Salicylic acid improves arbuscular mycorrhizal symbiosis, and chickpea growth and yield by modulating carbohydrate metabolism under salt stress

Insights into salicylic acid-mediated redox homeostasis, carbohydrate metabolism and secondary metabolite involvement in improvement of photosynthetic performance, enzyme activities, ionomics, and yield in different varieties of Abelmoschus esculentus

Salicylic acid (SA) is a well-known signaling molecule and phenolic plant hormone. However, the optimal concentration of SA required for beneficial effects may vary across different plant species. The objective of this study was to investigate the effects of salicylic acid (SA) on two different varieties of Abelmoschus esculentus (Sakata-713 and Neelam) in order to determine the optimal concentration of SA and its impact on the growth, physiology, and biochemical processes of the plants. We conducted an experiment applying different SA concentrations (0, 10-4, 10-5, 10-6, 10-7 M) at 25 days after sowing (DAS) and evaluated various plant parameters at different stages. To evaluate various parameters sampling was performed at 30 and 45 DAS; yield traits were calculated at 60 DAS. The results indicate that SA application increased cell division, trichome number, chlorophyll content, photosynthesis, gas exchange traits, and elemental status which further boosted plants growth and yield traits. SA application stimulated activity of several enzymes that participate in carboxylation/decarboxylation homeostasis (carbonic anhydrase), nitrogen metabolism (nitrate reductase), Calvin cycle (Rubisco), TCA cycle (succinate dehydrogenase and fumarase) and secondary metabolism (phenylalanine lyase). A gradual increase in the production of secondary metabolites (total phenol, total flavonoid, anthocyanin) and carbon metabolism (total reducing sugars, starch, glucose, fructose, sucrose) was observed. Notably, SA treatment also played a vital role in maintaining a balanced equilibrium between reactive oxygen species (ROS) and the scavenging system (catalase, peroxidase, superoxide dismutase). Based on our results, the optimal concentration of SA was determined to be 10-5 M, as it yielded the most favourable outcomes among the different concentrations tested. Moreover, when comparing the two varieties of okra, Sakata-713 exhibited a more promising response to SA treatment compared to Neelam.

Drought Stress Tolerance in Vegetables: The Functional Role of Structural Features, Key Gene Pathways, and Exogenous Hormones

The deleterious effects of drought stress have led to a significant decline in vegetable production, ultimately affecting food security. After sensing drought stress signals, vegetables prompt multifaceted response measures, eventually leading to changes in internal cell structure and external morphology. Among them, it is important to highlight that the changes, including changes in physiological metabolism, signal transduction, key genes, and hormone regulation, significantly influence drought stress tolerance in vegetables. This article elaborates on vegetable stress tolerance, focusing on structural adaptations, key genes, drought stress signaling transduction pathways, osmotic adjustments, and antioxidants. At the same time, the mechanisms of exogenous hormones such as abscisic acid (ABA), jasmonic acid (JA), salicylic acid (SA), and ethylene (ET) toward improving the adaptive drought tolerance of vegetables were also reviewed. These insights can enhance the understanding of vegetable drought tolerance, supporting vegetable tolerance enhancement by cultivation technology improvements under changing climatic conditions, which provides theoretical support and technical reference for innovative vegetable stress tolerance breeding and food security.

Models of mycorrhizal colonization patterns and strategies induced by biostimulator treatments in Zea mays roots

Agronomic inputs and technologies, especially fertilizers, act on the evolution of the symbiotic partnership between arbuscular mycorrhizal fungi and cultivated plants. The use of the MycoPatt method for the assessment of mycorrhizas in maize roots leads to the extraction of large parameter databases with an increased resolution over the colonization mechanism. The application of a biostimulator treatment on plants acted toward a reduction of root permissiveness for mycorrhizas. The phenomenon was noticeable through an increased colonization variability that overlapped with plant nutritional needs. The annual characteristic of the plant was highlighted by the simultaneous presence of arbuscules and vesicles, with a high share of arbuscules in the advanced phenophases. Colonized root parts presented numerous arbuscule-dominated areas in all phenophases, which indicated a continuous formation of these structures and an intense nutrient transfer between partners. Mycorrhizal maps showed the slowing effect of the biostimulators on colonization, with one phenophase delay in the case of biostimulated plants compared to the ones without biostimulators. The forecast models presented gradual colonization in plants without biostimulators, with the expansion of new hyphal networks. The use of biostimulators on plants exhibited a lower permissiveness for new colonization areas, and the mechanism relies on hyphae developed in the former phenophases.

