Plant Responses to Heat Stress and Mechanism of Heat Stress Tolerance

 

Plants are fixed, so they must encounter abiotic stresses such as drought, salinity, extreme temperatures and more. These stressors alter their growth and development, and reduce crop productivity. Heat stress can be defined as rise in temperature beyond a threshold level for a period of time sufficient to cause irreversible damage to plant growth and development. In general, a transient elevation in temperature, usually 10–15 °C above ambient, is considered heat stress. High temperatures affect plant growth at all developmental stages however, anthesis and grain filling stage is more susceptible. High temperature may adversely affect photosynthesis, respiration, water relations and membrane stability and also modulate levels of hormones and primary and secondary metabolites. The major physiological injuries observed under elevated temperatures include scorching of leaves and stems, leaf abscission and senescence, shoot and root growth inhibition or fruit damage, which consequently lead to decreased plant productivity.

The changes in ambient temperature are sensed by plants with a complicated set of sensors positioned in various cellular compartments. All macromolecules can serve as heat stress sensors. The heat stress sensors have ability to sense heat directly (due to a change in conformation) or indirectly. They trigger signaling pathways that ultimately result in expression of heat stress responsive genes. Several cascades of kinases and heat stress transcription factors (HSF) play roles in pathways leading from heat stress perception to heat induced gene expression. Components of the plasma membrane act as a primary heat sensors and initiate heat stress signal transduction pathways. The increased fluidity of the membrane leads to activation of lipid-based signaling cascades and to an increased Ca²⁺ influx and cytoskeletal reorganization. Signaling between these routes leads to the production of osmolytes and antioxidants in response to heat stress. Other adaptive changes include maintenance of membrane stability, scavenging of ROS, production of antioxidants, accumulation and adjustment of compatible solutes, induction of mitogen-activated protein kinase (MAPK) and calcium-dependent protein kinase (CDPK) cascades. Heat-shock proteins (Hsps) and antioxidant enzymes are important in encountering heat stress in plants. The heat-shock response is characterized by repression of normal cellular protein synthesis and induction of Hsp synthesis. Reactive oxygen species involving several pathways such as water-water cycle, Halliwell-Asada, glutathione peroxidase, Haber-Weiss and Fenton reactions helps in protecting plants against toxic radicals which otherwise could cause damage to lipophilic protein. The phytohormones, such as auxin, abscisic acid (ABA), brassinosteroids (BRs), cytokinin (CK), salicylic acid (SA), jasmonate (JA), and ethylene (ET) play a key role in integrating environmental stimuli and endogenous signals to regulate plant defensive response to various abiotic stresses, including heat. Other mechanism plants follow for flourishing under higher prevailing temperatures includes ion transporters, late embryogenesis abundant (LEA) proteins and osmoprotectants. The stress responsive mechanism is established by an initial stress signal that may be in the form of ionic and osmotic effect or changes in the membrane fluidity. This helps to re–establish homeostasis and to protect and repair damaged proteins and membranes

The general defense against heat stress includes cuticle as outermost shield, unsaturated fatty acids (UFAs) as membrane modulator and oxylipin precursor, RS scavengers that govern RS homeostasis, molecular chaperones that stabilize proteins and subcellular structures (e.g., membrane), as well as compatible solutes that act more than osmoprotectants. ROS when overproduced in the plants, they can readily attack various biomolecules encompassing carbohydrates, lipids, proteins, and nucleic acids, leading to oxidative catastrophe including enhanced photoinhibition and membrane lesions. In order to cope with these problems, plants have developed a sophisticated ROS scavenging system utilizing both non-enzymatic and enzymatic means. Many metabolites possess antioxidant properties, such as betalains, carotenoids, flavonoids, and vitamin E. Specialized enzymes comprise superoxide dismutases (SODs), catalases (CATs), and various peroxidases (PODs). SODs convert O₂^•- into H₂O₂ for further reduction to water by CATs and PODs. Besides, the ascorbate-glutathione (ASA-GSH) cycle required for ascorbate peroxidase (APX) involves dehydroascorbate reductase (DHAR), monodehydroascorbate reductase (MDHAR) and glutathione reductase (GR). Other enzymes, such as glutathione S-transferase (GST) and ferritins, also partake in detoxification.

Research has shown that when seedlings are shifted to temperatures five or more degrees above optimal growing temperatures, synthesis of most normal proteins and mRNAs is repressed, and transcription and translation of a small set of "heat shock proteins" (HSPs) is initiated. The heat shock response is not unique to plants. It was first discovered in Drosophila in the 1960s and has been described in a wide range of organisms including Escherichia coli, Saccharomyces cerevisiae, and humans. Heat shock proteins (HSPs) are well-known anti-stress proteins present in plants. They are induced to facilitate protein folding, assembly, transport, and degradation.  HSPs family is a universal salvation system employed by virtually all living organisms to counteract all detrimental conditions that can induce protein damage, wherein they function to prevent aggregation of denatured proteins, assist in their refolding or present them to lysosomes or proteasomes for proteolysis, thereby restoring cellular homeostasis. According to the molecular weight, there are five conserved HSP classes, namely HSP100/Clp, HSP90, HSP70/DnaK, HSP60/Chaperonin, and small HSP (sHSP). HSP70 is the most conserved one across different species, which consists of an N-terminal ATPase domain and a C-terminal substrate-binding domain. Binding and release of hydrophobic peptides rely on hydrolysis and recycling of ATP, which require the assistance of its co-chaperones including J-domain proteins (HSP40/DnaJ) that stimulate ATPase activity, and nucleotide exchange factors (NEFs) that promote release of ADP and binding of fresh ATP. Higher plants are characterized by the presence of at least 20 types of sHsps, but one species could contain 40 types of these sHsps. In Arabidopsis thaliana sHsps are divided into three subclasses:

