Shagun Bali

Role of various hormones in photosynthetic responses of green plants under environmental stresses.

Environmental stress includes adverse factors like water deficit, high salinity, enhanced temperature and heavy metals etc. These stresses alter the normal growth and metabolic processes of plants including photosynthesis. Major photosynthetic responses under various stresses include inhibition of photosystems (I and II), changes in thylakoid complexes, decreased photosynthetic activity and modifications in structure and functions of chloroplasts etc. Various defense mechanisms are triggered inside the plants in response to these stresses that are regulated by plant hormones or plant growth regulators. These phytohormones include abscisic acid, auxins, cytokinins, ethylene, brassinosteroids, jasmonates and salicylic acid etc. The present review focuses on stress protective effects of plants hormones on the photosynthetic responses.

Jasmonic acid-induced tolerance to root-knot nematodes in tomato plants through altered photosynthetic and antioxidative defense mechanisms

Plant parasitic nematodes cause severe damage to cultivated crops globally. Management of nematode population is a major concern as chemicals used as nematicides have negative impact on the environment. Natural plant products can be safely used for the control of nematodes. Among various plant metabolites, plant hormones play an essential role in developmental and physiological processes and also assist the plants to encounter stressful conditions. Keeping this in mind, the present study was designed to evaluate the effect of jasmonic acid (JA) on the growth, pigments, polyphenols, antioxidants, osmolytes, and organic acids under nematode infection in tomato seedlings. It was observed that nematode inoculation reduced the growth of seedlings. Treatment with JA improved root growth (32.79%), total chlorophylls (71.51%), xanthophylls (94.63%), anthocyanins (37.5%), and flavonoids content (21.11%) when compared to inoculated seedlings alone. The JA application enhanced the total antioxidant capacity (lipid- and water-soluble antioxidants) by 38.23 and 34.37%, respectively, in comparison to infected seedlings. Confocal studies revealed that there was higher accumulation of glutathione in hormone-treated seedlings under nematode infection. Treatment with JA increased total polyphenols content (74.56%) in comparison to nematode-infested seedlings. JA-treated seedlings also enhanced osmolyte and organic acid contents under nematode stress. Overall, treatment with JA improved growth, enhanced pigment levels, modulated antioxidant content, and enhanced osmolyte and organic acid content in nematode-infected seedlings.

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.

Interaction of Salicylic Acid with Plant Hormones in Plants Under Abiotic Stress

Plants are exposed to a number of abiotic stresses like salinity, heavy metals, temperature, drought, etc. which have adverse effects on their growth and yield. They have well-developed mechanisms which recognize various stress signals and manage the plants to grow under these stresses. Phytohormones play a major role in stress protection in plants by intervening growth, nutrient distribution, development, and source/sink transitions. In plants, interaction between various phytohormones results in positive and negative cross talk that play an essential role in response to abiotic stresses. Their biosynthetic pathways and mechanisms of action are interlinked. A complex hormone signaling and their ability to interact with each other make them optimal candidates for negotiating defense responses. Salicylic acid (SA) is an important plant growth regulator which regulates various physiological processes such as seed development, seed establishment, cell growth, senescence etc. in plants. The interaction of SA with other hormones like auxins, gibberellins, abscisic acid, ethylene, cytokinin, polyamines, jasmonic acid, and brassinosteroids play an important role in fine-tuning the network of immune response against abiotic stress.

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.

Brassinosteroids: Improving Crop Productivity and Abiotic Stress Tolerance

Brassinosteroids (BRs), a group of phyto-steroidal hormones, play crucial role in a wide spectrum of biochemical, physiological, growth, and developmental processes in plants. Recent molecular and physiological studies have further revealed the efficacy of BRs in improving the crop yield and productivity by controlling several genes regulated at cellular and sub-cellular levels. BRs also regulate several important agronomic traits like rhizogenesis, senescence, abscission, flower and fruit development, fruit ripening, flower sex expression, yield and quality of seed, grain, and fiber in horticultural and cash crops. These eco-friendly, nontoxic, and biosafe steroidal phytohormones when applied at specific dose at specific stage of development of specific crop enhanced quantity and quality of field-grown crops. Moreover, BRs also possess anti-ecdysteroidal, antiviral, and antifungal properties, and thus are considered as potential substitute to conventional pesticide, insecticide, and herbicide. Therefore, the present book chapter will update the knowledge of BRs in improving crop yield by ameliorating abiotic stresses such as salinity, drought, extreme temperatures, heavy metals, and pesticides. Besides, the current research has highlighted BRs-mediated cell elongation in vegetative organs as well as meristem homeostasis in plants. Various studies on BRs-related mutants indicated that steady BRs signaling is required for the optimal root growth, which further emphasized the indispensable roles of BRs in the regulation of the cell-cycle progression and differentiation in plants. Furthermore, the current chapter focuses on the exogenous application of effective doses of BRs to stress-affected plants as better and simple alternative approach for protecting plants from environmental stresses in comparison to the regular plant breeding practices.

