Exogenous Brassinolide Alleviates Salt Stress in Malus hupehensis Rehd. by Regulating the Transcription of NHX-Type Na+(K+)/H+ Antiporters

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

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

RNAi-Mediated Suppression of OsBBTI5 Promotes Salt Stress Tolerance in Rice

This study explores the impact of RNAi in terms of selectively inhibiting the expression of the OsBBTI5 gene, with the primary objective of uncovering its involvement in the molecular mechanisms associated with salt tolerance in rice. OsBBTI5, belonging to the Bowman-Birk inhibitor (BBI) family gene, is known for its involvement in plant stress responses. The gene was successfully cloned from rice, exhibiting transcriptional self-activation in yeast. A yeast two-hybrid assay confirmed its specific binding to OsAPX2 (an ascorbate peroxidase gene). Transgenic OsBBTI5-RNAi plants displayed insensitivity to varying concentrations of 24-epibrassinolide in the brassinosteroid sensitivity assay. However, they showed reduced root and plant height at high concentrations (10 and 100 µM) of GA3 immersion. Enzyme activity assays revealed increased peroxidase (POD) and superoxide dismutase (SOD) activities and decreased malondialdehyde (MDA) content under 40-60 mM NaCl. Transcriptomic analysis indicated a significant upregulation of photosynthesis-related genes in transgenic plants under salt stress compared to the wild type. Notably, this study provides novel insights, suggesting that the BBI gene is part of the BR signaling pathway, and that OsBBTI5 potentially enhances stress tolerance in transgenic plants through interaction with the salt stress-related gene OsAPX2.

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

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

External application of brassinolide enhances cold resistance of tea plants (Camellia sinensis L.) by integrating calcium signals

MAIN CONCLUSION

Exogenous brassinolide can activate the expression of key genes in the calcium signalling pathway to enhance cold resistance of tea plants. Brassinolide is an endogenous sterol phytohormone containing multiple hydroxyl groups that has the important function of improving plant cold resistance and alleviating freeze damage. To explore the molecular mechanism of how brassinolide improves the cold resistance of tea plants, "Qiancha 1" was used as the material, and the method of spraying brassinolide on the leaves was adopted to explore its effects on the tea plants under 4 °C low-temperature treatment. The results showed that brassinolide can significantly increase the protective enzyme activity of tea plants under cold stress and reduce cold damage. At the transcriptome level, brassinolide significantly enhanced the expression of key genes involved in calcium signal transduction, Calmodulin (CaM), Calcium-dependent protein kinase (CDPK), calcineurin B-like protein (CBL) and calmodulin-binding transcriptional activators (CAMTA), which then activated the downstream key genes transcriptional regulator CBF1 (CBF1) and transcription factor ICE1 (ICE1) during cold induction. Quantitative real-time PCR (qRT‒PCR) results showed that the expression of these genes was significantly induced after treatment with brassinolide, especially CaM and CBF1. When calcium signalling was inhibited, the upregulated expression of CBF1 and ICE1 disappeared, and when CAMTA was knocked down, the expression of other genes under cold stress was also significantly reduced. The above results indicate that brassinolide combined with the calcium signalling pathway can improve the cold resistance of tea plants. This study provides a new theoretical basis for the study of the cold resistance mechanism of brassinolide.

MdHB7-like positively modulates apple salt tolerance by promoting autophagic activity and Na+ efflux

Salt stress adversely affects the yield and quality of crops and limits their geographical distribution. Studying the functions and regulatory mechanisms of key genes in the salt stress response is important for breeding crops with enhanced stress resistance. Autophagy plays an important role in modulating the tolerance of plants to various types of abiotic stressors. However, the mechanisms underlying salt-induced autophagy are largely unknown. Cation/Ca2+ exchanger proteins enhance apple salt tolerance by inhibiting Na+ accumulation but the mechanism underlying the response to salt stress remains unclear. Here, we show that the autophagy-related gene MdATG18a modulated apple salt tolerance. Under salt stress, the autophagic activity, proline content, and antioxidant enzyme activities were higher and Na+ accumulation was lower in MdATG18a-overexpressing transgenic plants than in control plants. The use of an autophagy inhibitor during the salt treatment demonstrated that the regulatory function of MdATG18a depended on autophagy. The yeast-one-hybrid assay revealed that the homeodomain-leucine zipper (HD-Zip) transcription factor MdHB7-like directly bound to the MdATG18a promoter. Transcriptional regulation and genetic analyses showed that MdHB7-like enhanced salt-induced autophagic activity by promoting MdATG18a expression. The analysis of Na+ efflux rate in transgenic yeast indicated that MdCCX1 expression significantly promoted Na+ efflux. Promoter binding, transcriptional regulation, and genetic analyses showed that MdHB7-like promoted Na+ efflux and apple salt tolerance by directly promoting MdCCX1 expression, which was independent of the autophagy pathway. Overall, our findings provide insight into the mechanism underlying MdHB7-like-mediated salt tolerance in apple through the MdHB7-like-MdATG18a and MdHB7-like-MdCCX1 modules. These results will aid future studies on the mechanisms underlying stress-induced autophagy and the regulation of stress tolerance in plants.

Melatonin enhances KCl salinity tolerance by maintaining K+ homeostasis in Malus hupehensis

Large amounts of potash fertilizer are often applied to apple (Malus domestica) orchards to enhance fruit quality and yields, but this treatment aggravates KCl-based salinity stress. Melatonin (MT) is involved in a variety of abiotic stress responses in plants. However, its role in KCl stress tolerance is still unknown. In the present study, we determined that an appropriate concentration (100 μm) of MT significantly alleviated KCl stress in Malus hupehensis by enhancing K+ efflux out of cells and compartmentalizing K+ in vacuoles. Transcriptome deep-sequencing analysis identified the core transcription factor gene MdWRKY53, whose expression responded to both KCl and MT treatment. Overexpressing MdWRKY53 enhanced KCl tolerance in transgenic apple plants by increasing K+ efflux and K+ compartmentalization. Subsequently, we characterized the transporter genes MdGORK1 and MdNHX2 as downstream targets of MdWRKY53 by ChIP-seq. MdGORK1 localized to the plasma membrane and enhanced K+ efflux to increase KCl tolerance in transgenic apple plants. Moreover, overexpressing MdNHX2 enhanced the KCl tolerance of transgenic apple plants/callus by compartmentalizing K+ into the vacuole. RT-qPCR and LUC activity analyses indicated that MdWRKY53 binds to the promoters of MdGORK1 and MdNHX2 and induces their transcription. Taken together, our findings reveal that the MT-WRKY53-GORK1/NHX2-K+ module regulates K+ homeostasis to enhance KCl stress tolerance in apple. These findings shed light on the molecular mechanism of apple response to KCl-based salinity stress and lay the foundation for the practical application of MT in salt stress.

