Salt stress reduces root meristem size by nitric oxide-mediated modulation of auxin accumulation and signaling in Arabidopsis

Genome-wide analysis of the homeodomain-leucine zipper family in Lotus japonicus and the overexpression of LjHDZ7 in Arabidopsis for salt tolerance

The homeodomain-leucine zipper (HD-Zip) family participates in plant growth, development, and stress responses. Here, 40 HD-Zip transcription factors of Lotus japonicus were identified and gave an overview of the phylogeny and gene structures. The expression pattern of these candidate genes was determined in different organs and their response to abiotic stresses, including cold, heat, polyethylene glycol and salinity. The expression of the LjHDZ7 was strongly induced by abiotic stress, especially salt stress. Subsequently, LjHDZ7 gene was overexpressed in Arabidopsis. The transgenic plants grew obviously better than Col-0 plants under salt stress. Furthermore, LjHDZ7 transgenic lines accumulated higher proline contents and showed lower electrolyte leakage and MDA contents than Col-0 plants under salt stress. Antioxidant activities of the LjHDZ7 overexpression lines leaf were significantly higher than those of the Col-0 plants under salt stress. The concentration of Na⁺ ion in LjHDZ7 overexpression lines was significantly lower than that of Col-0 in leaf and root parts. The concentration of K⁺ ion in LjHDZ7 overexpression lines was significantly higher than that of Col-0 in the leaf parts. Therefore, these results showed that overexpression of LjHDZ7 increased resistance to salt stress in transgenic Arabidopsis plants, and certain genes of this family can be used as valuable tools for improving abiotic stresses.

Chemical priming enhances plant tolerance to salt stress

Salt stress severely limits the productivity of crop plants worldwide and its detrimental effects are aggravated by climate change. Due to a significant world population growth, agriculture has expanded to marginal and salinized regions, which usually render low crop yield. In this context, finding methods and strategies to improve plant tolerance against salt stress is of utmost importance to fulfill food security challenges under the scenario of the ever-increasing human population. Plant priming, at different stages of plant development, such as seed or seedling, has gained significant attention for its marked implication in crop salt-stress management. It is a promising field relying on the applications of specific chemical agents which could effectively improve plant salt-stress tolerance. Currently, a variety of chemicals, both inorganic and organic, which can efficiently promote plant growth and crop yield are available in the market. This review summarizes our current knowledge of the promising roles of diverse molecules/compounds, such as hydrogen sulfide (H2S), molecular hydrogen, nitric oxide (NO), hydrogen peroxide (H2O2), melatonin, chitosan, silicon, ascorbic acid (AsA), tocopherols, and trehalose (Tre) as potential primers that enhance the salinity tolerance of crop plants.

Effects of Exogenous Melatonin on Root Physiology, Transcriptome and Metabolome of Cotton Seedlings under Salt Stress

Root systems are the key organs through which plants absorb water and nutrients and perceive the soil environment and thus are easily damaged by salt stress. Melatonin can alleviate stress-induced damage to roots. The present study investigated the effects of exogenous melatonin on the root physiology, transcriptome and metabolome of cotton seedlings under salt stress. Salt stress was observed to damage the cell structure and disorder the physiological system of cotton seedling roots. After subjecting melatonin-soaked seeds to salt stress, the activities of SOD, CAT and POD in cotton seedling roots increased by 10–25%, 50–60% and 50–60%, respectively. The accumulation of H₂O₂ and MDA were significantly decreased by 30–60% and 30–50%, respectively. The contents of soluble sugar, soluble protein and K⁺ increased by 15–30%, 15–30% and 20–50%, respectively, while the Na⁺ content was significantly reduced. Melatonin also increased auxin (by 20–40%), brassinosteroids (by 5–40%) and gibberellin (by 5–35%) and promoted melatonin content and root activity. Exogenous melatonin maintained the integrity of root cells and increased the number of organelles. Transcriptomic and metabolomic results showed that exogenous melatonin could mitigate the salt-stress-induced inhibition of plant root development by regulating the reactive oxygen species scavenging system; ABC transporter synthesis; plant hormone signal transduction, endogenous melatonin gene expression; and the expression of the transcription factors MYB, TGA and WRKY33. These results provide a new direction and empirical basis for improving crop salt tolerance with melatonin.

Iron oxide nanoparticles impart cross tolerance to arsenate stress in rice roots through involvement of nitric oxide

The growth and development patterns of crop plants are being seriously threatened by arsenic (As) contamination in the soil, and it also acts as a major hurdle in crop productivity. This study focuses on arsenate As(V) mediated toxicity in rice plants. Further, among the different type of NPs, iron oxide nanoparticles (FeO NPs) display a dose-dependent effect but their potential role in mitigating As(V) stress is still elusive. FeO NPs (500 μM) play a role in imparting cross-tolerance against As(V) induced toxicity in rice. Growth attributes, photosynthetic performance, nutrient contents and biochemical parameters were significantly altered by As(V). But FeO NPs rescued the negative consequences of As(V) by restricting its entry with the possible involvement of NO in rice roots. Moreover, results related with gene expression of NO(OsNoA1 and OsNIA1) and proline metabolism were greatly inhibited by As(V) toxicity. But, FeO NPs reversed the toxic effect of As(V) by improving proline metabolism and stimulating NO mediated up-regulation of antioxidant enzymes particularly glutathione-S-transferase which may be possible reasons for the reduction of As(V) toxicity in rice roots. Overall, it can be stated that FeO NPs may act as an As(V) barrier to restrict the As(V) uptake by roots and have the ability to confer cross tolerance by modulating various morphological, biochemical and molecular characteristics with possible intrinsic involvement of NO.

PIN and PILS family genes analyses in Chrysanthemum seticuspe reveal their potential functions in flower bud development and drought stress

Auxin affects almost all plant growth and developmental processes. The PIN-FORMED (PIN) and PIN-LIKES (PILS) family genes determine the direction and distribution gradient of auxin flow by polar localization on the cell membrane. However, there are no systematic studies on PIN and PILS family genes in chrysanthemum. Here, 18 PIN and 13 PILS genes were identified in Chrysanthemum seticuspe. The evolutionary relationships, physicochemical properties, conserved motifs, cis-acting elements, chromosome localization, collinearity, and expression characteristics of these genes were analyzed. CsPIN10a, CsPIN10b, and CsPIN10c are unique PIN genes in C. seticuspe. Expression pattern analysis showed that these genes had different tissue specificities, and the expression levels of CsPIN8, CsPINS1, CsPILS6, and CsPILS10 were linearly related to the developmental period of the flower buds. In situ hybridization assay showed that CsPIN1a, CsPIN1b, and CsPILS8 were expressed in floret primordia and petal tips, and CsPIN1a was specifically expressed in the middle of the bract primordia, which might regulate lateral expansion of the bracts. CsPIN and CsPILS family genes are also involved in drought stress responses. This study provides theoretical support for the cultivation of new varieties with attractive flower forms and high drought tolerance.

Halophyte Nitraria billardieri CIPK25 mitigates salinity-induced cell damage by alleviating H2O2 accumulation

The plant-specific module of calcineurin B-like proteins (CBLs) and CBL-interacting protein kinases (CIPKs) play a crucial role in plant adaptation to different biotic and abiotic stresses in various plant species. Despite the importance of the CBL-CIPK module in regulating plant salt tolerance, few halophyte CIPK orthologs have been studied. We identified NbCIPK25 in the halophyte Nitraria billardieri as a salt-responsive gene that may improve salt tolerance in glycophytes. Sequence analyses indicated that NbCIPK25 is a typical CIPK family member with a conserved NAF motif, which contains the amino acids: asparagine, alanine, and phenylalanine. NbCIPK25 overexpression in salt-stressed transgenic Arabidopsis seedlings resulted in enhanced tolerance to salinity, a higher survival rate, longer newly grown roots, more root meristem cells, and less damaged root cells in comparison to wild-type (WT) plants. H2O2 accumulation and malondialdehyde (MDA) content were both deceased in NbCIPK25-transgenic plants under salt treatment. Furthermore, their proline content, an important factor for scavenging reactive oxygen species, accumulated at a significantly higher level. In concordance, the transcription of genes related to proline accumulation was positively regulated in transgenic plants under salt condition. Finally, we observed a stronger auxin response in salt-treated transgenic roots. These results provide evidence for NbCIPK25 improving salt tolerance by mediating scavenging of reactive oxygen species, thereby protecting cells from oxidation and maintaining plant development under salt stress. These findings suggest the potential application of salt-responsive NbCIPK25 for cultivating glycophytes with a higher salt tolerance through genetic engineering.

