"Review of literatureperoxidation and a free radical scavenger (Ashraf and Foolad, 2007; Trovato et al.,1 2008). Accumulation of proline can be achieved by overexpressing ? -pyrroline-5- carboxylate (P5C) reductase (P5CR) enzyme, as found in tobacco (Konstantinova et al.,2002). Transgenic petunia plants expressing AtP5Cs gene has accumulated high prolineand exhibited tolerance to drought stress (Yamada et al., 2005). The accumulation ofelevated proline content was also reported by Hsieh et al., (2002) in transgenic tomatoplants expressing Arabidopsis AtCBF1 gene under water stress conditions.Soluble Sugars Another physiological response of plants to water stress is a synthesis of sugarswhich protect plant cells from dehydration by maintaining osmotic equilibrium.Understress conditions, non-structural carbohydrates like sucrose, hexoses and sugar alcoholsaccumulate in varying amounts in different plant species. The accumulation of sugars in plants plays important roles in osmoticadjustment by maintaining turgor potential of the cell. A strong correlation betweenaccumulation of sugars and osmotic stress tolerance has been widely reported,including transgenic experiments (Taji et al., 2002). Soluble sugars function as typicalosmoprotectants by stabilizing cellular membranes and maintaining cell turgor (Blackand Pritchard, 2002). Sugar accumulation is facilitated by the expression of two genesnamely sucrose synthase and sucrose phosphate synthase (Ingram et al., 1997).Sugars are known for their role as products of hydrolytic processes, substrates inbiosynthesis processes, energy production and signal for metabolic regulation(Gibson, 2005). It was reported that during dehydration a resurrection plant,Craterostigma plantagineum produces an unusual 8 carbon sugar octulose which israpidly converted into sucrose (Norwood et al., 2000). Garg et al., (2002) have reported13 Review of literaturethat accumulation of sugars like trehalose improves the drought tolerant capacity oftransgenic rice plants. Proteins and chaperonesWater stress has a profound effect on plant metabolism and results in areduction in protein synthesis. Water stress can cause denaturation and dysfunctioningof many proteins. The broad category of proteins required for protein homeostasis(chaperones) includes Protein L-Isoaspartyl Methyl Transferase (PIMT), Heat ShockProteins (HSPs), Binding Proteins (BiPs), Late Embryogenesis Abundant (LEA)proteins, myo-inositol 1-phosphate synthase (MIPS), polyamines, proteases andproteinase inhibitors, scavenging and detoxifying enzymes water channel proteins likeaquaporins. These proteins have determining roles in protection and alleviation ofeffects of stress at the cellular level. HSPs and LEA proteins help to protect againststress by controlling the proper folding and conformation of both structural (i.e. cellmembrane) and functional (i.e. enzymes) proteins (Kotak et al., 2007). Heat-shock proteins (HSP) Hsps were first identified as proteins that are strongly induced by heatshock. They help in folding, assembly, translocation and degradation of proteins andin a broad array of normal cellular processes. They also function in thestabilization of proteins and membranes and can assist in protein refolding understress conditions.These HSPs are suggested to act as molecular chaperones in proteinquality control ( Kotak et al., 2007). Hsps are usually undetectable under normalgrowth conditions but can be induced by environmental stress and developmentalstimuli. Accumulation of Hsps coincides with the acquisition of stress tolerance. Fivemajor families of Hsps are recognized:the Hsp70 (DnaK) family; the chaperonins(GroEL and Hsp60); the Hsp90 family; the Hsp100 (Clp) family; and the small Hsp14 Review of literature(sHsp) family. There are several reports on the function and involvement of theseproteins under stress. Overexpression of Hsp70 genes induces thermal tolerance andincrease resistance to environmental stresses (Wang, et al., 2004). Hsps were found tobe expressed in seeds of Arabidopsis in response to desiccation stress.However, thereduction in the expression of Hsp proteins was found in desiccation-sensitiveArabidopsis mutant seeds, suggesting the protective role of Hsp in abiotic stresstolerance (Wehmeyer and Vierling, 2000). Sun et al., (2001) has found that thetransgenic Arabidopsis plants overexpressing AtHSP17 have accumulated high levels ofAtHSP17 gene which helps in imparting tolerance towards drought and salinity stress. Late Embryogenesis Abundant proteins (LEA)LEA proteins are low molecular weight (10-30 kDa) proteins, found in bothplants and animals. Plant LEA proteins were first characterized in cotton and wheat(Cuming, 1999). Most of the LEA proteins are hydrophilic due to the biased amino acidcomposition (Wise and Tunnacliffe, 2004). LEA proteins have been classified intodifferent groups group1 to group5 based on conserved structural features (Dure andVerma, 1993). LEA proteins involve in protecting higher plants from damage caused bydrought and cold shock.LEA and dehydrins are the most abundant plant proteins produced during lateembryogenesis or in response to drought, low temperature, salinity and ABA in manyplants including Arabidopsis, wheat, barley and rice (Close, 1997). LEA proteins areknown to be induced by osmotic stress or exogenous application of ABA (Ingram andBartels, 1996; Campalans et al., 2001). LEA proteins are known to exhibit anti- aggregation activity under water stress conditions (Tunnacliffe et al., 2005). LEAproteins accumulate in high levels in both prokaryotes and eukaryotic organisms. Theyare known to express in vegetative organs during water deficit, suggesting a protective15 "