Functional identification of ANR genes in apple (Malus halliana) that reduce saline-alkali stress tolerance

As one of the major abiotic stresses restricting the development of global agriculture, saline-alkali stress causes osmotic stress, ion poisoning, ROS damage and high pH damage, which seriously restrict sustainable development of fruit industry. Therefore, it is essential to develop and cultivate saline-alkali-resistant apple rootstocks to improve the yield and quality of apples in China. Based on transcriptome data, MhANR (LOC114827797), which is significantly induced by saline-alkali stress, was cloned from Malus halliana. The physicochemical properties, evolutionary relationships and cis-acting elements were analysed. Subsequently, the tolerance of MhANR overexpression in Arabidopsis thaliana, tobacco, and apple calli to saline-alkali stress was verified through genetic transformation. Transgenic plants contained less Chl a, Chl b and proline, SOD, POD and CAT activity, and higher relative electrical conductivity (REC) compared to WT plants under saline-alkali stress. In addition, expression of saline-alkali stress-related genes in overexpressed apple calli were also lower than in WT calli, including the antioxidant genes (MhSOD and MhCAT^), the Na+ transporter genes (MhCAX5, MhCAX5, MhSOS1, MhALT1), and the H+ -ATPase genes (MhAHA2 and MhAHA8), while expression of the K+ transporter genes (MhSKOR and MhNHX4) were higher. Expression of MhANR reduced tolerance of A. thaliana, tobacco, and apple calli to saline-alkali stress by regulating osmoregulatory substances, chlorophyll content, antioxidant enzyme activity, and expression of saline-alkali stress-related genes. This research provides a theoretical basis for cultivating apple rootstocks with effective saline-alkali stress tolerance.

Integrated Analysis of Transcriptome and Metabolome Reveals Molecular Mechanisms of Rice with Different Salinity Tolerances

Rice is a crucial global food crop, but it lacks a natural tolerance to high salt levels, resulting in significant yield reductions. To gain a comprehensive understanding of the molecular mechanisms underlying rice's salt tolerance, further research is required. In this study, the transcriptomic and metabolomic differences between the salt-tolerant rice variety Lianjian5 (TLJIAN) and the salt-sensitive rice variety Huajing5 (HJING) were examined. Transcriptome analysis revealed 1518 differentially expressed genes (DEGs), including 46 previously reported salt-tolerance-related genes. Notably, most of the differentially expressed transcription factors, such as NAC, WRKY, MYB, and EREBP, were upregulated in the salt-tolerant rice. Metabolome analysis identified 42 differentially accumulated metabolites (DAMs) that were upregulated in TLJIAN, including flavonoids, pyrocatechol, lignans, lipids, and trehalose-6-phosphate, whereas the majority of organic acids were downregulated in TLJIAN. The interaction network of 29 differentially expressed transporter genes and 19 upregulated metabolites showed a positive correlation between the upregulated calcium/cation exchange protein genes (OsCCX2 and CCX5_Ath) and ABC transporter gene AB2E_Ath with multiple upregulated DAMs in the salt-tolerant rice variety. Similarly, in the interaction network of differentially expressed transcription factors and 19 upregulated metabolites in TLJIAN, 6 NACs, 13 AP2/ERFs, and the upregulated WRKY transcription factors were positively correlated with 3 flavonoids, 3 lignans, and the lipid oleamide. These results suggested that the combined effects of differentially expressed transcription factors, transporter genes, and DAMs contribute to the enhancement of salt tolerance in TLJIAN. Moreover, this study provides a valuable gene-metabolite network reference for understanding the salt tolerance mechanism in rice.

Melatonin promotes Al3+ compartmentalization via H+ transport and ion gradients in Malus hupehensis

Soil acidification in apple (Malus domestica) orchards results in the release of rhizotoxic aluminum ions (Al3+) into soil. Melatonin (MT) participates in plant responses to abiotic stress; however, its role in AlCl3 stress in apple remains unknown. In this study, root application of MT (1 μM) substantially alleviated AlCl3 stress (300 μM) in Pingyi Tiancha (Malus hupehensis), which was reflected by higher fresh and dry weight, increased photosynthetic capacity, and longer and more roots compared with plants that did not receive MT treatment. MT functioned mainly by regulating vacuolar H+/Al3+ exchange and maintaining H+ homeostasis in the cytoplasm under AlCl3 stress. Transcriptome deep sequencing analysis identified the transcription factor gene SENSITIVE TO PROTON RHIZOTOXICITY 1 (MdSTOP1) was induced by both AlCl3 and MT treatments. Overexpressing MdSTOP1 in apple increased AlCl3 tolerance by enhancing vacuolar H+/Al3+ exchange and H+ efflux to the apoplast. We identified 2 transporter genes, ALUMINUM SENSITIVE 3 (MdALS3) and SODIUM HYDROGEN EXCHANGER 2 (MdNHX2), as downstream targets of MdSTOP1. MdSTOP1 interacted with the transcription factor NAM ATAF and CUC 2 (MdNAC2) to induce MdALS3 expression, which reduced Al toxicity by transferring Al3+ from the cytoplasm to the vacuole. Furthermore, MdSTOP1 and MdNAC2 coregulated MdNHX2 expression to increase H+ efflux from the vacuole to the cytoplasm to promote Al3+ compartmentalization and maintain cation balance in the vacuole. Taken together, our findings reveal an MT-STOP1 + NAC2-NHX2/ALS3-vacuolar H+/Al3+ exchange model for the alleviation of AlCl3 stress in apple, laying a foundation for practical applications of MT in agriculture.

How Plants Tolerate Salt Stress

Soil salinization inhibits plant growth and seriously restricts food security and agricultural development. Excessive salt can cause ionic stress, osmotic stress, and ultimately oxidative stress in plants. Plants exclude excess salt from their cells to help maintain ionic homeostasis and stimulate phytohormone signaling pathways, thereby balancing growth and stress tolerance to enhance their survival. Continuous innovations in scientific research techniques have allowed great strides in understanding how plants actively resist salt stress. Here, we briefly summarize recent achievements in elucidating ionic homeostasis, osmotic stress regulation, oxidative stress regulation, and plant hormonal responses under salt stress. Such achievements lay the foundation for a comprehensive understanding of plant salt-tolerance mechanisms.

SLs signal transduction gene CsMAX2 of cucumber positively regulated to salt, drought and ABA stress in Arabidopsis thaliana L

Recent studies have demonstrated that strigolactones (SLs) participate in the regulation of stress adaptation, however, the mechanisms remain elusive. MAX2 (MORE AXILLARY GROWTH2) is the key gene in the signal transduction pathway of SLs. This study aimed to clone and functionally characterize the CsMAX2 gene of cucumber in Arabidopsis. The results showed that the expression levels of the CsMAX2 gene changed significantly after salt, drought, and ABA stresses in cucumber. Moreover, the overexpression of CsMAX2 promoted stress tolerance and increased the germination rate and root length of Arabidopsis thaliana. Meanwhile, the content of chlorophyll increased and malondialdehyde decreased in CsMAX2 OE lines under salt and drought stresses. Additionally, the expression levels of stress-related marker genes, especially AREB1 and COR15A, were significantly upregulated under salt stress, while the expression levels of all genes were upregulated under drought stress, except ABI4 and ABI5 genes. The level of NCED3 continued to rise under both salt and drought stresses. In addition, D10 and D27 gene expression level also showed a continuous increase under ABA stress. The result suggested the interaction between SL and ABA in the process of adapting to stress. Overall, CsMAX2 could positively regulate salt, drought, and ABA stress resistance, and this process correlated with ABA transduction.