Jasmonic acid induced changes in physio-biochemical attributes and ascorbate-glutathione pathway in Lycopersicon esculentum under lead stress at different growth stages

Lead (Pb) is one of most toxic heavy metals that adversely affect growth and developmental in plants. It becomes necessary to explore environment safe strategies to ameliorate its toxic effects. Phytohormones play an imperative role in regulating stress protection in plants. Jasmonic acid (JA) is recognized as a potential phytohormone which mediates immune and growth responses to enhance plant survival under stressful environment. The present study was undertaken to evaluate the effect of JA on the growth, metal uptake, gaseous exchange parameters, and on the contents of pigments, osmolytes, and metal chelating compounds in tomato plants under Pb stress during different stages of growth (in 30-, 45-, and 60-day-old plants). We observed a decrease in shoot and root lengths under Pb stress. Treatment of JA improved the shoot and root lengths in the Pb-treated plants. The Pb uptake was increased with the increasing concentrations of Pb, however, seeds pretreated with JA reduced the Pb uptake by the plants. The chlorophyll and carotenoid contents increased by JA treatment in plants under Pb stress. Pre-soaking of seeds in JA, improved gaseous exchange parameters, such as internal CO2 concentration, net photosynthetic rate, stomatal conductance, and transpiration rate under Pb stress. JA enhanced the enzyme activity of ascorbate-glutathione cycle and reduced H2O2 concentration in Pb-treated plants. The contents of osmolyte and metal chelating compounds (total thiols, and non-protein and protein-bound thiols) were increased with the increase in Pb stress. In seeds primed with JA, the contents of osmolytes and metal chelating compounds were further increased in the Pb-treated plants. Our results suggested that treatment of JA ameliorated the toxic effects of Pb stress by reducing the Pb uptake and improving the growth, photosynthetic attributes, activity of ascorbate-glutathione cycle and increasing the contents of osmolytes and metal chelating compounds in the tomato plants.

Potential of Endophytic Bacteria in Heavy Metal and Pesticide Detoxification

Heavy metal (HM) and pesticide contamination in the soil is of major concern in the present era. Both of these contaminants disturb soil microflora and adversely affect the growth and development of plants. The soil contamination can be reduced by ecofriendly techniques. The use of endophytic bacteria (EB) in the rhizosphere is one such technique where EB reduce the HM and pesticide contaminants in the soil. They can efficiently reduce the HM and pesticide concentration in the soil by enhancing the phytoremediating efficiency of plants. Moreover, EB can also degrade the pesticides in soil by producing various hormones and enzymes which ultimately result in promotion of the growth of plants. Hence, keeping in mind the efficiency of EB in reducing the HM and pesticide contamination in soil, the present review gives a detailed view of HM and pesticide detoxification by these bacteria.

Gene Silencing: A Novel Cellular Defense Mechanism Improving Plant Productivity under Environmental Stresses

Pests and pathogens have become tolerant to the use of conventional methods to improve agricultural production. This has resulted in low yields of crops worldwide. Currently, genetic engineering is being employed to enhance yields both quantitatively and qualitatively. RNA interference is one such novel cellular process in functional genomics. It is triggered by short double-stranded RNA molecules referred to as small interfering RNAs that target and degrade complementary mRNAs and are controlled by an RNA-induced silencing complex. RNAi is rapidly becoming an important methodology for analyzing sequence functions in eukaryotes and even holds promise for the development of therapeutic gene silencing. Therefore, in this chapter, an endeavor has been made to concisely consolidate the available information on RNAi, its mode of action, and its utilization in crop improvement under changing climatic conditions.