Combined effect of salicylic acid and potassium mitigates drought stress through the modulation of physio-biochemical attributes and key antioxidants in wheat

Surviving the stress: Understanding the molecular basis of plant adaptations and uncovering the role of mycorrhizal association in plant abiotic stresses

Environmental stresses severely impair plant growth, resulting in significant crop yield and quality loss. Among various abiotic factors, salt and drought stresses are one of the major factors that affect the nutrients and water uptake by the plants, hence ultimately various physiological aspects of the plants that compromises crop yield. Continuous efforts have been made to investigate, dissect and improve plant adaptations at the molecular level in response to drought and salinity stresses. In this context, the plant beneficial microbiome presents in the rhizosphere, endosphere, and phyllosphere, also referred as second genomes of the plant is well known for its roles in plant adaptations. Exploration of beneficial interaction of fungi with host plants known as mycorrhizal association is one such special interaction that can facilitates the host plants adaptations. Mycorrhiza assist in alleviating the salinity and drought stresses of plants via redistributing the ion imbalance through translocation to different parts of the plants, as well as triggering oxidative machinery. Mycorrhiza association also regulates the level of various plant growth regulators, osmolytes and assists in acquiring minerals that are helpful in plant's adaptation against extreme environmental stresses. The current review examines the role of various plant growth regulators and plants' antioxidative systems, followed by mycorrhizal association during drought and salt stresses.

Advances in Roles of Salicylic Acid in Plant Tolerance Responses to Biotic and Abiotic Stresses

A multitude of biotic and abiotic stress factors do harm to plants by bringing about diseases and inhibiting normal growth and development. As a pivotal signaling molecule, salicylic acid (SA) plays crucial roles in plant tolerance responses to both biotic and abiotic stresses, thereby maintaining plant normal growth and improving yields under stress. In view of this, this paper mainly discusses the role of SA in both biotic and abiotic stresses of plants. SA regulates the expression of genes involved in defense signaling pathways, thus enhancing plant immunity. In addition, SA mitigates the negative effects of abiotic stresses, and acts as a signaling molecule to induce the expression of stress-responsive genes and the synthesis of stress-related proteins. In addition, SA also improves certain yield-related photosynthetic indexes, thereby enhancing crop yield under stress. On the other hand, SA acts with other signaling molecules, such as jasmonic acid (JA), auxin, ethylene (ETH), and so on, in regulating plant growth and improving tolerance under stress. This paper reviews recent advances in SA's roles in plant stress tolerance, so as to provide theoretical references for further studies concerning the decryption of molecular mechanisms for SA's roles and the improvement of crop management under stress.

Salicylic Acid- and Potassium-Enhanced Resilience of Quinoa (Chenopodium quinoa Willd.) against Salinity and Cadmium Stress through Mitigating Ionic and Oxidative Stress

Salinity and cadmium (Cd) contamination of soil are serious environmental issues threatening food security. This study investigated the role of salicylic acid (SA) and potassium (K) in enhancing the resilience of quinoa against the combined stress of salinity and Cd. Quinoa plants were grown under NaCl (0, 200 mM) and Cd (0, 100 µM) stress, with the addition of 0.1 mM SA and 10 mM K, separately or in combination. The joint stress of Cd and NaCl caused >50% decrease in plant growth, chlorophyll contents, and stomatal conductance compared to the control plants. The higher accumulation of Na and Cd reduced the uptake of K in quinoa tissues. The joint stress of salinity and Cd caused an 11-fold increase in hydrogen peroxide and 13-fold increase in thiobarbituric acid reactive substances contents, and caused a 61% decrease in membrane stability. An external supply of 0.1 mM SA and 10 mM K helped plants to better adapt to salinity and Cd stress with less of a reduction in plant biomass (shoot 19% and root 24%) and less accumulation of Na and Cd in plant tissues. The activities of superoxide dismutase (SOD), catalase (CAT), peroxidase (POD), and ascorbate peroxidase (APX) were enhanced by 11-fold, 10-fold, 7.7-fold, and 7-fold, respectively, when SA and K were applied together to the plants subjected to the joint stress of Cd and salinity. Based on the values of the bioconcentration factor (>1), the translocation factor (<1), and the higher tolerance index, it was clear that Cd-contaminated, salty soils could be stabilized with quinoa under the combined supply of SA and K.