Comprehensive Analysis of Differentially Expressed Genes and Epigenetic Modification-Related Expression Variation Induced by Saline Stress at Seedling Stage in Fiber and Oil Flax, Linum usitatissimum L

The ability of different germplasm to adapt to a saline-alkali environment is critical to learning about the tolerance mechanism of saline-alkali stress in plants. Flax is an important oil and fiber crop in many countries. However, its molecular tolerance mechanism under saline stress is still not clear. In this study, we studied morphological, physiological characteristics, and gene expression variation in the root and leaf in oil and fiber flax types under saline stress, respectively. Abundant differentially expressed genes (DEGs) induced by saline stress, tissue/organ specificity, and different genotypes involved in plant hormones synthesis and metabolism and transcription factors and epigenetic modifications were detected. The present report provides useful information about the mechanism of flax response to saline stress and could lead to the future elucidation of the specific functions of these genes and help to breed suitable flax varieties for saline/alkaline soil conditions.

ELO2 Participates in the Regulation of Osmotic Stress Response by Modulating Nitric Oxide Accumulation in Arabidopsis

The ELO family is involved in synthesizing very-long-chain fatty acids (VLCFAs) and VLCFAs play a crucial role in plant development, protein transport, and disease resistance, but the physiological function of the plant ELO family is largely unknown. Further, while nitric oxide synthase (NOS)-like activity acts in various plant environmental responses by modulating nitric oxide (NO) accumulation, how the NOS-like activity is regulated in such different stress responses remains misty. Here, we report that the yeast mutant Δelo3 is defective in H₂O₂-triggered cell apoptosis with decreased NOS-like activity and NO accumulation, while its Arabidopsis homologous gene ELO2 (ELO HOMOLOG 2) could complement such defects in Δelo3. The expression of this gene is enhanced and required in plant osmotic stress response because the T-DNA insertion mutant elo2 is more sensitive to the stress than wild-type plants, and ELO2 expression could rescue the sensitivity phenotype of elo2. In addition, osmotic stress-promoted NOS-like activity and NO accumulation are significantly repressed in elo2, while exogenous application of NO donors can rescue this sensitivity of elo2 in terms of germination rate, fresh weight, chlorophyll content, and ion leakage. Furthermore, stress-responsive gene expression, proline accumulation, and catalase activity are also repressed in elo2 compared with the wild type under osmotic stress. In conclusion, our study identifies ELO2 as a pivotal factor involved in plant osmotic stress response and reveals its role in regulating NOS-like activity and NO accumulation.

Inactivation of the entire Arabidopsis group II GH3s confers tolerance to salinity and water deficit

Indole-3-acetic acid (IAA) controls a plethora of developmental processes. Thus, regulation of its concentration is of great relevance for plant performance. Cellular IAA concentration depends on its transport, biosynthesis and the various pathways for IAA inactivation, including oxidation and conjugation. Group II members of the GRETCHEN HAGEN 3 (GH3) gene family code for acyl acid amido synthetases catalysing the conjugation of IAA to amino acids. However, the high degree of functional redundancy among them has hampered thorough analysis of their roles in plant development. In this work, we generated an Arabidopsis gh3.1,2,3,4,5,6,9,17 (gh3oct) mutant to knock out the group II GH3 pathway. The gh3oct plants had an elaborated root architecture, showed an increased tolerance to different osmotic stresses, including an IAA-dependent tolerance to salinity, and were more tolerant to water deficit. Indole-3-acetic acid metabolite quantification in gh3oct plants suggested the existence of additional GH3-like enzymes in IAA metabolism. Moreover, our data suggested that 2-oxindole-3-acetic acid production depends, at least in part, on the GH3 pathway. Targeted stress-hormone analysis further suggested involvement of abscisic acid in the differential response to salinity of gh3oct plants. Taken together, our data provide new insights into the roles of group II GH3s in IAA metabolism and hormone-regulated plant development.

Coordination of plant growth and abiotic stress responses by tryptophan synthase β subunit 1 through modulation of tryptophan and ABA homeostasis in Arabidopsis

To adapt to changing environments, plants have evolved elaborate regulatory mechanisms balancing their growth with stress responses. It is currently unclear whether and how the tryptophan (Trp), the growth-related hormone auxin, and the stress hormone abscisic acid (ABA) are coordinated in this trade-off. Here, we show that tryptophan synthase β subunit 1 (TSB1) is involved in the coordination of Trp and ABA, thereby affecting plant growth and abiotic stress responses. Plants experiencing high salinity or drought display reduced TSB1 expression, resulting in decreased Trp and auxin accumulation and thus reduced growth. In comparison with the wild type, amiR-TSB1 lines and TSB1 mutants exhibited repressed growth under non-stress conditions but had enhanced ABA accumulation and stress tolerance when subjected to salt or drought stress. Furthermore, we found that TSB1 interacts with and inhibits β-glucosidase 1 (BG1), which hydrolyses glucose-conjugated ABA into active ABA. Mutation of BG1 in the amiR-TSB1 lines compromised their increased ABA accumulation and enhanced stress tolerance. Moreover, stress-induced H₂O₂ disrupted the interaction between TSB1 and BG1 by sulfenylating cysteine-308 of TSB1, relieving the TSB1-mediated inhibition of BG1 activity. Taken together, we revealed that TSB1 serves as a key coordinator of plant growth and stress responses by balancing Trp and ABA homeostasis.

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.

CkREV Enhances the Drought Resistance of Caragana korshinskii through Regulating the Expression of Auxin Synthetase Gene CkYUC5

As a common abiotic stress, drought severely impairs the growth, development, and even survival of plants. Here we report a transcription factor, Caragana korshinskii REVOLUTA(CkREV), which can bidirectionally regulate the expression of the critical enzyme gene CkYUC5 in auxin synthesis according to external environment changes, so as to control the biosynthesis of auxin and further enhance the drought resistance of plants. Quantitative analysis reveals that the expression level of both CkYUC5 and AtYUC5 is down-regulated after C. korshinskii and Arabidopsis thaliana are exposed to drought. Functional verification of CkREV reveals that CkREV up-regulates the expression of AtYUC5 in transgenic A. thaliana under common conditions, while down-regulating it under drought conditions. Meanwhile, the expression of CkYUC5 is also down-regulated in C. korshinskii leaves instantaneously overexpressing CkREV. We apply a dual-luciferase reporter system to discover that CkREV can bind to the promoter of CkYUC5 to regulate its expression, which is further proved by EMSA and Y1H esxperiments. Functional verification of CkREV in C. korshinskii and transgenic A. thaliana shows that CkREV can regulate the expression of CkYUC5 and AtYUC5 in a contrary way, maintaining the equilibrium of plants between growth and drought resisting. CkREV can positively regulate the expression of CkYUC5 to promote auxin synthesis in favor of growth under normal development. However, CkREV can also respond to external signals and negatively regulate the expression of CkYUC5, which inhibits auxin synthesis in order to reduce growth rate, lower water demands, and eventually improve the drought resistance of plants.

Enhanced Salt Tolerance of Torreya grandis Genders Is Related to Nitric Oxide Level and Antioxidant Capacity

Nitric oxide (NO), a bioactive molecule, is often involved in the regulation of physiological and biochemical processes in stressed plants. However, the effects of NO donors on dioecious plants remain unclear. Using a pot experiment, female and male Torreya grandis were used to study the role of sex and NO in salt stress tolerance. In the present study, female and male T. grandis seedlings pretreated with an NO donor (sodium nitroprusside, SNP) were exposed to salt stress, and then leaf relative water content (RWC), photosynthetic pigments, chlorophyll fluorescence parameters, NO and glutathione levels, oxidative damage, and antioxidant enzyme activities were investigated. Female T. grandis plants had better tolerance to salinity, as they were characterized by significantly higher RWC, pigment content, and photochemical activities of photosystem II (PSII) and fewer negative effects associated with higher nitrate reductase (NR) activity and NO content. Pretreatment with an NO donor further increased the endogenous NO content and NR activity of both female and male T. grandis plants compared with salt treatment. Moreover, pretreatment with an NO donor alleviated salt-induced oxidative damage of T. grandis, especially in male plants, as indicated by reduced lipid peroxidation, through an enhanced antioxidant system, including proline and glutathione accumulation, and increased antioxidant enzyme activities. However, the ameliorating effect of the NO donor was not effective in the presence of the NO scavenger (Nω-nitro-L-arginine methyl ester, L-name). In conclusion, enhanced salt tolerance in T. grandis plants is related to nitric oxide levels and the supply of NO donors is an interesting strategy for alleviating the negative effect of salt on T. grandis. Our data provide new evidence to contribute to the current understanding of NO-induced salt stress tolerance.

