Plants Environmental and biological responses are important factors affecting crop plant's developmental and physiological events. The term stress is generally assigned to any physical adaptation that upset homeostasis. In this scenario, heat shock proteins (hsps) play a role in tolerance to a diversity of biotic and abiotic stresses by upholding plant cell working and endurance during stresses or help them in recovering from stresses. Heat shock proteins are molecular chaperons acting as crucial machinery that contribute to cellular homeostasis under both favorable and unfavorable growth circumstances. They can play a fundamental role in protecting plants against stress by restoring normal protein conformation and consequently cellular homeostasis. Newly synthesized proteins are present in moderately stable form, so these marginally unfolded proteins are more prone to protein aggregates formation and pose an unremitting risk (Chiti and Dobson, 2009). However, the immature proteins/or nascent protein chains are folded into their native states (folded). Though already folded proteins are also constantly in equilibrium with their marginally unfolded forms (Selmer and Liljas, 2008) because during stress conditions, this equilibrium is shifted to the favor of unfolded proteins.

 

A cell has protein quality control system that prevents the cell from damage posed by unfolded protein aggregates (Hartl et al., 2011). Molecular chaperones are the key players of this system that help in to maintain the native state of proteins thus prevent the formation of protein aggregates of unfolded proteins. Some of these are specialized in the folding of newly synthesized proteins whereas some are assisted in refolding of partly folded or misfolded proteins and this is energy (ATP) demanding process (Mayer, 2010). Other molecular chaperones are also played an imperative role in the degradation of unfolded proteins by directing these to degradation machinery (Dougan et al., 2002). Unfolded protein degradation is also a vital factor of protein quality control system (Tyedmers et al., 2010). Molecular chaperones are categorized into different families, on the basis of molecular size, phylogeny, structure and functions and ATP requirement (Arrigo et al., 2007). Various molecular chaperones are heat shock proteins because their expressions are induced or up-regulated during heat stress. Though other stresses can also up-regulate the heat shock protein (hsp) expression, whereas numerous hsps are housekeeping proteins and their expression is constitutive in a cell. Among these, Hsp100, 90, 70, 60, and small heat shock proteins are extensively studied families (Narberhaus, 2002).

 

Responses to thermal alterations are diverse, ranging from behavioral to molecular inflections. However the basic cellular stress program that is activated in response to a momentous increase of temperature is known as the heat shock response (HSR). Heat shock response is an important mechanism that maintains cellular homeostasis, when a cell is facing the distressing effects from stressors. It is a transient response which is triggered by a variety of stressors and provides protection to vital cellular proteins against damage imposed (Katschinski et al., 2004). In addition to this, heat shock response is the highly conserved known genetic system yet (Lindquist and Craig, 1988). However, numerous aspects of the heat shock response are universal but certain facets are varied among organisms.

 

This response includes a variety of processes such as the activation of hsp gene, blockage of cell cycle, HSPs accumulation and repression and the acquirement of thermal tolerance (Parsell and Lindquist, 1993). In a cell, the extent of heat shock response is based on certain variables including the heat shock temperature, the rate of temperature rise, and the duration of heat exposure (Katschinski et al., 2004).

 

This is anyway of the reality that Hsps or chaperons are recognized to be expressed in plants not only when they experience high temperature stress but also in response to a series of other environmental abuses such as water, salinity, osmotic, freezing and oxidative stresses. Heat shock proteins perform also a crucial part in the etiology of several diseases and during pathogen attacks; it may also provide a strong signal to activate host resistance response (Jindal, 1996; Feige et al., 1996) with essential task in the homeostasis of cell. They become particularly imperative in disease condition and also when a cell has to fight with traumatic situations. Heat shock proteins bind to unfolded proteins and utilize the energy generated from ATP hydrolysis. That’s why all heat shock proteins have highly conserved ATP binding terminals for binding to ATP. Among heat shock proteins, the Hsp70 family is the main molecular chaperone system present in expressosome of almost all organisms. Whilst small heat shock proteins (sHsps) are generally embodied by class I /cytosolic those are diverse in plants, having key job of avoiding irreversible protein aggregation. In other words, it could be said that small heat shock proteins (sHsps) are abundantly present with foremost task of preventing irrevocable protein aggregation and their expression is increased in response to array of metabolic cues. Therefore in this review, small heat shock protein is addressed briefly.

 

In plants, variety of sHsps is fascinating and account of their chaperone activity is imperative to comprehend plant resistance to stresses. The name small heat shock protein is given to this superfamily due to its molecular weight ranging between 12- 42 kDa (McHaourab et al., 2009). Small heat shock proteins (sHsps) bind the unfolding proteins, but they cannot actively refold them and ATP is not required in this process. While sHsps grasp proteins in a refolding competent state and prevent their aggregation (Basha et al., 2010).Being a part of cellular homeostasis network, sHsps present integral role in plants.