Exogenously-Sourced Salicylic Acid Imparts Resilience towards Arsenic Stress by Modulating Photosynthesis, Antioxidant Potential and Arsenic Sequestration in Brassica napus Plants

In the current study, salicylic acid (SA) assesses the physiological and biochemical responses in overcoming the potential deleterious impacts of arsenic (As) on Brassica napus cultivar Neelam. The toxicity caused by As significantly reduced the observed growth and photosynthetic attributes and accelerated the reactive oxygen species (ROS). Plants subjected to As stress revealed a significant (p ≤ 0.05) reduction in the plant growth and photosynthetic parameters, which accounts for decreased carbon (C) and sulfur (S) assimilation. Foliar spray of SA lowered the oxidative burden in terms of hydrogen peroxide (H2O2), superoxide anion (O2•-), and lipid peroxidation in As-affected plants. Application of SA in two levels (250 and 500 mM) protected the Brassica napus cultivar from As stress by enhancing the antioxidant capacity of the plant by lowering oxidative stress. Among the two doses, 500 mM SA was most effective in mitigating the adverse effects of As on the Brassica napus cultivar. It was found that SA application to the Brassica napus cultivar alleviated the stress by lowering the accumulation of As in roots and leaves due to the participation of metal chelators like phytochelatins, enhancing the S-assimilatory pathway, carbohydrate metabolism, higher cell viability in roots, activity of ribulose 1, 5-bisphosphate carboxylase (Rubisco), and proline metabolism through the active participation of γ-glutamyl kinase (GK) and proline oxidase (PROX) enzyme. The current study shows that SA has the capability to enhance the growth and productivity of B. napus plants cultivated in agricultural soil polluted with As and perhaps other heavy metals.

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.

Physiological Biochemistry-Combined Transcriptomic Analysis Reveals Mechanism of Bacillus cereus G2 Improved Salt-Stress Tolerance of Glycyrrhiza uralensis Fisch. Seedlings by Balancing Carbohydrate Metabolism

Salt stress severely threatens the growth and productivity of Glycyrrhiza uralensis. Previous results found that Bacillus cereus G2 enhanced several carbohydrate contents in G. uralensis under salt stress. Here, we analyzed the changes in parameters related to growth, photosynthesis, carbohydrate transformation, and the glycolysis Embden-Meyerhof-Parnas (EMP) pathway-tricarboxylic acid (TCA) cycle by G2 in G. uralensis under salt stress. Results showed that G2 helped G. uralensis-accumulating photosynthetic pigments during photosynthesis, which could further increase starch, sucrose, and fructose contents during carbohydrate transformation. Specifically, increased soluble starch synthase (SSS) activity caused to higher starch content, which could induce α-amylase (AM) and β-amylase (BM) activities; increased sucrose content due to the increase of sucrose synthase (SS) activity through upregulating the gene-encoding SS, which decreased cell osmotic potential, and consequently, induced invertase and gene-encoding α-glucosidase that decomposed sucrose to fructose, ultimately avoided further water loss; increased fructose content-required highly hexokinase (HK) activity to phosphorylate in G. uralensis, thereby providing sufficient substrate for EMP. However, G2 decreased phosphofructokinase (PFK) and pyruvate kinase (PK) activities during EMP. For inducing the TCA cycle to produce more energy, G2 increased PDH activity that enhanced CA content, which further increased isocitrate dehydrogenase (ICDH) activity and provided intermediate products for the G. uralensis TCA cycle under salt stress. In sum, G2 could improve photosynthetic efficiency and carbohydrate transformation to enhance carbohydrate products, thereby releasing more chemical energy stored in carbohydrates through the EMP pathway-TCA cycle, finally maintain normal life activities, and promote the growth of G. uralensis under salt stress.

Comparative transcriptomic and metabolic profiling provides insight into the mechanism by which the autophagy inhibitor 3-MA enhances salt stress sensitivity in wheat seedlings

BACKGROUND

Salt stress hinders plant growth and production around the world. Autophagy induced by salt stress helps plants improve their adaptability to salt stress. However, the underlying mechanism behind this adaptability remains unclear. To obtain deeper insight into this phenomenon, combined metabolomics and transcriptomics analyses were used to explore the coexpression of differentially expressed-metabolite (DEM) and gene (DEG) between control and salt-stressed wheat roots and leaves in the presence or absence of the added autophagy inhibitor 3-methyladenine (3-MA).

RESULTS

The results indicated that 3-MA addition inhibited autophagy, increased ROS accumulation, damaged photosynthesis apparatus and impaired the tolerance of wheat seedlings to NaCl stress. A total of 14,759 DEGs and 554 DEMs in roots and leaves of wheat seedlings were induced by salt stress. DEGs were predominantly enriched in cellular amino acid catabolic process, response to external biotic stimulus, regulation of the response to salt stress, reactive oxygen species (ROS) biosynthetic process, regulation of response to osmotic stress, ect. The DEMs were mostly associated with amino acid metabolism, carbohydrate metabolism, phenylalanine metabolism, carbapenem biosynthesis, and pantothenate and CoA biosynthesis. Further analysis identified some critical genes (gene involved in the oxidative stress response, gene encoding transcription factor (TF) and gene involved in the synthesis of metabolite such as alanine, asparagine, aspartate, glutamate, glutamine, 4-aminobutyric acid, abscisic acid, jasmonic acid, ect.) that potentially participated in a complex regulatory network in the wheat response to NaCl stress. The expression of the upregulated DEGs and DEMs were higher, and the expression of the down-regulated DEGs and DEMs was lower in 3-MA-treated plants under NaCl treatment.

