Abiotic stresses hamper plant growth and productivity. Climate change and agricultural malpractices like excessive use of fertilizers and pesticides have aggravated the effects of abiotic stresses on crop productivity, and degraded the ecosystem. There is an urgent need for environment-friendly management techniques such as the use of arbuscular mycorrhizal fungi (AMF) for enhancing crop productivity. AMF are commonly known as bio-fertilizers. Moreover, it is widely believed that the inoculation of AMF provides tolerance to host plants against various stressful situations like heat, salinity, drought, metals and extremes of temperature. AMF may both assist host plants in up-regulation of tolerance mechanisms, and prevent the down-regulation of key metabolic pathways. AMF, being natural root symbionts, provide essential plant inorganic nutrients to host plants, thereby improving growth and yield under unstressed and stressed regimes. The role of AMF as a bio-fertilizer can potentially strengthen plants’ adaptability to changing environment. Thus, further research focusing on the AMF-mediated promotion of crop quality and productivity is needed. The present review provides a comprehensive up-to-date knowledge on AMF and their influence on host plants at various growth stages, their advantages and applications, and consequently the importance of the relationships of different plant nutrients with AMF.

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Role of Arbuscular Mycorrhizal Fungi in Plant Growth Regulation: Implications in Abiotic Stress Tolerance.

Drought stress, being the inevitable factor that exists in various environments without recognizing borders and no clear warning thereby hampering plant biomass production, quality, and energy. It is the key important environmental stress that occurs due to temperature dynamics, light intensity, and low rainfall. Despite this, its cumulative, not obvious impact and multidimensional nature severely affects the plant morphological, physiological, biochemical and molecular attributes with adverse impact on photosynthetic capacity. Coping with water scarcity, plants evolve various complex resistance and adaptation mechanisms including physiological and biochemical responses, which differ with species level. The sophisticated adaptation mechanisms and regularity network that improves the water stress tolerance and adaptation in plants are briefly discussed. Growth pattern and structural dynamics, reduction in transpiration loss through altering stomatal conductance and distribution, leaf rolling, root to shoot ratio dynamics, root length increment, accumulation of compatible solutes, enhancement in transpiration efficiency, osmotic and hormonal regulation, and delayed senescence are the strategies that are adopted by plants under water deficit. Approaches for drought stress alleviations are breeding strategies, molecular and genomics perspectives with special emphasis on the omics technology alteration i.e., metabolomics, proteomics, genomics, transcriptomics, glyomics and phenomics that improve the stress tolerance in plants. For drought stress induction, seed priming, growth hormones, osmoprotectants, silicon (Si), selenium (Se) and potassium application are worth using under drought stress conditions in plants. In addition, drought adaptation through microbes, hydrogel, nanoparticles applications and metabolic engineering techniques that regulate the antioxidant enzymes activity for adaptation to drought stress in plants, enhancing plant tolerance through maintenance in cell homeostasis and ameliorates the adverse effects of water stress are of great potential in agriculture.

Drought Stress Impacts on Plants and Different Approaches to Alleviate Its Adverse Effects.

The present study was carried out to investigate the beneficial role of exogenous application of salicylic acid (1 mM SA) and nitric oxide (100 μM NO) in preventing the oxidative damage in Vigna angularis triggered by salinity stress. Salinity (100 mM NaCl) stress reduced growth, biomass accumulation, chlorophyll synthesis, photosynthesis, gas exchange parameters, and photochemical efficiency (Fv/Fm) significantly. Exogenous application of SA and NO was affective in enhancing these growth and photosynthetic parameters. Salinity stress reduced relative water content over control. Further, the application of SA and NO enhanced the synthesis of proline, glycine betaine, and sugars as compared to the control as well as NaCl treated plants contributing to the maintenance of tissue water content. Exogenous application of SA and NO resulted in up-regulation of the antioxidant system. Activities of enzymatic antioxidants including superoxide dismutase (SOD), catalase (CAT), ascorbate peroxidase (APX), dehydroascorbate reductase (DHAR), and glutathione reductase (GR), as well as the content of non-enzymatic components, were more in SA + NO treated seedlings as compared to control and salinity stressed counterparts resulting in significant alleviation of the NaCl mediated oxidative damage. Content of nitrogen, potassium, and calcium increased due to SA and NO under normal conditions and NaCl stress conditions while as Na and Cl content reduced significantly.

https://www.mdpi.com/2218-273X/10/1/42/pdf

Influence of Exogenous Salicylic Acid and Nitric Oxide on Growth, Photosynthesis, and Ascorbate-Glutathione Cycle in Salt Stressed Vigna angularis.