1.   Subclass CI represented by six proteins

2.   Subclass CII represented by two genes

3.   Subclass CIII represented by one gene

There are six groups of genes that encode for the sHsps. The expression of genes for these sHsps is limited in the absence of environmental stress and occurs in some stages of growth and development of plants such as embryogenesis, germination, development of pollen grains, and fruit ripening. The transcription of these genes is controlled by regulatory proteins called heat stress transcription factors (Hsfs) located in the cytoplasm in an inactive state. Molecular pathway leading to the expression of genes to synthesize heat-shock proteins is composed of several mechanisms such as mechanism of sensing temperature that is connected to the mechanism of signal transfer to Hsfs where the activation of gene expression occurs by binding to the heat shock element (HSE) in DNA. Heat shock element is a specific recognition sequence located in the region of gene activator in DNA.

In recent decades, exogenous application of protectant such as osmoprotectants, phytohormones, signaling molecules, trace elements, etc., have shown beneficial effect on plants grown under heat tolerance as these protectants has growth promoting and antioxidant capacity. Accumulation of osmolytes such as proline, glycine betaine and trehalose is a well–known adaptive mechanism in plants against abiotic stress conditions including heat tolerance. Since heat sensitive plants apparently lack the ability to accumulate these substances, heat tolerance in such plants can be improved by exogenous application of osmoprotectants. Proline and glycine betaine application considerably reduced the H₂O₂ production, improved the accumulation of soluble sugars and protected the developing tissues from heat stress effects. Exogenous proline and glycine betaine application also improved the K⁺ and Ca₂⁺ contents, and increased the concentrations of free proline, glycine betaine and soluble sugars which rendered the buds more tolerant to heat tolerance. Identically, exogenous applications of several phytohormones were found to be effective in mitigating heat stress in plants. Results of some important studies demonstrated that phytohormones induced amelioration of heat tolerance stress in Brassica juncea and also found that soaking seeds in 100μM IAA, 100μM GA, 50 and 100μM Kinetin and 0.5 and 1μM ABA were effective for mitigating the effect of heat stress (47 ± 0.5 °C). The significant observation was that both growth promoting and growth retarding hormones were effective in mitigation of heat stress effects. The role of growth promoting hormone in the mitigation of heat stress was at a concentration which was otherwise lethal or toxic to its growth seedling stage. Salicylic acid is a plant hormone found to be an effective protectant under heat stress.

The oxidative damages in abscisic acid (ABA) treated plants were also much lower than non–treated plants under heat stress condition which was indicated by reduced MDA and H₂O₂ contents. In the contrary, inhibitor of ABA biosynthesis, fluridone (FLU) reverted the actions induced by ABA which suggest a clear role of ABA in mitigating heat–induced damages. When grape seedlings were treated with 50μM jasmonic acid (JA) solution, it was observed that JA could extenuate the change of stress under heat stress (42°C). This protection was accompanied by the upregulation of antioxidant enzyme’s (SOD, CAT and POD) activity compared with these untreated under heat stress. Similarly, the effect of different concentrations of 24–epibrassinolide (24–EBL) on growth and antioxidant enzyme of mustard (B. juncea) seedlings was studied and found that polyamine provides protection to plant from high temperature stress in different ways. Structure and function of the photosynthetic apparatus can be regulated effectively by PAs. Polyamines are able to maintain thermostability of thylakoid membranes under heat thus increase photosynthetic efficiency. In addition, under stressful circumstances, special type of small organic compounds may accrue to act as osmoprotectants against dehydration, scavengers of RS, and/or stabilizers of proteins and membranes. Examples of these small organic molecules are sugars, amino acids and their derivatives such as raffinose, trehalose, inositol, mannitol, proline (Pro), and glycine betaine (GB). They are electrically neutral, highly soluble and with low toxicity that can even mount up to fairly high concentrations inside cells with few perturbations.

Osmoprotectants are low molecular weight organic compounds primarily accumulated in response  to  osmotic  stresses  in  diverse  taxa  including  plants. These molecules increase the osmotic pressure in the cytoplasm, thereby maintaining driving gradient for both water uptake and turgor pressure. Apart from osmotic adjustment, these compounds are reported to function as scavengers of reactive oxygen species (ROS), having chaperone-like activity and help in metabolic detoxification. In addition, osmoprotectants play an essential role in stabilizing proteins and membranes during oxidative damage by stress-induced ROS outburst. Osmoprotectants are thought to counteract osmotic imbalance by reducing cell’s osmotic potential and thereby maintaining turgor pressure under conditions of low water potential and high ionic strength. They also function to protect or replace the water shell around proteins and stabilize protein complexes and membranes.

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