CONCLUSION

3-MA enhanced the salt stress sensitivity of wheat seedlings by inhibiting the activity of the roots and leaves, inhibiting autophagy in the roots and leaves, increasing the content of both H₂O₂ and O₂•-, damaged photosynthesis apparatus and changing the transcriptome and metabolome of salt-stressed wheat seedlings.

Arbuscular mycorrhizae and silicon alleviate arsenic toxicity by enhancing soil nutrient availability, starch degradation and productivity in Cajanus cajan (L.) Millsp

Arsenic (As) pollution of soil reduces the growth and reproductive potential of plants. Silicon (Si) and arbuscular mycorrhizal (AM) fungi play significant roles in alleviating adverse effects of As stress. However, studies are scant regarding alleviative effects of Si in pigeonpea (Cajanus cajan L. Millsp.) because legumes are considered low Si-accumulators. We investigated the individual as well as synergistic potential of Si with two AM species (M1-Claroideoglomus etunicatum and M2-Rhizoglomus intraradices) in modulating soil properties, thereby improving growth and productivity of pigeonpea genotype Pusa 2001 grown in AsV and AsIII challenged soils. Both As species hampered the establishment of AM symbiosis, thus, reducing nutrient uptake, growth and yield, with AsIII more toxic than AsV. Exogenously applied Si and AM species enhanced soil glomalin and phosphatases activity, hence decreased metal bioavailability in soil, increased plant nutrient acquisition, biomass and chlorophylls; with maximum benefits provided by M2, closely followed by Si and least by M1. These amendments boosted the activities of starch hydrolytic enzymes (α-, β-amylase, starch phosphorylase) in plants, along with a simultaneous increase in total soluble sugars (TSS). This enhanced sugar accumulation directly led to improved reproductive attributes, more efficiently by M2 and Si than by M1. Moreover, there was a substantial increase in proline biosynthesis due to significantly enhanced activities of its biosynthetic enzymes. Additionally, combined applications of Si and AM, especially +Si+M2, complemented each other where AM enhanced Si uptake, while Si induced mycorrhization, suggesting their mutual and beneficial roles in ameliorating metal(loid) toxicity and achieving sustainability in pigeonpea production under As stress.

The role of halophytic nanoparticles towards the remediation of degraded and saline agricultural lands

Salinity is one of the major causes of abiotic stress that leads to a reduction in crop yield. One strategy to alleviate and improve crop yield is to use halophytes. These types of plants naturally produce bioactive secondary metabolites, proteins, carbohydrates, and biopolymers that are involved in specialized physiological adaptation mechanisms to alleviate soil salinity. These traits could be leveraged and, in turn, be the focus of future breeding programs aimed to improve salinity resistance in traditional crops. Recently, the field of nanotechnology has gained the attention of researchers involved in agricultural science and associated disciplines. However, information on salinity tolerance mechanisms of halophytes, based on nanoparticles in agricultural crop plants, is limited. Recently, the use of selected halophytic-based nanoparticles has shown to improve crop performance by enhancing the plants' ion flux, improving water efficiency, root hydraulic movement in the favor of plant photosynthesis, the production of proteins involved in oxidation-reduction reactions, reactive oxygen species (ROS) detoxification, and hormonal signaling pathways under stress. Therefore, the aim of this review is to highlight the application of halophytic nanoparticles in alleviating salt stress in plants by understanding the mechanisms of plant growth, water relation, ion flux, photosynthesis, and the antioxidant defense system. This review also addresses uncertainties, ecotoxicological concerns, and associated drawbacks of nanoparticles on the environment. Future research perspectives with respect to the sustainable usage of nanoparticles in saline agriculture have also been presented.

Salicylic acid affects mycorrhizal features, antioxidant enzyme activities and seed yield of linseed under water-deficit stress in open-field conditions

