Nitric oxide is involved in brassinosteroid-induced alternative respiratory pathway in Nicotiana benthamiana seedlings' response to salt stress

Brassinosteroid enhances salt tolerance via S-nitrosoglutathione reductase and nitric oxide signaling pathway in mangrove Kandelia obovata

Brassinosteroid (BR) has been shown to modulate plant tolerance to various stresses. S-nitrosoglutathione reductase (GSNOR) is involved in the plant response to environment stress by fine-turning the level of nitric oxide (NO). However, whether GSNOR is involved in BR-regulated Na+ /K+ homeostasis to improve the salt tolerance in halophyte is unknown. Here, we firstly reported that high salinity increases the expression of BR-biosynthesis genes and the endogenous levels of BR in mangrove Kandelia obovata. Then, salt-induced BR triggers the activities and gene expressions of GSNOR and antioxidant enzymes, thereafter decrease the levels of malondialdehyde, hydrogen peroxide. Subsequently, BR-mediated GSNOR negatively regulates NO contributions to the reduction of reactive oxygen species generation and induction of the gene expression related to Na+ and K+ transport, leading to the decrease of Na+ /K+ ratio in the roots of K. obovata. Finally, the applications of exogenous BR, NO scavenger, BR biosynthetic inhibitor and GSNOR inhibitor further confirm the function of BR. Taken together, our result provides insight into the mechanism of BR in the response of mangrove K. obovata to high salinity via GSNOR and NO signaling pathway by reducing oxidative damage and modulating Na+ /K+ homeostasis.

24-Epibrassinolide Reduces Drought-Induced Oxidative Stress by Modulating the Antioxidant System and Respiration in Wheat Seedlings

Brassinosteroids (BRs) represent a group of plant signaling molecules with a steroidal skeleton that play an essential role in plant adaptation to different environmental stresses, including drought. In this work, the effect of pretreatment with 0.4 µM 24-epibrassinolide (EBR) on the oxidant/antioxidant system in 4-day-old wheat seedlings (Triticum aestivum L.) was studied under moderate drought stress simulated by 12% polyethylene glycol 6000 (PEG). It was revealed that EBR-pretreatment had a protective effect on wheat plants as evidenced by the maintenance of their growth rate, as well as the reduction in lipid peroxidation and electrolyte leakage from plant tissues under drought conditions. This effect was likely due to the ability of EBR to reduce the stress-induced accumulation of reactive oxygen species (ROS) and modulate the activity of antioxidant enzymes. Meanwhile, EBR pretreatment enhanced proline accumulation and increased the barrier properties of the cell walls in seedlings by accelerating the lignin deposition. Moreover, the ability of EBR to prevent a drought-caused increase in the intensity of the total dark respiration and the capacity of alternative respiration contributes significantly to the antistress action of this hormone.

Promotion of Ca2+ Accumulation in Roots by Exogenous Brassinosteroids as a Key Mechanism for Their Enhancement of Plant Salt Tolerance: A Meta-Analysis and Systematic Review

Brassinosteroids (BRs), the sixth major phytohormone, can regulate plant salt tolerance. Many studies have been conducted to investigate the effects of BRs on plant salt tolerance, generating a large amount of research data. However, a meta-analysis on regulating plant salt tolerance by BRs has not been reported. Therefore, this study conducted a meta-analysis of 132 studies to elucidate the most critical physiological mechanisms by which BRs regulate salt tolerance in plants from a higher dimension and analyze the best ways to apply BRs. The results showed that exogenous BRs significantly increased germination, plant height, root length, and biomass (total dry weight was the largest) of plants under salt stress. There was no significant difference between seed soaking and foliar spraying. However, the medium method (germination stage) and stem application (seedling stage) may be more effective in improving plant salt tolerance. BRs only inhibit germination in Solanaceae. BRs (2 μM), seed soaking for 12 h, and simultaneous treatment with salt stress had the highest germination rate. At the seedling stage, the activity of Brassinolide (C28H48O6) was higher than that of Homobrassinolide (C29H50O6), and post-treatment, BRs (0.02 μM) was the best solution. BRs are unsuitable for use in the germination stage when Sodium chloride is below 100 mM, and the effect is also weakest in the seedling stage. Exogenous BRs promoted photosynthesis, and antioxidant enzyme activity increased the accumulation of osmoregulatory and antioxidant substances and reduced the content of harmful substances and Na+, thus reducing cell damage and improving plant salt tolerance. BRs induced the most soluble protein, chlorophyll a, stomatal conductance, net photosynthetic rate, Glutathione peroxidase, and root-Ca2+, with BRs causing Ca2+ signals in roots probably constituting the most important reason for improving salt tolerance. BRs first promoted the accumulation of Ca2+ in roots, which increased the content of the above vital substances and enzyme activities through the Ca2+ signaling pathway, improving plant salt tolerance.

