Trichoderma harzianum enhances antioxidant defense of tomato seedlings and resistance to water deficit

Some plant-symbiotic strains of the genus Trichoderma colonize roots and induce profound changes in plant gene expression that lead to enhanced growth, especially under biotic and abiotic stresses. In this study, we tested the hypothesis that one of the protective mechanisms enhanced by T. harzianum T22 colonization is the antioxidant defense mechanism. Having established that strain T22 modulates the expression of the genes encoding antioxidant enzymes, the status of antioxidant defense of tomato seedlings in response to colonization by T22 and water deficit was investigated. Total ascorbate or glutathione levels were not affected by either stimuli, but under water deficit, antioxidant pools became more oxidized (lower ratios of reduced to oxidized forms), whereas colonized plants maintained redox state as high as or higher than unstressed and untreated plants. The enhanced redox state of colonized plants could be explained by their higher activity of ascorbate and glutathione-recycling enzymes, higher activity of superoxide dismutase, catalase, and ascorbate peroxidase, in both root and shoot throughout the experiment. Similar enzymes were induced in uncolonized plants in response to water-deficit stress but to a lower extent when compared with colonized plants. This orchestrated enhancement in activity of reactive oxygen species (ROS)-scavenging pathways in colonized plants in response to stress supports the hypothesis that enhanced resistance of colonized plants to water deficit is at least partly due to higher capacity to scavenge ROS and recycle oxidized ascorbate and glutathione, a mechanism that is expected to enhance tolerance to abiotic and biotic stresses.     

Glycinebetaine and abiotic stress tolerance in plants

The accumulation of osmolytes like glycinebetaine (GB) in cell is known to protect organisms against abiotic stresses via osmoregulation or osmoprotection. Transgenic plants engineered to produce GB accumulate very low concentration of GB, which might not be sufficient for osmoregulation. Therefore, other roles of GB like cellular macromolecule protection and ROS detoxification have been suggested as mechanisms responsible for abiotic stress tolerance in transgenic plants. In addition, GB influences expression of several endogenous genes in transgenic plants. The new insights gained about the mechanism of stress tolerance in GB accumulating transgenic plants are discussed.     

Reactive oxygen species and temperature stresses: A delicate balance between signaling and destruction

Temperature stress can have a devastating effect on plant metabolism, disrupting cellular homeostasis, and uncoupling major physiological processes. A direct result of stress-induced cellular changes is the enhanced accumulation of toxic compounds in cells that include reactive oxygen species (ROS). Although a considerable amount of work has shown a direct link between ROS scavenging and plant tolerance to temperature stress, recent studies have shown that ROS could also play a key role in mediating important signal transduction events. Thus, ROS, such as superoxide (O₂^–), are produced by NADPH oxidases during abiotic stress to activate stress-response pathways and induce defense mechanisms. The rates and cellular sites of ROS production during temperature stress could play a central role in stress perception and protection. ROS levels, as well as ROS signals, are thought to be controlled by the ROS gene network of plants. It is likely that in plants this network is interlinked with the different networks that control temperature stress acclimation and tolerance. In this review paper, we attempt to summarize some of the recent studies linking ROS and temperature stress in plants and propose a model for the involvement of ROS in temperature stress sensing and defense.     

Potassium-Induced Regulation of Cellular Antioxidant Defense and Improvement of Physiological Processes in Wheat under Water Deficit Condition

Drought is the most common form of abiotic stress that reduces plant growth and productivity. It causes plant injuries through elevated production of reactive oxygen species (ROS). Potassium (K) is a vital plant nutrient that notably ameliorates the detrimental effect of drought stress in the plant. A pot experiment was conducted at the Laboratory of Plant Stress Responses, Faculty of Agriculture, Kagawa University, Japan, under controlled environment of green house to explore the role of K in mitigating drought severity in wheat (Triticum asevitum L.) seedlings. Three days after germination, seedlings were exposed to three water regimes viz., 100, 50, and 20% field capacity (FC) for 21 days. Potassium was adjusted in Hoagland nutrient solution at 0, 6 and 12 mM concentration and applied to pot instead of normal water. Results show that, water deficit stress notably reduced plant growth, biomass accumulation, leaf relative water content (RWC) along with reduced photosynthetic pigments. Increased amount of biochemical stress markers viz., malondialdehyde (MDA), hydrogen peroxide (H₂O₂), methylglyoxal (MG), proline (Pro) as well as an impaired antioxidant defense system were observed in drought affected wheat plants. On the contrary, K supplementation resulted in improvement of biochemical and physiological parameters that worked behind in improving growth and development of the wheat plants. In addition, enzymes of ascorbateglutathione (AsA-GSH) cycle were also enhanced by supplemented K that accelerated the ROS detoxification process in plant. Although glyoxalse system did not performed well till MG was detoxified might following another short stepped pathways. Our results revealed that drought stressed plants showed better performances in terms of biochemical and physiological attributes, antioxidant defense and glyoxalase system, as well as ROS detoxification due to K supplementation with better performance at 12 mM K added in 50% FC growing condition.     