Root growth direction in simulated microgravity is modulated by a light avoidance mechanism mediated by flavonols

In a microgravity environment, without any gravitropic signal, plants are not able to define and establish a longitudinal growth axis. Consequently, absorption of water and nutrients by the root and exposure of leaves to sunlight for efficient photosynthesis is hindered. In these conditions, other external cues can be explored to guide the direction of organ growth. Providing a unilateral light source can guide the shoot growth, but prolonged root exposure to light causes a stress response, affecting growth and development, and also affecting the response to other environmental factors. Here, we have investigated how the protection of the root from light exposure, while the shoot is illuminated, influences the direction of root growth in microgravity. We report that the light avoidance mechanism existing in roots guides their growth towards diminishing light and helps establish the proper longitudinal seedling axis in simulated microgravity conditions. This process is regulated by flavonols, as shown in the flavonoid-accumulating mutant transparent testa 3, which shows an increased correction of the root growth direction in microgravity, when the seedling is grown with the root protected from light. This finding may improve the efficiency of water and nutrient sourcing and photosynthesis under microgravity conditions, as they exist in space, contributing to better plant fitness and biomass production in space farming enterprises, necessary for space exploration by humans.

Auxin response factors in plant adaptation to drought and salinity stress

Salinity and drought stresses affect plant growth worldwide and limit crop production. Auxin is crucial in regulating plants' salinity and drought stress adaptative response. As a chemical messenger, auxin influences gene expression through a family of functionally distinct transcription factors, the DNA-binding AUXIN RESPONSE FACTORS (ARFs). Various studies have revealed the important roles of ARFs in regulating drought and salinity stress responses in plants. Different ARFs regulate soluble sugar content, promote root development, and maintain chlorophyll content under drought and saline stress conditions to help plants adapt to these stresses. The functional characterization of ARFs pertaining to the regulation of drought and salinity stress responses is still in its infancy. Interestingly, the small RNA-mediated post-transcriptional regulation of ARF expression has been shown to influence plant responses to both stresses. The current knowledge on the diverse roles of ARFs in conferring specificity to auxin-mediated drought and salinity stress responses has not been reviewed to date. In this review, we summarize the recent research concerning the role of ARFs in response to drought and salinity stresses: gene expression patterns, functional characterization, and post-transcriptional regulation under drought and salinity stresses. We have also reviewed the modulation of ARF expression by other molecular regulators in the context of drought and salt stress signaling.

Nitric Oxide Improves Salt Tolerance of Cyclocarya paliurus by Regulating Endogenous Glutathione Level and Antioxidant Capacity

Cyclocarya paliurus is commonly used to treat diabetes in China. However, the natural habitats of C. paliurus are typically affected by salt stress. Previous studies showed that nitric oxide (NO) level was related to salt tolerance of C. paliurus, and its synthesis was induced by exogenous hydrogen sulfide. However, the effects of different NO donors in alleviating the negative effect of salt stress are still unclear. In the present study, C. paliurus seedlings pretreated with three NO donors (S-nitroso-N-acetylpenicillamine, SNAP and S-nitrosoglutathione, GSNO and sodium nitroprusside, SNP) were exposed to salt stress, and then, the total biomass, chlorophyll fluorescence parameters, NO and glutathione levels, oxidative damage, and antioxidant enzyme activities were investigated. The results showed that pretreatment of NO donors maintained chlorophyll fluorescence and attenuated the loss of plant biomass under salt stress, and the best performance was observed in C. paliurus under SNP treatment. We also found that pretreatment of NO donors further increased the endogenous NO content and nitrate reductase (NR) activity compared with salt treatment. Moreover, pretreatment with NO donors, especially SNP, alleviated salt-induced oxidative damage, as indicated by lowered lipid peroxidation, through an enhanced antioxidant system including glutathione accumulation and increased antioxidant enzyme activities. The supply of NO donors is an interesting strategy for alleviating the negative effect of salt on C. paliurus. Our data provide new evidence contributing to the current understanding of NO-induced salt stress tolerance.

ORESARA 15, a PLATZ transcription factor, controls root meristem size through auxin and cytokinin signalling-related pathways

An optimal size of post-embryonic root apical meristem (RAM) is achieved by a balance between cell division and differentiation. Despite extensive research, molecular mechanisms underlying the coordination of cell division and differentiation are still fragmentary. Here, we report that ORESARA 15 (ORE15), an Arabidopsis PLANT A/T-RICH SEQUENCE-AND ZINC-BINDING PROTEIN (PLATZ) transcription factor preferentially expressed in the RAM, determines RAM size. Primary root length, RAM size, cell division rate, and stem cell niche activity were reduced in an ore15 loss-of-function mutant but enhanced in an activation-tagged line overexpressing ORE15, compared with wild type. ORE15 forms mutually positive and negative feedback loops with auxin and cytokinin signalling, respectively. Collectively, our findings imply that ORE15 controls RAM size by mediating the antagonistic interaction between auxin and cytokinin signalling-related pathways.

A waste to energy technology for Enrichment of biomethane generation: A review on operating parameters, types of biodigesters, solar assisted heating systems, socio economic benefits and challenges

Anaerobic Digestion (AD) is one of the promising wastestoenergy (WtE) technologies that convert organic wastes to useful gaseous fuel (biogas). In this process methane is produced in the presence of methanogens (bacteria). The survival and activities of methanogens are based on several parameters such as pH, temperature, organic loading rate, types of biodigester. Moreover, these parameters influence the production of biogas in terms of yield and composition. Maintaining an appropriate temperaturefor AD is highly critical and energy intensive. This study reviews the various hybrid technologies assistedbio gas production schemes particularly from renewable energy sources. Also discuss the direct and indirect solar assisted bio-digester impacts and recommendation to improve its performance. In addition, the performance analysis Solar Photovoltaic (PV) and thermal collector assisted bio gas plants; besides their impact on the performance of anaerobic digesters. Since opportunities of solar energy are attractive, the effective utilization of the same is selected for the discussion. Besides, the various constraints that affect the yield and composition of biogas are also evaluated along with the current biogas technologies and the biodigesters. The environmental benefits, challenges and socio-economic factors are also discussed for the successful implementation of various technologies.

Low Salt Treatment Results in Plant Growth Enhancement in Tomato Seedlings

Climate change together with excessive fertilization and poor water quality can affect soil quality and salinization. In plants, high salinity causes osmotic stress, ionic toxicity, and oxidative stress. Consequently, salt stress limits plant development, growth, productivity, and yield. Tomatoes are a very common agricultural product, and some cultivars can partially tolerate salinity. However, most studies are focused on salt excess, which does not necessarily extrapolate on how plants develop in soils with low concentrations of salts. Thus, this study characterizes plant growth and the development of different salt concentrations from 25 to 200 mM in Solanum lycopersicum cv. Moneymaker. Tomato seedlings grown in Murashige and Skoog medium supplied with different NaCl concentrations (0, 25, 50, 75, 100, 125, 150, 175, and 200 mM) showed that low salt concentrations (25 and 50 mM) have a positive impact on lateral root development. This was further observed in physiological parameters such as shoot length, primary root length, and proliferation of lateral roots versus controls. Interestingly, no significant changes in Na⁺ concentration were observed in 25 mM NaCl in roots or shoots versus controls. Overall, our results suggest that non-toxic salt concentrations can have a positive impact on plant development.