Brassinolide promotes interaction between chloroplasts and mitochondria during the optimization of photosynthesis by the mitochondrial electron transport chain in mesophyll cell protoplasts of Arabidopsis thaliana

The current experimental data unveils the role of brassinolide (BL), a phytohormone of class brassinosteroids (BRs), in augmenting the cross-talk between the mitochondrial electron transport chain (mETC) and chloroplasts to strengthen the efficiency of the Calvin-Benson cycle (CBC) for higher assimilation of carbon dioxide in the mesophyll cell protoplasts (MCP) of Arabidopsis thaliana. The outcome of total respiration (TR) and photosynthetic carbon assimilation (PCA) was monitored as O₂ uptake under dark and NaHCO₃-dependent O₂ evolution under light, respectively, after pre-incubation of MCP at a broad spectrum of BL concentration from 0.05 pM to 5 pM at 25 °C and optimum light intensity of 1000 μmol m^-2 s^-1. The addition of optimal concentration (0.5 pM) of BL to MCP stimulated the (i) TR, (ii) PCA, and (iii) para-benzoquinone-dependent O₂ evolution (PSII activity). Further, in response to BL, the enzyme activity or transcript levels of redox-regulated CBC enzymes and glucose-6-phosphate raised considerably. Also, the addition of BL to MCP remarkably accelerated the capacity of the cytochrome oxidase (COX) and alternative oxidase (AOX) pathways concurrently with an increase in total cellular pyruvate and reactive oxygen species (ROS) levels. Besides, malate valve components (Malate, Chl-MDH, M-MDH) increased in response to BL. At the same time, the cellular redox ratios of pyridine nucleotides (NADPH and NADH) were kept low in the presence of BL. However, BL could not keep up the CBC activity of photosynthesis along with its associated light-activated enzymes/transcripts when mETC through COX or AOX pathway is restricted by antimycin A (AA) or salicylhydroxamic acid (SHAM), respectively. In contrast, adding BL to MCP under restricted mETC showed aggravation in total cellular ROS, pyruvate, malate, and redox ratio of pyridine nucleotides with a concomitant increase in transcripts associated with malate valve and antioxidant systems. These results suggest that BL enhances the PCA by coordinating in cross-talk of chloroplasts and mitochondria to regulate the cellular redox ratio or ROS through the involvement of COX and AOX pathways along with the malate valve and antioxidant systems.

Nitric oxide, calmodulin and calcium protein kinase interactions in the response of Brassica napus to salinity stress

Involvement of nitric oxide (NO) in plant metabolism and its connection with phytohormones has not been fully described, thus information about the role of this molecule in signalling pathways remains fragmented. In this study, the effects of NO on calmodulin (CAM), calcium protein kinase (CPK), content of phytohormones and secondary metabolites in canola plants under salinity stress were investigated. We applied 100 μM sodium nitroprusside as an NO source to canola plants grown under saline (100 mM NaCl) and non-saline conditions at the vegetative stage. Plant growth was negatively affected by salinity, but exogenous NO treatment improved growth. NO caused a significant increase in activity of CAT, SOD and POX through their enhanced gene expression in stressed canola. Salinity-responsive genes, namely CAM and CPK, were induced by NO in plants grown under salinity. NO application enhanced phenolic compounds, such as gallic acid and coumaric acid and flavonoid compound,s catechin, diadzein and kaempferol, in plants subjected to salinity. NO treatment enhanced abscisic acid and brassinosteroids but decreased auxin and gibberellin in stressed canola plants. The impacts of NO in improving stress tolerance in canola required CAM and CPK. Also, NO signalling re-established the phytohormone balance and resulted in enhanced tolerance to salt stress. Furthermore, NO improved salinity tolerance in canola by increasing enzymatic and non-enzymatic antioxidant content.

