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Osmotic stress: Osmotic treatments using sorbitol induce high levels of Put and ADC in detached oat leaves (Flores and Galston, 1984). Spd and Spm show a dramatic decrease. Bouchereau et al. (1999) reports that osmotica with widely different assimilation routes, such as sorbitol, mannitol, proline, betaine and sucrose, all induce a rise in Put. These changes are coincident with measurable signs of stress, such as wilting and protein loss. Tiburcio et al. in 1995 reported that when peeled oat leaves are incubated with sorbitol in the dark, they lose chlorophyll and senesce rapidly. Senescence could be delayed by including Spm in the incubation medium. The senescence- retarding effect of Spm was correlated with increase in the incorporation of labeled precursors into proteins, RNA and DNA. They also concluded that osmotic shock in the dark induces an activation of the pathway catalyzed by ADC. Borrell et al. (1996) have reported the regulation of ADC synthesis by Spm in osmotically-stressed oat leaves using a polyclonal antibody to oat ADC and a cDNA clone encoding oat ADC. Treatment with Spm in combination with osmotic-stress resulted in increased steady-state levels of ADC mRNA, yet the levels of ADC activity decreased. This absence of correlation has been explained by the fact that Spm inhibits processing of the ADC proenzyme, which results in increased levels of this inactive ADC form and a subsequent decrease in the ADC- processed form (Boucherau et al., 1999). They also studied that in osmotically-stressed oat leaves, degradation of cytochrome thylakoid proteins and the enzyme Rubisco can be avoided by addition of Spm to the incubation medium. Thus post-translational regulation of ADC synthesis by Spm may be important in explaining its anti-senescence properties. Interestingly, Masgrau et al. (1997) concluded that the overexpression of oat ADC in tobacco resulted in similar detrimental effects to those observed by ADC activation induced by osmotic-stress in the homologous oat leaf stem (chlorosis and necrosis). Therefore, optimum levels of polyamines are necessary for the proper growth and development of the plants (Bouchereau et al., 1999). Recently, Liu et al. in 2010 have investigated the changes in the content and the form of polyamines (PAs) in the leaves of two wheat (Triticum aestivum L.) cultivar seedlings, differing in drought tolerance, under the osmotic stress by Polyethylene glycol (PEG) 6000 treatment. The results suggested that free-Spd, -Spm and PIS-bound Put (perchloric acid insoluble bound putrescine) facilitated the osmotic stress tolerance of wheat seedlings. The important roles of reactive oxygen species in the relationship between ethylene and polyamines (PA's) have also been investigated in leaves of spring wheat seedlings under root osmotic stress (Li et al., 2010).
Hypoxia: There has been lot of work done by Reggiani's grup on the role of polyamines under the hypoxia stress conditions. Reggiani et al., (1990) reported that that are many examples available where plant shoots and seedlings of different Gramineae species, when subjected to lack of oxygen, provide evidence of an association between tolerance and the capacity to accumulate Put. Species such as rice and barnyard grass which are adapted to germinate in an oxygen deprived environment, showed a greater capacity of Put accumulation than the anoxia-intolerant species (Reggiani and Bartani, 1989). This consideration supports the hypothesis for a role of Put as a protective compound against hypoxia (Reggiani and Bartani, 1990; Bouchereau et al., 1999). Reggiani et al. in 1989 has reported that Put is required for the anaerobic elongation of rice coleoptiles but it has no effect on aerobic elongation of rice coleoptiles where auxin is active. This group has also concluded that with a decrease in oxygen concentration, the conjugated Put became predominant in comparison with the free forms (80% at 0.3% oxygen) and that there is a negative correlation between Put accumulation (specially under conjugated forms) and shoot elongation (reggiani and Bartini, 1989; Bouchereau et al., 1999). On the other hand, the results of Lee et al., (1996) have indicated that increase in the activies of ADC and ODC and Put levels are essential for the elongation of Scirpus shoots grown under submergence.
Damage by Ozone: Ozone, the protective gas in the upper atmosphere is known to protect us from the harmful UV rays of the sun. But it is known to have serious effects on the vegetation. Experiments are ongoing throughout the world in this respect. According to Heagle et al. (1989) O3-stress can lead to a significant decline in net photosynthesis, cause leaf injury and accelerate senescence, even when applied at low levels. Reaction to this stress triggers many biochemical changes in the plants like increase in ABA, peroxidases, phenolic compounds, ethylene and polyamines which form a part of the plant self defense mechanism. Rowland- Bamford et al. in 1989 observed that the ADC activity in the ozone treated barley leaves increased before the damage became apparent. Many more examples have been quoted by Bouchereau et al. (1999) supporting the protective role of the polyamines during the ozone damage. Though the exact mechanism is not clear, but there can be a possibility of PA's being involved in the free radical scavenging (Bors et al., 1989). This is also supported by the fact that the levels of superoxide radical formed enzymatically with xanthine oxidase or chemically from riboflavin or pyrogallol were inhibited in vitro by Put, Spd or Spm at 10-50 mM (Drolet et al., 1986). Also, superoxide radical protection was inhibited by PAs when added to microsomal membrane preparations. These findings have been also supported by the fact that PA's tend to inhibit lipid peroxidation (Tang and Newton, 2005; Zhao and Yang, 2008). These conclusions were however disputed by the findings of Langebartels et al. (1991) as mentioned by Bouchereau et al. (1999). Leaf injury, caused by O3 in the tobacco cultivar Bel W3, could be prevented by feeding Put, Spd or Spm through the root. These exogenous treatments were correlated with a 2- to 3-fold increase in soluble conjugated Put and Spd (monocaffeoyl forms). Conjugated Put and Spd associated with cell wall and membrane fractions were increased 4- to 6-fold. When free PAs were assayed in vitro for their radical-scavenging properties, very low rate constants were found. On the other hand, PA conjugates had relatively high rate constants. It was thus concluded that free PAs could not account for the protection against O3 damage. But assuming their role in the ozone damage, it was suggested that the protective effect of exogenous free PAs was mediated by their prior conversion to conjugated forms. Consistent with this hypothesis, it was found that monocaffeoyl Put, an effective scavenger of oxyradicals, was present in the apoplastic fluid of tobacco leaves exposed to O3 (Langebartels et al., 2003). The results Navakoudis et al. (2003) also supports these findings according to which the enhanced atmospheric ozone is the accumulation of polyamines, generally observed as an increase in putrescine level, and in particular its bound form to thylakoid membranes. A study by Schraudner et al. (1990) also discovered a relationship between ethylene emission and PA biosynthesis was found in O3-treated potato and tobacco plants, the leaves of which show early senescence in response to the pollutant. In the presence of O3, all compounds of ethylene biosynthetic pathway in tobacco leaves were up-regulated. Put and Spd levels also increased, as did Ornithine Decarboxylase (ODC) activity (Bouchereau et al., 1999).
