Parvaiz Ahmad

Reactive oxygen species, antioxidants and signaling in plants

Several reactive oxygen species (ROS) are continuously produced in plants as byproducts of many metabolic reactions, such as photosynthesis, photo respiration and respiration, Depending on the nature of the ROS species, some are highly toxic and rapidly detoxified by various cellular enzymatic and nonenzymatic mechanisms. Oxidative stress occurs when there is a serious imbalance between the production of ROS and antioxidative defence. ROS participate in signal transduction, but also modify cellular components and cause damage. ROS is highly reactive molecules and can oxidize all types of cellular components. Various enzymes involved in ROS-scavenging have been manipulated and over expressed or down regulated. An overview of the literature is presented in terms of primary antioxidant free radical scavenging and redox signaling in plant cells. Special attention is given to ROS and ROS-anioxidant interaction as a metabolic interface for different types of signals derived from metabolisms and from the changing environment.

Effect of salt stress on growth and biochemical parameters of Pisum sativum L.

Abstract The effect of salinity on some physio-biochemical parameters in plants of pea (Pisum sativum L. cv. EC 33866) has been investigated. Plants were subjected to four salt treatments, 50, 100, 150 and 200 mM NaCl, for 30 days in sand culture and the physiological responses were measured. Salinity affected all of the considered parameters. Thus, high NaCl concentrations caused a great reduction in growth parameters such as fresh and dry weight of leaves and roots, but the leaf number was less affected. These changes were associated with a decrease in the relative water content and the K+ concentrations. The proline and sugar content was increased, but nitrate reductase activity and chlorophyll was found to decrease in leaves. The significance of organic solute accumulation in relation to osmotic adjustment has been discussed.

Salinity Stress and Arbuscular Mycorrhizal Symbiosis in Plants

Soil salinity is one of the main abiotic stresses, which restrict the plant growth and development, and therefore causes major threat to crop productivity. To minimize the crop loss, plant biologists are searching alternatives to develop salt-tolerant crop plants through different means such as plant breeding and genetic engineering. These approaches are successful and presently under use in developed countries but are too costly for the developing countries. In the present scenarios, various methods are utilized all over the globe to mitigate the adverse effect of salinity. One of the commonly used methods is to use beneficial bacteria and fungi that colonize with the plant roots and ultimately alleviate the salinity stress in plants. Among them, application of arbuscular mycorrhizal (AM) fungi has been found to be very effective in mitigating the salinity stress and also improves the crop yield. Various parameters were thoroughly studied and reported in the literature and have demonstrated positive effect in plants subjected to AM fungi under different environmental stress including saline stress. This is attributed to increased antioxidative activities, osmolytes and osmoprotectants in tolerant plants. Deficiency of mineral content in plants under salt stress is compensated by the use of AM fungi as the hyphae of AM fungi acquire minerals in abundance and thereby prevent the plants against deleterious effects of salinity stress. Efficient use of AM fungi can bring the wonders in the field of agriculture with improved yield of crop under salt stress and soil health. Besides this, identification of novel genes that regulate and maintain the biosynthesis of proline and water status in plant cells will help plant researchers to utilize this beneficial interaction in more sustainable ways. The chapter will throw light on the use of AM fungal association in alleviating salt stress in plants and how to exploit this for improved productivity under growth-limited conditions.

Chapter 3 – Arbuscular Mycorrhiza in Crop Improvement under Environmental Stress

Plants are continuously confronted by a variety of environmental stresses at various stages in their development. Extremely harse environmental conditions lead to oxidative stress because of the toxicities of certain ions and the overproduction of reactive oxygen species. Various stress-tolerance strategies are triggered that help plants to overcome/mitigate the stress-induced deleterious effects. These include morphological adaptations, osmoregulation, and enhanced activities of enzymatic as well as nonenzymatic antioxidants. To some extent, these strategies do help plants withstand unfavorable environmental conditions; however, in order to meet the food needs of the increasing world population, we have to develop/implement strategies to induce enhanced stress tolerance potential in crop plants. In connection with this, exploiting the biological means to induce enhanced stress tolerance for sustainable crop productivity is one of the promising means. Arbuscular mycorrhizal fungi (AMF) form symbiotic associations with the majority of the plants, altering root morphology and physiology as well. AMF-infected plants show efficient nutrient uptake and have higher enzymatic activities. The review in this chapter encompasses the general information about AMF and their possible role in stress mitigation.

Polyamines: Role in Plants Under Abiotic Stress

Environmental changes, irrespective of source, cause a variety of stresses in plants. These stresses affect the growth and development and trigger a series of morphological, physiological, biochemical and molecular changes in plants. Abiotic stress is the primary cause of crop loss worldwide. The most challenging job before the plant biologists is the development of stress tolerant plants and maintenance of sufficient yield of crops in this changing environment. Polyamines can be of great use to enhance stress tolerance in such crop plants. Polyamines are small organic polycations present in all organisms and have a leading role in cell cycle, expression of genes, signaling, plant growth and development and tolerance to a variety of abiotic stresses. High accumulation of polyamines (putrescine, spermidine and spermine) in plants during abiotic stress has been well documented and is correlated with increased tolerance to abiotic stress. Genetic engineering of PA biosynthetic genes in crop plants is the way to create tolerance against different stresses. The present review throws light on the role of polyamines in plants.