Strigolactone Alleviates the Adverse Effects of Salt Stress on Seed Germination in Cucumber by Enhancing Antioxidant Capacity

Strigolactones (SLs), as a new phytohormone, regulate various physiological and biochemical processes, and a number of stress responses, in plants. In this study, cucumber 'Xinchun NO. 4' is used to study the roles of SLs in seed germination under salt stress. The results show that the seed germination significantly decreases with the increase in the NaCl concentrations (0, 1, 10, 50, and 100 mM), and 50 mM NaCl as a moderate stress is used for further analysis. The different concentrations of SLs synthetic analogs GR24 (1, 5, 10, and 20 μM) significantly promote cucumber seed germination under NaCl stress, with a maximal biological response at 10 μM. An inhibitor of strigolactone (SL) synthesis TIS108 suppresses the positive roles of GR24 in cucumber seed germination under salt stress, suggesting that SL can alleviate the inhibition of seed germination caused by salt stress. To explore the regulatory mechanism of SL-alleviated salt stress, some contents, activities, and genes related to the antioxidant system are measured. The malondialdehyde (MDA), H₂O₂, O₂^-, and proline contents are increased, and the levels of ascorbic acid (AsA) and glutathione (GSH) are decreased under salt stress conditions, while GR24 treatment reduces MDA, H₂O₂, O₂^-, and proline contents, and increases AsA and GSH contents during seed germination under salt stress. Meanwhile, GR24 treatment enhances the decrease in the activities of antioxidant enzymes caused by salt stress [superoxide dismutase (SOD), peroxidase (POD), catalase (CAT) and ascorbate peroxidase (APX)], following which antioxidant-related genes SOD, POD, CAT, APX, and GRX2 are up-regulated by GR24 under salt stress. However, TIS108 reversed the positive effects of GR24 on cucumber seed germination under salt stress. Together, the results of this study revealed that GR24 regulates the expression levels of genes related to antioxidants and, therefore, regulates enzymatic activity and non-enzymatic substances and enhances antioxidant capacity, alleviating salt toxicity during seed germination in cucumber.

Genome-wide identification and expression analysis of BZR gene family and associated responses to abiotic stresses in cucumber (Cucumis sativus L.)

BACKGROUND

BRASSINAZOLE-RESISTANT (BZR) is a class of specific transcription factor (TFs) involved in brassinosteroid (BR) signal transduction. The regulatory mechanism of target genes mediated by BZR has become one of the key research areas in plant BR signaling networks. However, the functions of the BZR gene family in cucumber have not been well characterized.

RESULTS

In this study, six CsBZR gene family members were identified by analyzing the conserved domain of BES1 N in the cucumber genome. The size of CsBZR proteins ranges from 311 to 698 amino acids and are mostly located in the nucleus. Phylogenetic analysis divided CsBZR genes into three subgroups. The gene structure and conserved domain showed that the BZR genes domain in the same group was conserved. Cis-acting element analysis showed that cucumber BZR genes were mainly involved in hormone response, stress response and growth regulation. The qRT-PCR results also confirmed CsBZR response to hormones and abiotic stress.

CONCLUSION

Collectively, the CsBZR gene is involved in regulating cucumber growth and development, particularly in hormone response and response to abiotic stress. These findings provide valuable information for understanding the structure and expression patterns of BZR genes.

Evaluation of Amino Acid Profiles of Rice Genotypes under Different Salt Stress Conditions

Amino acids are building blocks of proteins that are essential components of a wide range of metabolic pathways in plant species, including rice species. Previous studies only considered changes in the amino acid content of rice under NaCl stress. Here, we evaluated profiles of essential and non-essential amino acids in four rice genotype seedlings in the presence of three types of salts, namely NaCl, CaCl₂, and MgCl₂. Amino acid profiles in 14-day-old rice seedlings were determined. The total essential and non-essential amino acid contents in cultivar Cheongcheong were considerably increased upon NaCl and MgCl₂ application, whereas total amino acids were increased upon NaCl, CaCl₂, and MgCl₂ application in the cultivar Nagdong. The total amino acid content was significantly lower in the salt-sensitive cultivar IR28 and salt-tolerant Pokkali under different salt stress conditions. Glycine was not detected in any of the rice genotypes. We observed that cultivars with the same origin respond similarly to each other under salinity stress conditions: cultivars Cheongcheong and Nagdong were found to show increased total amino acid content, whereas the content in foreign cultivars IR28 and Pokkali was found to decrease. Thus, our findings showed that the amino acid profile of each rice cultivar might depend on the origin, immune level, and genetic makeup of the respective cultivar.

Genome-wide identification and expression analysis of the HD2 protein family and its response to drought and salt stress in Gossypium species

Histone deacetylase 2 (HD2) proteins play an important role in the regulation of gene expression. This helps with the growth and development of plants and also plays a crucial role in responses to biotic and abiotic stress es. HD2s comprise a C₂H₂-type Zn²⁺ finger at their C-terminal and an HD2 label, deacetylation and phosphorylation sites, and NLS motifs at their N-terminal. In this study, a total of 27 HD2 members were identified, using Hidden Markov model profiles, in two diploid cotton genomes (Gossypium raimondii and Gossypium arboretum) and two tetraploid cotton genomes (Gossypium hirsutum and Gossypium barbadense). These cotton HD2 members were classified into 10 major phylogenetic groups (I-X), of which group III was found to be the largest with 13 cotton HD2 members. An evolutionary investigation showed that the expansion of HD2 members primarily occurred as a result of segmental duplication in paralogous gene pairs. Further qRT-PCR validation of nine putative genes using RNA-Seq data suggested that GhHDT3D.2 exhibits significantly higher levels of expression at 12h, 24h, 48h, and 72h of exposure to both drought and salt stress conditions compared to a control measure at 0h. Furthermore, gene ontology, pathways, and co-expression network study of GhHDT3D.2 gene affirmed their significance in drought and salt stress responses.

Exogenous 24-epibrassinolide promoted growth and nitrogen absorption and assimilation efficiency of apple seedlings under salt stress

Introduction

High salinity significantly hampers global agricultural productivity. Plants typically undergo lower nitrogen utilization efficiency (NUE) under salt stress. As an active byproduct from brassinolide biosynthesis, 24-epibrassinolide (EBR) is involved in regulating the stress-treated plant N absorption and assimilation. However, the exogenous EBR application effects' on N absorption and assimilation in apple exposed to the salt-stressed condition remains unclear.

Methods

We sprayed exogenous EBR (0.2 mg L^-1) on apple dwarf rootstock (M9T337) seedlings (growing hydroponically) under salt (NaCl) stress in a growth chamber. We analyzed the seedling development, photosynthesis and its-mediated C fixation, N ( NO 3 - ) absorption and assimilation in reponse to exogenous EBR application under salt stress.