Gene Expression Analysis of Potato (Solanum tuberosum L.) Lipoxygenase Cascade and Oxylipin Signature under Abiotic Stress

The metabolism of polyunsaturated fatty acids through the lipoxygenase-catalyzed step and subsequent reactions is referred to as the lipoxygenase (LOX) pathway. The components of this system, such as jasmonates, are involved in growth, development and defense reactions of plants. In this report, we focus on dynamics of expression of different LOX pathway genes and activities of target enzymes with three abiotic stress factors: darkness, salinity and herbicide toxicity. To obtain a more complete picture, the expression profiles of marker genes for salicylic acid, abscisic acid, ethylene, auxin and gibberellin-dependent signaling systems under the same stresses were also analyzed. The gene expression in Solanum tuberosum plants was analyzed using qRT-PCR, and we found that the LOX-cascade-related genes responded to darkness, salinity and herbicide toxicity in different ways. We detected activation of a number of 9-LOX pathway genes; however, in contrast to studies associated with biotic stress (infection), the 9-divinyl ether synthase branch of the LOX cascade was inhibited under all three stresses. GC-MS analysis of the oxylipin profiles also showed the main activity of the 9-LOX-cascade-related enzymes after treatment with herbicide and darkness.

Heat Stress Reduces Root Meristem Size via Induction of Plasmodesmal Callose Accumulation Inhibiting Phloem Unloading in Arabidopsis

The intercellular transport of sugars, nutrients, and small molecules is essential for plant growth, development, and adaptation to environmental changes. Various stresses are known to affect the cell-to-cell molecular trafficking modulated by plasmodesmal permeability. However, the mechanisms of plasmodesmata modification and molecules involved in the phloem unloading process under stress are still not well understood. Here, we show that heat stress reduces the root meristem size and inhibits phloem unloading by inducing callose accumulation at plasmodesmata that connect the sieve element and phloem pole pericycle. Furthermore, we identify the loss-of-function of CALLOSE SYNTHASE 8 (CalS8), which is expressed specifically in the phloem pole pericycle, decreasing the plasmodesmal callose deposition at the interface between the sieve element and phloem pole pericycle and alleviating the suppression at root meristem size by heat stress. Our studies indicate the involvement of callose in the interaction between root meristem growth and heat stress and show that CalS8 negatively regulates the thermotolerance of Arabidopsis roots.

Salt-Specific Gene Expression Reveals Elevated Auxin Levels in Arabidopsis thaliana Plants Grown Under Saline Conditions

Soil salinization is increasing globally, driving a reduction in crop yields that threatens food security. Salinity stress reduces plant growth by exerting two stresses on plants: rapid shoot ion-independent effects which are largely osmotic and delayed ionic effects that are specific to salinity stress. In this study we set out to delineate the osmotic from the ionic effects of salinity stress. Arabidopsis thaliana plants were germinated and grown for two weeks in media supplemented with 50, 75, 100, or 125 mM NaCl (that imposes both an ionic and osmotic stress) or iso-osmolar concentrations (100, 150, 200, or 250 mM) of sorbitol, that imposes only an osmotic stress. A subsequent transcriptional analysis was performed to identify sets of genes that are differentially expressed in plants grown in (1) NaCl or (2) sorbitol compared to controls. A comparison of the gene sets identified genes that are differentially expressed under both challenge conditions (osmotic genes) and genes that are only differentially expressed in plants grown on NaCl (ionic genes, hereafter referred to as salt-specific genes). A pathway analysis of the osmotic and salt-specific gene lists revealed that distinct biological processes are modulated during growth under the two conditions. The list of salt-specific genes was enriched in the gene ontology (GO) term "response to auxin." Quantification of the predominant auxin, indole-3-acetic acid (IAA) and IAA biosynthetic intermediates revealed that IAA levels are elevated in a salt-specific manner through increased IAA biosynthesis. Furthermore, the expression of NITRILASE 2 (NIT2), which hydrolyses indole-3-acetonitile (IAN) into IAA, increased in a salt-specific manner. Overexpression of NIT2 resulted in increased IAA levels, improved Na:K ratios and enhanced survival and growth of Arabidopsis under saline conditions. Overall, our data suggest that auxin is involved in maintaining growth during the ionic stress imposed by saline conditions.

A novel mitochondrial protein is required for cell wall integrity, auxin accumulation and root elongation in Arabidopsis under salt stress

Maintenance of root elongation is beneficial for the growth and survival of plants under salt stress, but currently the cellular components involved in the regulation of root growth under high salinity are not fully understood. In this study, we identified an Arabidopsis mutant, rres1, which exhibited reduced root elongation under treatment of a variety of salts, including NaCl, NaNO3, KCl, and KNO3. RRES1 encodes a novel mitochondrial protein and its molecular function is still unknown. Under salt stress, the root meristem length was shorter in the rres1 mutant compared to the wild type, which was correlated with a reduced auxin accumulation in the mutant. Reactive oxygen species (ROS), as important signals that regulate root elongation, were accumulated to higher levels in the rres1 mutant than the wild type after salt treatment. Measurement of monosaccharides in the cell wall showed that arabinose and xylose contents were decreased in the rres1 mutant under salt stress, and application of boric acid, which is required for the crosslinking of pectic polysaccharide rhamnogalacturonan-II (RG-II), largely rescued the root growth arrest of the rres1 mutant, suggesting that RRES1 participates in the maintenance of cell wall integrity under salt stress. GUS staining assay indicated that the RRES1 gene was expressed in leaves and weakly in root tip under normal conditions, but its expression was dramatically increased in leaves and roots after salt treatment. Together, our study reveals a novel mitochondrial protein that regulates root elongation under salt stress via the modulation of cell wall integrity, auxin accumulation, and ROS homeostasis.

SbbHLH85, a bHLH member, modulates resilience to salt stress by regulating root hair growth in sorghum

bHLH family proteins play an important role in plant stress response. However, the molecular mechanism regulating the salt response of bHLH is largely unknown. This study aimed to investigate the function and regulating mechanism of the sweet sorghum SbbHLH85 during salt stress. The results showed that SbbHLH85 was different from its homologs in other species. Also, it was a new atypical bHLH transcription factor and a key gene for root development in sweet sorghum. The overexpression of SbbHLH85 resulted in significantly increased number and length of root hairs via ABA and auxin signaling pathways, increasing the absorption of Na⁺. Thus, SbbHLH85 plays a negative regulatory role in the salt tolerance of sorghum. We identified a potential interaction partner of SbbHLH85, which was phosphate transporter chaperone PHF1 and modulated the distribution of phosphate, through screening a yeast two-hybrid library. Both yeast two-hybrid and BiFC experiments confirmed the interaction between SbbHLH85 and PHF1. The overexpression of SbbHLH85 led to a decrease in the expression of PHF1 as well as the content of Pi. Based on these results, we suggested that the increase in the Na⁺ content and the decrease in the Pi content resulted in the salt sensitivity of transgenic sorghum.

Mulberry based zinc nano-particles mitigate salinity induced toxic effects and improve the grain yield and zinc bio-fortification of wheat by improving antioxidant activities, photosynthetic performance, and accumulation of osmolytes and hormones

Salinity stress (SS) is a challenging abiotic stress that limits crop growth and productivity. Sustainable and cost effective methods are needed to improve crop production and decrease the deleterious impacts of SS. Zinc (Zn) nano-particles (NPs) have emerged as an important approach to regulating plant tolerance against SS. However, the mechanisms of SS tolerance mediated by Zn-NPs are not fully explained. Thus, this study was performed to explore the role of Zn-NPs (seed priming and foliar spray) in reducing the deleterious impacts of SS on wheat plants. The study comprised different SS levels: control, 6 and 12 dS m^-1, and different Zn-NPs treatments: control, seed priming (40 ppm), foliar spray (20 ppm), and their combination. Salinity stress markedly reduced plant growth, biomass, and grain yield. This was associated with enhanced electrolyte leakage (EL), malondialdehyde (MDA), hydrogen peroxide (H₂O₂), sodium (Na), chloride (Cl) accumulation, reduced photosynthetic pigments, relative water contents (RWC), photosynthetic rate (Pn), transpiration rate (Tr), stomata conductance (Gs), water use efficiency (WUE), free amino acids (FAA), total soluble protein (TSP), indole acetic acid (IAA), gibberellic acid (GA), and nutrients (Ca, Mg, K, N, and P). However, the application of Zn-NPs significantly improved the yield of the wheat crop, which was associated with reduced abscisic acid (ABA), MDA, H₂O₂ concentration, and EL, owing to improved antioxidant activities, and an increase in RWC, Pn, Tr, WUE, and the accumulation of osmoregulating compounds (proline, soluble sugars, TSP, and FAA) and hormones (GA and IAA). Furthermore, Zn-NPs contrasted the salinity-induced uptake of toxic ions (Na and Cl) and increased the uptake of Ca, K, Mg, N, and P. Additionally, Zn-NPs application substantially increased the wheat grain Zn bio-fortification. Our results support previous findings on the role of Zn-NPs in wheat growth, yield, and grain Zn bio-fortification, demonstrating that beneficial effects are obtained under normal as well as adverse conditions, thanks to improved physiological activity and the accumulation of useful compounds. This sets the premise for general use of Zn-NPs in wheat, to which aim more experimental evidence is intensively being sought. Further studies are needed at the genomic, transcriptomic, proteomic, and metabolomic level to better acknowledge the mechanisms of general physiological enhancement observed with Zn-NPs application.