Potassium fertilizer improves drought stress alleviation potential in sesame by enhancing photosynthesis and hormonal regulation

Soil-potassium (K) low availability and drought stress are limiting factors to crop productivity in arid and semiarid regions. A pot experiment with four K soil supplies (0, 60, 120 and 180 K₂O kg ha^-1) and exposed to drought stress with 50 ± 5% field capacity was performed to investigate the function of K in protecting sesame plants from the adverse effects of drought based on the related physio-biochemical traits. The water stress was applied during flowering by withholding water for 6 days, and then rewatering to a well-watered level (75 ± 5% field capacity). Results showed that drought stress substantially reduced leaf relative water content (RWC), stomatal conductance (G_s), transpiration rate (T_r), photosynthetic rate (P_n), maximum PSII yield (F_v/F_m), and actual quantum yield of PSII (Ф_PSII), leading to greater non-photochemical quenching (qN) and stomatal limitation (L_s), thereby resulting in a decreased yield in contrast with well-watered sesame plants. Incidentally, K was more effective in promoting yield production under drought stress relative to well-watered conditions, and the optimal K application was 120 kg ha^-1, which primarily attributed to the enhanced photosynthetic and plant water retaining ability. Specifically, plants receiving K supply showed greater leaf gas exchange traits, higher F_v/F_m and Ф_PSII values, and superior water use efficiency as compared to K-deficiency plants in both water regimes. Moreover, K can ameliorate the adverse effects of drought by improving salicylic acid (SA) while conversely decreasing abscisic acid (ABA) and jasmonic acid (JA) concentrations that are involved in controlling stomatal closure. It is noted that significant correlations between the seed yield, gas exchange parameters, and aforementioned endogenous hormones were observed. In conclusion, the K application can improve the sesame plant's potential to maintain functionality regarding photosynthetic response and phytohormone regulation under drought stress, and ultimately, enhancing the sesame's productivity.

Appropriate Application Methods for Salicylic Acid and Plant Nutrients Combinations to Promote Morpho-Physiological Traits, Production, and Water Use Efficiency of Wheat under Normal and Deficit Irrigation in an Arid Climate

Freshwater shortage and inadequate nutrient management are the two major challenges for sustainable wheat production in arid agro-ecosystems. Relatively little is known about the positive roles of the application methods for the combination of salicylic acid (SA) and plant nutrients in sustaining wheat production under arid climatic conditions. A two-year field study was undertaken to assess the impact of seven treatments for the integrated application of SA, macronutrients, and micronutrients on the morpho-physiological traits, yield, and irrigation water use efficiency (IWUE) of wheat subjected to full (FL) and limited (LM) irrigation regimes. The results showed that the LM regime caused a significant reduction in different plant growth traits, relative water content, chlorophyll pigments, yield components, and yield, while a significant increase was observed in IWUE. The sole application of SA or co-application with micronutrients through soil did not significantly affect the studied traits under the FL regime, while they achieved some improvement over untreated plants under the LM regime. Based on the different multivariate analyses, the soil and foliar applications for the combinations of SA and micronutrients, as well as a foliar application for the combinations of SA, macronutrients, and micronutrients were identified as an efficient option for mitigating the negative impacts of water deficit stress and enhancing the growth and production of wheat under normal conditions. In conclusion, the results obtained herein indicated that the co-application of SA and macro- and micronutrients is an effective option to greatly enhance and improve the growth and production of wheat crops in water-scarce countries of arid regions, such as Saudi Arabia, while an appropriate application method for this combination was required for positive effects.

Drought tolerance of Aspergillus violaceofuscus and Bacillus licheniformis and their influence on tomato growth and potassium uptake in mica amended tropical soils under water-limiting conditions

Drought is a significant abiotic stress that alters plant physiology and ultimately affects crop productivity. Among essential plant nutrients, potassium (K) is known to mitigate the deleterious effect of drought on plant growth. If so, K addition or inoculation of potassium solubilizing microorganisms (KSMs) that are tolerant to drought should promote plant growth during water stress. Therefore, in this study, K solubilizing Aspergillus violaceofuscus and Bacillus licheniformis, isolated from saxicolous environments, were tested for their capacity to tolerate drought using different molecular weights (~4000, 6000, and 8000 Da), and concentrations (0, 250, 500, 750, 1000, and 1250 mg/L) of polyethylene glycol (PEG) under in vitro conditions. The results showed that high concentrations (750 and 1000 mg/L) of PEG with different molecular weight considerably improved bacterial cell numbers/fungal biomass and catalase (CAT) and proline activities. Moreover, the ability of KSMs alone or in combination to impart drought tolerance and promote plant growth in the presence and absence of mica (9.3% K₂O) supplementation was tested in Alfisol and Vertisol soil types under greenhouse conditions. The results revealed that the tomato plants inoculated with KSMs individually or dually with/without mica improved the physiological and morphological traits of the tomato plants under drought. Generally, tomato plants co-inoculated with KSMs and supplemented with mica were taller (2.62 and 3.38-fold) and had more leaf area (2.03 and 1.98-fold), total root length (3.26 and 8.86-fold), shoot biomass (3.87 and 3.93-fold), root biomass (9.00 and 7.24-fold), shoot K content (3.08 and 3.62-fold), root K content (3.39 and 2.03-fold), relative water content (1.51 and 1.27-fold), CAT activity (2.11 and 2.14-fold), proline content (3.41 and 3.28-fold), and total chlorophyll content (1.81 and 1.90-fold), in unsterilized Alfisol and Vertisol soil types, respectively, than uninoculated ones. Dual inoculation of the KSMs along with mica amendment, also improved the endorrhizal symbiosis of tomato plants more than their individual inoculation or application in both soil types. These findings imply that the A. violaceofuscus and B. licheniformis isolates are promising as novel bioinoculants for improving crop growth in water-stressed and rainfed areas of the tropics in the future.