Results

The findings demonstrated that NaCl stress greatly hampered seedlings' root growth and that exogenous EBR application obviously alleviated this growth suppression. Exogenous EBR-treated plants under NaCl stress displayed the more ideal root morphology and root activity, stronger salt stress tolerance and photosynthetic capacity as well as higher C- and N-assimilation enzyme activities, NO 3 - ion flow rate and nitrate transporter gene expression level than did untreated plants. Furthermore, the results of isotope labeling noted that exogenous EBR application also enhanced ¹³C-photoassimilate transport from leaves to roots and ¹⁵ NO 3 - transport from roots to leaves under NaCl stress.

Conclusion

Our findings imply that exogenous EBR application, through strengthening photosynthesis, C- and N-assimilation enzyme activities, nitrate absorption and transport as well as synchronized optimizing the distribution of seedlings' C and N, has a fundamental role in improving NUE in apple rootstock seedlings under salt stress.

The apple 14-3-3 gene MdGRF6 negatively regulates salt tolerance

The 14-3-3 (GRF, general regulatory factor) regulatory proteins are highly conserved and are widely distributed throughout the eukaryotes. They are involved in the growth and development of organisms via target protein interactions. Although many plant 14-3-3 proteins were identified in response to stresses, little is known about their involvement in salt tolerance in apples. In our study, nineteen apple 14-3-3 proteins were cloned and identified. The transcript levels of Md14-3-3 genes were either up or down-regulated in response to salinity treatments. Specifically, the transcript level of MdGRF6 (a member of the Md14-3-3 genes family) decreased due to salt stress treatment. The phenotypes of transgenic tobacco lines and wild-type (WT) did not affect plant growth under normal conditions. However, the germination rate and salt tolerance of transgenic tobacco was lower compared to the WT. Transgenic tobacco demonstrated decreased salt tolerance. The transgenic apple calli overexpressing MdGRF6 exhibited greater sensitivity to salt stress compared to the WT plants, whereas the MdGRF6-RNAi transgenic apple calli improved salt stress tolerance. Moreover, the salt stress-related genes (MdSOS2, MdSOS3, MdNHX1, MdATK2/3, MdCBL-1, MdMYB46, MdWRKY30, and MdHB-7) were more strongly down-regulated in MdGRF6-OE transgenic apple calli lines than in the WT when subjected to salt stress treatment. Taken together, these results provide new insights into the roles of 14-3-3 protein MdGRF6 in modulating salt responses in plants.

New Insight into Plant Saline-Alkali Tolerance Mechanisms and Application to Breeding

Saline-alkali stress is a widespread adversity that severely affects plant growth and productivity. Saline-alkaline soils are characterized by high salt content and high pH values, which simultaneously cause combined damage from osmotic stress, ionic toxicity, high pH and HCO₃^-/CO₃^2- stress. In recent years, many determinants of salt tolerance have been identified and their regulatory mechanisms are fairly well understood. However, the mechanism by which plants respond to comprehensive saline-alkali stress remains largely unknown. This review summarizes recent advances in the physiological, biochemical and molecular mechanisms of plants tolerance to salinity or salt- alkali stress. Focused on the progress made in elucidating the regulation mechanisms adopted by plants in response to saline-alkali stress and present some new views on the understanding of plants in the face of comprehensive stress. Plants generally promote saline-alkali tolerance by maintaining pH and Na⁺ homeostasis, while the plants responding to HCO₃^-/CO₃^2- stress are not exactly the same as high pH stress. We proposed that pH-tolerant or sensitive plants have evolved distinct mechanisms to adapt to saline-alkaline stress. Finally, we highlight the areas that require further research to reveal the new components of saline-alkali tolerance in plants and present the current and potential application of key determinants in breed improvement and molecular breeding.

Exogenous brassinosteroids alleviate calcium deficiency-induced tip-burn by maintaining cell wall structural stability and higher photosynthesis in mini Chinese Cabbage

Tip-burn has seriously affected the yield, quality and commodity value of mini Chinese cabbage. Calcium (Ca²⁺) deficiency is the main cause of tip-burn. In order to investigate whether exogenous brassinosteroids (BRs) can alleviate tip-burn induced by calcium (Ca²⁺) deficiency and its mechanism, in this study, Ca²⁺ deficiency in nutrient solution was used to induced tip-burn, and then distilled water and BRs were sprayed on leaves to observe the tip-burn incidence of mini Chinese cabbage. The tip-burn incidence and disease index, leaf area, fluorescence parameters (Fv/Fm, NPQ, qP andφPSII) and gas exchange parameters (Tr, Pn, Gs and Ci), pigment contents, cell wall components, mesophyll cell ultrastructure and the expression of genes related to chlorophyll degradation were measured. The results showed that exogenous BRs reduced the tip-burn incidence rate and disease index of mini Chinese cabbage, and the tip-burn incidence rate reached the highest on the ninth day after treatment. Exogenous BRs increased the contents of cellulose, hemifiber, water-soluble pectin in Ca²⁺ deficiency treated leaves, maintaining the stability of cell wall structure. In addition, BRs increased photosynthetic rate by increasing the activities of Ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) and fructose 1,6-bisphosphatase (FBPase) related to Calvin cycle, maintaining relatively complete chloroplast structure and higher chlorophyll content via down-regulating the expression of BrPPH1 and BrPAO1 genes related to chlorophyll degradation. In conclusion, exogenous BRs alleviated calcium deficiency-induced tip-burn by maintaining cell wall structural stability and higher photosynthesis.

Brassinolide improves the tolerance of Malus hupehensis to alkaline stress

Malus hupehensis is one of the most widely used apple rootstocks in china but is severely damaged by alkaline soil. Alkaline stress can cause more serious harmful effects on apple plants than salt stress because it also induces high pH stress except for ion toxicity, osmotic stress, and oxidative damage. Brassinolide (BL) plays important roles in plant responses to salt stress. However, its role and function mechanism in apple plants in response to alkaline stress has never been reported. This study showed that applying exogenous 0.2 mg/L BL significantly enhanced the resistance of M. hupehensis seedlings to alkaline stress. The main functional mechanisms were also explored. First, exogenous BL could decrease the rhizosphere pH and promote Ca²⁺ and Mg²⁺ absorption by regulating malic acid and citric acid contents and increasing H⁺ excretion. Second, exogenous BL could alleviate ion toxicity caused by alkaline stress through enhancing Na⁺ efflux and inhibiting K⁺ expel and vacuole compartmentalization. Last, exogenous BL could balance osmotic stress by accumulating proline and reduce oxidative damage through increasing the activities of antioxidant enzymes and antioxidants contents. This study provides an important theoretical basis for further analyzing the mechanism of exogenous BL in improving alkaline tolerance of apple plants.