The auxin signaling pathway to its PIN transporters: insights based on a meta-analysis of auxin-induced transcriptomes

Active polar transport of the plant hormone auxin carried out by its PIN transporters is a key link in the formation and maintenance of auxin distribution, which, in turn, determines plant morphogenesis. The plasticity of auxin distribution is largely realized through the molecular genetic regulation of the expression of its transporters belonging to the PIN-FORMED (PIN) protein family. Regulation of auxin-response genes occurs through the ARF-Aux/ IAA signaling pathway. However, it is not known which ARF-Aux/IAA proteins are involved in the regulation of PIN gene expression by auxin. In Arabidopsis thaliana, the PIN, ARF, and Aux/IAA families contain a larger number of members; their various combinations are possible in realization of the signaling pathway, and this is a challenge for understanding the mechanisms of this process. The use of high-throughput sequencing data on auxin-induced transcriptomes makes it possible to identify candidate genes involved in the regulation of PIN expression. To address this problem, we created an approach for the meta-analysis of auxin-induced transcriptomes, which helped us select genes that change their expression during the auxin response together with PIN1, PIN3, PIN4 and PIN7. Possible regulators of ARF-Aux/ IAA signaling pathway for each of the PINs under study were identif ied, and so were the aspects of their regulatory circuits both common for groups of PIN genes and specif ic for each PIN gene. Reconstruction of gene networks and their analysis predicted possible interactions between genes and served as an additional conf irmation of the pathways obtained in the meta-analysis. The approach developed can be used in the search for gene expression regulators in other genomewide data.

Exogenous melatonin mitigates salinity-induced damage in olive seedlings by modulating ion homeostasis, antioxidant defense, and phytohormone balance

Melatonin (MEL) is a ubiquitous molecule with pleiotropic roles in plant adaption to stress. In this study, we investigated the effects of foliar spray of 100 and 200 μM MEL on the biochemical and physiological traits linked with the growth performance of olive seedlings exposed to moderate (45 mM NaCl) and severe (90 mM NaCl) salinity. Both salt stress conditions caused a considerable reduction in leaf relative water content and the contents of photosynthetic pigments (carotenoids, chlorophylls a and b, and total chlorophylls), K⁺ and Ca⁺² , while the contents of Na⁺ and the activities of antioxidant enzymes increased. In addition, salt-stressed olive seedlings showed high accumulations of hydrogen peroxide (H₂ O₂ ), malondialdehyde (MDA), and electrolyte leakage (EL), indicating that olive seedlings suffered from salinity-induced oxidative damage. In contrast, MEL application revived the growth of olive seedlings, including shoot height, root length and biomass under salt stress conditions. MEL protected the photosynthetic pigments and decreased the Na⁺ /K⁺ ratio under both moderate and severe salt stresses. Furthermore, MEL induced the accumulations of proline, total soluble sugars, glycine betaine, abscisic acid, and indole acetic acid in salt-stressed olive seedlings, which showed a positive correlation with improved leaf water status and biomass. MEL application also increased the activities of catalase, superoxide dismutase, ascorbate peroxidase, and peroxidase in salt-stressed seedlings, resulting in lower levels of H₂ O₂ , MDA, and EL in these plants. Taken together, MEL mitigates salinity through its roles in various biochemical and physiological processes, thereby representing a promising agent for application in crop protection.

Osmotic stress represses root growth by modulating the transcriptional regulation of PIN-FORMED3

Osmotic stress influences root system architecture, and polar auxin transport (PAT) is well established to regulate root growth and development. However, how PAT responds to osmotic stress at the molecular level remains poorly understood. In this study, we explored whether and how the auxin efflux carrier PIN-FORMED3 (PIN3) participates in osmotic stress-induced root growth inhibition in Arabidopsis (Arabidopsis thaliana). We observed that osmotic stress induces a HD-ZIP II transcription factor-encoding gene HOMEODOMAIN ARABIDOPSIS THALIANA2 (HAT2) expression in roots. The hat2 loss-of-function mutant is less sensitive to osmotic stress in terms of root meristem growth. Consistent with this phenotype, whereas the auxin response is downregulated in wild-type roots under osmotic stress, the inhibition of auxin response by osmotic stress was alleviated in hat2 roots. Conversely, transgenic lines overexpressing HAT2 (Pro35S::HAT2) had shorter roots and reduced auxin accumulation compared with wild-type plants. PIN3 expression was significantly reduced in the Pro35S::HAT2 lines. We determined that osmotic stress-mediated repression of PIN3 was alleviated in the hat2 mutant because HAT2 normally binds to the promoter of PIN3 and inhibits its expression. Taken together, our data revealed that osmotic stress inhibits root growth via HAT2, which regulates auxin activity by directly repressing PIN3 transcription.

A rice transcription factor, OsMADS57, positively regulates high salinity tolerance in transgenic Arabidopsis thaliana and Oryza sativa plants

MADS-box transcription factors (TFs) play indispensable roles in various aspects of plant growth, development as well as in response to environmental stresses. Several MADS-box genes have been reported to be involved in the salt tolerance in different plant species. However, the role of the transcription factor OsMADS57 under salinity stress is still unknown. Here, the results of this study showed that OsMADS57 was mainly expressed in roots and leaves of rice plants (Oryza sativa). Gene expression pattern analysis revealed that OsMADS57 was induced by NaCl. Overexpression of OsMADS57 in both Arabidopsis thaliana (A. thaliana) and rice could improve their salt tolerance, which was demonstrated by higher germination rates, longer root length and better growth status of overexpression plants than wild type (WT) under salinity conditions. In contrast, RNA interference (RNAi) lines of rice showed more sensitivity towards salinity. Moreover, less reactive oxygen species (ROS) accumulated in OsMADS57 overexpressing lines when exposed to salt stress, as measured by 3, 3'-diaminobenzidine (DAB) or nitroblue tetrazolium (NBT) staining. Further experiments exhibited that overexpression of OsMADS57 in rice significantly increased the tolerance ability of plants to oxidative damage under salt stress, mainly by increasing the activities of antioxidative enzymes such as superoxide dismutase (SOD) and peroxidase (POD), reducing malonaldehyde (MDA) content and improving the expression of stress-related genes. Taken together, these results demonstrated that OsMADS57 plays a positive role in enhancing salt tolerance by activating the antioxidant system.

Uncovering How Auxin Optimizes Root Systems Architecture in Response to Environmental Stresses

Since colonizing land, plants have developed mechanisms to tolerate a broad range of abiotic stresses that include flooding, drought, high salinity, and nutrient limitation. Roots play a key role acclimating plants to these as their developmental plasticity enables them to grow toward more favorable conditions and away from limiting or harmful stresses. The phytohormone auxin plays a key role translating these environmental signals into developmental outputs. This is achieved by modulating auxin levels and/or signaling, often through cross talk with other hormone signals like abscisic acid (ABA) or ethylene. In our review, we discuss how auxin controls root responses to water, osmotic and nutrient-related stresses, and describe how the synthesis, degradation, transport, and response of this key signaling hormone helps optimize root architecture to maximize resource acquisition while limiting the impact of abiotic stresses.

PIN-mediated polar auxin transport regulations in plant tropic responses

Tropisms, growth responses to environmental stimuli such as light or gravity, are spectacular examples of adaptive plant development. The plant hormone auxin serves as a major coordinative signal. The PIN auxin exporters, through their dynamic polar subcellular localizations, redirect auxin fluxes in response to environmental stimuli and the resulting auxin gradients across organs underlie differential cell elongation and bending. In this review, we discuss recent advances concerning regulations of PIN polarity during tropisms, focusing on PIN phosphorylation and trafficking. We also cover how environmental cues regulate PIN actions during tropisms, as well as the crucial role of auxin feedback on PIN polarity during bending termination. Finally, the interactions between different tropisms are reviewed to understand plant adaptive growth in the natural environment.