The research aims were to study salicylic acid (SA) effects on mycorrhiza [hyphal width (HW), vesicle diameter (VD) and mycorrhizal colonization (MC)] and interaction between them on greenness index (GI), drought tolerance index (DTI), antioxidant enzymes activities, and seed yield of linseed under drought. A factorial experiment was conducted in an open-field place with mycorrhiza [non-inoculation, Funneliformis mosseae (FM), and Rhizoglomus intraradices (RI)], SA (250 μM and non-SA), and irrigation levels [100%, 70%, and 40% field capacity (FC)] as treatments. Severe drought increased VD, MC, superoxide dismutase (SOD), ascorbate peroxidase (APX), and peroxidase activities while decreased GI, DTI, and yield. The RI-linseed had higher MC, GI, SOD, and glutathione reductase (GR) activities, but FM-linseed had greater VD and yield under drought. Inoculated linseed with both mycorrhiza showed a reduction in DTI and yield under SA than non-SA. In RI-linseed, SA increased GI, MC, HW, VD, catalase and GR, but decreased in FM-plants. Mycorrhiza (particularly RI) alleviated drought (40% FC)-caused negative effects on linseed via the improvement of SOD, APX, and GI. Regardless of other treatments, SA had negative effects on HW and VD, but SA effects varied depending on mycorrhizal species so that SA increased HW, VD, and MC in RI. Due to the positive correlation between MC and HW, SA reduces FM colonization by reducing the HW of FM. Totally, SA along with RI species can mitigate the harmful effects of drought and improve tolerance via increasing MC, HW, VD, catalase, peroxidase, and GR activities.

Arbuscular mycorrhizal fungi improve growth, essential oil, secondary metabolism, and yield of tobacco (Nicotiana tabacum L.) under drought stress conditions

Drought is a major environmental threat limiting worldwide crop production. Drought stress affects the tobacco quality and yield; therefore, the current research studies were undertaken to investigate the effectiveness of arbuscular mycorrhizal fungi (AMF) under drought stress on morphological and biochemical attributes of tobacco (Nicotiana tabacum L. variety Yunyan 87). AMF-inoculated and AMF-non-inoculated plants were maintained in a greenhouse and irrigated with a half-strength Hoagland solution (100 mL pot-1) once a week. At harvesting, the plant height, number of leaves, fresh and dry weights, mycorrhizal colonization, and concentration of leaf photosynthetic pigments and photosynthetic rate were measured. Data were statistically analyzed by ANOVA and the principal component (PCA) analyses. The effect of root colonization significantly increased biomass production and essential oil accumulation. Results showed that drought at mild and severe stressed levels significantly affected tobacco growth by decreasing plant height, biomass, and a number of leaves. However, inoculation of AMF considerably increased plant height, fresh and dry weights, chlorophyll (a, b), total chlorophyll, and carotenoid content by 43.84, 40.87 and 49.76, 185.29, 325.60, 173.12, and 211.49%, respectively. Compared with non-inoculated plants, AMF inoculation significantly enhanced the essential oil yield and the uptake of nitrogen, phosphorus, and potassium with the increase of 257.36, 102.71, and 90.76, 62.32, and 84.51%, respectively, in mild drought + AMF-treated plants. Similarly, the antioxidant enzymatic activity, glomalin-related soil protein (GRSP), and accumulation of phenols and flavonoids and osmolytes content were also significantly improved in inoculated plants under drought stress. Additionally, AMF inoculation significantly upregulated the lipoxygenase (LOX) and phenylalanine ammonia-lyase (PAL) enzymes by 197 and 298.44% under drought conditions. These findings depicted that the symbiotic association of AMF improved the overall growth pattern and secondary metabolism in tobacco plants under severe drought stress conditions and may be used as an approaching source of important drugs in the field of pharmacology.

Traversing arbuscular mycorrhizal fungi and Pseudomonas fluorescens for carrot production under salinity

Carrot is a vital supply of dietary fiber, vitamins, and carotenoids, and it is also rich in antioxidants and minerals. Soil salinity significantly reduces the yield and quality of carrots. Mycorrhiza inoculum (AMF) is known to improve morphological and biochemical traits of vegetables even under saline conditions. But the role of AMF in combating soil salinity effect in carrot is not studied in detail. Therefore here, in the first set, carrot seeds are inoculated with microbes in a pot experiment under polyhouse condition. In total, we applied 7 treatments with different combinations of Mycorrhiza inoculum (Glomus mosseae (G_m) and Gigaspora gigantea (G_g)) and phosphate solubilizing bacteria (Pseudomonas fluroscens (P_f)). In pot experiment study the best two treatments were the combination of G_m + P_f + G_G and P_f + G_G. Both of these treatments were selected for validation under the open field conditions. Primarily, there seems to be a promising opportunity for AMF application to carrots under pot culture as well as under field trials because of promising effect towards morphological parameters, especially root weight, and disparities in nutrients and metabolites. Overall, our study highlights mycorrhizal fungi and other microbes' efficacy in achieving a successful carrot production under salinity stress.