Nitric oxide-induced salt stress tolerance in plants: ROS metabolism,signaling, and molecular interactions

Nitric oxide (NO), a non-charged, small, gaseous free-radical, is a signaling molecule in all plant cells. Several studies have proposed multifarious physiological roles for NO, from seed germination to plant maturation and senescence. Nitric oxide is thought to act as an antioxidant, quenching ROS during oxidative stress and reducing lipid peroxidation. NO also mediates photosynthesis and stomatal conductance and regulates programmed cell death, thus providing tolerance to abiotic stress. In mitochondria, NO participates in the electron transport pathway. Nitric oxide synthase and nitrate reductase are the key enzymes involved in NO-biosynthesis in aerobic plants, but non-enzymatic pathways have been reported as well. Nitric oxide can interact with a broad range of molecules, leading to the modification of protein activity, GSH biosynthesis, S-nitrosylation, peroxynitrite formation, proline accumulation, etc., to sustain stress tolerance. In addition to these interactions, NO interacts with fatty acids to form nitro-fatty acids as signals for antioxidant defense. Polyamines and NO interact positively to increase polyamine content and activity. A large number of genes are reprogrammed by NO; among these genes, proline metabolism genes are upregulated. Exogenous NO application is also shown to be involved in salinity tolerance and/or resistance via growth promotion, reversing oxidative damage and maintaining ion homeostasis. This review highlights NO-mediated salinity-stress tolerance in plants, including NO biosynthesis, regulation, and signaling. Nitric oxide-mediated ROS metabolism, antioxidant defense, and gene expression and the interactions of NO with other bioactive molecules are also discussed. We conclude the review with a discussion of unsolved issues and suggestions for future research.     

Stress-induced synthesis of proline confers tolerance to water deficit in transgenic wheat

Water deficit is one of the main abiotic factors that affect spring wheat planted in subtropical regions. Accumulation of proline appears to be a promising approach to maintain the productivity of plants under stress condition. However, morphological alterations and growth reduction are observed in transgenic plants carrying genes coding for osmoprotectants controlled by constitutive promoters. We report here the effects of water deficit on wheat plants transformed with the Vigna aconitifolia Delta(1)-pyrroline-5-carboxylate synthetase (P5CS) cDNA that encodes the key regulatory enzyme in proline biosynthesis, under the control of a stress-induced promoter complex-AIPC. Transgenic wheat plants submitted to 15 days of water shortage presented a distinct response. We have found that drought resulted in the accumulation of proline. The tolerance to water deficit observed in transgenic plants was mainly due to protection mechanisms against oxidative stress and not caused by osmotic adjustment.     

Plant sugars are crucial players in the oxidative challenge during abiotic stress: extending the traditional concept

Plants suffering from abiotic stress are commonly facing an enhanced accumulation of reactive oxygen species (ROS) with damaging as well as signalling effects at organellar and cellular levels. The outcome of an environmental challenge highly depends on the delicate balance between ROS production and scavenging by both enzymatic and metabolic antioxidants. However, this traditional classification is in need of renewal and reform, as it is becoming increasingly clear that soluble sugars such as disaccharides, raffinose family oligosaccharides and fructans – next to their associated metabolic enzymes – are strongly related to stress‐induced ROS accumulation in plants. Therefore, this review aims at extending the current concept of antioxidants functioning during abiotic stress, with special focus on the emanate role of sugars as true ROS scavengers. Examples are given based on their cellular location, as different organelles seem to exploit distinct mechanisms. Moreover, the vacuole comes into the picture as important player in the ROS signalling network of plants. Elucidating the interplay between the mechanisms controlling ROS signalling during abiotic stress will facilitate the development of strategies to enhance crop tolerance to stressful environmental conditions.     