Enhancement of cucumber resistance under salt stress by 2, 4-epibrassinolide lactones

This study investigated the effects of exogenous 2, 4-epibrassinolide lactone (EBR) on the growth, photosynthetic pigments, antioxidant defense system, ion homeostasis, MAPK cascade and key genes of SOS signaling pathway of cucumber seedlings under salt stress using cucumber "Xinchun 4" as the test material. The experiment was set up with four treatments: foliar spraying of distilled water (CK), 50 mmol.L^-1 NaCl (NaCl), 50 mmol.L^-1 NaCl+foliar spray of 0.02 μmol.L^-1 EBR (EBR+NaCl), and 50 mmol.L^-1 NaCl+foliar spray of 24 μmol.L^-1 Brassinazole (BRZ) (BRZ+NaCl). The results showed that EBR+NaCl treatment significantly increased plant height, above-ground fresh weight, total root length, total root surface area, average rhizome and photosynthetic pigment content compared to NaCl treatment. Meanwhile, compared with NaCl treatment, EBR+NaCl treatment significantly increased superoxide dismutase, catalase and ascorbate peroxidase (SOD, CAT and APX) activities, significantly promoted the accumulation of osmoregulatory substances (soluble sugars and proline), and thus effectively reduced malondialdehyde (MDA) content and relative electrical conductivity of cucumber leaves. Exogenous spraying of EBR also significantly reduced Na⁺/K⁺ under NaCl stress, effectively alleviating the toxic effects of Na⁺ ions. In addition, exogenous EBR induced the up-regulated expression of CsMAPK3, CsMAPK4, CsMAPK6 and CsMAPK9 genes in the MAPK cascade signaling pathway and CsSOS1, CsSOS2 and CsSOS3 genes in the SOS signaling pathway to enhance salt tolerance in cucumber under NaCl stress. Therefore, exogenous spraying EBR may effectively reduce the damage of salt stress on cucumber seedlings by improving antioxidant capacity, maintaining ion homeostasis and activating salt-tolerant related signaling pathways, which might promote the growth of cucumber seedlings and the establishment of root system morphology. This study provides a reference for EBR to improve the salt tolerance of cucumber.

Comparative physiological and coexpression network analyses reveal the potential drought tolerance mechanism of peanut

BACKGROUND

Drought stress has negative effects on plant growth and productivity. In this study, a comprehensive analysis of physiological responses and gene expression was performed. The responses and expressions were compared between drought-tolerant (DT) and drought-sensitive (DS) peanut varieties to investigate the regulatory mechanisms and hub genes involved in the impact of drought stress on culture.

RESULTS

The drought-tolerant variety had robust antioxidative capacities with higher total antioxidant capacity and flavonoid contents, and it enhanced osmotic adjustment substance accumulation to adapt to drought conditions. KEGG analysis of differentially expressed genes demonstrated that photosynthesis was strongly affected by drought stress, especially in the drought-sensitive variety, which was consistent with the more severe suppression of photosynthesis. The hub genes in the key modules related to the drought response, including genes encoding protein kinase, E3 ubiquitin-protein ligase, potassium transporter, pentatricopeptide repeat-containing protein, and aspartic proteinase, were identified through a comprehensive combined analysis of genes and physiological traits using weighted gene co-expression network analysis. There were notably differentially expressed genes between the two varieties, suggesting the positive roles of these genes in peanut drought tolerance.

CONCLUSION

A comprehensive analysis of physiological traits and relevant genes was conducted on peanuts with different drought tolerances. The findings revealed diverse drought-response mechanisms and identified candidate genes for further research.

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.

Brassinolide alleviates Fe deficiency-induced stress by regulating the Fe absorption mechanism in Malus hupehensis Rehd

Exogenous brassinolide promotes Fe absorption through mechanism I strategy, thus improving the tolerance of Malus hupehensis seedlings to Fe deficiency stress. Iron (Fe) deficiency is a common nutritional disorder that results in decreased yield and poor fruit quality in apple production. As a highly active synthetic analog of brassinosteroids, brassinolide (BL) plays numerous roles in plant responses to abiotic stresses. However, its role in Fe deficiency stress in apple plants has never been reported. Herein, we found that the exogenous application of 0.2 mg L^-1 BL could significantly enhance the tolerance of apple seedlings to Fe deficiency stress and result in a low etiolation rate and a high photosynthetic rate. The functional mechanisms of this effect were also explored. We found that first, exogenous BL could improve Fe absorption through the mechanism I strategy. BL induced the activity of H⁺-ATPase and the expression of MhAHA family genes, resulting in rhizosphere acidification. Moreover, BL could enhance the activity of Fe chelate reductase and absorb Fe through direct binding with the E-box of the MhIRT1 or MhFRO2 promoter via the transcription factors MhBZR1 and MhBZR2. Second, exogenous BL alleviated osmotic stress by increasing the contents of osmolytes (proline, solution proteins, and solution sugar) and scavenged reactive oxygen species by improving the activities of antioxidant enzymes. Lastly, exogenous BL could cooperate with other endogenous plant hormones, such as indole-3-acetic acid, isopentenyl adenosine, and gibberellic acid 4, that respond to Fe deficiency stress indirectly. This work provided a theoretical basis for the application of exogenous BL to alleviate Fe deficiency stress in apple 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.

Physiological and Transcriptomic Analysis provide Molecular Insight into 24-Epibrassinolide mediated Cr(VI)-Toxicity Tolerance in Pepper Plants

The ever-increasing industrial activities over the decades have generated high toxic metals such as chromium (Cr) that hampers plant growth and development. To counter Cr-toxicity, plants have evolved complex defensive systems including hormonal crosstalk with various signaling pathways. 24-epibrassinolide (24-EBR) lowers oxidative stress and alleviates Cr(VI)-toxicity in plants. In this study, the concealed BR-mediated influences on Cr(VI)-stress tolerance were explored by transcriptome analysis in the Capsicum annuum. Results revealed a linkage between plant development under Cr(VI)-stress and the mitigating effect of 24-epibrassinolide and brassinazole. Growth inhibition, chlorophyll degradation, and a significant rise of malondialdehyde (MDA) were observed after 40 mg/L Cr(VI) treatment in Brz supplemented seedlings, whereas 24-EBR supplemented seedlings exhibited commendatory effect. Comparative transcriptome analysis showed that the expression levels of 6687 genes changed (3846 up-regulated and 2841 downregulated) under Cr(VI)-stress with Brz supplementation. Whereas the expression levels of only 1872 genes changed under Cr(VI)-stress with 24-EBR supplementation (1223 up-regulated and 649 downregulated). The functional categories of the differentially expressed genes (DEGs) by gene ontology (GO) revealed that drug transport, defense responses, and drug catabolic process were the considerable enrichments between 24-EBR and Brz supplemented seedlings under Cr(VI)-stress. Furthermore, auxin signaling, glutathione metabolism, ABC transporters, MAPK pathway, and 36 heavy metal-related genes were significantly differentially expressed components between Cr(VI)-stress, 24-EBR, and Brz supplemented seedlings. Overall, our data demonstrate that employing 24-EBR can commendably act as a growth stimulant in plants subjected to Cr(VI)-stress by modulating the physiological and defense regulatory system.