Facultative symbiosis with a saprotrophic soil fungus promotes potassium uptake in American sweetgum trees

Several species of soil free-living saprotrophs can sometimes establish biotrophic symbiosis with plants, but the basic biology of this association remains largely unknown. Here, we investigate the symbiotic interaction between a common soil saprotroph, Clitopilus hobsonii (Agaricomycetes), and the American sweetgum (Liquidambar styraciflua). The colonized root cortical cells were found to contain numerous microsclerotia-like structures. Fungal colonization led to increased plant growth and facilitated potassium uptake, particularly under potassium limitation (0.05 mM K⁺ ). The expression of plant genes related to potassium uptake was not altered by the symbiosis, but colonized roots contained the transcripts of three fungal genes with homology to K⁺ transporters (ACU and HAK) and channel (SKC). Heterologously expressed ChACU and ChSKC restored the growth of a yeast K⁺ -uptake-defective mutant. Upregulation of ChACU transcript under low K⁺ conditions (0 and 0.05 mM K⁺ ) compared to control (5 mM K⁺ ) was demonstrated in planta and in vitro. Colonized plants displayed a larger accumulation of soluble sugars under 0.05 mM K⁺ than non-colonized plants. The present study suggests reciprocal benefits of this novel tree-fungus symbiosis under potassium limitation mainly through an exchange of additional carbon and potassium between both partners.

Exogenous Sodium Nitroprusside Mitigates Salt Stress in Lentil (Lens culinaris Medik.) by Affecting the Growth, Yield, and Biochemical Properties

Soil salinity disrupts the physiological and biochemical processes of crop plants and ultimately leads to compromising future food security. Sodium nitroprusside (SNP), a contributor to nitric oxide (NO), holds the potential to alleviate abiotic stress effects and boost tolerance in plants, whereas less information is available on its role in salt-stressed lentils. We examined the effect of exogenously applied SNP on salt-stressed lentil plants by monitoring plant growth and yield-related attributes, biochemistry of enzymes (superoxide dismutase (SOD), catalase (CAT), and peroxidase (POD)) amassing of leaf malondialdehyde (MDA) and hydrogen peroxide (H2O2). Salinity stress was induced by NaCl application at concentrations of 50 mM (moderate salinity) and 100 mM (severe salinity), while it was alleviated by SNP application at concentrations of 50 µM and 100 µM. Salinity stress severely inhibited the length of roots and shoots, the relative water content, and the chlorophyll content of the leaves, the number of branches, pods, seeds, seed yield, and biomass per plant. In addition, MDA, H2O2 as well as SOD, CAT, and POD activities were increased with increasing salinity levels. Plants supplemented with SNP (100 µM) showed a significant improvement in the growth- and yield-contributing parameters, especially in plants grown under moderate salinity (50 mM NaCl). Essentially, the application of 100 µM SNP remained effective to rescue lentil plants under moderate salinity by regulating plant growth and biochemical pathways. Thus, the exogenous application of SNP could be developed as a useful strategy for improving the performance of lentil plants in salinity-prone environments.

Relative roles of Arbuscular Mycorrhizae in establishing a correlation between soil properties, carbohydrate utilization and yield in Cicer arietinum L. under As stress

Accumulation of As (metalloid) degrades soil by negatively affecting the activities of soil enzymes, which in turn reduce growth and yield of the inhabiting plant. Arbuscular mycorrhizal (AM) symbiosis can impart metalloid tolerance in plants by secreting glomalin-related soil protein (GRSP) which binds with As or inertly adsorb in the extraradical mycelial surface. However, profitable use of AM requires selection of the most efficient combination of host plant and fungal species. The current study, therefore designed to study the efficacy of 3 a.m. fungal species: Rhizoglomus intraradices (Ri), Funneliformis mosseae (Fm) and Claroideoglomus claroideum (Cc) in imparting arsenate As(V) and arsenite As(III) stress tolerance in Cicer arietinum (chickpea) genotypes (G) - relatively metalloid tolerant- HC 3 and sensitive- C 235. Roots were found to be more severly affected as compared to shoots which resulted into a major decline in uptake of nutrients, chlorophyll concentrations and yield with As(III) inducing more toxic effects than As(V). HC 3 established more effective mycorrhizal symbiosis and was able to extract higher nutrients from the soil than C 235. Ri was most beneficial in improving plant biomass, carbohydrate utilization and productivity followed by Fm and Cc which could be due to its capability to initiate highest percent colonization and least metalloid uptake in roots through higher glomalin production in the soil. Moreover, Ri was highly efficient in improving soil enzymes activities-phosphatases (PHAs), β-glucosidase (BGA) and invertase (INV), thereby, imparting metalloid tolerance in chickpea genotypes. The results suggested use of Ri-chickpea symbiosis as a promising strategy for ameliorating As stress in chickpea.

