Changes in Wheat P5CS Gene Expression in Response to Salt Stress in Wheat

Abstract

Drought and high salt are the most important adverse environmental factors that cause osmotic stress, negatively impacting plant growth and crop productivity. To maintain a stable intracellular environment in the presence of ex- ternal environmental stresses, many plants increase their cellular osmotic potential through accumulation of intracellular organic osmolytes such as proline, glycine betaine, mannitol and trehalose (Wang et al., 2007; Zhu, 2002). Proline is an osmo-protecting molecule that accumulates in many organisms, including bacteria, fungi, and plants, in response to low water stress and salinity (Claussen, 2005; Kumar et al., 2003). Proline accumulation in plants not only increases cell potential but also stabilizes proteins, membranes, and subcellular structures (Verslues et al., 2006). It also protects cells against oxidative damage by reactive oxygen species (Borsani et al., 2005; Sharma and Dietz, 2006; Vendruscolo et al., 2007).
In higher plants, proline is synthesized from glutamate or arginine/ ornithine. The pathway via arginine/ ornithine is not important for proline synthesis during osmotic stress (Hu et al., 1992). The first two steps of proline biosynthesis from glutamate are known to be catalyzed by a bifunctional enzyme Δ1-pyrroline-5-carboxylate synthetase (P5CS), which is a rate-limiting enzyme in the pathway, triggering both γ-glutamyl kinase (γ-GK) and glutamic-γ-semialdehyde dehydrogenase (GSA-DH) activities (Yoshiba et al., 1995).
In common with bacteria (Smith et al., 1984), proline controls the γ-GK activity of P5CS by feedback in plants (Hong et al., 2000). In higher plants, P5CS is encoded by a nuclear gene that was cloned from Vigna aconitifolia (Hu et al., 1992), Arabidopsis thaliana (Strizhov et al., 1997), Medicaho truncatula (Armengaud et al., 2004), Medicaho sativa (Ginzberg et al., 1998), and Lycopersicon esculentum (Fujita et al., 1998) and other species. The expression patterns of different P5CS genes in plants have been studied under various stress conditions. For example, the gene VaP5CS was highly expressed in leaves and roots of salt stressed V. aconitifolia (Hu et al., 1992). The AtP5CS1 gene was expressed in most plant organs and tissues in Arabidopsis and was upregulated by dehydration, high salinity, and abscisic acid (ABA) treatments, but was not expressed in dividing cell cultures in the absence of stress .Expression of AtP5CS2 in dividing cell cultures under stress induction was dependent on protein synthesis (Strizhov et al., 1997). In rice (Oryza sativa), expression of OsP5CS1 was induced by salt, drought, ABA, and cold, but not by heat treatment (Igarashi et al., 1997). RT-PCR analysis showed that the OsP5CS2 transcript was present in reproductive organs, especially in stamens (Hur et al., 2004). In response to NaCl stress, mRNA of tomPRO2 increased more than 3-fold, whereas the transcripts of tomPRO1 were undetectable (Fujita et al., 1998). Two P5CS genes of M. truncatula showed developmental and environment-specific features. MtP5CS1 in different organs had steady-state transcript levels that were well correlated with proline levels, whereas MtP5CS2 transcripts accumulated only in shoots of salt stressed plants (Armengaud et al., 2004). It appeared that MtP5CS1 acted as a developmental housekeeping enzyme responsible for the supply of proline to the reproductive organs, and MtP5CS2 acted as a shoot-specific osmoregulated isoform.