The Mechanisms Underlying Salt Resistance Mediated by Exogenous Application of 24-Epibrassinolide in Peanut

Peanut is one of the most important oil crops in the world, the growth and productivity of which are severely affected by salt stress. 24-epibrassinolide (EBL) plays an important role in stress resistances. However, the roles of exogenous EBL on the salt tolerance of peanut remain unclear. In this study, peanut seedlings treated with 150 mM NaCl and with or without EBL spray were performed to investigate the roles of EBL on salt resistance. Under 150 mM NaCl conditions, foliar application of 0.1 µM EBL increased the activity of catalase and thereby could eliminate reactive oxygen species (ROS). Similarly, EBL application promoted the accumulation of proline and soluble sugar, thus maintaining osmotic balance. Furthermore, foliar EBL spray enhanced the total chlorophyll content and high photosynthesis capacity. Transcriptome analysis showed that under NaCl stress, EBL treatment up-regulated expression levels of genes encoding peroxisomal nicotinamide adenine dinucleotide carrier (PMP34), probable sucrose-phosphate synthase 2 (SPS2) beta-fructofuranosidase (BFRUCT1) and Na⁺/H⁺ antiporters (NHX7 and NHX8), while down-regulated proline dehydrogenase 2 (PRODH). These findings provide valuable resources for salt resistance study in peanut and lay the foundation for using BR to enhance salt tolerance during peanut production.

Genome Assembly and Population Resequencing Reveal the Geographical Divergence of Shanmei (Rubus corchorifolius)

Rubus corchorifolius (Shanmei or mountain berry, 2n = 14) is widely distributed in China, and its fruits possess high nutritional and medicinal values. Here, we reported a high-quality chromosome-scale genome assembly of Shanmei, with contig size of 215.69 Mb and 26,696 genes. Genome comparison among Rosaceae species showed that Shanmei and Fupenzi (Rubus chingii Hu) were most closely related, followed by blackberry (Rubus occidentalis), and that environmental adaptation-related genes were significantly expanded in the Shanmei genome. Further resequencing of 101 samples of Shanmei collected from four regions in the provinces of Yunnan, Hunan, Jiangxi, and Sichuan in China revealed that the Hunan population of Shanmei possessed the highest diversity and represented the more ancestral population. Moreover, the Yunnan population underwent strong selection based on the nucleotide diversity, linkage disequilibrium, and historical effective population size analyses. Furthermore, genes from candidate genomic regions that showed strong divergence were significantly enriched in the flavonoid biosynthesis and plant hormone signal transduction pathways, indicating the genetic basis of adaptation of Shanmei to the local environment. The high-quality assembled genome and the variome dataset of Shanmei provide valuable resources for breeding applications and for elucidating the genome evolution and ecological adaptation of Rubus species.

Plant salt response: Perception, signaling, and tolerance

Salt stress is one of the significant environmental stressors that severely affects plant growth and development. Plant responses to salt stress involve a series of biological mechanisms, including osmoregulation, redox and ionic homeostasis regulation, as well as hormone or light signaling-mediated growth adjustment, which are regulated by different functional components. Unraveling these adaptive mechanisms and identifying the critical genes involved in salt response and adaption are crucial for developing salt-tolerant cultivars. This review summarizes the current research progress in the regulatory networks for plant salt tolerance, highlighting the mechanisms of salt stress perception, signaling, and tolerance response. Finally, we also discuss the possible contribution of microbiota and nanobiotechnology to plant salt tolerance.

Strigolactone alleviates the salinity-alkalinity stress of Malus hupehensis seedlings

Salinity-alkalinity stress can remarkably affect the growth and yield of apple. Strigolactone (SL) is a class of carotenoid-derived compounds that functions in stress tolerance. However, the effects and mechanism of exogenous SL on the salinity-alkalinity tolerance of apple seedlings remain unclear. Here, we assessed the effect of SL on the salinity-alkalinity stress response of Malus hupehensis seedlings. Results showed that treatment with 100 μM exogenous SL analog (GR24) could effectively alleviate salinity-alkalinity stress with higher chlorophyll content and photosynthetic rate than the apple seedlings without GR24 treatment. The mechanism was also explored: First, exogenous GR24 regulated the expression of Na⁺/K⁺ transporter genes and decreased the ratio of Na⁺/K⁺ in the cytoplasm to maintain ion homeostasis. Second, exogenous GR24 increased the enzyme activities of superoxide, peroxidase and catalase, thereby eliminating reactive oxygen species production. Third, exogenous GR24 alleviated the high pH stress by regulating the expression of H⁺-ATPase genes and inducing the production of organic acid. Last, exogenous GR24 application increased endogenous acetic acid, abscisic acid, zeatin riboside, and GA3 contents for co-responding to salinity-alkalinity stress indirectly. This study will provide important theoretical basis for analyzing the mechanism of exogenous GR24 in improving salinity-alkalinity tolerance of apple.

Heterologous Expression of the Melatonin-Related Gene HIOMT Improves Salt Tolerance in Malus domestica

Melatonin, a widely known indoleamine molecule that mediates various animal and plant physiological processes, is formed from N-acetyl serotonin via N-acetylserotonin methyltransferase (ASMT). ASMT is an enzyme that catalyzes melatonin synthesis in plants in the rate-determining step and is homologous to hydroxyindole-O-methyltransferase (HIOMT) melatonin synthase in animals. To date, little is known about the effect of HIOMT on salinity in apple plants. Here, we explored the melatonin physiological function in the salinity condition response by heterologous expressing the homologous human HIOMT gene in apple plants. We discovered that the expression of melatonin-related gene (MdASMT) in apple plants was induced by salinity. Most notably, compared with the wild type, three transgenic lines indicated higher melatonin levels, and the heterologous expression of HIOMT enhanced the expression of melatonin synthesis genes. The transgenic lines showed reduced salt damage symptoms, lower relative electrolyte leakage, and less total chlorophyll loss from leaves under salt stress. Meanwhile, through enhanced activity of antioxidant enzymes, transgenic lines decreased the reactive oxygen species accumulation, downregulated the expression of the abscisic acid synthesis gene (MdNCED3), accordingly reducing the accumulation of abscisic acid under salt stress. Both mechanisms regulated morphological changes in the stomata synergistically, thereby mitigating damage to the plants' photosynthetic ability. In addition, transgenic plants also effectively stabilized their ion balance, raised the expression of salt stress-related genes, as well as alleviated osmotic stress through changes in amino acid metabolism. In summary, heterologous expression of HIOMT improved the adaptation of apple leaves to salt stress, primarily by increasing melatonin concentration, maintaining a high photosynthetic capacity, reducing reactive oxygen species accumulation, and maintaining normal ion homeostasis.

