Abiotic Stresses in Crop Plants

Usha Chakraborty, Bishwanath Chakraborty

• Publisher: CAB International
• Year: 2015
• ISBN:
  9781789243666
• Topic: Biological Sciences
• Subtopic: Horticulture
• Index: Biological Sciences

Based on the biochemical and molecular mechanisms of tolerance of commonly encountered abiotic stresses in nature, this book covers the effect of increasing temperature, flood, drought, salinity, ozone and heavy metals such as arsenic and cadmium on plants. It discusses how these abiotic stresses can be managed in a cost-effective and eco-friendly way by utilising the alleviating mechanisms of microbes. Written in three sections, it considers each stress and their alleviation methods in detail, providing a rounded and vital resource on the subject for researchers and students of crop stress, management and biology.

1 Heat-Shock Proteins and Molecular Chaperones: Role in Regulation of Cellular Proteostasis and Stress Management

M. Kapoor^* and S.S. Roy

Department of Biological Sciences, The University of Calgary, Alberta, Canada

Abstract

The multiplicity of environmental and physiological stresses experienced by all organisms presents a formidable challenge to survival. Encounters with near-lethal temperature, extreme cold, pathogens and parasites, metabolic toxins, heavy metals, nutrient deficit, hypoxia and desiccation comprise some of the more common forms of stress that negatively impact all three domains of life. Many of these agents lead to protein unfolding and structural damage to intracellular organelles and cell membranes, and genome replication, transcriptional and translational machinery. The prime strategy to ameliorate the effect of adverse conditions relies upon the evolutionarily conserved stress response: the rapid and transient production of numerous defence-capable proteins, the molecular chaperones. The most prominent and extensively investigated amongst this group are the heat shock proteins (Hsps). Their accelerated synthesis and accumulation, immediately following hyperthermia, confers thermotolerance: the capacity to withstand subsequent exposure to lethal temperature and other metabolic insults. The appearance of aberrant, unfolded or mis-folded aggregation-prone proteins is a signal for mounting the heat shock-stress response. Many of the Hsps are molecular chaperones with vital functions in metabolic pathways, signal transduction, cell proliferation, differentiation and apoptosis even under permissive growth conditions. The accumulation of molecular chaperones under adverse conditions provides the basic strategy for stress management. Molecular chaperone families are classified into two general categories. The first comprises the ‘foldases’, including the ATP-dependent chaperonins, Hsp70, Hsp90 and Hsp110 families, involved in folding nascent polypeptides and refolding proteins unfolded as a result of stress. The second group, the ‘holdases’, sequester unfolded or partially folded proteins, which are subsequently processed by the foldases. The ubiquitous set of small Hsps (sHsps) represents the ATP-independent holdases that play a major role in protection against hyperthermia, oxidative stress and a variety of other abiotic stresses. In plants, sHsps have an important role in development of thermotolerance and adaptation to osmotic and high salinity stress. In addition, some subfamilies of plant sHsps are not heat shock-inducible but are expressed constitutively during specific developmental stages.

^*E-mail: [email protected]

1.1 Introduction

Numerous factors in the life of an organism elicit moderate to severe physiological/environmental stress. The most prevalent forms of stress experienced on a regular basis include hyperthermia, exposure to ultraviolet light, nutrient deficit, dehydration/drought and metabolic poisons, heavy metals, microbial pathogens and other toxic substances in the milieu. Consequently, during the course of evolution, a wide variety of strategies for managing potentially lethal effects of stress have been perfected in different organisms to counteract specific threats to survival. The best understood and the most extensively investigated strategy for protection against hyperthermal conditions, in prokaryotes and eukaryotes alike, is the evolutionarily conserved phenomenon referred to as the heat shock (stress) response. In addition to high temperature, a surfeit of reactive oxygen species (ROS) is a universal elicitor of stress response, while persistence of herbicides and toxins in the soil, salinity and bacterial and fungal infections also induce a powerful stress response in plants.

Hyperthermia and other stresses present a serious threat to survival by causing unfolding and mis-folding of proteins, resulting in disturbance of intracellular protein homoeostasis. As partially or completely unfolded/mis-folded proteins are intrinsically aggregation-prone, reversal of the process by refolding or removal of the offending proteins constitutes the prime strategy for stress management. Appearance of unfolded proteins in the cytosol acts as the principal signal for immediate deployment of the heat shock-stress response: elevated expression of a plethora of stress-inducible genes and the rapid synthesis of defence-capable proteins (Hsps) fortifies the target organism against adverse environmental conditions. Such a defence mechanism can react swiftly to a wide range of physiological and chemical challenges leading to protein unfolding and is encountered universally in all three biological kingdoms: the Eubacteria, Archaea and Eukarya. The Hsps, also known as molecular chaperones, are exquisitely designed for shielding the cellular machinery from damaged macromolecules. Although most Hsps are required at low levels during normal growth and metabolism, a dramatic up-regulation of their synthesis is necessitated under stress conditions, as molecular chaperones are required in stoichiometric amounts relative to the population of unfolded/mis-folded or aggregated polypeptides.