Drought Stress Induced Oxidative Damage and Antioxidants in Plants

Crop production is affected by a number of abiotic factors including temperature, salinity, drought, pesticides, pH of the soil, and heavy metals as they affect all metabolic activities of the plant. When the plant is unable to grow normally, it is said to be affected by water deficit. The mechanism involved in combating water stress is not clear in spite of the various research programs and practices. Therefore, it is of great importance to understand the molecular mechanism of water deficit in order to significantly enhance the production of crop plants and quality of the environment. As transcript profiling data are available, the genetic approaches are necessary to determine the changes in gene expression. In this chapter, we tried to describe the mechanism of drought resistance in plants on an antioxidant, physiological, enzymatic and proteomic basis.

Role of AM Fungi in Alleviating Drought Stress in Plants

Abiotic stress is increasing at an alarming rate throughout the globe. Drought stress is one of the major abiotic stresses responsible for hampering plant growth and development in many arid and semi-arid regions of the world. Drought stress disturbs the plant’s osmotic and ionic balance and hampers uptake of essential nutrients. Drought stress affects many physiological processes like gas exchange and water relations, pigments, organic solutes, lipid peroxidation, and electrolyte leakage, etc. Severe drought is also responsible for the generation of reactive oxygen species (ROS), which are deleterious to the normal functioning of the cell. However, plants are armed with certain antioxidants to defend themselves against these ROSs. Plants interact with certain microorganisms such as fungi that improve their performance during stress. Mycorrhizae is a close association between fungi and plant roots. Mycorrhizal association has shown to enhance crop growth, biomass, and mineral uptake under normal and drought conditions. This chapter throws light on the deleterious effects of drought stress and the beneficial effects of mycorrhizae in delaying or coping with toxic effects of drought stress and maintaining overall physiological balance.

Heavy Metal Stress: Plant Responses and Signaling

Abstract Environmental contamination by hazardous heavy metals (HMs) is increasing worldwide and poses a serious threat to plant production and human health. Subjection of plants to HM stress lead to a variety of morphological, physiological, and biochemical alterations or even death of plants. The prime drawback to developing HM stress-tolerant plants is the lack of information on in-depth molecular mechanisms and signaling events underlying plant responses to HM toxicity. Plants have array of defense mechanisms to counter the deleterious effects of HM stress. The primary defense mechanisms include the reduced absorption or their sequestration in root cells and secondary mechanisms include the binding of HM ions by phytochelatins, glutathione, and amino acids and modulation of antioxidant defense systems. In recent years, the dual role of reactive oxygen species in HM toxicity because both oxidative damage inducers and signaling molecules has been well-documented and recent developments in molecular biology have facilitated transcriptomics and proteomics studies to identify regulatory genes and signaling pathways implied in HM tolerance in plants. The integration of the current findings will be of extreme significance to dissecting the actual HM stress response network. In this review, we endeavor to describe the specific aspects of the molecular mechanisms of HM stress response and signaling that may contribute for better understanding to produce plants that have traits desirable for imparting HM stress tolerance as well as to provide scientific clues for the development of phytoremediation strategies.

Glutathione Metabolism in Plants under Environmental Stress

Glutathione is an essential component of the defense system in plants and human beings exposed to various environmental stresses. The protection mechanism involves metal sequestration and scavenging of reactive oxygen species by glutathione in plants facing environmental stress. It is a low molecular weight thiol peptide existing in both oxidized (GSSG) and reduced (GSH) forms. Glutathione reductase (GR) recycles the GSSG back to GSH, which is crucial for the functioning of the ascorbate–glutathione cycle as well as for the synthesis of phytochelatins (PCs). The GSSG/GSH redox system signals to specific responses under environmental stress. Another enzyme, glutathione-S-transferase (GST), has a significant role in intracellular detoxification of xenobiotics and toxic compounds produced within the cell. Glutathione is formed by the combination of γ-glutamate and cysteine via γ-glutamylcysteine synthetase (GSH1) through peptide bond and at the same time gets combined with glycine catalyzed by glutathione synthase. Genes such as GSTM1 and GSTP1 are known to regulate glutathione metabolism and homeostasis by production of glutathione recycling enzymes in plants. The transportation of GSH across the plasma membrane of plant cells contains both high-affinities as well as low-affinity GSH transport systems. Some ABA-type transporters similar to multiple drug resistance transporters (MRPs) (e.g. AtMRP1, AtMRP2, AtMRP3 and ATMRP5) transports GSH- metal and PC-metal complex though plasma membrane in Arabidopsis sp. The present chapter contains up-to-date information regarding the mechanism of the glutathione mediated protection system, metabolism, transport and biosynthesis of glutathione in plants.

Catalase: A Versatile Antioxidant in Plants

Catalase (CAT, 1.11.1.6) is an antioxidant enzyme present in all aerobic organisms. It is known to catalyze H2O2 into water and oxygen in an energy-efficient manner in the cells exposed to environmental stress. Catalase is located in all major sites of H2O2 production in the cellular environment (such as peroxisomes, mitochondria, cytosol and chloroplast) of higher plants. Multiple molecular forms of catalase isozymes indicate its versatile role within the plant system. The modulation of H2O2 by the catalase isozymes within specific cells or organelles at specific time and developmental phases directly or indirectly interferes with signal transduction in plants. Catalase isozymes CAT-1, CAT-2, CAT-3 have been encoded by structural genes Cat1, Cat2 and Cat3, respectively. The expression of cat gene shows time, species and stress specificity. Catalase deficiency in plants develops anomalies such as chlorosis and head sterility and sensitivity to normal photorespiratory conditions. The molecular phylogeny of plant catalase proteins also reveals the structure and functional links among a wide range of plant species. This chapter compiles the latest information on catalase structure, localization, biochemistry, genes and function in plants.