The protective role of exogenously applied kinetin (10 μM KN, a cytokinin) against the adverse effects caused by NaCl-induced (150 mM) stress in Solanum lycopersicum was investigated. Application of KN significantly enhanced growth and biomass production of normally grown plants (non-stressed) and also mitigated the adverse effect of NaCl on stressed plants to a considerable extent. Among the examined parameters, chlorophyll and carotenoid contents, photosynthetic parameters, components of the antioxidant system (both enzymatic and non-enzymatic), osmotica accumulation, and mineral uptake exhibited a significant increase following the application of KN. Furthermore, KN application reduced the generation of reactive free radical hydrogen peroxide, coupled with a significant reduction in lipid peroxidation and an increase in membrane stability. The activities of antioxidant enzymes, and glyoxylase system were found to be promoted in plants exposed to NaCl, and the activities were further promoted by KN application, thereby protecting S. lycopersicum plants against NaCl-induced oxidative damage. Further strengthening of the antioxidant system in KN supplied plants was ascribed to regulation of ascorbate-glutathione cycle, phenols and flavonoids in them. The levels of proline and glycine betaine increased considerably in KN-treated plants, thereby maintaining relative water content. Moreover, exogenous KN application reduced the inhibitory effects of NaCl on K+ and Ca2+ uptake, which resulted in a considerable reduction in tissue Na+/K+ ratio.

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Potential of exogenously sourced kinetin in protecting Solanum lycopersicum from NaCl-induced oxidative stress through up-regulation of the antioxidant system, ascorbate-glutathione cycle and glyoxalase system.

Selenium protects wheat seedlings against salt stress-mediated oxidative damage by up-regulating antioxidants and osmolytes metabolism.

Plants are confronted with a variety of environmenmtal stresses resulting in enhanced production of ROS. Plants require a threshold level of ROS for vital functions and any change in their concentration alters the entire physiology of plant. Delicate balance of ROS is maintained by an efficient functioning of intriguing indigenous defence system called antioxidant system comprising enzymatic and non enzymatic components. Down regulation of antioxidant system leads to ROS induced oxidative stress causing damage to important cellular structures and hence anomalies in metabolism. Proper mineral nutrition, in addition to other agricultural practices, forms an important part for growth and hence the yield. Potassium (K) is a key macro-element regulating growth and development through alterations in physiological and biochemical attributes. K has been reported to result into accumulation of osmolytes and augmentation of antioxidant components in the plants exposed to water and salt stress. In the present review an effort has been made to revisit the old findings and the current advances in research regarding the role of optimal, suboptimal and deficient K soil status on growth under normal and stressful conditions. Effect of K deficiency and sufficiency is discussed and the information about the K mediated antioxidant regulation and plant response is highlighted.

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Plant growth under water/salt stress: ROS production; antioxidants and significance of added potassium under such conditions.

The present investigation reports antioxidant and osmotic components of Marigold at different developmental stages and also presents influence of its leachates on germination and growth of wheat (Triticum aestivum L.) seedlings. Leaves of younger plants exhibit more phenols, tannins and flavonoids as compared to root and stem. Leaves and flowers of CV. PBG exhibited greater contents of total phenols and flavonoids in comparison to CV. PNG. Leaves and flowers had greater free amino acids and free proline as compared to stem and root. Sodium was more in roots than in stem and leaves, whereas potassium and nitrogen are greater in leaves and stem of younger plants. Leaves of CV. PBG accumulated greater contents of potassium, nitrogen and calcium in comparison to those of PNG. Activity of PAL, SOD and CAT was greater in older plants and in flowers as compared to leaves. Non-enzymatic antioxidant components like ascorbic acid and reduced glutathione also exhibited the same trend. Nevertheless, APX, GPX and GR activity was less in the older plants and more in flowers than leaves. APX and GR activity was higher in CV. PBG than CV. PNG, whereas the activity of GPX and GST was more in PNG in comparison to PBG. The two cultivars (PNG and PBG) of Tagetes erecta L., their different parts and developmental stages showed considerable variation in their antioxidant and osmotic constituents. Accumulation of various antioxidant and osmotic components reported above may probably help Tagetes erecta L. adapt to varying environments. Higher concentration of leachates of leaves and flowers of marigold reduced germination percentage and seedling growth of wheat, whereas treatments of leachates of lower concentration somewhat stimulated the growth of wheat seedlings. Flower leachates showed more pronounced effects than leaf leachates. Further experimentation may lead to identification of allelochemicals in these leachates which may pave way for exploring the possibility of their use along with synthetic herbicides/weedicides in order to minimize the toxicity of soil caused by the use of synthetic compounds.