Comparative Physiological and Proteomic Analyses Reveal the Mechanisms of Brassinolide-Mediated Tolerance to Calcium Nitrate Stress in Tomato

Secondary salinization caused by the overaccumulation of calcium nitrate [Ca(NO3)2] in soils due to excessive fertilization has become one of the major handicaps of protected vegetable production. Brassinolide, a bioactive plant steroid hormone, plays an important role in improving abiotic stress tolerance in plants. However, whether and how brassinolide (BR) can alleviate Ca(NO3)2 stress remains elusive. Here, we investigated the effects of exogenous BR on hydroponically grown tomato (Solanum lycopersicum L.) plants under Ca(NO3)2 stress through proteomics combined with physiological studies. Proteomics analysis revealed that Ca(NO3)2 stress affected the accumulation of proteins involved in photosynthesis, stress responses, and antioxidant defense, however, exogenous BR increased the accumulation of proteins involved in chlorophyll metabolism and altered the osmotic stress responses in tomatoes under Ca(NO3)2 stress. Further physiological studies supported the results of proteomics and showed that the exogenous BR-induced alleviation of Ca(NO3)2 stress was associated with the improvement of photosynthetic efficiency, levels of soluble sugars and proteins, chlorophyll contents, and antioxidant enzyme activities, leading to the reduction in the levels of reactive oxygen species and membrane lipid peroxidation, and promotion of the recovery of photosynthetic performance, energy metabolism, and plant growth under Ca(NO3)2 stress. These results show the importance of applying BR in protected agriculture as a means for the effective management of secondary salinization.

Potassium: A track to develop salinity tolerant plants

Salinity is one of the major constraints to plant growth and development across the globe that leads to the huge crop productivity loss. Salinity stress causes impairment in plant's metabolic and cellular processes including disruption in ionic homeostasis due to excess of sodium (Na⁺) ion influx and potassium (K⁺) efflux. This condition subsequently results in a significant reduction of the cytosolic K⁺ levels, eventually inhibiting plant growth attributes. K⁺ plays a crucial role in alleviating salinity stress by recasting key processes of plants. In addition, K⁺ acquisition and retention also serve as the perquisite trait to establish salt tolerant mechanism. In addition, an intricate network of genes and their regulatory elements are involved in coordinating salinity stress responses. Furthermore, plant growth regulators (PGRs) and other signalling molecules influence K⁺-mediated salinity tolerance in plants. Recently, nanoparticles (NPs) have also been found several implications in plants with respect to their roles in mediating K⁺ homoeostasis during salinity stress in plants. The present review describes salinity-induced adversities in plants and role of K⁺ in mitigating salinity-induced damages. The review also highlights the efficacy of PGRs and other signalling molecules in regulating K⁺ mediated salinity tolerance along with nano-technological perspective for improving K⁺ mediated salinity tolerance in plants.

Resveratrol improves the iron deficiency adaptation of Malus baccata seedlings by regulating iron absorption

BACKGROUND

Resveratrol (Res), a phytoalexin, has been widely reported to participate in plant resistance to fungal infections. However, little information is available on its role in abiotic stress, especially in iron deficiency stress. Malus baccata is widely used as apple rootstock in China, but it is sensitive to iron deficiency.

RESULTS

In this study, we investigated the role of exogenous Res in M. baccata seedings under iron deficiency stress. Results showed that applying 100 μM exogenous Res could alleviate iron deficiency stress. The seedlings treated with Res had a lower etiolation rate and higher chlorophyll content and photosynthetic rate compared with the apple seedlings without Res treatment. Exogenous Res increased the iron content in the roots and leaves by inducing the expression of MbAHA genes and improving the H+-ATPase activity. As a result, the rhizosphere pH decreased, iron solubility increased, the expression of MbFRO2 and MbIRT1 was induced, and the ferric-chelated reductase activity was enhanced to absorb large amounts of Fe2+ into the root cells under iron deficiency conditions. Moreover, exogenous Res application increased the contents of IAA, ABA, and GA3 and decreased the contents of DHZR and BL for responding to iron deficiency stress indirectly. In addition, Res functioned as an antioxidant that strengthened the activities of antioxidant enzymes and thus eliminated reactive oxygen species production induced by iron deficiency stress.

CONCLUSION

Resveratrol improves the iron deficiency adaptation of M. baccata seedlings mainly by regulating iron absorption.

Metabolomic Profiling of Dongxiang Wild Rice Under Salinity Demonstrates the Significant Role of Amino Acids in Rice Salt Stress

Dongxiang common wild rice is a precious rice germplasm resource for the study and improvement of salt tolerance in rice.The metabolism profile of Dongxiang wild rice (DXWR) under salinity was determined by high performance liquid chromatography-mass spectrometry (HPLC-MS) to find differential metabolites and screen potential biomarkers for salt-tolerant rice varieties. A global untargeted metabolism analysis showed 4,878 metabolites accumulated in seedlings of Dongxiang wild rice. Principal component analysis (PCA) and orthogonal partial least squares-discriminant analysis (OPLS-DA) results provided a clear metabolism discrimination between DXWR under control and DXWR under salinity. A total of 90 metabolites were significantly changed (49 upregulated and 41 downregulated) under salinity, of which the largest increase was in DL-2-Aminoadipic acid (27.08-fold) and the largest decrease was in L-Carnitine (0.014-fold). Amino acids and nuclear glycosides were mainly upregulated, while carbohydrates and organic acids were mainly downregulated in the salt-treated group. Among the top 10 upregulated metabolites, five kinds of differential metabolites were amino acids. According to the survival rates of the seedlings under salinity, we selected three backcross inbred lines of DXWR with survival rates above 80% as salt-tolerant progenies (pro-DS) and three backcross inbred lines with survival rates below 10% as non-salt-tolerant progenies (pro-NDS) for an amino acid change analysis. This analysis found that the change in L-Asparagine (2.59-fold) was the biggest between pro-DS and pro-NDS under salinity, revealing that the contents of L-Asparagine may be one of the indices we can use to evaluate the salt tolerance of rice varieties.

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.

Resveratrol Alleviates the KCl Salinity Stress of Malus hupehensis Rhed

Applying large amounts of potash fertilizer in apple orchards for high apple quality and yield aggravates KCl stress. As a phytoalexin, resveratrol (Res) participates in plant resistance to biotic stress. However, its role in relation to KCl stress has never been reported. Herein we investigated the role of Res in KCl stress response of Malus hupehensis Rehd., a widely used apple rootstock in China which is sensitive to KCl stress. KCl-stressed apple seedlings showed significant wilting phenotype and decline in photosynthetic rate, and the application of 100 μmol Res alleviated KCl stress and maintained photosynthetic capacity. Exogenous Res can strengthen the activities of peroxidase and catalase, thus eliminating reactive oxygen species production induced by KCl stress. Moreover, exogenous Res can decrease the electrolyte leakage by accumulating proline for osmotic balance under KCl stress. Furthermore, exogenous Res application can affect K+/Na+ homeostasis in cytoplasm by enhancing K+ efflux outside the cells, inhibiting Na+ efflux and K+ absorption, and compartmentalizing K+ into vacuoles through regulating the expression of K+ and Na+ transporter genes. These findings provide a theoretical basis for the application of exogenous Res to relieve the KCl stress of apples.

Regulation of Plant Responses to Salt Stress

Salt stress is a major environmental stress that affects plant growth and development. Plants are sessile and thus have to develop suitable mechanisms to adapt to high-salt environments. Salt stress increases the intracellular osmotic pressure and can cause the accumulation of sodium to toxic levels. Thus, in response to salt stress signals, plants adapt via various mechanisms, including regulating ion homeostasis, activating the osmotic stress pathway, mediating plant hormone signaling, and regulating cytoskeleton dynamics and the cell wall composition. Unraveling the mechanisms underlying these physiological and biochemical responses to salt stress could provide valuable strategies to improve agricultural crop yields. In this review, we summarize recent developments in our understanding of the regulation of plant salt stress.