Nitric oxide sensing revisited

Nitric oxide (NO) sensing is an ancient trait enabled by hemoproteins harboring a highly conserved Heme-Nitric oxide/OXygen (H-NOX) domain that operates throughout bacteria, fungi, and animal kingdoms including in humans, but that has long thought to be absent in plants. Recently, H-NOX-containing plant hemoproteins mediating crucial NO-dependent responses such as stomatal closure and pollen tube guidance have been reported. There are indications that the detection method that led to these discoveries will uncover many more heme-based NO sensors that operate as regulatory sites in complex proteins. Their characterizations will in turn offer a much more complete picture of plant NO responses at both the molecular and systems level.

Cross-Talk between Hydrogen Peroxide and Nitric Oxide during Plant Development and Responses to Stress

Nitric oxide (NO) and hydrogen peroxide (H₂O₂) are gradually becoming established as critical regulators in plants under physiological and stressful conditions. Strong spatiotemporal correlations in their production and distribution have been identified in various plant biological processes. In this context, NO and H₂O₂ act synergistically or antagonistically as signals or stress promoters depending on their respective concentrations, engaging in processes such as the hypersensitive response, stomatal movement, and abiotic stress responses. Moreover, proteins identified as potential targets of NO-based modifications include a number of enzymes related to H₂O₂ metabolism, reinforcing their cross-talk. In this review, several processes of well-characterized functional interplay between H₂O₂ and NO are discussed with respect to the most recent reported evidence on hypersensitive response-induced programmed cell death, stomatal movement, and plant responses to adverse conditions and, where known, the molecular mechanisms and factors underpinning their cross-talk.

Altered Root Growth, Auxin Metabolism and Distribution in Arabidopsis thaliana Exposed to Salt and Osmotic Stress

Salt and osmotic stress are the main abiotic stress factors affecting plant root growth and architecture. We investigated the effect of salt (100 mM NaCl) and osmotic (200 mM mannitol) stress on the auxin metabolome by UHPLC-MS/MS, auxin distribution by confocal microscopy, and transcript levels of selected genes by qRT-PCR in Arabidopsis thaliana ecotype Columbia-0 (Col-0) and DR5rev::GFP (DR5) line. During long-term stress (13 days), a stability of the auxin metabolome and a tendency to increase indole-3-acetic acid (IAA) were observed, especially during salt stress. Short-term stress (3 h) caused significant changes in the auxin metabolome, especially NaCl treatment resulted in a significant reduction of IAA. The data derived from auxin profiling were consistent with gene expressions showing the most striking changes in the transcripts of YUC, GH3, and UGT transcripts, suggesting disruption of auxin biosynthesis, but especially in the processes of amide and ester conjugation. These data were consistent with the auxin distribution observed in the DR5 line. Moreover, NaCl treatment caused a redistribution of auxin signals from the quiescent center and the inner layers of the root cap to the epidermal and cortical cells of the root elongation zone. The distribution of PIN proteins was also disrupted by salt stress; in particular, PIN2 was suppressed, even after 5 min of treatment. Based on our results, the DR5 line was more sensitive to the applied stresses than Col-0, although both lines showed similar trends in root morphology, as well as transcriptome and metabolome parameters under stress conditions.

Analysis of Phytohormone Signal Transduction in Sophora alopecuroides under Salt Stress

Salt stress seriously restricts crop yield and quality, leading to an urgent need to understand its effects on plants and the mechanism of plant responses. Although phytohormones are crucial for plant responses to salt stress, the role of phytohormone signal transduction in the salt stress responses of stress-resistant species such as Sophora alopecuroides has not been reported. Herein, we combined transcriptome and metabolome analyses to evaluate expression changes of key genes and metabolites associated with plant hormone signal transduction in S. alopecuroides roots under salt stress for 0 h to 72 h. Auxin, cytokinin, brassinosteroid, and gibberellin signals were predominantly involved in regulating S. alopecuroides growth and recovery under salt stress. Ethylene and jasmonic acid signals may negatively regulate the response of S. alopecuroides to salt stress. Abscisic acid and salicylic acid are significantly upregulated under salt stress, and their signals may positively regulate the plant response to salt stress. Additionally, salicylic acid (SA) might regulate the balance between plant growth and resistance by preventing reduction in growth-promoting hormones and maintaining high levels of abscisic acid (ABA). This study provides insight into the mechanism of salt stress response in S. alopecuroides and the corresponding role of plant hormones, which is beneficial for crop resistance breeding.

Auxin alters sodium ion accumulation and nutrient accumulation by playing protective role in salinity challenged strawberry

High salinity in soil affects the strawberry production and fruit quality. Auxin-primed plants have enhanced responses to soil salinization. In this study, we report that exogenous application of IAA can partially relieve stress responses of strawberry seedlings. Cytological analysis showed that the ultrastructure of root tip and leaf cells in strawberry seedlings were altered under high salinity condition, which was partially recovered after the application of IAA. The study showed that the ultrastructure of root tip and leaf cells in strawberry seedlings were altered under salt stress condition, which was partially recovered after the application of IAA. Exogenous IAA ameliorated deleterious effects on seedling growth under salinity were attributed to accelerated Na⁺ fluxes, decreased the contents of Na⁺ to maintain the ion homeostasis, protect root growth, and promote the absorption of nutrients for improved photosynthetic efficiency in strawberry.

Exogenous Putrescine Enhances Salt Tolerance and Ginsenosides Content in Korean Ginseng (Panax ginseng Meyer) Sprouts

The effect of exogenously applied putrescine (Put) on salt stress tolerance was investigated in Panax ginseng. Thirty-day-old ginseng sprouts were grown in salinized nutrient solution (150 mM NaCl) for five days, while the control sprouts were grown in nutrients solution. Putrescine (0.3, 0.6, and 0.9 mM) was sprayed on the plants once at the onset of salinity treatment, whereas control plants were sprayed with water only. Ginseng seedlings tested under salinity exhibited reduced plant growth and biomass production, which was directly interlinked with reduced chlorophyll and chlorophyll fluorescence due to higher reactive oxygen species (hydrogen peroxide; H2O2) and lipid peroxidation (malondialdehyde; MDA) production. Application of Put enhanced accumulation of proline, total soluble carbohydrate, total soluble sugar and total soluble protein. At the same time, activities of antioxidant enzymes like superoxide dismutase, catalase, ascorbate peroxidase, guaiacol peroxidase in leaves, stems, and roots of ginseng seedlings were increased. Such modulation of physio-biochemical processes reduced the level of H2O2 and MDA, which indicates a successful adaptation of ginseng seedlings to salinity stress. Moreover, protopanaxadiol (PPD) ginsenosides enhanced by both salinity stress and exogenous Put treatment. On the other hand, protopanaxatriol (PPT) ginsenosides enhanced in roots and reduced in leaves and stems under salinity stress condition. In contrast, they enhanced by exogenous Put application in all parts of the plants for most cases, also evidenced by principal component analysis. Collectively, our findings provide an important prospect for the use of Put in modulating salinity tolerance and ginsenosides content in ginseng sprouts.

Induction of S-nitrosoglutathione reductase protects root growth from ammonium toxicity by regulating potassium homeostasis in Arabidopsis and rice

Ammonium (NH4+) is toxic to root growth in most plants already at moderate levels of supply, but mechanisms of root growth tolerance to NH4+ remain poorly understood. Here, we report that high levels of NH4+ induce nitric oxide (NO) accumulation, while inhibiting potassium (K+) acquisition via SNO1 (sensitive to nitric oxide 1)/SOS4 (salt overly sensitive 4), leading to the arrest of primary root growth. High levels of NH4+ also stimulated the accumulation of GSNOR (S-nitrosoglutathione reductase) in roots. GSNOR overexpression improved root tolerance to NH4+. Loss of GSNOR further induced NO accumulation, increased SNO1/SOS4 activity, and reduced K+ levels in root tissue, enhancing root growth sensitivity to NH4+. Moreover, the GSNOR-like gene, OsGSNOR, is also required for NH4+ tolerance in rice. Immunoblotting showed that the NH4+-induced GSNOR protein accumulation was abolished in the VTC1- (vitamin C1) defective mutant vtc1-1, which is hypersensititive to NH4+ toxicity. GSNOR overexpression enhanced vtc1-1 root tolerance to NH4+. Our findings suggest that induction of GSNOR increases NH4+ tolerance in Arabidopsis roots by counteracting NO-mediated suppression of tissue K+, which depends on VTC1 function.