The Role of Nitric Oxide in Plant Responses to Salt Stress

The gas nitric oxide (NO) plays an important role in several biological processes in plants, including growth, development, and biotic/abiotic stress responses. Salinity has received increasing attention from scientists as an abiotic stressor that can seriously harm plant growth and crop yields. Under saline conditions, plants produce NO, which can alleviate salt-induced damage. Here, we summarize NO synthesis during salt stress and describe how NO is involved in alleviating salt stress effects through different strategies, including interactions with various other signaling molecules and plant hormones. Finally, future directions for research on the role of NO in plant salt tolerance are discussed. This summary will serve as a reference for researchers studying NO in plants.

Ethylene-induced NbMYB4L is involved in resistance against tobacco mosaic virus in Nicotiana benthamiana

Several MYB transcription factors are known to play important roles in plant resistance to environmental stressors. However, the mechanism governing the involvement of MYBs in regulating tobacco mosaic virus (TMV) resistance in plants is still unclear. In this study, we found that not only is Nicotiana benthamiana MYB4-like involved in defence against TMV, but also that the ethylene pathway participates in MYB4L-mediated resistance. Transcription of NbMYB4L was up-regulated in N. benthamiana infected with TMV. Silencing of NbMYB4L led to intensified TMV replication, whereas overexpression of NbMYB4L induced significant resistance to TMV. Transcription of NbMYB4L was greater in 1-aminocyclopropanecarboxylic acid (ACC, ethylene precursor)-pretreated plants but lower when the ethylene signalling pathway was blocked during TMV infection. Gene expression analysis showed that the transcription of NbMYB4L was largely suppressed in ETHYLENE INSENSITIVE 3-like 1(EIL1)-silenced plants. The results of electrophoretic mobility shift assay and chromatin immunoprecipitation-quantitative PCR (ChIP-qPCR) experiments indicated that NbEIL1 could directly bind to two specific regions of the NbMYB4L promoter. Furthermore, a luciferase assay revealed that NbEIL1 significantly induced the reporter activity of the MYB4L promoter in N. benthamiana. These results point to NbEIL1 functioning as a positive regulator of NbMYB4L transcription in N. benthamiana against TMV. Collectively, our work reveals that EIL1 and MYB4L constitute a coherent feed-forward loop involved in the robust regulation of resistance to TMV in N. benthamiana.

Brassinosteroids in Plants: Crosstalk with Small-Molecule Compounds

Brassinosteroids (BRs) are known as the sixth type of plant hormone participating in various physiological and biochemical activities and play an irreplaceable role in plants. Small-molecule compounds (SMCs) such as nitric oxide (NO), ethylene, hydrogen peroxide (H₂O₂), and hydrogen sulfide (H₂S) are involved in plant growth and development as signaling messengers. Recently, the involvement of SMCs in BR-mediated growth and stress responses is gradually being discovered in plants, including seed germination, adventitious rooting, stem elongation, fruit ripening, and stress responses. The crosstalk between BRs and SMCs promotes plant development and alleviates stress damage by modulating the antioxidant system, photosynthetic capacity, and carbohydrate metabolism, as well as osmotic adjustment. In the present review, we try to explain the function of BRs and SMCs and their crosstalk in the growth, development, and stress resistance of plants.