Integrating Application Methods and Concentrations of Salicylic Acid as an Avenue to Enhance Growth, Production, and Water Use Efficiency of Wheat under Full and Deficit Irrigation in Arid Countries

As water deficit in arid countries has already become the norm rather than the exception, water conservation in crop production processes has become very critical. Therefore, it is urgent to develop feasible strategies to achieve this goal. Exogenous application of salicylic acid (SA) has been proposed as one of the effective and economical strategies for mitigating water deficit in plants. However, the recommendations concerning the proper application methods (AMs) and the optimal concentrations (Cons) of SA under field conditions seem contradictory. Here, a two-year field study was conducted to compare the effects of twelve combinations of AMs and Cons on the vegetative growth, physiological parameters, yield, and irrigation water use efficiency (IWUE) of wheat under full (FL) and limited (LM) irrigation regimes. These combinations included seed soaking in purified water (S₀), 0.5 mM SA (S₁), and 1.0 mM SA (S₂); foliar spray of SA at concentrations of 1.0 mM (F₁), 2.0 mM (F₂), and 3.0 mM (F₃); and combinations of S₁ and S₂ with F₁ (S₁F₁ and S₂F₁), F₂ (S₁F₂ and S₂F₂), and F₃ (S₁F₃ and S₂F₃). The results showed that the LM regime caused a significant reduction in all vegetative growth, physiological, and yield parameters, while it led to an increase in IWUE. The application of SA through seed soaking, foliar application, and a combination of both methods increased all of the studied parameters in all the evaluated times, resulting in higher values for all parameters than the treatment without SA (S₀). The multivariate analyses, including principal component analysis and heatmapping, identified the foliar application method with 1-3 mM SA alone or in combination with seed soaking with 0.5 mM SA as the best treatments for the optimal performance of wheat under both irrigation regimes. Overall, our results indicated that exogenous application of SA has the potential to greatly improve growth, yield, and IWUE under limited water application, while optimal coupling combinations of AMs and Cons were required for positive effects in field conditions.

Salicylic Acid's Impact on Growth, Photosynthesis, and Antioxidant Enzyme Activity of Triticum aestivum When Exposed to Salt

Recently, the application of salicylic acid (SA) for improving a plant's resistance to abiotic stresses has increased. A large part of the irrigated land (2.1% out of 19.5%) is severely affected by salinity stress worldwide. In 2020, total production of wheat (Triticum aestivum) was 761 million tons, representing the second most produced cereal after maize; therefore, research on its salinity tolerance is of world concern. Photosynthetic attributes such as net photosynthetic rate (P_N), stomatal conductance (gs), intercellular CO₂ concentration (Ci), and transpiration rate (E) were increased significantly by the application of SA. Salt stress increased antioxidant enzyme activity; however, SA further boosted their activity along with proline level. We conclude that SA interacts with meristematic cells, thereby triggering biochemical pathways conductive to the increment in morphological parameters. Further research is required to dissect the mechanisms of SA within the wheat plants under stress.