A sucrose transporter‐interacting protein disulphide isomerase affects redox homeostasis and links sucrose partitioning with abiotic stress tolerance

Sucrose accumulation in leaves in response to various abiotic stresses suggests a specific role of this disaccharide for stress tolerance and adaptation. The high‐affinity transporter StSUT1 undergoes substrate‐induced endocytosis presenting the question as to whether altered sucrose accumulation in leaves in response to stresses is also related to enhanced endocytosis or altered activity of the sucrose transporter. StSUT1 is known to interact with several stress‐inducible proteins; here we investigated whether one of the interacting candidates, StPDI1, affects its subcellular localization in response to stress: StPDI1 expression is induced by ER‐stress and salt. Both proteins, StSUT1 and StPDI1, were found in the detergent resistant membrane (DRM) fraction, and this might affect internalization. Knockdown of StPDI1 expression severely affects abiotic stress tolerance of transgenic potato plants. Analysis of these plants does not reveal modified subcellular localization or endocytosis of StSUT1, but rather a disturbed redox homeostasis, reduced detoxification of reactive oxygen species and effects on primary metabolism. Parallel observations with other StSUT1‐interacting proteins are discussed. The redox status in leaves seems to be linked to the sugar status in response to various stress stimuli and to play a role in stress tolerance.     

Role of AP2/EREBP transcription factors in gene regulation during abiotic stress.

Crop plants are exposed to many types of abiotic stress during their life cycle. Water deficit derived from drought, low temperature or high salt concentration in the soil, is one of the most common environmental stresses that affects growth and development of plants through alterations in metabolism and gene expression. Adaptation to these conditions may involve passive tolerance or active homeostatic mechanisms for maintaining water balance. Active responses occur at different levels in the plant and may represent a concomitant protection against other types of stress such as pathogen attack. Many morphological and physiological adaptations to water stress are under the control of the plant hormone abscisic acid and involve specific activation of target genes that in one way or another protect cells against water deficit or participate in the regulation of the drought response. Here, we discuss recent advances in our understanding of drought adaptation mediated by specific changes in gene expression and the role of AP2/EREBP nuclear factors in these processes.     

A rice calcium-dependent protein kinase OsCPK12 oppositely modulates salt-stress tolerance and blast disease resistance

Calcium-dependent protein kinases (CDPKs) regulate the downstream components in calcium signaling pathways. We investigated the effects of overexpression and disruption of an Oryza sativa (rice) CDPK (OsCPK12) on the plant's response to abiotic and biotic stresses. OsCPK12-overexpressing (OsCPK12-OX) plants exhibited increased tolerance to salt stress. The accumulation of hydrogen peroxide (H(2) O(2) ) in the leaves was less in OsCPK12-OX plants than in wild-type (WT) plants. Genes encoding reactive oxygen species (ROS) scavenging enzymes (OsAPx2 and OsAPx8) were more highly expressed in OsCPK12-OX plants than in WT plants, whereas the expression of the NADPH oxidase gene, OsrbohI, was decreased in OsCPK12-OX plants compared with WT plants. Conversely, a retrotransposon (Tos17) insertion mutant, oscpk12, and plants transformed with an OsCPK12 RNA interference (RNAi) construct were more sensitive to high salinity than were WT plants. The level of H(2) O(2) accumulation was greater in oscpk12 and OsCPK12 RNAi plants than in the WT. These results suggest that OsCPK12 promotes tolerance to salt stress by reducing the accumulation of ROS. We also observed that OsCPK12-OX seedlings had increased sensitivity to abscisic acid (ABA) and increased susceptibility to blast fungus, probably resulting from the repression of ROS production and/or the involvement of OsCPK12 in the ABA signaling pathway. Collectively, our results suggest that OsCPK12 functions in multiple signaling pathways, positively regulating salt tolerance and negatively modulating blast resistance.     

Avenues of the membrane transport system in adaptation of plants to abiotic stresses

Abstract

Abiotic stress imposed by many factors such as: extreme water regimes, adverse temperatures, salinity, and heavy metal contamination result in severe crop yield losses worldwide. Plants must be able to quickly respond to these stresses in order to adapt to their growing conditions and minimize metabolic losses. In this context, transporter proteins play a vital role in regulating stress response mechanisms by facilitating movement of a variety of molecules and ions across the plasma membrane in order to maintain fundamental cellular processes such as ion homeostasis, osmotic adjustment, signal transduction, and detoxification. Aquaporins play a crucial role in alleviating abiotic stress by transporting water and other small molecules to maintain cellular homeostasis. Similarly, other transporter families such as CDF, ZIP, ABC, NHX, HKT, SWEETs, TMTs, and ion channels also contribute to abiotic stress tolerance. Hormones and other signaling molecules are necessary to coordinate responses across different tissues and to precisely regulate molecular trafficking. The present review highlights the current understanding of how membrane transporters orchestrate stress responses in plants. It also provides insights about the importance of these sensing and adaptive mechanisms for ensuring improved sustainable crop production during unfavorable conditions. Finally, this review discusses future prospects for the use of computational tools in constructing signaling networks to improve our understanding of the behavior of transporters under abiotic stress.     