Insights Into Microbially Induced Salt Tolerance and Endurance Mechanisms (STEM) in Plants

Salt stress threatens the achievement of sustainable global food security goals by inducing secondary stresses, such as osmotic, ionic, and oxidative stress, that are detrimental to plant growth and productivity. Various studies have reported the beneficial roles of microbes in ameliorating salt stress in plants. This review emphasizes salt tolerance and endurance mechanisms (STEM) in microbially inoculated (MI) plants that ensure plant growth and survival. Well-established STEM have been documented in MI plants and include conglomeration of osmolytes, antioxidant barricading, recuperating nutritional status, and ionic homeostasis. This is achieved via involvement of P solubilization, siderophore production, nitrogen fixation, selective ion absorption, volatile organic compound production, exopolysaccharide production, modifications to plant physiological processes (photosynthesis, transpiration, and stomatal conductance), and molecular alterations to alter various biochemical and physiological processes. Salt tolerance and endurance mechanism in MI plants ensures plant growth by improving nutrient uptake and maintaining ionic homeostasis, promoting superior water use efficiency and osmoprotection, enhancing photosynthetic efficiency, preserving cell ultrastructure, and reinforcing antioxidant metabolism. Molecular research in MI plants under salt stress conditions has found variations in the expression profiles of genes such as HKT1, NHX, and SOS1 (ion transporters), PIPs and TIPs (aquaporins), RBCS, RBCL (RuBisCo subunits), Lipoxygenase2 [jasmonic acid (JA) signaling], ABA (abscisic acid)-responsive gene, and APX, CAT, and POD (involved in antioxidant defense). Proteomic analysis in arbuscular mycorrhizal fungi-inoculated plants revealed upregulated expression of signal transduction proteins, including Ca²⁺ transporter ATPase, calcium-dependent protein kinase, calmodulin, and energy-related proteins (NADH dehydrogenase, iron-sulfur protein NADH dehydrogenase, cytochrome C oxidase, and ATP synthase). Future research should focus on the role of stress hormones, such as JA, salicylic acid, and brassinosteroids, in salt-stressed MI plants and how MI affects the cell wall, secondary metabolism, and signal transduction in host plants.

Hydrogen peroxide as a signalling molecule in plants and its crosstalk with other plant growth regulators under heavy metal stress

Hydrogen peroxide (H₂O₂) acts as a significant regulatory component interrelated with signal transduction in plants. The positive role of H₂O₂ in plants subjected to myriad of abiotic factors has led us to comprehend that it is not only a free radical, generated as a product of oxidative stress, but also helpful in the maintenance of cellular homeostasis in crop plants. Studies over the last two centuries has indicated that H₂O₂ is a key molecule which regulate photosynthesis, stomatal movement, pollen growth, fruit and flower development and leaf senescence. Exogenously-sourced H₂O₂ at nanomolar levels functions as a signalling molecule, facilitates seed germination, chlorophyll content, stomatal opening, and delays senescence, while at elevated levels, it triggers oxidative burst to organic molecules, which could lead to cell death. Furthermore, H₂O₂ is also known to interplay synergistically or antagonistically with other plant growth regulators such as auxins, gibberellins, cytokinins, abscisic acid, jasmonic acid, ethylene and salicylic acid, nitric oxide and Ca²⁺ (as signalling molecules), and brassinosteroids (steroidal PGRs) under myriad of environmental stresses and thus, mediate plant growth and development and reactions to abiotic factors. The purpose of this review is to specify accessible knowledge on the role and dynamic mechanisms of H₂O₂ in mediating growth responses and plant resilience to HM stresses, and its crosstalk with other significant PGRs in controlling various processes. More recently, signal transduction by mitogen activated protein kinases and other transcription factors which attenuate HM stresses in plants have also been dissected.

Alleviation of Salt Stress in Upland Rice (Oryza sativa L. ssp. indica cv. Leum Pua) Using Arbuscular Mycorrhizal Fungi Inoculation

Arbuscular mycorrhizal fungi (AMF) symbionts not only promote the growth of host plant but also alleviate abiotic stresses. This study aimed to investigate the putative role of AMF in salt stress regulation of upland pigmented rice cv. Leum Pua (LP) comparing with Pokkali salt tolerant (positive check). In general, LP is a variety of glutinous rice that contains anthocyanin pigment in the black pericarp, due to which it possesses high antioxidant activities compared to non-pigmented rice. Pot experiment was conducted to evaluate the impact of inoculated AMF, Glomus etunicatum (GE), Glomus geosporum (GG), and Glomus mosseae (GM) strains, in the LP plantlets subjected to 0 (control) or 150 mM NaCl (salt stress) for 2 weeks in comparison with Pokkali (a salt tolerant rice cultivar), which was maintained as a positive check. Root colonization percentage under NaCl conditions ranged from 23 to 30%. Na⁺ content in the flag leaf tissues was increased to 18-35 mg g^-1 DW after exposure to 150 mM NaCl for 14 days in both inoculated and un-inoculated LP plants, whereas Na:K ratio was very low in cv. Pokkali. Interestingly, sucrose content in the flag leaf tissues of un-inoculated LP plants under salt stress was increased significantly by 50 folds over the control as an indicator of salt stress response, whereas it was unchanged in all AMF treatments. Fructose and free proline in GE inoculated plants under salt stress were accumulated over control by 5.75 and 13.59 folds, respectively, for osmotic adjustment of the cell, thereby maintaining the structure and functions of chlorophyll pigments, F_v/F_m, Φ_PSII, and stomatal function. Shoot height, flag leaf length, number of panicles, panicle length, panicle weight, and 100-grain weight in GE inoculated plants of cv. LP under salt stress were maintained similar to cv. Pokkali. Interestingly, cyanidin-3-glucoside (C3G) and peonidin-3-glucoside (P3G) in the pericarp of cv. LP were regulated by GE inoculation under salt stress conditions. In summary, AMF-inoculation in rice crop is a successful alternative approach to reduce salt toxicity, maintain the yield attributes, and regulate anthocyanins enrichment in the pericarp of grains.

