Resham Sharma

Redox homeostasis in plants under abiotic stress: role of electron carriers, energy metabolism mediators and proteinaceous thiols

Contemporaneous presence of both oxidized and reduced forms of electron carriers is mandatory in efficient flux by plant electron transport cascades. This requirement is considered as redox poising that involves the movement of electron from multiple sites in respiratory and photosynthetic electron transport chains to molecular oxygen. This flux triggers the formation of superoxide, consequently give rise to other reactive oxygen species (ROS) under adverse environmental conditions like drought, high or low temperature, heavy metal stress etc. that plants owing during their life span. Plant cells synthesize ascorbate, an additional hydrophilic redox buffer, which protect the plants against oxidative challenge. Large pools of antioxidants also preside over the redox homeostasis. Besides, tocopherol is a liposoluble redox buffer, which efficiently scavenges the ROS like singlet oxygen. In addition, proteinaceous thiol members such as thioredoxin, peroxiredoxin and glutaredoxin, electron carriers and energy metabolism mediators phosphorylated (NADP) and non-phosphorylated (NAD+) coenzyme forms interact with ROS, metabolize and maintain redox homeostasis.

LEA Proteins in Salt Stress Tolerance

In late embryogenesis, the water content of living cell is reduced tremendously that leads to a state of dehydration and thus, might impose severe irreparable damage to cellular and macromolecular structures. However, the mature orthodox seeds can withstand severe desiccation due to role of osmoprotectants viz., reducing sugars, prolines, glycinebetaines or Late Embryogenesis Abundant (LEA) proteins. These operate on the virtue of intrinsic molecular mechanisms that alleviate multiple abiotic stresses in plants such as protein desiccation, membrane degradation, salt stress and cold and chilling stress. The LEA proteins are a group of versatile, adaptive, hydrophilic proteins considerably defined as ‘molecular shields’ for their anti-stress properties attributable to partial or complete structural randomness. On the basis of their amino acid composition and sequencing, LEA proteins have been clubbed into seven groups that are further sub divided into a number of protein sub families. Out of these, Group 2 LEA proteins called the ‘Dehydrins’ are of prime importance in the plant kingdom. The latent and unique stress remediating characteristics of this class of proteins has been further enhanced by transgenic studies, wherein the target LEA genes have been identified, sequenced to understand their molecular role in plants. Further investigations into the behavior of LEA proteins and mode of their regulation in stressed plants will facilitate in elucidating the function of LEA proteins. The present chapter reviews the versatility and role of LEA proteins in plant stress protection.

Aquaporins: Role Under Salt Stress in Plants

In living cells, water flows via apoplastic path (across cell walls), symplastic path (from cell to cell by plasmodesmata) or transcellular path (traversing the cell membranes). Detailed studies on water relations revealed the essential class of water channel proteins known as aquaporins (AQPs) that facilitate water-conductance. These are membrane channels with a conserved structure and play a crucial role in transport of water, solutes such as urea, boric acid, silicic acid or gases (ammonia, carbon dioxide). AQPs exhibit a high isoform multiplicity, and transport activity can be regulated by multiple mechanisms viz. protein abundance, regulation of transcript, subcellular trafficking or cytosolic protons. Besides, AQPs mediate the regulation of water transport in response to various environmental cues and hence play an important role in stressed conditions thereby making them an essential class of plant proteins. The present review highlights the structural diversity, regulation of AQPs and physiological roles of AQPs in plant stress tolerance to environmental stimuli.

Responses of Phytochelatins and Metallothioneins in Alleviation of Heavy Metal Stress in Plants: An Overview

Abstract Heavy metal detoxification in plants is a phenomenon resulting from complex interactions among interconnected physiological pathways and defense shunts leading to reactive oxygen species scavenging and subsequent protection of cellular vitals. These signaling pathways involve cross-talk between a number of antioxidant compounds including two main groups of amino acid rich metal chelators, namely the phytochelatins (PCs) and metallothioneins (MTs). This book chapter traces the mechanism of metal tolerance and detoxification strategies possessed by these biological molecules in addition to their biosynthesis, roles played and genetic aspects involved in their course of action. The isolation, characterization of PC and MT genes involved in metal compartmentalization and their successful induction in other plants is a much more recent application because this is of immense importance to the world of agronomics. Genetic validation and success for the same has been reported widely in this decade and many prominent reports have been included in the text to highlight this. Extending this vast information about the PC and MT gene pool at the proteomic level is gaining a lot of momentum currently and shall remain the future line of investigation for understanding metal resistance pathways at the cellular as well as subcellular level.