Integration of polyamines with other molecules during stress conditions
Polyamines affect several physiological processes in plants by activating the biosynthesis of signaling molecules like NO, H2O2; they affect ABA synthesis and signaling and are involved in Ca2+ homeostasis and ion channel signaling during the abiotic stress conditions Figure 1 summarizes this information.
Abscissic acid ABA) is an anti-transpirant that reduces water loss through stomatal pores on the leaf surface in response to water deficit, resulting in the redistribution and accumulation of ABA in guard cells and finally closure of the stomata (Bray 1997). Many authors (Liu et al. 2000; An et al. 2008; Alcazar et al., 2010) have reported that Put, Spd and Spm also regulate stomatal responses by reducing their aperture and inducing closure and that Put modulates ABA biosynthesis in response to abiotic stress. Thus polyamines are involved in the ABA mediated stress responses which affect the stomatal closure. Polyamines are also linked with reactive oxygen species (ROS) and NO signaling as amino oxidases during the catabolic process generate H2O2 which is a ROS (associated with plant defense and abiotic stress) and also there are evidences in which polyamines are reported to enhance the production of NO (Tun et al., 2006). NO is also known to enhance the salt stress tolerance in plants by regulating the content and proportions of the different types of free polyamines (Fan et al., 2010). According to Neill et al. (2008), both H2O2 and NO are involved in the regulation of stomatal movements in response to ABA, in such a way that NO generation depends on H2O2 production. Thus, altogether, the available data indicate that polyamines, ROS (H2O2) and NO act synergistically in promoting ABA responses in guard cells (Alcazar et al., 2010).
Polyamines are positively charged compounds, which can interact electrostatically with negatively charged proteins, including ion channels. Indeed, polyamines at their physiological
concentration block the fast-activating vacuolar (FV) cation channel in a charge-dependent manner (Spm, 4+ > Spd 3+ >> Put 2+), at both whole-cell and single-channel level, thus indicating a direct blockage of the channel by polyamines (Bruggemann et al. 1998). Aording to Alacazar et al. (2010), in response to different abiotic stresses, such as potassium deficiency, Put levels are increased drastically (reaching millimolar concentrations), whereas the levels of Spd and Spm are not significantly affected and this increase of Put may significantly reduce FV channel activity. In the similar manner, during another kind of stess, i.e., salinity, Bruggemann et al. (1988) has reported that all PA levels increase, and the enhanced Spm concentration probably blocks FV channel activity. These observations can be explained by the fact that as in animal and bacterial cells, polyamines in plants may thus modulate ion channel activities through direct binding to the channel proteins and/or their associated membrane components (Delavega and Delcour 1995; Johnson 1996; Alcazar et al., 2010). Phosphorylation and dephosphorylation of ion channel proteins are closely related to their activities. Thus, polyamines could also affect protein kinase and/or phosphatase activities to regulate ion channel functions (Bethke and Jones 1997; Michard et al. 2005; Alcazar et al., 2010). However, Zhao et al. (2007) points out that for elucidating the moleculat mechanisms underlying polyamine action, identification of ion channel structural elements and/or receptor molecules regulated by polyamines would be of great importance.
Polyamines also tend to maintain Ca2+ homeostasis. Several examples have been reported by Alcazar et al. (2010). Yamaguchi et al. (2006, 2007) proposed that the protective role of Spm against high salt and drought stress is a consequence of altered control of Ca2+ allocation through regulating Ca2+permeable channels. The increase in cytoplasmic Ca2+results in prevention of Na+/K+ entry into the cytoplasm, enhancement of Na+/K+ influx to the vacuole or suppression of Na+/K+ release from the vacuole, which in turn increases salt tolerance (Yamaguchi et al. 2006; Kusano et al. 2007; Alcazar et al., 2010). Thus polyamines have a definite role to play to main the calcium homeostasis during the stressed conditions.
Conclusions and Future Perspectives
Polyamines have now been considered as secondary messangers in addition to being known as vital plant regulators (Liu et al., 2007). Although the exact mechanism of action of polyamines during the stressful conditions is not known, genetic tools have been played useful; traditional quantitative trait locus (QTL) mapping (Alonso-Blanco et al. 2009) and by genome-wide association mapping (Nordborg and Weigel 2008) can be used for the identification of the genes underlying the mode of action and regulation of polyamines (Alcazar et al., 2010). Cloning of these genes would be another added advantage as these could be used in the same way as farm chemicals to alleviate or mitigate stress derived injury for crop protection. Transfer of such technology to the other crops will help in creating germplasms which would be better adapted to the harsh stressful conditions and thus contributing towards enhanced agricultural productivity.