Amino Acids Proline and Amides

It has been reported that amino acids (such as alanine, arginine, glycine, serine, leucine, and valine, the nonprotein amino acids citrulline and orni-thine (Orn)), together with the imino acid Pro, and the amides such as glutamine and asparagine are accumulated in higher plants under salinity and drought stress (Dubey 1997; Mansour 2000). Pro is known to occur widely in higher plants and can be accumulated in considerable amounts in response to salt stress, water deficit, and other abiotic stresses (Ali et al. 1999; Kavi Kishore et al. 2005; Koca et al. 2007; Ahmad and Sharma 2008) . The Pro concentration is meta-bolically controlled. This imino acid is synthesized in plastids and cytoplasm while degraded to L-glutamate (Glu) in mitochondria. There are two different precursors of Pro in plants: Glu and Orn (Fig. 1.3). Pro is synthesized from Glu via glutamic-g-semialdehyde (GSA) and A'-pyrroline-5-carboxylate (P5C). P5C synthase (P5CS) catalyses the conversion of Glu to P5C, followed by P5C reductase (P5CR), which reduces P5C to Pro (Ashraf and Foolad 2007) . The other precursor for Pro biosynthesis is Orn, which is transaminated to P5C by a mitochon-drial Orn-g-aminotransferase (OAT) enzyme (Verbruggen and Hermans 2008). In the reverse reaction, Pro is metabolized to Glu in a feedback manner, via P5C and GSA with the aid of Pro dehydrogenase followed by P5C dehydrogenase (P5CDH) (Wang et al. 2003a).

The contribution of Glu and Orn pathways to stress-induced Pro synthesis differs between species, and it has been shown that stress-tolerant plants are able to accumulate Pro in higher concentrations than stress-sensitive plants. Slama et al. (2008) showed a positive correlation between Pro accumulation and tolerance to salt, drought, and the combined effects of these stresses. Osmotic stress (particularly mannitol stress) led to a considerable increase of the Pro concentration in the obligatory halophyte Sesuvium portulacastrum, while the contents in soluble sugars and in Na+ remained unchanged. In drought-stressed plants, the concentration of K+, Na+, Cl-, and Pro, as well as ornithine-8-aminotransferase (8-OAT) activity increased significantly. Inversely, Pro dehydrogenase activity was impaired. Re-watering leads to a recovery of these parameters at values close to those of plants permanently irrigated with 100% of field capacity. The presence of NaCl and mannitol in the culture medium (ionic and osmotic stress) led to a significant increase of the Na+ and Pro concentration in the leaves, but it had no effect on leaf soluble sugar content. Slama et al. (2007a, b) assumed that the ability of NaCl to improve plant performance under mannitol-induced water stress is caused by an improved osmotic adjustment through Na+ and Pro accumulation, which is coupled with the maintenance of the photosyn-thetic activity. Similarly, the Pro concentration in the roots of salt tolerant alfalfa plants rapidly doubled under salt stress and was significantly higher than in salt sensitive genotypes (Petrusa and Winicov 1997) , In addition to its role as an osmolyte for osmotic adjustment, Pro contributes to stabilizing subcellular structures (membranes and proteins) by forming clusters with water molecules which attach to proteins and membranes and prevent their denaturation (Koca et al. 2007; Ashraf and Foolad 2007; Lee et al. 2008). Due to its protective function on membranes it can also improve cell water status and ion homeostasis (Gadallah 1999; Gleeson et al. 2005), and it can scavenge free radicals and buffer cellular redox potential under stress conditions (Koca et al. 2007;

Ashraf and Foolad 2007; Lee et al. 2008). Pro is also involved in alleviation of cytoplasmic aci-dosis and sustaining NADP+/NADPH ratios at required levels for metabolism (Hare and Cress 1997), thus supporting redox cycling (Babiychuk et al. 1995).

Transgenic approaches proved an enhancement of plant stress tolerance via overproduction of Pro. For instance, transgenic tobacco (N. tabacum), overexpressing the p5cs gene that encodes P5CS, produced 10- to 18-fold more Pro and exhibited better tolerance under salt stress (Kavi Kishor et al. 2005). In Arabidopsis, the overexpression of an antisense Pro dehydrogenase cDNA resulted in an increased accumulation of Pro and a constitutive tolerance to freezing and a higher salt tolerance (Nanjo et al. 2003). Similarly, Borsani et al. (2005) reported that the Arabidopsis P5CDH (A'-pyrroline-5-carboxylate dehydrogenase) and SRO5, an overlapping gene of unknown function in the antisense orientation, produced two types of siRNAs, 24-nt siRNA and 21-nt siRNA. In fact, they compared the levels of salt stress-induced Pro accumulation in various mutant plants (dcl2, sgs3, rdr6, and nrpd1a) which lacked SRO5-P5CDH nat-siRNAs and cleavage of the P5CDH transcript, Pro accumulation was not significantly induced by salt stress or was induced to a lesser extent than in the corresponding wild type. This result is consistent with their inability to down-regulate P5CDH under stress, thereby causing a continued Pro catabolism and reduced Pro accumulation. In contrast, the dcl1 and rdr2 mutants, which were able to degrade P5CDH mRNA, had the same Pro level as the wild type under salt stress. The wild-type level of Pro accumulation in dcl1 indicates that although the 21-nt P5CDH nat-siRNAs were not produced, the 24-nt SRO5-P5CDH nat-siRNA alone was sufficient to cause the downregulation of P5CDH (Fig. 1.4).

An alternative approach to improve plant stress tolerance is the exogenous application of Pro, which can lead to either osmoprotection or cryoprotection. For example, in various plant species growing under salt stress, among them the halophyte Allenrolfea occidentals, exogenous application of Pro led to a higher osmopro-tection and an increased growth (Yancey 1994).

Fig. 1.4 Diagram of phased processing of SRO5-P5CDH nat-siRNAs and its role in a salt-stress regulatory loop (Borsani et al. 2005)

Fig. 1.4 Diagram of phased processing of SRO5-P5CDH nat-siRNAs and its role in a salt-stress regulatory loop (Borsani et al. 2005)

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