Exogenous application of brassinosteroids regulates tobacco leaf size and expansion via modulation of endogenous hormones content and gene expression

Brassinosteroids (BR) play diverse roles in the regulation of plant growth and development. BR promotes plant growth by triggering cell division and expansion. However, the effect of exogenous BR application on the leaf size and expansion of tobacco is unknown. Tobacco seedlings are treated with different concentrations of exogenous 2,4-epibrassinolide (EBL) [control (CK, 0 mol L^-1), T1 (0.5 × 10^-7 mol L^-1), and T2 (0.5 × 10^-4 mol L^-1)]. The results show that T1 has 17.29% and T2 has 25.99% more leaf area than control. The epidermal cell area is increased by 24.40% and 17.13% while the number of epidermal cells is 7.06% and 21.06% higher in T1 and T2, respectively, relative to control. So the exogenous EBL application improves the leaf area by increasing cell numbers and cell area. The endogenous BR (7.5 times and 68.4 times), auxin (IAA) (4.03% and 25.29%), and gibberellin (GA₃) contents (84.42% and 91.76%) are higher in T1 and T2, respectively, in comparison with control. Additionally, NtBRI1, NtBIN2, and NtBES1 are upregulated showing that the brassinosteroid signaling pathway is activated. Furthermore, the expression of the key biosynthesis-related genes of BR (NtDWF4), IAA (NtYUCCA6), and GA₃ (NtGA3ox-2) are all upregulated under EBL application. Finally, the exogenous EBL application also upregulated the expression of cell growth-related genes (NtCYCD3;1, NtARGOS, NtGRF5, NtGRF8, and NtXTH). The results reveal that the EBL application increases the leaf size and expansion by promoting the cell expansion and division through higher BR, IAA, and GA₃ contents along with the upregulation of cell growth-related genes. The results of the study provide a scientific basis for the effect of EBL on tobacco leaf growth at morphological, anatomical, biochemical, and molecular levels.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s12298-021-00971-x.

Evaluating and Screening of Agro-Physiological Indices for Salinity Stress Tolerance in Wheat at the Seedling Stage

Soil salinity is a worldwide issue that affects wheat production. A comprehensive understanding of salt-tolerance mechanisms and the selection of reliable screening indices are crucial for breeding salt-tolerant wheat cultivars. In this study, 30 wheat genotypes (obtained from a rapid selection of 96 original varieties) were chosen to investigate the existing screening methods and clarify the salinity tolerance mechanisms in wheat. Ten-day-old seedlings were treated with 150 mM NaCl. Eighteen agronomic and physiological parameters were measured. The results indicated that the effects of salinity on the agronomic and physiological traits were significant. Salinity stress significantly decreased K⁺ content and K⁺/Na⁺ ratio in the whole plant, while the leaf K⁺/Na⁺ ratio was the strongest determinant of salinity tolerance and had a significantly positive correlation with salt tolerance. In contrast, salinity stress significantly increased Na⁺ concentration and relative gene expression (TaHKT1;5, TaSOS1, and TaAKT1-like). The Na⁺ transporter gene (TaHKT1;5) showed a significantly greater increase in expression than the K⁺ transporter gene (TaAKT1-like). We concluded that Na⁺ exclusion rather than K⁺ retention contributed to an optimal leaf K⁺/Na⁺ ratio. Furthermore, the present exploration revealed that, under salt stress, tolerant accessions had higher shoot water content, shoot dry weight and lower stomatal density, leaf sap osmolality, and a significantly negative correlation was observed between salt tolerance and stomatal density. This indicated that changes in stomata density may represent a fundamental mechanism by which a plant may optimize water productivity and maintain growth under saline conditions. Taken together, the leaf K⁺/Na⁺ ratio and stomatal density can be used as reliable screening indices for salt tolerance in wheat at the seedling stage.

Exogenous Strigolactones alleviate KCl stress by regulating photosynthesis, ROS migration and ion transport in Malus hupehensis Rehd

AIMS

In recent years, the application of large amounts of potash fertilizer in apple orchards leads to worsening KCl stress. Strigolactone (SL), as a novel phytohormone, reportedly participates in plant tolerance to NaCl and drought stresses. However, the underlying mechanism and the effects of exogenous SL on the KCl stress of apple seedlings remain unclear.

METHODS

We sprayed different concentrations of exogenous SL on Malus hupehensis Rehd. under KCl stress and measured the physiological indexes like, photosynthetic parameter, content of ROS, osmolytes and mineral element. In addition, the expressions of KCl-responding genes and SL-signaling genes were also detected and analyzed.

RESULTS

Application of exogenous SL protected the chlorophyll and maintained the photosynthetic rate of apple seedlings under KCl stress. Exogenous SL strengthened the enzyme activities of peroxidase and catalase, thereby eliminating reactive oxygen species production induced by KCl stress, promoting the accumulation of proline, and maintaining osmotic balance. Exogenous SL expelled K⁺ outside of the cytoplasm and compartmentalized K⁺ into the vacuole, increased the contents of Na⁺, Mg²⁺, Fe²⁺, and Mn²⁺ in the cytoplasm to maintain the ion homeostasis under KCl stress.

CONCLUSIONS

Exogenous SL can regulate photosynthesis, ROS migration and ion transport in apple seedlings to alleviate KCl stress.

Phylogenetic Diversity and Physiological Roles of Plant Monovalent Cation/H+ Antiporters

The processes of plant nutrition, stress tolerance, plant growth, and development are strongly dependent on transport of mineral nutrients across cellular membranes. Plant membrane transporters are key components of these processes. Among various membrane transport proteins, the monovalent cation proton antiporter (CPA) superfamily mediates a broad range of physiological and developmental processes such as ion and pH homeostasis, development of reproductive organs, chloroplast operation, and plant adaptation to drought and salt stresses. CPA family includes plasma membrane-bound Na⁺/H⁺ exchanger (NhaP) and intracellular Na⁺/H⁺ exchanger NHE (NHX), K⁺ efflux antiporter (KEA), and cation/H⁺ exchanger (CHX) family proteins. In this review, we have completed the phylogenetic inventory of CPA transporters and undertaken a comprehensive evolutionary analysis of their development. Compared with previous studies, we have significantly extended the range of plant species, including green and red algae and Acrogymnospermae into phylogenetic analysis. Our data suggest that the multiplication and complexation of CPA isoforms during evolution is related to land colonisation by higher plants and associated with an increase of different tissue types and development of reproductive organs. The new data extended the number of clades for all groups of CPAs, including those for NhaP/SOS, NHE/NHX, KEA, and CHX. We also critically evaluate the latest findings on the biological role, physiological functions and regulation of CPA transporters in relation to their structure and phylogenetic position. In addition, the role of CPA members in plant tolerance to various abiotic stresses is summarized, and the future priority directions for CPA studies in plants are discussed.