Neha Handa

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.

Selenium: An Antioxidative Protectant in Plants Under Stress

Abstract Selenium (Se), the sister element of sulfur, has gained importance recently as an essential trace element in the plant systems. It uses the transporters of sulfur for its uptake and similar biochemical pathways for its metabolism, which leads to its incorporation in various biomolecules. The wide range of beneficial effects of Se on plants has been established. Low concentrations of Se have been proven to enhance seed germination, growth, photosynthesis, respiratory potential, yield, etc. Its role in protecting the plants against various types of biotic and abiotic stresses has become an area of active research. Various studies have revealed its direct effect on antioxidative defense system thereby increasing the potential of the plants to combat the stressful conditions. Despite of its well documented positive effects, Se still is an intermediate between being beneficial or harmful because of its toxic effects at higher concentrations. Therefore, concentration of Se to be used, is still a matter of contemplation and research.

Modulation of antioxidative defense expression and osmolyte content by co-application of 24-epibrassinolide and salicylic acid in Pb exposed Indian mustard plants

The study focuses on potential of combined pre-soaking treatment of 24-Epibrassinolide (EBL) and Salicylic acid (SA) in alleviating Pb phytotoxicity in Brassica juncea L. plants. The seeds after treatment with combination of both the hormones were sown in mixture of soil, sand and manure (3:1:1) and were exposed to Pb concentrations (0.25mM, 0.50mM and 0.75mM). After 30 days of growth, the plants were harvested and processed, for quantification of various metabolites. It was found that pre-sowing of seeds in combination of EBL and SA, mitigated the adverse effects of metal stress by modulating antioxidative defense response and enhanced osmolyte contents. Dry matter content and heavy metal tolerance index were enhanced in response to co-application of EBL and SA. The levels of superoxide anions, hydrogen peroxide and malondialdehyde were lowered by the combined treatment of hormones. Enhancement in activities of guaiacol peroxidase, catalase, glutathione reductase and glutathione-s-transferase were recorded. Contents of glutathione, tocopherol and ascorbic acid were also enhanced in response to co-application of both hormones. Expression of POD, CAT, GR and GST1 genes were up-regulated whereas SOD gene was observed to be down-regulated. Contents of proline, trehalose and glycine betaine were also reported to be elevated as a result of treatment with EBL+SA. The results suggest that co-application of EBL+SA may play an imperative role in improving the antioxidative defense expression of B. juncea plants to combat the oxidative stress generated by Pb toxicity.

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.

Antifungal and Antioxidant Profile of Ethnomedicinally Important Liverworts (Pellia endivaefolia and Plagiochasma appendiculatum) Used by Indigenous Tribes of District Reasi: North West Himalayas

The present investigation has been an attempt to evaluate antifungal and antioxidant potential of two ethno botanically important liverworts viz, Plagiochasma appendiculatum and Pellia endivaefolia. The methanolic extract was tested against three fungal species viz. Alternaria alternata, Aspergillus flavus and Aspergillus niger. Plagiochasma sp. proved to be more effective against all tested fungi; especially, A. flavus. Similarly Pellia sp. inhibited A. alternata the most. The antioxidant system viz. antioxidants and antioxidant enzymes were also evaluated. The values were enough closer to those of higher plants. Guaiacol peroxidase and catalase activity of Plagiochasma sp. was found to be higher than that of Pellia sp. All other evaluated parameters like superoxide dismutase, ascorbic acid, proline, glutathione and total phenols were higher in Pellia sp. as compared to Plagiochasma sp. This is the first report regarding the antioxidant system of these two liverworts.

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.

ROS Signaling in Plants Under Heavy Metal Stress

Contamination of soil with toxic heavy metals is a major reason for retarded growth of crops and harmful effects on human health. Cultivation of large number of agricultural crops in contaminated soil is a major concern of environmentalist in the present times. Increased level of heavy metals can enter in to the food chain and may available for human consumption. Metal toxicity-induced oxidative stress eventually leads to refrained enzyme activities due to displacement of essential cofactors with other metal ions and blocking of functional groups such as carboxyl, histidyl and thiol, and proteins. Oxidative burst releases large quantities of reactive oxygen species (ROS) such as superoxide anion, hydrogen peroxide, hydroxyl radical, singlet oxygen, etc., which is one of the primary response of plants to heavy metal stress. Production of ROS is an inherent feature of plant cell and contributes to the process of oxidative damage leading to cell death. Its production is restricted to several cellular compartments such as mitochondria, chloroplast, and peroxisomes etc. ROS production leads to alteration of several physiological processes including degradation of enzymes, proteins, and amino acids and change in structure of cells. ROS are well described as secondary messengers in variety of cellular processes including acclimatization of cells to stress conditions. The signaling of ROS as a result of oxidative damage is regulated by several other signaling cascades which are interlinked. Their role has been studied under various stress conditions specifically heavy metals which leads to production of NO, H2O2, synthetic electrophilic compounds, lipid peroxidation molecules, etc.

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.