Advanced Material Against Human (Including Covid-19) and Plant Viruses: Nanoparticles As a Feasible Strategy

The SARS-CoV-2 virus outbreak revealed that these nano-pathogens have the ability to rapidly change lives. Undoubtedly, SARS-CoV-2 as well as other viruses can cause important global impacts, affecting public health, as well as, socioeconomic development. But viruses are not only a public health concern, they are also a problem in agriculture. The current treatments are often ineffective, are prone to develop resistance, or cause considerable adverse side effects. The use of nanotechnology has played an important role to combat viral diseases. In this review three main aspects are in focus: first, the potential use of nanoparticles as carriers for drug delivery. Second, its use for treatments of some human viral diseases, and third, its application as antivirals in plants. With these three themes, the aim is to give to readers an overview of the progress in this promising area of biotechnology during the 2017-2020 period, and to provide a glance at how tangible is the effectiveness of nanotechnology against viruses. Future prospects are also discussed. It is hoped that this review can be a contribution to general knowledge for both specialized and non-specialized readers, allowing a better knowledge of this interesting topic.

24-Epibrassinolide alleviates the toxic effects of NaCl on photosynthetic processes in potato plants

Brassinosteroids are promising agents for alleviating the negative effects of salinity on plants, but the mechanism of their protective action is far from being understood. We investigated the effect of pretreatment with 24-epibrassinolide (24-EBL) on the photosynthetic and physiological parameters of potato plants under progressive salinity stress caused by root application of 100 mM NaCl. Salinity clearly inhibited primary photosynthetic processes in potato plants by reducing the contents of photosynthetic pigments, photosynthetic electron transport and photosystem II (PSII) maximal and effective quantum yields. These negative effects of salinity on primary photosynthetic processes were mainly due to toxic ionic effects on the plant's ability to oxidize the plastoquinone pool. Pretreatment with 24-EBL alleviated this stress effect and allowed the maintenance of plastoquinone pool oxidation and the efficiency of photosystem II photochemistry to be at the same levels as those in unstressed plants; however, the pretreatment did not affect the photosynthetic pigment content. 24-EBL pretreatment clearly alleviated the decrease in leaf osmotic potential under salinity stress. The stress-induced increases in lipid peroxidation and proline contents were not changed under brassinosteroid pretreatment. However, 24-EBL pretreatment increased the peroxidase activity and improved the K⁺/Na⁺ ratio in potato leaves, which were likely responsible for the protective 24-EBL action under salt stress.

How Plant Hormones Mediate Salt Stress Responses

Salt stress is one of the major environmental stresses limiting plant growth and productivity. To adapt to salt stress, plants have developed various strategies to integrate exogenous salinity stress signals with endogenous developmental cues to optimize the balance of growth and stress responses. Accumulating evidence indicates that phytohormones, besides controlling plant growth and development under normal conditions, also mediate various environmental stresses, including salt stress, and thus regulate plant growth adaptation. In this review, we mainly discuss and summarize how plant hormones mediate salinity signals to regulate plant growth adaptation. We also highlight how, in response to salt stress, plants build a defense system by orchestrating the synthesis, signaling, and metabolism of various hormones via multiple crosstalks.

Silencing of HvGSK1.1-A GSK3/SHAGGY-Like Kinase-Enhances Barley (Hordeum vulgare L.) Growth in Normal and in Salt Stress Conditions

Glycogen synthase kinase 3 (GSK3) is a highly conserved kinase present in all eukaryotes and functions as a key regulator of a wide range of physiological and developmental processes. The kinase, known in land plants as GSK3/SHAGGY-like kinase (GSK), is a key player in the brassinosteroid (BR) signaling pathway. The GSK genes, through the BRs, affect diverse developmental processes and modulate responses to environmental factors. In this work, we describe functional analysis of HvGSK1.1, which is one of the GSK3/SHAGGY-like orthologs in barley. The RNAi-mediated silencing of the target HvGSK1.1 gene was associated with modified expression of its paralogs HvGSK1.2, HvGSK2.1, HvGSK3.1, and HvGSK4.1 in plants grown in normal and in salt stress conditions. Low nucleotide similarity between the silencing fragment and barley GSK genes and the presence of BR-dependent transcription factors’ binding sites in promoter regions of barley and rice GSK genes imply an innate mechanism responsible for co-regulation of the genes. The results of the leaf inclination assay indicated that silencing of HvGSK1.1 and the changes of GSK paralogs enhanced the BR-dependent signaling in the plants. The strongest phenotype of transgenic lines with downregulated HvGSK1.1 and GSK paralogs had greater biomass of the seedlings grown in normal conditions and salt stress as well as elevated kernel weight of plants grown in normal conditions. Both traits showed a strong negative correlation with the transcript level of the target gene and the paralogs. The characteristics of barley lines with silenced expression of HvGSK1.1 are compatible with the expected phenotypes of plants with enhanced BR signaling. The results show that manipulation of the GSK-encoding genes provides data to explore their biological functions and confirm it as a feasible strategy to generate plants with improved agricultural traits.