In the eukaryotes, pathogen attack and several genetic/physiological factors also cause a substantial build-up of unfolded proteins in the endoplasmic reticulum (ER), the lumen of ER being the locale of synthesis of secretory and membrane-specific proteins. Perturbations in the redox status, calcium homoeostasis or post-translational modifications of secretory proteins, can result in substandard local folding capacity culminating in the accumulation of unfolded or mis-folded ER macromolecules. To counter this cytotoxic hazard, a robust surveillance system – designated the unfolded protein response (UPR) – conserved in plant, fungal and mammalian species, is launched. UPR is critical for adjustment of ER homoeostasis under stress elicited by mis-folded proteins (Walter and Ron, 2011). This system implements an immediate cessation of normal protein synthesis and activation of a preferred set of genes encoding chaperone and co-chaperone proteins, affording protection by induction of the ER-specific degradation system, or apoptosis as a last resort. Fortuitously these chaperones also promote resumption of proper folding (Lai et al., 2006). Irreversibly damaged ER proteins are moved to the cytoplasm and subjected to degradation by the ERAD system (ER-associated degradation).

Persisting ER stress is linked to several metabolic disorders, such as obesity, diabetes, diseases of the liver and atherosclerosis. Avenues are being explored to develop therapeutic approaches targeting specific components of the UPR for treatment of human diseases (reviewed in Lee and Ozcan, 2014). Insightful analyses of the heat shock response, protein folding, aggregation, macromolecular assemblies, and structure and function of molecular chaperones are available in recent reviews (Pearl and Prodromou, 2006; Hartl and Hayer-Hartl, 2009; Richter et al., 2010; Tyedmers et al., 2010; Waters, 2013). The following is a brief overview of commonly encountered environmental stresses and properties and structural features of selected, typical molecular chaperones that respond to them.

1.2 Molecular Chaperones: Functions and Properties

During the last two decades a large number of molecular chaperone families (exceeding 100) have been recorded, found in virtually every compartment/organelle of the cell – the endoplasmic reticulum, the cytosol, mitochondria, chloroplasts and nucleus. Molecular chaperones span an ever-expanding, wide-ranging class of proteins, with a majority of the members being present at a basal level throughout the life cycle; their rate of synthesis is accelerated manifold, immediately and transiently under stress-inducing stimuli. As stated in the Introduction, with an increase in the population of unfolded and aggregated proteins during growth at super-optimal temperatures or elevated ROS levels, a bank of defensive proteins (Hsps and other stress proteins) is vital for survival. In the immediate term, Hsps aid survival by conducting repair/reversal of damage and subsequently protect the organism from potential lethality by conferring tolerance/adaptation towards other potent abiotic and biotic stresses.

In eukaryotic cells some of the major consequences of hyperthermia include defects in the cytoskeleton by disruption of intermediate actin and tubulin networks, erroneous localization of organelles, fragmentation of ER and the Golgi, changes in membrane fluidity, deficit of mitochondria, aggregation of ribosomal proteins and defective processing of ribosomal RNA (Nover et al., 1989; Richter et al., 2010; Toivola et al., 2010). As stated in the foregoing, Hsps and their constitutively expressed cognates are essential, even under stress-free conditions, for the assembly of macromolecular complexes, trans-membrane trafficking of proteins, development and differentiation, cell cycle signalling pathways, regulation of gene expression and apoptosis.

Molecular chaperones also play vital roles in normal metabolism by regulating crucial steps in DNA replication and repair, maintenance of genome integrity, chromatin architecture, membrane stability, ribosome biogenesis, metabolic pathways, signal transduction, control of cell proliferation, differentiation and apoptosis (Frydman, 2001; Calderwood et al., 2006). They intercede at every step of protein biogenesis and maturation, from the emergence of the nascent polypeptide chain to the conclusion of its synthesis and the final design of a stable three-dimensional native structure. Stress-inducible proteins sustain the conformational integrity of structural proteins and enzymes by enabling requisite folding of nascent and partially unfolded polypeptides as well as by directing the timely degradation of irreparably impaired proteins.