Physiological aspects of raffinose family oligosaccharides in plants: protection against abiotic stress

Abiotic stresses resulting from water deficit, high salinity or periods of drought adversely affect plant growth and development and represent major selective forces during plant evolution. The raffinose family oligosaccharides (RFOs) are synthesised from sucrose by the subsequent addition of activated galactinol moieties donated by galactinol. RFOs are characterised as compatible solutes involved in stress tolerance defence mechanisms, although evidence also suggests that they act as antioxidants, are part of carbon partitioning strategies and may serve as signals in response to stress. The key enzyme and regulatory point in RFO biosynthesis is galactinol synthase (GolS), and an increase of GolS in expression and activity is often associated with abiotic stress. It has also been shown that different GolS isoforms are expressed in response to different types of abiotic stress, suggesting that the timing and accumulation of RFOs are controlled for each abiotic stress. However, the accumulation of RFOs in response to stress is not universal and other functional roles have been suggested for RFOs, such as being part of a carbon storage mechanism. Transgenic Arabidopsis plants with increased galactinol and raffinose concentrations had better ROS scavenging capacity, while many sugars have been shown in vitro to have antioxidant activity, suggesting that RFOs may also act as antioxidants. The RFO pathway also interacts with other carbohydrate pathways, such as that of O‐methyl inositol (OMI), which shows that the functional relevance of RFOs must not be seen in isolation to overall carbon re‐allocation during stress responses.     

Unraveling salt stress signaling in plants

Salt stress is a major environmental factor limiting plant growth and productivity. A better understanding of the mechanisms mediating salt resistance will help researchers design ways to improve crop performance under adverse environmental conditions. Salt stress can lead to ionic stress, osmotic stress and secondary stresses, particularly oxidative stress, in plants. Therefore,to adapt to salt stress, plants rely on signals and pathways that re-establish cellular ionic, osmotic, and reactive oxygen species(ROS) homeostasis. Over the past two decades, genetic and biochemical analyses have revealed several core stress signaling pathways that participate in salt resistance. The Salt Overly Sensitive signaling pathway plays a key role in maintaining ionic homeostasis,via extruding sodium ions into the apoplast. Mitogenactivated protein kinase cascades mediate ionic, osmotic,and ROS homeostasis. SnR K2(sucrose nonfermenting1-related protein kinase 2) proteins are involved in maintaining osmotic homeostasis. In this review, we discuss recent progress in identifying the components and pathways involved in the plant's response to salt stress and their regulatory mechanisms. We also review progress in identifying sensors involved in salt-induced stress signaling in plants.     

Multifaceted Role of Salicylic Acid in Combating Cold Stress in Plants: A Review

Plants face different types of stresses, including biotic and abiotic stresses. Among various abiotic stress, low-temperature stress alters various morphological, cytological, physiological, and other biochemical processes in plants. To thrive in such condition’s plants must adopt some strategy. Out of various strategies, the approach of using plant growth regulators (PGRs) gained a prominent role in the alleviation of multiple stresses. Salicylic acid, application triggers tolerance to both biotic and abiotic stresses via regulation of various morpho-physiological, cytological, and biochemical attributes. SA is shown to alleviate and regulate the various cold-induced changes. Both endogenous and exogenously applied SA show an imperative role in the alleviation of cold-induced changes by activating multiple signaling pathways like ABA-dependent or independent pathway, Ca²⁺ signaling pathway, mitogen-activated protein kinase (MAPKs) pathway, reactive oxygen species (ROS), and reactive nitrogen species (RNS) pathways. Activation of these pathways leads to the amelioration of the cold-induced changes by increasing production of antioxidants, osmolytes, HSPs and other cold-responsive proteins like LEA, dehydrins, AFPs, PR proteins, and various other proteins. This review describes the tolerance of cold stress by SA in plants through the involvement of different stress signaling pathways.