 

During thermal stresses particularly in heat stress, expression level of sHsps is at higher rate as compared to Hsp70, the most abundant protein in eukaryotes (Waters et al., 1996). In plants, there are numerous genes encoded for sHsps whereas in Escherichia coli and Sacchromyces cereviseae only two genes are encoded for sHsps viz., IbpA and IbpB and hsp 26 and hsp 42 respectively. In Arabidopsis thaliana, 19 genes are encoded for sHsps (Scharf et al., 2001 and Siddique et al., 2008). In plants 11 subfamilies of sHsps have been documented in literature and among these six are cytosolic and five are organelle-localized. one in peroxisomes, one in the endoplasmic reticulum, one in chloroplasts, and two in mitochondria (Waters et al., 2007). Organelle localized sHsps are almost unique to plants, with just one known exception of a mitochondrial sHsp in Drosophilla melanogaster (Basha et al., 2010).

 

In plants, small heat shock proteins are the most important stress responsive proteins. Under normal physiological condition sHsps are mostly not detectable in plants; while they are induced under different biotic and abiotic stresses i.e. drought, salinity, low or high temperature (Hamilton and Heckathorn, 2001; Scharf et al., 2001; Zhang et al., 2008) and even on metal exposure (Swindell et al., 2007). Small heat shock proteins represent the common and well-known superfamily of molecular chaperones in almost all organisms. Their expression is generally up-regulated during heat shock and manifestation of their essential role is maintaining the cell homeostasis under adverse conditions (de Jong et al., 1998; Haslbeck et al., 2005).

 

All members of sHsps share conserved region of ~90 residues close to their C-termini, named α-crystallin domain (core domain) however their flanking N-termini and C-termini are variable in sequence and length (Kappe et al., 2003). The interaction between sHsps and non-native state protein substrates is characterized by substrates promiscuity, high capacity, and ATP-independence (Haslbeck et al., 2005). In arabidopsis, a chloroplast localized sHsp; Hsp 21(At Hsp 25.3) has orthologs in all higher plants. Highly conserved and relatively long region of N-termini containing methionine rich domain is the characteristics of this shsp. This methionine on oxidation, obsolete the molecular chaperon activity of hsp21 that can be restored by the activity of sulfoxide reductase (Hartl et al., 2011). In comparison to other extensively studied molecular chaperons such as the members of Hsp60 and Hsp70 families (Mayer, 2010), the molecular mechanism of sHsps is not cleared. This is mainly accredited to their enormous diversity and heterogeneity, in terms of both oligomeric structure and substrate binding, making them technically challenging to study (Eyles and Gierasch, 2010).

 

Expression of heat shock protein genes is regulated at different levels i-e., synthesis of mRNA as well as its stability and translation process efficiency (Katschinski et al., 2004). The synthesis of hsp mRNA is regulated by transcription factors known as heat shock factors (HSFs) that are activated in response to stress. Some organisms possess only one Heat shock factor such as yeast and Drosophila whereas higher eukaryotes own multiple HSFs (Cotto and Morimoto, 1999). The Heat Shock Factors has four isoforms HSF1, HSF2, HSF3 and HSF4 that have evolved in vertebrates and plants and are activated under different conditions. HSF1, the vertebrate counterpart of the HSF present in yeast and Drosophila, is induced by any stressors (Akerfelt et al., 2010). On the other hand, HSF2 does not exhibit stress inducible heat shock transcription, but rather is triggered during embryogenesis, spermatogenesis and erythroid differentiation (Ostling et al., 2007). HSF3, an avian specific HSF, retort to harsh and continual stress (Pirkkala et al., 2001). Although HSF4, unlike the other isoforms, constitutively join to DNA, and has been put forwarded being negative regulator of the heat shock response (Nakai et al., 1997). Trimerization of monomeric HSF is achieved upon stresses that enhance its DNA-binding affinity (Westwood et al., 1991). Then this HSF in its active state (homotrimer) recognizes and binds to highly conserved cis-acting upstream DNA sequence elements, known as heat shock elements (HSEs) (Morimoto, 1993). The domain of heat shock element is comprised of inverted repeat sequence, nGAAn and is located in the upstream 5´ region of heat shock protein genes (Fernandes et al., 1994). Promoter regions of heat shock protein genes contain at least one HSE and the binding of heat shock factors to it regulates the transcription of these genes as well as mediates the response to stressors (Mosser et al., 1990).

 

As already mentioned in this review that almost all genes are regulated by heat shock factors and contain at least one HSE in their promoter regions. Though, a number of hsp genes such as Hsp26 and Hsp104 also contain another regulatory element, known as stress responsive element (STRE) in their promoter regions, indicating the potential overlapping between the heat shock and other stress responses (Amoros and Estruch, 2001). The STRE is hsf1- independent control element comprises of 5 base pair sequence functionally active in both orientations (CCCCT or AGGGG) (Wang, 2012). During traumatic conditions, cell requires stress responsive proteins such as heat shock proteins or chaperons, then repression of heat shock factor is terminated and hsf is dissociated from inhibitory regulators and become trimerize and gains the ability to bind DNA sequence for initiating the transcription of hsp genes (Conde et al 2005).

 

When heat shock proteins are synthesized in adequate amount for binding to unfolded protein, then inhibitory chaperons (negative regulator) bind to HSF (trimeric state) and results in its dissociation to HSE by reverting hsf to its inactive state (monomeric) (Shamovsky and Nudler, 2008).

 

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