Prior to the completion of polypeptide synthesis, hydrophobic segments of nascent chains, released from the translational apparatus, display a strong tendency to associate with each other. The intracellular environment in the cytosol is extremely crowded; the high local concentration of macromolecules – estimated at ~300–400 mg ml⁻¹ – provides congenial conditions for self- and cross-aggregation of proteins. In such a milieu, the primary target of molecular chaperones is the linear polypeptide chain exiting from the ribosome in the process of synthesis. Molecular chaperones sequester the newly synthesized hydrophobic patches – which would normally be buried in the interior of the mature folded protein – thereby precluding the intra- or inter-molecular aggregation of exposed surfaces while guiding the proper folding of the polypeptide and subsequent assembly into biologically functional oligomers or macromolecular complexes. Physico-chemical studies, in conjunction with Cryo-electron microscopy (Cryo-EM) and other imaging techniques, demonstrate protein aggregates to be either amorphous – typically seen in bacterial inclusion bodies – or amyloid-like in nature (Tyedmers et al., 2010). Aggregates of endogenous proteins form in bacteria under heat or oxidative stress conditions, as well as in host cells over-expressing recombinant proteins. In the latter situation, preferential high level synthesis and build-up of heterologous polypeptides mimics an internal stress condition, when the host cell may lack the capacity to garner adequate quantities of chaperones. It is noteworthy that the abundance of bacterial proteins predisposed to thermal unfolding – with structural features amenable to aggregate formation – may form up to 1.5 to 3% of total protein contingent in Escherichia coli (Winkler et al., 2010).

Under heat stress or otherwise unfavourable conditions, chaperones overcome the devastating effects of damage either directly by refolding of mis-folded, non-native proteins or by channelling the malformed proteins towards degradation by energy-dependent protease systems (Young et al., 2004). Some molecular chaperones mediate macromolecular trafficking across membranes while others assist in remediation of damaged multi-component assemblages. In all encounters, the chaperones engage only transiently with their substrates and readily dissociate upon conclusion of the folding/refolding/disaggregation reactions. Many molecular chaperones are endowed with the facility to differentiate between the native and non-native states of proteins. In coordination with the protease system, they provide a robust scheme of ‘quality control’ to cleanse the cell of dysfunctional proteins (Bukau et al., 2006). Molecular chaperones often form complex interacting networks in cooperation with some of the other major chaperones and co-chaperones (reviewed in Söti et al., 2005). Co-chaperones perform critical functions in facilitating substrate recognition and selectivity, and assist its productive binding to the chaperone protein. In the case of chaperones that utilize ATP binding and hydrolysis (e.g. Hsp70 and 90 families), their co...

Table of contents

• Cover
• Half Title
• Title
• Copyright
• Contents
• Contributors
• Introduction
• About the Editors
• 1 Heat-Shock Proteins and Molecular Chaperones: Role in Regulation of Cellular Proteostasis and Stress Management
• 2 Heat Response, Senescence and Reproductive Development in Plants
• 3 Ethylene, Nitric Oxide and Haemoglobins in Plant Tolerance to Flooding
• 4 Monitoring the Activation of Jasmonate Biosynthesis Genes for Selection of Chickpea Hybrids Tolerant to Drought Stress
• 5 Genetic Engineering of Crop Plants to Sustain Drought Tolerance
• 6 Physiology and Biochemistry of Salt Stress Tolerance in Plants
• 7 Sugarcane (Saccharum sp.) Salt Tolerance at Various Developmental Levels
• 8 The Impact of Ozone Pollution on Plant Defence Metabolism: Detrimental Effects on Yield and Quality of Agricultural Crops
• 9 Potentiality of Ethylene in Sulfur-Mediated Counteracting Adverse Effects of Cadmium in Plants
• 10 Heavy Metal and Metalloid Stress in Plants: The Genomics Perspective
• 11 Influence of Arsenic and Phosphate on the Growth and Metabolism of Cultivated Plants
• 12 Plant Responses to Abiotic Stresses in Sustainable Agriculture
• 13 Interactive Role of Polyamines and Reactive Oxygen Species in Stress Tolerance of Plants
• 14 Indirect and Direct Benefits of the Use of Trichoderma harzianum Strain T-22 in Agronomic Plants Subjected to Abiotic and Biotic Stresses
• 15 Role of Microorganisms in Alleviation of Abiotic Stresses for Sustainable Agriculture
• Index
• Back