Green leaf volatile (Z)-3-hexeny-1-yl acetate reduces salt stress in peanut by affecting photosynthesis and cellular redox homeostasis

Green leaf volatiles (GLVs) are released by plants when they encounter biotic stress, but their functions in the response to abiotic stress have not been determined. We have previously shown that exogenous application of (Z)-3-hexeny-1-yl acetate (Z-3-HAC), a kind of GLV, could alleviate salt stress in peanut (Arachis hypogaea L.) seedlings; however, notably little is known concerning the transcription regulation mechanisms of Z-3-HAC. In this study, we comprehensively characterized the transcriptomes and physiological indices of peanut seedlings exposed to Z-3-HAC and/or salt stress. Analysis of transcriptome data showed that 1420 genes were upregulated in the seedlings primed with Z-3-HAC under salt stress compared with the non-primed treatment. Interestingly, these genes were significantly enriched in the photosynthetic and ascorbate metabolism-related categories, as well as several plant hormone metabolism pathways. The physiological data revealed that Z-3-HAC significantly increased the net photosynthetic rate, SPAD value, plant height and shoot biomass compared with the non-primed peanut seedlings under salt stress. A significantly higher ratio of K⁺ :Na⁺ , reduced-to-oxidized glutathione (GSH:GSSG), and ascorbate-to-dehydroascorbate (AsA:DHA) were also observed for the plants primed with Z-3-HAC compared with the salt stress control. Meanwhile, Z-3-HAC significantly increased the activity of enzymes in the AsA-GSH cycle. Taken together, these results highlight the importance of Z-3-HAC in protecting peanut seedlings against salt stress by affecting photosynthesis, cellular redox homeostasis, K⁺ :Na⁺ homeostasis, and phytohormones.

Interactive role of exogenous 24 Epibrassinolide and endogenous NO in Brassica juncea L. under salinity stress: Evidence for NR-dependent NO biosynthesis

The present study unravels origin of nitric oxide (NO) and the interaction between 24-Epibrassinolide (EBL) and nitrate reductase (NR) for NO production in Indian mustard (Brassica juncea L.) under salinity stress. Two independent experiments were performed to check whether (i) Nitrate reductase or Nitric oxide synthase takes part in the biosynthesis of endogenous NO and (ii) EBL has any regulatory effect on NR-dependent NO biosynthesis in the alleviation of salinity stress. Results revealed that NR-inhibitor tungstate significantly (P ≤ 0.05) decreased the NR activity and endogenous NO content, while NOS inhibitor l-NAME did not influence NO biosynthesis and plant growth. Under salinity stress, inhibition in NR activity decreased the activities of antioxidant enzymes, increased H₂O₂, MDA, protein carbonyl content and caused DNA damage, implying that antioxidant defense might be related to NO signal. EBL supplementation enhanced the NR activity but did not influence NOS activity, suggesting that NR was involved in endogenous NO production. EBL supplementation alleviated the inhibitory effects of salinity stress and improved the plant growth by enhancing nutrients, photosynthetic pigments, compatible osmolytes, and performance of AsA-GSH cycle. It also decreased the superoxide ion accumulation, leaf epidermal damages, cell death, DNA damage, and ABA content. Comet assay revealed significant (P ≤ 0.05) enhancement in tail length and olive tail moment, while flow cytometry did not showed any significant (P ≤ 0.05) changes in genome size and ploidy level under salinity stress. Moreover, EBL supplementation increased the G6PDH activity and S-nitrosothiol content which further boosted the antioxidant responses under salinity stress. Taken together, these results suggested that NO production in mustard occurred in NR-dependent manner and EBL in association with endogenous NO activates the antioxidant system to counter salinity stress.