Interactive effects of polyamines and arbuscular mycorrhiza in modulating plant biomass, N2 fixation, ureide, and trehalose metabolism in Cajanus cajan (L.) Millsp. genotypes under nickel stress

Nickel (Ni) is an essential micronutrient but considered toxic for plant growth when present in excess in the soil. Polyamines (PAs) and arbuscular mycorrhiza (AM) play key roles in alleviating metal toxicity in plants. Present study compared the roles of AM and PAs in improving rhizobial symbiosis, ureide, and trehalose (Tre) metabolism under Ni stress in Cajanus cajan (pigeon pea) genotypes (Pusa 2001, AL 201). The results documented significant negative impacts of Ni on plant biomass, especially roots, more in AL 201 than Pusa 2001. Symbiotic efficiency with Rhizobium and AM declined under Ni stress, resulting in reduced AM colonization, N₂ fixation, and ureide biosynthesis. This decline was proportionate to increased Ni uptake in roots and nodules. Put-reduced Ni uptake improved plant growth and functional efficiency of nodules and ureides synthesis, with higher positive effects than other PAs. However, AM inoculations were most effective in enhancing nodulation, nitrogen fixing potential, and Tre synthesis under Ni toxicity. Combined applications of AM with respective PAs, especially +Put+AM, were highly beneficial in alleviating Ni-induced nodule senescence by arresting leghemoglobin degradation and improving functional efficiency of nodules by boosting Tre metabolism, especially in Pusa 2001. The study suggested use of Put along with AM as a promising approach in imparting Ni tolerance to pigeon pea plants.

SA and AM symbiosis modulate antioxidant defense mechanisms and asada pathway in chickpea genotypes under salt stress

Salt stress disturbs redox homeostasis by perturbing equilibrium between generation and removal of reactive oxygen species (ROS), which alters the normal metabolism of plants through membrane damage, lipid peroxidation and denaturation of proteins. Salicylic acid (SA) seed priming and arbuscular mycorrhizal (AM) fungi impart salt tolerance in legumes by maintaining redox balance. The present investigation focused on the relative and combined applications of SA and Rhizoglomus intraradices in scavenging ROS in Cicer arietinum L. (chickpea) genotypes (salt tolerant-PBG 5, relatively sensitive-BG 256) subjected to salt stress. Despite the enhanced antioxidant mechanisms under salt stress, ROS (superoxide, O₂- and hydrogen peroxide, H₂O₂) accumulation increased significantly and induced lipid peroxidation and lipoxygenase (LOX) activities, which disrupted membrane stability, more in BG 256 than PBG 5. Salt stress also caused redox imbalance by lowering ascorbate/dehydroascorbate (ASA/DHA) and reduced glutathione/oxidized glutathione (GSH/GSSG) ratios, indicating that redox-homeostasis was crucial for salt-tolerance. Exogenous SA was more promising in reducing ROS-generation and lipid-peroxidation, which provided higher membrane stability as compared to AM inoculation. Although, the enzymatic antioxidants were more active in SA treated plants, yet, AM inoculation outperformed in increasing reformative enzyme activities of Foyer-Halliwell-Asada cycle, which resulted in higher plant biomass in a genotype-dependent manner. SA increased AM root colonization and provided functional complementarity to R. intraradices and thereby strengthening antioxidant defense mechanisms through their cumulative contribution. The study suggested the use of +SA+AM as an eco-friendly tool in imparting salt tolerance in chickpea genotypes subjected to long-term salinity.

