THE ROLE OF PHOSPHOLIPASE C IN SIGNAL TRANSDUCTION DURING DROUGHT AND COLD STRESSES IN WHEAT (Triticum aestivum L.)

Abstract

Environmental stresses are one of the major challenges to crop production. Molecular biologists have investigated these problems through the study of the genes whose expression could be changed in response to stress with the hypothesis that these genes may contribute to tolerance to these stresses. Zhu (2002) stated that salt and drought stress signal transduction consists of ionic and osmotic homeostasis signaling pathways, detoxification response pathways, and growth regulation pathways. Some phospholipids systems are activated by osmotic stress, generating a diverse array of messenger molecules, some of them function upstream of the osmotic stress-activated protein kinases. Also, ABA biosynthesis is regulated by osmotic stress. Both ABA dependent and independent osmotic stress signaling start with the modification of the expressed transcription factors, leading to the expression of early response transcriptional activators, which then activate downstream, stress tolerance effect or genes. Many cold responsive genes have been identified and characterized since the 1980’s. More recently DNA microarrays analysis using a 24 K GeneChips representing about 24000 Arabidopsis genes have been used to characterize changes in gene expression in response to cold treatment (Zhu et al., 2007). One study identified 655 genes cold upregulated and 284 as cold downregulated genes after chilling treatment (Lee et al., 2005). Many of the early cold responsive genes encode transcription factors that probably activate the genes that are induced after longer periods of exposure to the cold. Only one transcription factor was down regulated during early cold stress, suggesting that cold responses in plants are started by transcriptional activation and not by repression of genes. To investigate the interaction of proteins that may have signalling or regulatory function, protein-protein interaction was studied with yeast 2 hybrid analysis (Tardif et al., 2007). Phospholipases are sets of enzymes which have the ability to hydrolize phospholipids. They have different forms in plants, including phosphlipases C, D and A. Phospho-lipase C and D have important roles in signal transduction. The general structure of phospholipids consists of two fatty acyl chains esterified to a glycerol backbone at the sn-1 and sn-2 positions, and a phosphate at sn-3 position to which a variable head group (R) is attached. Phos-phatidylglycerol plays a specific role in photosynthesis (Sakurai et al., 2007) by helping in the organization and function of thylakoid membranes in plant cells. The first cell change during stress injury is an alteration in the structure and function of cell membranes. In many plants, alterations in lipids, particularly in phospholipids and sterols were very clear due to water stress (Liljenberg, 1992) or salt treatment (Kuiper, 1984). These alterations in the membrane seem to control the fluidity and the surrounding proteins, which influences membrane functions such as bilayer permeability carrier-mediated transport and the activity of membrane bound enzymes including ATPase activity (Surjus and Durand, 1996). In plants, G-proteins are involved in the regulation of ion channel and abscisic acid signalling, modulation of cell proliferation, and in many other processes such as seed germination, shoot and root growth, and stomatal regulation. Phospholipase C (PLC) has been shown to function as an intercellular effector molecule in plants for the α-subunit of pea heterotrimeric G-proteins whose activity is regulated by salinity stress (Misra et al., 2007). Gα protein interacts with pea phos-pholipase-C (PLC) at the calcium-binding domain (C2) leading to increasing GTPase activity, thus the signal transduction can be reduced by PLC association with the Gα-protein (Tuteja, 2007). The main objective of this study was to characterize the expression profiles of three full length cDNA clones from two phosphogyceride specific phospholipase C’s wheat genes under drought and cold stresses.