Salicylic Acid Protects Sweet Potato Seedlings from Drought Stress by Mediating Abscisic Acid-Related Gene Expression and Enhancing the Antioxidant Defense System

China has the largest sweet potato planting area worldwide, as well as the highest yield per unit area and total yield. Drought is the most frequently encountered environmental stress during the sweet potato growing season. In this study, we investigated salicylic acid (SA)-mediated defense mechanisms under drought conditions in two sweet potato varieties, Zheshu 77 and Zheshu 13. Drought stress decreased growth traits, photosynthetic pigments and relative water contents, as well as the photosynthetic capability parameters net photosynthetic rate, stomatal conductance and transpiration rate, whereas it increased reactive oxygen species production, as well as malondialdehyde and abscisic acid contents. The application of SA to drought-stressed plants reduced oxidative damage by triggering the modulation of antioxidant enzyme activities and the maintenance of optimized osmotic environments in vivo in the two sweet potato varieties. After SA solution applications, NCED-like3 expression was downregulated and the abscisic acid contents of drought-stressed plants decreased, promoting photosynthesis and plant growth. Thus, foliar spraying an appropriate dose of SA, 2.00-4.00 mg·L^-1, on drought-stressed sweet potato varieties may induce resistance in field conditions, thereby increasing growth and crop yield in the face of increasingly frequent drought conditions.

Effects of Combined Application of Salicylic Acid and Proline on the Defense Response of Potato Tubers to Newly Emerging Soft Rot Bacteria (Lelliottia amnigena) Infection

Potato soft rot, caused by the pathogenic bacterium Lelliottia amnigena (Enterobacter amnigenus), is a serious and widespread disease affecting global potato production. Both salicylic acid (SA) and proline (Pro) play important roles in enhancing potato tuber resistance to soft rot. However, the combined effects of SA and Pro on defense responses of potato tubers to L. amnigena infection remain unknown. Hence, the combined effects of SA and Pro in controlling newly emerging potato soft rot bacteria were investigated. Sterilized healthy potato tubers were pretreated with 1.5 mM SA and 2.0 mM Pro 24 h before an inoculation of 0.3 mL of L. amnigena suspension (3.69 × 107 CFU mL−1). Rotting was noticed on the surfaces of the hole where the L. amnigena suspension was inoculated. Application of SA and Pro with L. amnigena lowered the activity of pectinase, protease, pectin lyase, and cellulase by 64.3, 77.8, 66.4 and 84.1%, and decreased malondialdehyde and hydrogen peroxide contents by 77.2% and 83.8%, respectively, compared to the control. The activities of NADPH oxidase, superoxide dismutase, peroxide, catalase, polyphenol oxidase, phenylalanine ammonia-lyase, cinnamyl alcohol dehydrogenase, 4-coumaryl-CoA ligase and cinnamate-4-hydroxylase were increased in the potato tubers with combined treatments by 91.4, 92.4, 91.8, 93.5, 94.9, 91.3, 96.2, 94.7 and 97.7%, respectively, compared to untreated stressed tubers. Six defense-related genes, pathogenesis-related protein, tyrosine-protein kinase, Chitinase-like protein, phenylalanine ammonia-lyase, pathogenesis-related homeodomain protein, and serine protease inhibitor, were induced in SA + Pro treatment when compared with individual application of SA or Pro. This study indicates that the combined treatment of 1.5 mM SA and 2.0 mM Pro had a synergistic effect in controlling potato soft rot caused by a newly emerging bacterium.

Plants' Physio-Biochemical and Phyto-Hormonal Responses to Alleviate the Adverse Effects of Drought Stress: A Comprehensive Review

Water, a necessary component of cell protoplasm, plays an essential role in supporting life on Earth; nevertheless, extreme changes in climatic conditions limit water availability, causing numerous issues, such as the current water-scarce regimes in many regions of the biome. This review aims to collect data from various published studies in the literature to understand and critically analyze plants' morphological, growth, yield, and physio-biochemical responses to drought stress and their potential to modulate and nullify the damaging effects of drought stress via activating natural physiological and biochemical mechanisms. In addition, the review described current breakthroughs in understanding how plant hormones influence drought stress responses and phytohormonal interaction through signaling under water stress regimes. The information for this review was systematically gathered from different global search engines and the scientific literature databases Science Direct, including Google Scholar, Web of Science, related studies, published books, and articles. Drought stress is a significant obstacle to meeting food demand for the world's constantly growing population. Plants cope with stress regimes through changes to cellular osmotic potential, water potential, and activation of natural defense systems in the form of antioxidant enzymes and accumulation of osmolytes including proteins, proline, glycine betaine, phenolic compounds, and soluble sugars. Phytohormones modulate developmental processes and signaling networks, which aid in acclimating plants to biotic and abiotic challenges and, consequently, their survival. Significant progress has been made for jasmonates, salicylic acid, and ethylene in identifying important components and understanding their roles in plant responses to abiotic stress. Other plant hormones, such as abscisic acid, auxin, gibberellic acid, brassinosteroids, and peptide hormones, have been linked to plant defense signaling pathways in various ways.