Heat Shock Proteins (HSPs)
Heat shock proteins (HSP) are a family of proteins that are produced by cells in response to various stressfirl conditions. They were first described in relation to heat stress (Ritossa, 1962), but now well studied that these are also expressed during other stresses and also under unstressed conditions. Hsps are also called stress proteins or stress induced proteins (Lindquist and Crig, 1988; Morimoto et al, 1994; Gupta et al., 2010). All organisms respond to high temperature by modifying their gene expression pattern and turning on the heat shock gene. Heat stress activates the heat shock factors which in turn bind to heat shock elements/ promoters. Heat shock factors then up regulates the heat shock genes (Schoffl et al, 1998; von Koskull-Doring et al, 2007; Scharf et al, 2012). Heat shock proteins are the product of these heat shock genes.
Heat stress affects many basic physiological processes such as photosynthesis, respiration, and water relations (Wahid et al, 2007). Many researchers have suggested the five classes of HSPs characterized by their activities as molecular chaperones according to their approximate molecular weight viz., HsplOO, Hsp90, Hsp70, Hsp60, and small heat-shock proteins (Schlesinger, 1990; Schoffl et al, 1998; Kotak et al, 2007). Further Gupta et al (2010) classified the heat-shock proteins into families according to their, amino acid sequence, molecular weight, homologies and functions: HsplOO family, Hsp90 family Hsp70 family, Hsp60 family, and the small Hsp family. It has been reported that HSPs function as molecular chaperone, regulating the folding and accumulation of proteins as well as localization and degradation hi all plants and animal species (Feder and Hofmann, 1999 Schulze- Lefert, 2004; Panaretou and Zliai, 2008; Hu et al, 2009; Gupta et al, 2010). These HSPs, as chaperones, prevent the irreversible aggregation of other proteins and participate in refolding proteins during heat stress conditions (Tripp et al, 2009).
HSP60 is also known as chaperonin. It has been reported that HSP60 play important role in assisting rubisco (Wang et al, 2004). Some studies suggested that this class might participate in folding and aggregation of many proteins that were transported to organelles such as chloroplasts and mitochondria ((Lubben et al, 1989). HSP70 function as chaperones for newly synthesized proteins to prevent their accumulation as aggregates and folds in a correct way during their transfer to their final destination (Sung et al, 2001; Su and Li, 2008). In addition, Hsp70 and sHsps primarily act as molecular chaperone and play a crucial role in protecting plant cell from the damaging effects of heat stress (Rouch et al, 2004). Hsp90 binds with Hsp70 in many chaperone complexes and has important role in signaling protein function and trafficking (Pratt and Toft, 2003). One exclusive function of HSP100 class is the reactivation of aggregated proteins (Parsed and Lindquist, 1993) by resolubilization of non-functional protein aggregates and also helping to degrade irreversibly damaged polypeptides (Bosl et al, 2006; Kim et al, 2007). It has been suggested that HSP100 class also participates in facilitating the normal situation of the organism after severe stress (Gurley, 2000).
Plants have evolved many biochemical and molecular mechanisms in response to adverse environmental effects. One of the well studied stress responses is accumulation of osmolytes in plant cell during stress. Osmolytes are compounds that accumulate themselves in higher concentration under stress condition and protect the plants from the adverse effect of stress. Osmolytes protect enzyme and membrane integrity, along with adaptive roles in mediating osmotic adjustment in plants grown under water stress conditions. Transgenic plants over expressing the genes involved in the biosynthesis of these osmolytes have improved the tolerance to osmotic stress in many crops.
Important compatible osmolytes are proline, glycine betaine, sugars and sugar alcohols, which play important role in abiotic stress tolerance (Bohnert and Jensen, 1996; Rajam et al, 1998; Bohnert and Shen, 1999; Kumar et al, 2006). These osmolytes do not have any adverse effect on normal cellular functions even when present at high concentration (Yancy et al, 1982; Serraj and Sinclair, 2002). Accumulation of these molecules at higher concentration helps plants to preserve water within cells and protects cellular compartments from injury caused by dehydration or maintains turgor pressure during water stress. Furthermore, these molecules stabilize the structure and function of certain macromolecules, signaling functions or induction of adaptive pathways and scavenge reactive oxygen species (Hasegawa et al., 2000; Chen and Murata, 2002). Though, the molecular and cellular interactions of these solutes are not completely understood.