Variation in Phenolic Compounds, Antioxidant Activity and Osmotica of Different Cultivars of Tagetes erecta L. at Different Growth Stages and Effect of its Leachates on Germination and Growth of Wheat ( Triticum aestivum L.)

Prosopis juliflora (Sw.) DC. is an exotic plant having potential to grow in drought, salinity, extremes of soil pH and heavy metal stress, that is the stress conditions in which the growth and productivity of other plants are most adversely affected. P. juliflora (Sw.) DC. is having phytochemicals with varying biological activities. Nevertheless, invasive alien species pose a threat to ecosystem functioning, biodiversity, water availability and beauty of natural areas over the world. The current report encompasses phytochemical analysis and enzyme assay at different developmental stages of P. juliflora (Sw.) DC. A variety of phytochemicals and enzymes including those involved in antioxidant and free-radical scavenging activities contribute to the survival of the plant under adverse conditions. Activity of the enzymes involved in antioxidants activity was greater in older leaves probably helping the plants withstanding the deleterious effects of oxidative stresses increasing with age. However, accumulation of secondary metabolites (tannins, total phenols and phytic acid) in P. juliflora (Sw.) DC. is greater in younger leaves and internodes in comparison to older ones. Sodium and calcium percentage increases, whereas potassium and chloride decreases with age in leaves as well as internodes. P. juliflora can help for the removal of sodium and chloride and therefore making it useful in improvement/reclamation of such soils, as it can grow in soils rich in such salts. Manoeuvring the antioxidant systems by altering the regulatory processes controlling their expression may provide an additional means of improvement.

A Note on Phyto Analysis of P. juliflora (Swartz) DC.

Experiments were conducted to evaluate the effectivity of Tagetes erecta L. leachates on various growth, physiological, and biochemical parameters of wheat at different stages of growth. Results suggested that Triticum aestivum L. seedlings/plants when exposed to higher concentrations of marigold leachates (10%, 20%, and 30% w/v of fresh parts and 5% and 10% w/v of dry parts) exhibited enhanced lipid peroxidation along with an increase in the activity of protease and phenylalanine ammonia lyase. Treatment with higher concentrations of leachates of fresh (30% w/v) and dry (10% w/v) T. erecta upregulated the activity of superoxide dismutase, catalase, ascorbate peroxidase, guaiacol peroxidase, glutathione S-transferase, and glutathione reductase and also increased the non-enzymatic components of antioxidant defense such as glutathione, ascorbic acid, and total phenols along with osmotic constituents comprising free proline, free sugars, and free amino acids in wheat. The growth and yield attributes of wheat exhibited a slight increase at treatments with lower concentrations (1% w/v) of dry leachates, whereas a decrease was recorded at higher concentrations (10% w/v). In general, treatments with flower leachates (higher concentrations) showed greater influence as compared with those with leaf leachates. Identification and understanding the mechanism of function of allelochemicals in these leachates may pave a way for further experimentation on Tagetes erecta L crop while it is cultivated and decomposed in the field.

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Differential activity of wheat antioxidant defense system and alterations in the accumulation of osmolytes at different developmental stages as influenced by marigold (Tagetes erecta L.) leachates

Environmental stresses, both abiotic and biotic, affect the survival and productivity of plants. Extreme environmental conditions hamper plant metabolism by initiating excess generation of toxic molecules like reactive oxygen species (ROS), resulting in reduced growth and yield performance. Environmental stresses effect photosynthesis by influencing water and ion balance, electron transport in thylakoids and CO2 supply by affecting stomatal functioning and the carbon reduction cycle. Water stress is one of the major environmental factors the world over, particularly in arid and semi-arid areas, limiting plant growth and yield significantly. Photosynthetic regulation under salinity stress is controlled by the transcription pattern of several genes involved in chlorophyll biosynthesis, the light signalling pathway and ROS scavenging. Proline is another important osmolyte involved in growth regulation under stress. Crop plants accumulating increased proline content exhibit greater stress tolerance.

Surendra Argal

Papers

Plant growth under water/salt stress: ROS production; antioxidants and significance of added potassium under such conditions.

Variation in Phenolic Compounds, Antioxidant Activity and Osmotica of Different Cultivars of Tagetes erecta L. at Different Growth Stages and Effect of its Leachates on Germination and Growth of Wheat ( Triticum aestivum L.)

A Note on Phyto Analysis of P. juliflora (Swartz) DC.

Differential activity of wheat antioxidant defense system and alterations in the accumulation of osmolytes at different developmental stages as influenced by marigold (Tagetes erecta L.) leachates

Role of Osmolytes in Regulation of Growth and Photosynthesis under Environmental Stresses