The putative role of endogenous nitric oxide in brassinosteroid-induced antioxidant defence system in pepper (Capsicum annuum L.) plants under water stress

Brassinosteroids (BRs) have been rarely tested for their effective roles in mitigation of deleterious effects of water stress (WS) on plants. In addition, the contribution of nitric oxide (NO) in BR-improved plant tolerance to water stress needs to be elucidated. So, a trial was carried out to uncover the contribution of NO in BR-induced tolerance of pepper plants to WS. For well-watered and water-stressed plants, soil water availability was sustained at 80% and 40% of the full water storage capacity, respectively. BR (24-epibrassinolide, EB; 1.0 μM) was sprayed to the leaves of both well-watered and water stressed-pepper plants every two days for 10 days prior to the initiation of stress treatment. After starting WS treatment, cPTIO was sprayed to plant leaves twice a week for four weeks. Water stress caused a reduced plant growth and oxidative stress, but the application of EB increased plant growth and reversed the oxidative stress. The EB treatment increased endogenous NO and reinforced antioxidant defence systems, but the cPTIO application reversed the NO levels, downregulated the antioxidant defence systems, and aggravated oxidative damages caused by WS. These results show that EB-induced NO generation and NO-mediated antioxidant defence systems might be crucial mechanisms for EB-improved tolerance of pepper plants to WS. So, both EB and NO jointly are responsible for achieving improved tolerance of pepper plants to water stress.

Priming With the Green Leaf Volatile (Z)-3-Hexeny-1-yl Acetate Enhances Salinity Stress Tolerance in Peanut (Arachis hypogaea L.) Seedlings

Green leaf volatiles play vital roles in plant biotic stress; however, their functions in plant responses to abiotic stress have not been determined. The aim of this study was to investigate the possible role of (Z)-3-hexeny-1-yl acetate (Z-3-HAC), a kind of green leaf volatile, in alleviating the salinity stress of peanut (Arachis hypogaea L.) seedlings and the underlying physiological mechanisms governing this effect. One salt-sensitive and one salt-tolerant peanut genotype were primed with 200 μM Z-3-HAC at the 4-week-old stage before they were exposed to salinity stress. Physiological measurements showed that the primed seedlings possessed higher relative water content, net photosynthetic rate, maximal photochemical efficiency of photosystem II, activities of the antioxidant enzymes, and osmolyte accumulation under salinity conditions. Furthermore, the reactive oxygen species, electrolyte leakage, and malondialdehyde content in the third fully expanded leaves were significantly lower than in nonprimed plants. Additionally, we found that application of Z-3-HAC increased the total length, surface area, and volume of the peanut roots under salinity stress. These results indicated that the green leaf volatile Z-3-HAC protects peanut seedlings against damage from salinity stress through priming for modifications of photosynthetic apparatus, antioxidant systems, osmoregulation, and root morphology.

Nitric Oxide as a Downstream Signaling Molecule in Brassinosteroid-Mediated Virus Susceptibility to Maize Chlorotic Mottle Virus in Maize

Maize chlorotic mottle virus (MCMV) infection causes growth abnormalities in maize. Transcriptome sequencing was conducted to compare the global gene expression of MCMV-inoculated plants with that of mock-inoculated plants. Data analyses showed that brassinosteroid (BR)-associated genes were upregulated after MCMV infection. Exogenous 2,4-epibrassinolide (BL) or brassinazole (BRZ) applications indicated that BR pathway was involved in the susceptibility to MCMV infection. In addition, treatment of BL on maize induced the accumulation of nitric oxide (NO), and the changes of NO content played positive roles in the disease incidence of MCMV. Moreover, MCMV infection was delayed when the BL-treated plants were applied with NO scavenger, which suggested that BR induced the susceptibility of maize to MCMV infection in a NO-dependent manner. Further investigation showed the maize plants with knock-down of DWARF4 (ZmDWF4, a key gene of BR synthesis) and nitrate reductase (ZmNR, a key gene of NO synthesis) by virus-induced gene silencing displayed higher resistance to MCMV than control plants. Taken together, our results suggest that BR pathway promotes the susceptibility of maize to MCMV in a NO-dependent manner.

