Role of Chloroplast Heatshock Proteins Involved in Abiotic Stress Tolerance in Wheat

ROLE OF CHLOROPLAST HEATSHOCK PROTEINS INVOLVED IN ABIOTIC STRESS TOLERANCE IN WHEAT.

INTRODUCTION:

Plants being sessile organisms have to survive with different environmental factors. All the natural factors including climatic, biotic and abiotic put the plants under stress conditions. Furthermore, anthropogenic activities are responsible to put extra stress on plants by changing the temperature and concentration of metals & ions etc. in soil (Al-Whaibi 2011).

The field crops are routinely exposed to combination of abiotic stresses. Co-occurrence of stresses is more lethal rather than a single stress condition. For example, drought and heat stress occurs in field simultaneously (Maestri et al. 2002; Mittler 2006). Cereals such as wheat (Triticum aestivum) and rice (Oryza sativa), vegetables such as potato (Solanum tuberosum) are suceptible to the abiotic stresses. Human population is increasing exponentially, so there is need to increase the production of stress tolerant varieties to minimize the food insecurity in society (Vasquez-Robinet 2007; Maestri et al. 2002). Cereal grains have been the principal component of human diet for thousands of years and have played a major role in shaping human civilization. Around the world, wheat is important staple critical to daily survival of billions of people. More than 50% of world daily caloric intake is derived directly from cereal grain consumption (Awika 2011).

Wheat is adversely affected by abiotic stresses. Among different abiotic stress, drought stress and salinity are the biggest threats to crop growth and yield. One of the major and harmful abiotic stresses is soil salinity. The salt stress is effecting the wheat on large scale and responsible to decrease the quality and quantity of yield (Yamaguchi and Blumwald 2005). Drought stress brings about a reduction in growth rate, stem elongation, leaf expansion and stomatal movements. Furthermore, it causes changes in a number of physiological and biochemical processes governing plant growth and productivity(Alexieva et al. 2001).

Photosystem II is one of the most thermo labile processes and heat sensitive membrane complex affected by abiotic stress. The function of photosystem II is ATP synthesis and involved in electron transport chain (Haq et al. 2013). Abiotic stresses are also cause of malfunction of proteins. Plants respond to stress by enhanced synthesis of  certain group of proteins designated as heat shock proteins (Schlesinger 1990). Heat shock proteins maintain the cellular homeostasis and plant survival during abiotic stress. The HSPs help the othes proteins to stabilize their functional native conformation during stress conditions (Wang et al. 2004; Park and Seo 2015).

Historically, the heat shock proteins were originally identified and described in 1960s by the work of an Italian geneticist Ferruccio Ritossa in Drosophila melanogaster. The proteins production was originally described and detected during puffing of chromosome after exposure to heat. Different other stress factors such as 2, 4-dinitrophenol, and salicylate also led to increase in synthesis of proteins (Al-Whaibi 2011).

Now the HSPs are well-known to be induced by a varied variety of stresses, including exposure to saline soil, drought conditions, cold, UV light, wound healing, tissue remodeling, heavy metals, organic toxic substances or biotic stresses. Thus, many stresses other than heat can induce expression of hsp genes (Park and Seo 2015; Haq et al. 2013).

Different kinds of HSPs are well known in different organisms. In plants, the five different classes of HSPs are characterized with respect to their amino acid sequence, molecular weight, activities: (1) Hsp100 (2) Hsp90 (3) Hsp70-chaperons (4) Hsp60/ Hsp10- the chaperonins        (5) small heat-shock proteins- crystalline proteins (Gupta et al. 2010; Wang et al. 2004; Chauhan et al. 2012).

In plants, small heat shock proteins are produced as result of other abiotic stresses such as drought, salinity and heavy metals etc. These proteins are further divided in 11 classes and characterized by a-crystalline domain (Sun et al. 2002). During stress, small heat shock proteins are induced at different developmental stages such as embryogenesis, seed germination and fruit development (Chauhan et al. 2012). There are two  classes of sHSPs that localize to the cytosol (classes I and II), and distinct classes of organelle-localized sHSPs found in the endoplasmic reticulum, the mitochondrion, or the chloroplast (Wehmeyer and Vierling 2000).

Heat shock proteins are also reported in chloroplasts as these are vital sites of protein transport and import. The hsp70 homologs were found in chloroplast by the use of immunoblotting techniques. Before this the hsp70 homologs were only reported in the mitochondria and endoplasmic reticulum (Marshall et al. 1990). Chloroplast small heat shock proteins have three basic domains: N-terminal domain (methionine rich), crystalline and C-terminal domain (Chen and Vierling 1991).

Chloroplastic sHsp26 is exceedingly inducible in wheat tissues representing all growth stages, such as seedling, root, panicle and developing and mature seeds showed remarkably high expressions (Chauhan et al. 2011).

The chloroplast small heat shock proteins (Cp-sHSPs) are known to protect photosystem II in plants during heat stress (Heckathorn et al. 1998) and thylakoid membranes during different abiotic stresses in chenopodium album (Haq et al. 2013). These proteins interact physically with the photosystem II by altering their state from slouble to insoluble (Chauhan et al. 2012). Chloroplast small Hsps (sHsps) protect photosynthetic electron transport (Phet) during heat, oxidative, and photoinhibitory stress (Heckathorn et al. 2004)

All Hsps are characterized by the presence of a carboxylic terminal called heat-shock Domain. The Heat-shock proteins which have molecular weights ranging from 10 to 200 KD are characterized as chaperones. These are involved in signal induction process during stress and chaperonins assist the folding of nascent and stress destabilized proteins (Al-Whaibi 2011; Nitnavare et al. 2016).

Wheat Chloroplastic sHSP, TaHSP26, have been reported to confer the heat stress tolerance both in Arabidopsis and Triticum aestivum The HSP26 is induced after heat stress in the floral and vegetative parts. This protein is also involved in seed development. 60% increase in the protein levels for HSP26 in durum wheat under heat stress have also been reported (Laino et al. 2010; Chauhan et al. 2012).