Improving Performance of Salt-Grown Crops by Exogenous Application of Plant Growth Regulators

Soil salinity is one of the major abiotic stresses restricting plant growth and development. Application of plant growth regulators (PGRs) is a possible practical means for minimizing salinity-induced yield losses, and can be used in addition to or as an alternative to crop breeding for enhancing salinity tolerance. The PGRs auxin, cytokinin, nitric oxide, brassinosteroid, gibberellin, salicylic acid, abscisic acid, jasmonate, and ethylene have been advocated for practical use to improve crop performance and yield under saline conditions. This review summarizes the current knowledge of the effectiveness of various PGRs in ameliorating the detrimental effects of salinity on plant growth and development, and elucidates the physiological and genetic mechanisms underlying this process by linking PGRs with their downstream targets and signal transduction pathways. It is shown that, while each of these PGRs possesses an ability to alter plant ionic and redox homeostasis, the complexity of interactions between various PGRs and their involvement in numerous signaling pathways makes it difficult to establish an unequivocal causal link between PGRs and their downstream effectors mediating plants' adaptation to salinity. The beneficial effects of PGRs are also strongly dependent on genotype, the timing of application, and the concentration used. The action spectrum of PGRs is also strongly dependent on salinity levels. Taken together, this results in a rather narrow "window" in which the beneficial effects of PGR are observed, hence limiting their practical application (especially under field conditions). It is concluded that, in the light of the above complexity, and also in the context of the cost-benefit analysis, crop breeding for salinity tolerance remains a more reliable avenue for minimizing the impact of salinity on plant growth and yield. Further progress in the field requires more studies on the underlying cell-based mechanisms of interaction between PGRs and membrane transporters mediating plant ion homeostasis.

Dynamics of Auxin and Cytokinin Metabolism during Early Root and Hypocotyl Growth in Theobroma cacao

The spatial location and timing of plant developmental events are largely regulated by the well balanced effects of auxin and cytokinin phytohormone interplay. Together with transport, localized metabolism regulates the concentration gradients of their bioactive forms, ultimately eliciting growth responses. In order to explore the dynamics of auxin and cytokinin metabolism during early seedling growth in Theobroma cacao (cacao), we have performed auxin and cytokinin metabolite profiling in hypocotyls and root developmental sections at different times by using ultra-high-performance liquid chromatography-electrospray tandem mass spectrometry (UHPLC-MS/MS). Our work provides quantitative characterization of auxin and cytokinin metabolites throughout early root and hypocotyl development and identifies common and distinctive features of auxin and cytokinin metabolism during cacao seedling development.

Overexpression of Cassava MeAnn2 Enhances the Salt and IAA Tolerance of Transgenic Arabidopsis

Annexins are a superfamily of soluble calcium-dependent phospholipid-binding proteins that have considerable regulatory effects in plants, especially in response to adversity and stress. The Arabidopsis thaliana AtAnn1 gene has been reported to play a significant role in various abiotic stress responses. In our study, the cDNA of an annexin gene highly similar to AtAnn1 was isolated from the cassava genome and named MeAnn2. It contains domains specific to annexins, including four annexin repeat sequences (I–IV), a Ca²⁺-binding sequence, Ca²⁺-independent membrane-binding-related tryptophan residues, and a salt bridge-related domain. MeAnn2 is localized in the cell membrane and cytoplasm, and it was found to be preferentially expressed in the storage roots of cassava. The overexpression of MeAnn2 reduced the sensitivity of transgenic Arabidopsis to various Ca²⁺, NaCl, and indole-3-acetic acid (IAA) concentrations. The expression of the stress resistance-related gene AtRD29B and auxin signaling pathway-related genes AtIAA4 and AtLBD18 in transgenic Arabidopsis was significantly increased under salt stress, while the Malondialdehyde (MDA) content was significantly lower than that of the control. These results indicate that the MeAnn2 gene may increase the salt tolerance of transgenic Arabidopsis via the IAA signaling pathway.

Arabidopsis phosphatidylinositol 4-phosphate 5-kinase genes PIP5K7, PIP5K8, and PIP5K9 are redundantly involved in root growth adaptation to osmotic stress

Phosphatidylinositol 4-phosphate 5-kinase (PIP5K) produces phosphatidylinositol (4,5)-bisphosphate (PtdIns(4,5)P₂ ), a signaling phospholipid critical for various cellular processes in eukaryotes. The Arabidopsis thaliana genome encodes 11 PIP5K genes. Of these, three type B PIP5K genes, PIP5K7, PIP5K8, and PIP5K9, constitute a subgroup highly conserved in land plants, suggesting that they retain a critical function shared by land plants. In this study, we comprehensively investigated the biological functions of the PIP5K7-9 subgroup genes. Reporter gene analyses revealed their preferential expression in meristematic and vascular tissues. Their YFP-fusion proteins localized primarily to the plasma membrane in root meristem epidermal cells. We selected a mutant line that was considered to be null for each gene. Under normal growth conditions, neither single mutants nor multiple mutants of any combination exhibited noticeable phenotypic changes. However, stress conditions with mannitol or NaCl suppressed main root growth and reduced proximal root meristem size to a greater extent in the pip5k7pip5k8pip5k9 triple mutant than in the wild type. In root meristem epidermal cells of the triple mutant, where plasma membrane localization of the PtdIns(4,5)P₂ marker P24Y is impaired to a large extent, brefeldin A body formation is retarded compared with the wild type under hyperosmotic stress. These results indicate that PIP5K7, PIP5K8, and PIP5K9 are not required under normal growth conditions, but are redundantly involved in root growth adaptation to hyperosmotic conditions, possibly through the PtdIns(4,5)P₂ function promoting plasma membrane recycling in root meristem cells.

Polyamine homeostasis modulates plasma membrane- and tonoplast-associated aquaporin expression in etiolated salt-stressed sunflower (Helianthus annuus L.) seedlings

Salt stress adversely affects plants by causing osmotic and ionic imbalance. Cellular osmotic adjustment occurs by modulation of water fluxes. Polyamines (PAs) are often advocated to be involved in osmoregulation during stressful conditions, and thus, they serve as potential "osmolytes." Aquaporins (AQPs), the water-transporting channels, are expected to play crucial roles in osmoregulation. Present investigations on etiolated sunflower seedlings demonstrate a possible correlation between PA homeostasis and maintenance of water balance, as a function of modulation of the abundance of two major AQP subfamilies: PIP2 (plasma membrane intrinsic protein 2) and TIP1 (tonoplast intrinsic protein 1). Salt stress (120 mM NaCl) restricts growth of sunflower seedlings and induces reduction in relative water content (RWC). This accompanies enhanced abundance of PIP2s and TIP1s in seedling roots and that of TIP1s in cotyledons, as revealed by Western blot analysis of AQP isoforms and also their imaging by confocal laser scanning microscopy (CLSM). Raising seedlings in the presence of 500 μM of DFMA (DL-α-difluoromethylarginine) or DFMO (DL-α-difluoromethylornithine), which are potent inhibitors of PA biosynthesis enzymes (arginine decarboxylase (ADC) and ornithine decarboxylase (ODC), respectively), significantly promotes root extension, irrespective of NaCl stress, and results in further lowering of salt-induced reduction in RWC in roots and cotyledons. This correlates with enhanced accumulation of both PIP2s and TIP1s in seedling roots, but not in cotyledons. Present work, therefore, implicates PA homeostasis in the maintenance of water status of sunflower seedlings, possibly via regulation of abundance and distribution of AQP isoforms associated with the plasma membrane and tonoplast.