Hydrogen Peroxide and Nitric Oxide Crosstalk Mediates Brassinosteroids Induced Cold Stress Tolerance in Medicago truncatula

Brassinosteroids (BRs) play pivotal roles in modulating plant growth, development, and stress responses. In this study, a Medicago truncatula plant pretreated with brassinolide (BL, the most active BR), enhanced cold stress tolerance by regulating the expression of several cold-related genes and antioxidant enzymes activities. Previous studies reported that hydrogen peroxide (H₂O₂) and nitric oxide (NO) are involved during environmental stress conditions. However, how these two signaling molecules interact with each other in BRs-induced abiotic stress tolerance remain largely unclear. BL-pretreatment induced, while brassinazole (BRZ, a specific inhibitor of BRs biosynthesis) reduced H₂O₂ and NO production. Further, application of dimethylthiourea (DMTU, a H₂O₂ and OH- scavenger) blocked BRs-induced NO production, but BRs-induced H₂O₂ generation was not sensitive to 2-phenyl-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (PTIO, a scavenger of NO). Moreover, pretreatment with DMTU and PTIO decreased BL-induced mitochondrial alternative oxidase (AOX) and the photosystem capacity. However, pretreatment with PTIO was found to be more effective than DMTU in reducing BRs-induced increases in Valt, Vt, and MtAOX1 gene expression. Similarly, BRs-induced photosystem II efficiency was found in NO dependent manner than H₂O₂. Finally, we conclude that H₂O₂ was involved in NO generation, whereas NO was found to be crucial in BRs-induced AOX capacity, which further contributed to the protection of the photosystem under cold stress conditions in Medicago truncatula.

Mitochondrial alternative oxidase-dependent autophagy involved in ethylene-mediated drought tolerance in Solanum lycopersicum

Mitochondrial alternative oxidase (AOX) is involved in a large number of plant physiological processes, such as growth, development and stress responses; however, the exact role of AOX in response to drought remains unclear. In our study, we provide solid evidences that the activated AOX capacity positively involved in ethylene-induced drought tolerance, in tomato (Solanum lycopersicum), accompanied by the changing level of hydrogen peroxide (H2 O2 ) and autophagy. In AOX1a-RNAi plants, the ethylene-induced drought tolerance was aggravated and associated with decreasing level of autophagy. The H2 O2 level was relatively higher in AOX1a-RNAi plants, whereas it was lower in AOX1a-overexpressing (35S-AOX1a-OE) plants after 1-(aminocarbonyl)-1-cyclopropanecarboxylic acid (ACC) pretreatment in the 14th day under drought stress. Interestingly, the accumulation of autophagosome was accompanied by the changing level of reactive oxygen species (ROS) in AOX transgenic tomato under drought stress whether or not pretreated with ACC. Pharmacological scavenging of H2 O2 accumulation in AOX1a-RNAi (aox19) stimulated autophagy acceleration under drought stress, and it seems that AOX-dependent ROS signalling is critical in triggering autophagy. Lower levels of ROS signalling positively induce autophagy activity, whereas higher ROS level would lead to rapid programmed cell death (PCD), especially in ethylene-mediated drought tolerance. Moreover, ethylene-induced autophagy during drought stress also can be through ERF5 binding to the promoters of ATG8d and ATG18h. These results demonstrated that AOX plays an essential role in ethylene-induced drought tolerance and also played important roles in mediating autophagy generation via balancing ROS level.