Chapter 19 – Prospects of Field Crops for Phytoremediation of Contaminants

Anthropogenic activities have led to increased pollution of soil all over the world. These pollutants can be either organic (e.g., PCBs, PAHs, fertilizers, pesticides) or inorganic pollutants including various heavy metals (e.g., Cd, Cu, As, Zn, Hg, Pb). Phytoremediation is a green technology in which plants are used to clean up pollutants from water and soil. This environmentally friendly and cost-effective technology is now focusing on higher plants with large biomass that have a high tolerance to pollutants. Due to low shoot and root growth of hyperaccumulator plants, phytoremediation study has moved toward the high biomass species such as herbaceous field crops. Field crops may have low metal concentrations, but they compensate this with their high biomass yield. Various amendments, such as use of chelating agents, plant growth-promoting bacteria, plant growth-promoting hormones, and mycorrhizae, can be used to increase the phytoremediation potential of field crops. Molecular techniques used to produce transgenic plants also show promise for the efficient use of field crops for phytoremediation. Thus, due to the higher growth potential of field crops compared to hyperaccumulators, phytoremediation efficiency should be thought of as a future significant remediation tool.

Lignins and Abiotic Stress: An Overview

Lignin is a major carbon sink in the biosphere accounting for about 30 % of total carbon sequestered in terrestrial plants. Being the second most abundant polymer on earth, it is a complex 3-dimensional polymer which is the principal structural component of plant cell wall. The phenylpropanoid pathway is responsible for biosynthesis of a variety of products that include lignin flavonoids and hydroxycinnamic acid conjugates. The phenylpropanoid metabolism has attracted significant research attention as lignin is a limiting factor in a number of agroindustrial processes like chemical pulping, forage digestibility and the processing of lignocellulosic plant biomass to bioethanol. Further, many functions of lignins and related products make the phenylpropanoid pathway essential to the health and survival of plants by providing resistance from abiotic and biotic stresses. These polymers play crucial role in plethora of ecological and biological functions which include shaping of wood characteristics, mechanical support in plants and most importantly stress management (biotic and abiotic stresses). Since lignins act synergistically in a number of agricultural processes, viz. crop production, vigour and disease resistance, thus insights into both the biosynthetic pathway and biodegradation of lignins are of prime significance. Due to the urgent requirement of upregulation and downregulation of lignin genes, focus has been drawn on the genetic engineering of its biosynthetic pathway. This proposed book chapter lays intensive focus on abiotic stress management through lignins by drawing a comparison between the process of lignification of plants under normal conditions as opposed to plants subjected to a variety of abiotic stresses such as drought, flooding, UV rays, heat, chilling and freezing and heavy metal stress.

Microbial Siderophores in Metal Detoxification and Therapeutics: Recent Prospective and Applications

Siderophores are small molecular weight metal scavengers which are released by plants, plant growth-promoting bacterial strains and fungi into the rhizosphere. These molecules have been widely reported as Fe3+ carriers under poor iron ion mobilization; however, recently they are being exposed for affinity towards other metal ions such as copper, zinc, etc. highlighting their phytoremedial potential. They are also effective anti-pathogenic agents, important signals towards oxidative stress and new age therapeutics. To understand the mechanism by which these moieties solubilize metal ions at both genetic and protein levels is the crux of our studies as these are extremely versatile molecules having myriad applications in the fields of agriculture, physiology, drug therapy, diagnosis, etc. Additionally, this paper also covers the biosynthesis and classification of microbial siderophores and their roles in plant and animal physiology.

Chapter 17 – Osmolyte Dynamics: New Strategies for Crop Tolerance to Abiotic Stress Signals

Osmolytes and osmoprotectants have long been identified as pivotal abiotic stress busters because they help plants overcome extremely harsh environmental conditions by constant cellular homeostatic monitoring. This group mainly consists of sugars, polyols, amino acids, and betaines. Together, they shield plants by exercising a number of physiological responses such as membrane integrity strengthening, enzymatic/antioxidant activity balancing, and water adjustments under various abiotic stresses (e.g., temperature fluctuations, water deficit, salinity, heavy metals) and, more recently, pesticide exposure. Crop plant cultivation across the globe needs such comprehensive protection strategies against a vast array of ever-changing environmental conditions by developing tolerance at genetic and molecular levels. Currently, this is being achieved via osmoprotectant genetic engineering of plant genomes and exogenous application of antistress agents to crop plants to obtain high-yield, stress-resistant varieties. A newer modification to this two-dimensional approach is osmoprotectant activation via “omics”—an integral approach comprised of genomics, metabolomics, and transcriptomics. This chapter gives a cumulative account of their biosynthesis, classification, and functioning with respect to transgene induction, exogenous application, and omic approaches for an improved agricultural dimension.