Heat Shock Proteins: Role and Mechanism of Action
Suneha Goswami1, Kavita Dubey1, Khushboo Singh1, Gyanendra K. Rai2 and Ranjeet Ranjan Kumar1
division of Biochemistiy, IARI, New Delhi 2School of Biotechnology,
Sher-e- Kashmir University of Agricultural Sciences and Technology’ of Jammu-180009 (J&K)
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Abstract
Plants as sessile organisms are exposed to persistently changing stress factors. The primary stresses such as drought, salinity, cold and hot temperatures and chemicals are interconnected in then effects on plants. These factors cause damage to the plant cell and lead to secondary stresses such as osmotic and oxidative stresses. Plants cannot avoid the exposure to these factors but adapt morphologically and physiologically by some other mechanisms. Almost all stresses induce the production of a group of proteins called heat-shock proteins (Hsps) or stress-induced proteins. The induction of transcription of these proteins is a common phenomenon in all living things. These proteins are grouped in plants into five classes according to their approximate molecular weight: (1) HsplOO, (2) Hsp90, (3) Hsp70, (4) Hsp60 and (5) small heat-shock proteins (sHsps). Higher plants have at least 20 sHsps and there might be 40 kinds of these sHsps in one plant species. It is believed that this diversification of these proteins reflects an adaptation to tolerate the heat stress. Transcription of heat-shock protein genes is controlled by regulatory proteins called heat stress transcription factors (Hsfs). Plants show at least 21 Hsfs with each one having its role in regulation, but they also cooperate in all phases of periodical heat stress responses (triggering, maintenance and recovery). There are more than 52 plant species (including crop ones) that have been genetically engineered for different traits such as yield, herbicide and insecticide resistance and some metabolic changes. In conclusion, major heat-shock proteins have some kind of related roles in solving the problem of misfolding and aggregation, as well as their role as chaperones.
Keywords: Chaperones, Heat-shock proteins, Transcription factors, Stress, Heat stress, Heat tolerance
Introduction
The constant flow of energy through all biological organisms provides the dynamic driving force for the maintenance of biological processes such as cellular biosynthesis and transport. The maintenance of steady-state results in a metastable condition called homeostasis.Any undesired modulation disrupting the homeostasis is known as biological stress. Plants as sessile organisms are exposed to persistently changing environmental stress factors. Biological stress in plants divided into two categories: abiotic and biotic stress. Abiotic stress is a physical stress (e.g. temperature, drought, chemical, light, salt etc.) that the environment may impose on the plant. Biotic stress is a biological insult, (e.g., insects, pests, pathogens etc.) to which a plant may be exposed during its lifetime.
Abiotic stresses especially heat, drought and salinity stresses are the major problems in agriculture. They significantly affect the growth of plants and productivity of crops. It is considered as the major cause of >50% reduction in average yield of major crops. Heat stress is turning out to be a major problem in cultivation of various crops like wheat. Of late, a drastic decrease in the total seed setting and yield has been observed in many wheat growing regions of India mainly due to the terminal heat stress. The problem of heat stress is likely to exacerbate with the global climate change adding to the exasperations of the stake- holders. Heat stress has been shown to influence photosynthesis, cellular and sub-cellular membrane components, seed setting, protein content and antioxidant enzyme activity; thereby significantly limits the crop productivity (Georgieva, 1999). Besides mitigating the heat stress, crop productivity under the stress may be enhanced by adaptation strategies.
Numerous heat responsive proteins have been identified from different crop species. However, the expression pattern of these genes and proteins under heat stress are still not clear. Different stress associated proteins have been identified from crops like rice, maize, Arabidopsis etc. and their characterization has also been carried out in response to different stresses.
Heat Stress
The primaiy str esses such as high temperature, dr ought, salinity, cold and chemicals are interconnected in their effects on plants. These factors cause damage to the plant cell and lead to secondary stresses such as osmotic and oxidative stresses. Plants cannot avoid the exposure to these factors but adapt morphologically and physiologically by some other mechanisms.Heat stress as well as other stresses can trigger some mechanisms of defense such as the expression of str ess associated chaperones, the heat shock proteins (HSPs),that was not expressed under “normal” conditions (Kumar et ah, 2016). Almost all stresses induce the production of a group of proteins called heat-shock proteins (HSPs) or Stress-induced proteins. Heat stress/shock response is a universal phenomenon and heat shock proteins (HSPs) form the most crucial defense system in all living systems at the cellular level (Katschinski, 2004). The cyto-protective effects of HSPs were attributed primarily to their chaperone activities, which minimize the proteotoxicity induced by the accumulation of unfolded or denatured proteins upon stress (Katschinski, 2004). HSP synthesis is tightly regulated by different members of heat shock transcription factors (Hsfs) at transcriptional level (Morimoto, 1998). Hsfs alone can function in the maintenance of cellular homeostasis that include regulation of cell cycle, cell proliferation, redox homeostasis, cell death mechanisms etc. (Katschinski, 2004; Sreedhar et ah, 2006).