Brassinosteroids: A Promising Option in Deciphering Remedial Strategies for Abiotic Stress Tolerance in Rice

Rice is an important staple crop as it feeds about a half of the earth's population. It is known to be sensitive to a range of abiotic stresses which result in significant decline in crop productivity. Recently, the use of phytohormones for abiotic stress amelioration has generated considerable interest. Plants adapt to various environmental stresses by undergoing series of changes at physiological and molecular levels which are cooperatively modulated by various phytohormones. Brassinosteroids (BRs) are a class of naturally occurring steroidal phytohormones, best known for their role in plant growth and development. For the past two decades, greater emphasis on studies related to BRs biosynthesis, distribution and signaling has resulted in better understanding of BRs function. Recent advances in the use of contemporary genetic, biochemical and proteomic tools, with a vast array of accessible biological resources has led to an extensive exploration of the key regulatory components in BR signaling networks, thus making it one of the most well-studied hormonal pathways in plants. The present review highlights the advancements of knowledge in BR research and links it with its growing potential in abiotic stress management for important crop like rice.

Effect of 24-epibrassinolide on Brassica napus alternative respiratory pathway, guard cells movements and phospholipid signaling under salt stress

Using Brassica napus roots we observed statistically significant increase in alternative respiratory pathway in response to exogenous 24-epibrassinolide (EBL) under optimal conditions and salinity. Also we observed activation of phospholipid signaling under the same conditions in response to EBL by measuring levels of lipid second messengers - diacylglycerol (DAG) and phosphatidic acid (PA). We found that brassinosteroids cause closure of stomata in isolated leaf disks while inhibitors of alternative oxidase cancelled these effects. This study demonstrates that BRs activate total respiration rate, alternative respiratory pathway, production of PA and DAG, stimulate stomata closure and growth under optimal conditions and salinity. Also, specific inhibitor of brassinosteroids biosynthesis decreased alternative respiratory pathway and production of lipid messengers in rape plants.

Brassinosteroid Action in Plant Abiotic Stress Tolerance

Brassinosteroids (BRs) are a class of plant steroidal hormones that play essential roles in plant growth and development. Systematic studies had first been undertaken concomitantly to determine both the effects of exogenous BR on stress phenotypes of Arabidopsis thaliana and Brassica napus (rapeseed) seedlings and the expression of stress marker genes in BR-treated and untreated seedlings. When reproducible and convincing evidence of the role of BR in stress tolerance had been obtained, molecular mechanisms underlying the ability of BR to confer tolerance against heat, cold, drought, and salt stress, as well as pathogen resistance were studied with several molecular approaches and tools. The results of these studies have together provided valuable insights into how BRs, through their control of many basic cellular processes and stress responses, promote vigor in plants and prepare the plant to mount a dynamic response upon environmental challenges. Protocols to assess BR effects on abiotic stress tolerance in Arabidopsis and rapeseed seedlings are described here and they can be fine-tuned and adapted for other plant species.

Role of brassinosteroid signaling in modulating Tobacco mosaic virus resistance in Nicotiana benthamiana

Plant steroid hormones, brassinosteroids (BRs), play essential roles in plant growth, development and stress responses. However, mechanisms by which BRs interfere with plant resistance to virus remain largely unclear. In this study, we used pharmacological and genetic approaches in combination with infection experiments to investigate the role of BRs in plant defense against Tobacco Mosaic Virus (TMV) in Nicotiana benthamiana. Exogenous applied BRs enhanced plant resistance to virus infection, while application of Bikinin (inhibitor of glycogen synthase kinase-3), which activated BR signaling, increased virus susceptibility. Silencing of NbBRI1 and NbBSK1 blocked BR-induced TMV resistance, and silencing of NbBES1/BZR1 blocked Bikinin-reduced TMV resistance. Silencing of NbMEK2, NbSIPK and NbRBOHB all compromised BR-induced virus resistance and defense-associated genes expression. Furthermore, we found MEK2-SIPK cascade activated while BES1/BZR1 inhibited RBOHB-dependent ROS production, defense gene expression and virus resistance induced by BRs. Thus, our results revealed BR signaling had two opposite effects on viral defense response. On the one hand, BRs enhanced virus resistance through MEK2-SIPK cascade and RBOHB-dependent ROS burst. On the other hand, BES1/BZR1 inhibited RBOHB-dependent ROS production and acted as an important mediator of the trade-off between growth and immunity in BR signaling.