Characterization of Plant Growth-Promoting Traits and Inoculation Effects on Triticum durum of Actinomycetes Isolates under Salt Stress Conditions

This study aimed to characterize the halotolerant capability, in vitro, of selected actinomycetes strains and to evaluate their competence in promoting halo stress tolerance in durum wheat in a greenhouse experiment. Fourteen isolates were tested for phosphate solubilization, indole acetic acid, hydrocyanic acid, and ammonia production under different salt concentrations (i.e., 0, 0.25, 0.5, 0.75, 1, 1.25, and 1.5 M NaCl). The presence of 1-aminocyclopropane-1-carboxylate deaminase activity was also investigated. Salinity tolerance was evaluated in durum wheat through plant growth and development parameters: shoot and root length, dry and ash-free dry weight, and the total chlorophyll content, as well as proline accumulation. In vitro assays have shown that the strains can solubilize inorganic phosphate and produce indole acetic acid, hydrocyanic acid, and ammonia under different salt concentrations. Most of the strains (86%) had 1-aminocyclopropane-1-carboxylate deaminase activity, with significant amounts of α-ketobutyric acid. In the greenhouse experiment, inoculation with actinomycetes strains improved the morpho-biochemical parameters of durum wheat plants, which also recorded significantly higher content of chlorophylls and proline than those uninoculated, both under normal and stressed conditions. Our results suggest that inoculation of halotolerant actinomycetes can mitigate the negative effects of salt stress and allow normal growth and development of durum wheat plants.

Expression of soybean plant hemoglobin gene family under abiotic stresses

Many abiotic stresses induce the generation of nitric oxide (NO) in plant tissues, where it functions as a signal molecule in stress responses. Plants modulate NO by oxidizing it to NO₃ ^- with plant hemoglobin (GLB), because excess NO is toxic to cells. At least eight genes encoding GLB have been identified in soybean, in three clades: GLB1, GLB2, and GLB3. However, it is still unclear which GLB genes are responsible for NO regulation under abiotic stress in soybean. We exposed soybean roots to flooding, salt, and two NO donors-sodium pentacyanonitrosylferrate (III) dihydrate (SNP) and S-nitroso-N-acetyl-d,l-penicillamine (SNAP)-and analyzed expression of GLB genes. GmGLB1, one of two GLB1 genes of soybean, significantly responded to both SNP and SNAP, and its induction was almost completely repressed by a NO scavenger, 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide. GmGLB1 responded to flooding but not to salt, suggesting that it is responsible for NO regulation under NO-inducing abiotic stresses such as flooding. GmGLB3, one of two GLB3 genes of soybean, did not respond to NO donors at all but did respond to flooding, at a lower level than GmGLB1. These results suggest that flooding induces not only NO but also unknown factor(s) that induce GmGLB3 gene in soybean.

The Diverse Salt-Stress Response of Arabidopsis ctr1-1 and ein2-1Ethylene Signaling Mutants Is Linked to Altered Root Auxin Homeostasis

We explored the interplay between ethylene signals and the auxin pool in roots exposed to high salinity using Arabidopsisthaliana wild-type plants (Col-0), and the ethylene-signaling mutants ctr1-1 (constitutive) and ein2-1 (insensitive). The negative effect of salt stress was less pronounced in ctr1-1 individuals, which was concomitant with augmented auxin signaling both in the ctr1-1 controls and after 100 mM NaCl treatment. The R2D2 auxin sensorallowed mapping this active auxin increase to the root epidermal cells in the late Cell Division (CDZ) and Transition Zone (TZ). In contrast, the ethylene-insensitive ein2-1 plants appeared depleted in active auxins. The involvement of ethylene/auxin crosstalk in the salt stress response was evaluated by introducing auxin reporters for local biosynthesis (pTAR2::GUS) and polar transport (pLAX3::GUS, pAUX1::AUX1-YFP, pPIN1::PIN1-GFP, pPIN2::PIN2-GFP, pPIN3::GUS) in the mutants. The constantly operating ethylene-signaling pathway in ctr1-1 was linked to increased auxin biosynthesis. This was accompanied by a steady expression of the auxin transporters evaluated by qRT-PCR and crosses with the auxin transport reporters. The results imply that the ability of ctr1-1 mutant to tolerate high salinity could be related to the altered ethylene/auxin regulatory loop manifested by a stabilized local auxin biosynthesis and transport.

Nitric oxide function during oxygen deprivation in physiological and stress processes

Plants are aerobic organisms that have evolved to maintain specific requirements for oxygen (O2), leading to a correct respiratory energy supply during growth and development. There are certain plant developmental cues and biotic or abiotic stress responses where O2 is scarce. This O2 deprivation known as hypoxia may occur in hypoxic niches of plant-specific tissues and during adverse environmental cues such as pathogen attack and flooding. In general, plants respond to hypoxia through a complex reprogramming of their molecular activities with the aim of reducing the impact of stress on their physiological and cellular homeostasis. This review focuses on the fine-tuned regulation of hypoxia triggered by a network of gaseous compounds that includes O2, ethylene, and nitric oxide. In view of recent scientific advances, we summarize the molecular mechanisms mediated by phytoglobins and by the N-degron proteolytic pathway, focusing on embryogenesis, seed imbibition, and germination, and also specific structures, most notably root apical and shoot apical meristems. In addition, those biotic and abiotic stresses that comprise hypoxia are also highlighted.

Developmental roles of Auxin Binding Protein 1 in Arabidopsis thaliana

Auxin is a major plant growth regulator, but current models on auxin perception and signaling cannot explain the whole plethora of auxin effects, in particular those associated with rapid responses. A possible candidate for a component of additional auxin perception mechanisms is the AUXIN BINDING PROTEIN 1 (ABP1), whose function in planta remains unclear. Here we combined expression analysis with gain- and loss-of-function approaches to analyze the role of ABP1 in plant development. ABP1 shows a broad expression largely overlapping with, but not regulated by, transcriptional auxin response activity. Furthermore, ABP1 activity is not essential for the transcriptional auxin signaling. Genetic in planta analysis revealed that abp1 loss-of-function mutants show largely normal development with minor defects in bolting. On the other hand, ABP1 gain-of-function alleles show a broad range of growth and developmental defects, including root and hypocotyl growth and bending, lateral root and leaf development, bolting, as well as response to heat stress. At the cellular level, ABP1 gain-of-function leads to impaired auxin effect on PIN polar distribution and affects BFA-sensitive PIN intracellular aggregation. The gain-of-function analysis suggests a broad, but still mechanistically unclear involvement of ABP1 in plant development, possibly masked in abp1 loss-of-function mutants by a functional redundancy.

The NAC-type transcription factor CaNAC46 regulates the salt and drought tolerance of transgenic Arabidopsis thaliana

BACKGROUND

The NAC (NAM, ATAF1/ATAF2, and CUC2) transcription factors belong to a large family of plant-specific transcription factors in monocot and dicot species. These transcription factors regulate the expression of stress tolerance-related genes that protect plants from various abiotic stresses, including drought, salinity, and low temperatures.

RESULTS

In this study, we identified the CaNAC46 transcription factor gene in Capsicum annuum. Its open reading frame was revealed to comprise 921 bp, encoding a protein consisting of 306 amino acids, with an isoelectric point of 6.96. A phylogenetic analysis indicated that CaNAC46 belongs to the ATAF subfamily. The expression of CaNAC46 was induced by heat, cold, high salt, drought, abscisic acid, salicylic acid, and methyl jasmonate treatments. Thus, CaNAC46 may be important for the resistance of dry pepper to abiotic stresses. A subcellular localization analysis confirmed that CaNAC46 is localized in the nucleus. The overexpression of CaNAC46 improved the tolerance of transgenic Arabidopsis thaliana plants to drought and salt stresses. The CaNAC46-overexpressing lines had longer roots and more lateral roots than wild-type lines under prolonged drought and high salt stress conditions. Additionally, CaNAC46 affected the accumulation of reactive oxygen species (ROS). Moreover, CaNAC46 promoted the expression of SOD, POD, RD29B, RD20, LDB18, ABI, IAA4, and P5CS. The malondialdehyde contents were higher in TRV2-CaNAC46 lines than in wild-type plants in response to drought and salt stresses. Furthermore, the expression levels of stress-responsive genes, such as ABA2, P5CS, DREB, RD22, CAT, and POD, were down-regulated in TRV2-CaNAC46 plants.

CONCLUSIONS

Under saline and drought conditions, CaNAC46 is a positive regulator that activates ROS-scavenging enzymes and enhances root formation. The results of our study indicate CaNAC46 is a transcriptional regulator responsible for salinity and drought tolerance and suggest the abiotic stress-related gene regulatory mechanisms controlling this NAC transcription factor are conserved between A. thaliana and pepper.