Mitigation of Salinity Stress in Plants by Arbuscular Mycorrhizal Symbiosis: Current Understanding and New Challenges

Modern agriculture is facing twin challenge of ensuring global food security and executing it in a sustainable manner. However, the rapidly expanding salinity stress in cultivable areas poses a major peril to crop yield. Among various biotechnological techniques being used to reduce the negative effects of salinity, the use of arbuscular mycorrhizal fungi (AMF) is considered to be an efficient approach for bio-amelioration of salinity stress. AMF deploy an array of biochemical and physiological mechanisms that act in a concerted manner to provide more salinity tolerance to the host plant. Some of the well-known mechanisms include improved nutrient uptake and maintenance of ionic homeostasis, superior water use efficiency and osmoprotection, enhanced photosynthetic efficiency, preservation of cell ultrastructure, and reinforced antioxidant metabolism. Molecular studies in past one decade have further elucidated the processes involved in amelioration of salt stress in mycorrhizal plants. The participating AMF induce expression of genes involved in Na+ extrusion to the soil solution, K+ acquisition (by phloem loading and unloading) and release into the xylem, therefore maintaining favorable Na+:K+ ratio. Colonization by AMF differentially affects expression of plasma membrane and tonoplast aquaporins (PIPs and TIPs), which consequently improves water status of the plant. Formation of AM (arbuscular mycorrhiza) surges the capacity of plant to mend photosystem-II (PSII) and boosts quantum efficiency of PSII under salt stress conditions by mounting the transcript levels of chloroplast genes encoding antenna proteins involved in transfer of excitation energy. Furthermore, AM-induced interplay of phytohormones, including strigolactones, abscisic acid, gibberellic acid, salicylic acid, and jasmonic acid have also been associated with the salt tolerance mechanism. This review comprehensively covers major research advances on physiological, biochemical, and molecular mechanisms implicated in AM-induced salt stress tolerance in plants. The review identifies the challenges involved in the application of AM in alleviation of salt stress in plants in order to improve crop productivity.

Mitigation of Salinity Stress in Plants by Arbuscular Mycorrhizal Symbiosis: Current Understanding and New Challenges

Modern agriculture is facing twin challenge of ensuring global food security and executing it in a sustainable manner. However, the rapidly expanding salinity stress in cultivable areas poses a major peril to crop yield. Among various biotechnological techniques being used to reduce the negative effects of salinity, the use of arbuscular mycorrhizal fungi (AMF) is considered to be an efficient approach for bio-amelioration of salinity stress. AMF deploy an array of biochemical and physiological mechanisms that act in a concerted manner to provide more salinity tolerance to the host plant. Some of the well-known mechanisms include improved nutrient uptake and maintenance of ionic homeostasis, superior water use efficiency and osmoprotection, enhanced photosynthetic efficiency, preservation of cell ultrastructure, and reinforced antioxidant metabolism. Molecular studies in past one decade have further elucidated the processes involved in amelioration of salt stress in mycorrhizal plants. The participating AMF induce expression of genes involved in Na⁺ extrusion to the soil solution, K⁺ acquisition (by phloem loading and unloading) and release into the xylem, therefore maintaining favorable Na⁺:K⁺ ratio. Colonization by AMF differentially affects expression of plasma membrane and tonoplast aquaporins (PIPs and TIPs), which consequently improves water status of the plant. Formation of AM (arbuscular mycorrhiza) surges the capacity of plant to mend photosystem-II (PSII) and boosts quantum efficiency of PSII under salt stress conditions by mounting the transcript levels of chloroplast genes encoding antenna proteins involved in transfer of excitation energy. Furthermore, AM-induced interplay of phytohormones, including strigolactones, abscisic acid, gibberellic acid, salicylic acid, and jasmonic acid have also been associated with the salt tolerance mechanism. This review comprehensively covers major research advances on physiological, biochemical, and molecular mechanisms implicated in AM-induced salt stress tolerance in plants. The review identifies the challenges involved in the application of AM in alleviation of salt stress in plants in order to improve crop productivity.

Unraveling the Initial Plant Hormone Signaling, Metabolic Mechanisms and Plant Defense Triggering the Endomycorrhizal Symbiosis Behavior

Arbuscular mycorrhizal (AM) fungi establish probably one of the oldest mutualistic relationships with the roots of most plants on earth. The wide distribution of these fungi in almost all soil ecotypes and the broad range of host plant species demonstrate their strong plasticity to cope with various environmental conditions. AM fungi elaborate fine-tuned molecular interactions with plants that determine their spread within root cortical tissues. Interactions with endomycorrhizal fungi can bring various benefits to plants, such as improved nutritional status, higher photosynthesis, protection against biotic and abiotic stresses based on regulation of many physiological processes which participate in promoting plant performances. In turn, host plants provide a specific habitat as physical support and a favorable metabolic frame, allowing uptake and assimilation of compounds required for the life cycle completion of these obligate biotrophic fungi. The search for formal and direct evidences of fungal energetic needs raised strong motivated projects since decades, but the impossibility to produce AM fungi under axenic conditions remains a deep enigma and still feeds numerous debates. Here, we review and discuss the initial favorable and non-favorable metabolic plant context that may fate the mycorrhizal behavior, with a focus on hormone interplays and their links with mitochondrial respiration, carbon partitioning and plant defense system, structured according to the action of phosphorus as a main limiting factor for mycorrhizal symbiosis. Then, we provide with models and discuss their significances to propose metabolic targets that could allow to develop innovations for the production and application of AM fungal inocula.