt-tolerant tomato species (Lycopersicon pennellii, Carrel D'Arcy) but not in the salt-sensitive tomato species (L. esculentum). In both species, stress treatments decreased the levels of Put and Spd. The Spm levels did not decrease with salinity in L. pennellii over the salinization period, whereas they greatly decreased in L. esculentum. The effects of different NaCl concentrations on maize embryogenic cells derived from immature embryo cultures of a salt-sensitive inbred line (cv, w64) and a resistant hybrid (cv Arizona) have also been reported where increased salt concentration remarkably decresed the growth of the calluses and showed a significant increase in the total PA (Put, Spd) content, especially caused by a rise in Put. It has been reported by Bouchereau et al .(1999) that using the inhibitors of Put synthesis, the ADC pathway in tomato plants operates in both stress and control conditions, whereas the ODC pathway is prompted only in the stress conditions. This finding is further supported by the studies of Urano et al. (2003) who concluded that the expressions of the Arginine decarboxylase 2(ADC 2) and spermine synthases (SPMS) during the 24 hour stress treatment maintained and hence increased the levels of Put and Spm. Yamaguchi et al. in 2006 also suggested the protective role of Spm polyamine when its addition suppressed the salt senstivity in Spm deficient mutants. Bouchereau et al . (1999) suggests that polyamine responses to salt stress are also ABA-dependent, since both ADC2 and SPMS are induced by ABA. Infact, Alcazar et al. (2006) reports that stress-responsive, drought responsive (DRE), low temperature-responsive (LTR) and ABA-responsive elements (ABRE and/or ABRE-related motifs) are present in the promoters of the polyamine biosynthetic genes. This also reinforces the view that in response to drought and salt treatments, the expression of some of the genes involved in polyamine biosynthesis are regulated by ABA (Alcazar et al., 2010). The study of the Arabidopsis thaliana flowers by Tassoni et al. (2010) has also supported the hypothesis that polyamine levels (mainly Spm here) increase with the increase in the salt concentration and therefore contribute to the plant tolerance during the stressful conditions.