Genome-Wide Identification and Characterization of the HAK Gene Family in Quinoa (Chenopodium quinoa Willd.) and Their Expression Profiles under Saline and Alkaline Conditions

The high-affinity K+ transporter (HAK) family, the most prominent potassium transporter family in plants, which involves K+ transport, plays crucial roles in plant responses to abiotic stresses. However, the HAK gene family remains to be characterized in quinoa (Chenopodium quinoa Willd.). We explored HAKs in quinoa, identifying 30 members (CqHAK1-CqHAK30) in four clusters phylogenetically. Uneven distribution was observed across 18 chromosomes. Furthermore, we investigated the proteins' evolutionary relationships, physicochemical properties, conserved domains and motifs, gene structure, and cis-regulatory elements of the CqHAKs family members. Transcription data analysis showed that CqHAKs have diverse expression patterns among different tissues and in response to abiotic stresses, including drought, heat, low phosphorus, and salt. The expressional changes of CqHAKs in roots were more sensitive in response to abiotic stress than that in shoot apices. Quantitative RT-PCR analysis revealed that under high saline condition, CqHAK1, CqHAK13, CqHAK19, and CqHAK20 were dramatically induced in leaves; under alkaline condition, CqHAK1, CqHAK13, CqHAK19, and CqHAK20 were dramatically induced in leaves, and CqHAK6, CqHAK9, CqHAK13, CqHAK23, and CqHAK29 were significantly induced in roots. Our results establish a foundation for further investigation of the functions of HAKs in quinoa. It is the first study to identify the HAK gene family in quinoa, which provides potential targets for further functional study and contributes to improving the salt and alkali tolerance in quinoa.

Effects of microplastics on seed germination and seedling physiological characteristics of Spinacia oleracea under alkali stress

Microplastics, a type of new environmental pollutant, have received much attention for their negative effects on organisms and environment. We examined the effects of microplastics on seed germination and seedling physiological characteristics of spinach (Spinacia oleracea) under alkali stress, taking polystyrene microspheres with a diameter of 100 nm (200, 400, 800, 1600 mg·L-1) as the microplastic treatment, and mixed NaHCO3 and Na2CO3 as alkaline salt solution (5, 10, 20, 40 mmol·L-1) according to the molar ratio of 1:1. The results showed that the presence of MPs (≥400 mg·L-1) inhibited seed germination, and that the length of roots and shoots increased at low while decreased at high concentration of MPs. Different concentrations of alkali alone could inhibit seed germination, root and bud elongation. With the increases of MPs concentration, SOD activity of spinach seedlings gradually decreased, while POD activity firstly increased and then decreased, and chlorophyll content increased at low concentration (200 mg·L-1) and decreased significantly at medium and high concentration (≥400 mg·L-1). Different alkali stresses reduced chlorophyll content of spinach seedlings, and the effects on SOD and POD were 'promotion at low concentration and inhibition at high'. In the treatments of microplastics (200, 800 mg·L-1) and alkali (5, 20 mmol·L-1) combined exposure, germination of spinach seeds was inhibited, and chlorophyll content decreased. The activities of SOD and POD in spinach seedlings were reduced under the combined exposure except the treatment of 200 mg·L-1 MPs and 5 mmol·L-1 alkali. Compared to the alkali stress, the combination of low concentration of MPs (200 mg·L-1) and alkali could improve germination rate, germination index, germination vigor and vigor index of seeds, and significantly promoted the elongation of roots and shoots, while the addition of high concentration of MPs (800 mg·L-1) reduced the germination rate, germination index, germination vigor and vigor index of seeds and inhibited the growth of roots and buds. The different concentrations of combined exposure inhibited the activities of SOD and POD and decreased the content of chlorophyll in spinach seedlings.

Exogenous abscisic acid and sodium nitroprusside regulate flavonoid biosynthesis and photosynthesis of Nitraria tangutorum Bobr in alkali stress

Abscisic acid (ABA) and nitric oxide (NO) are involved in mediating abiotic stress-induced plant physiological responses. Nitraria tangutorum Bobr is a typical salinized desert plant growing in an arid environment. In this study, we investigated the effects of ABA and NO on N.tangutorum seedlings under alkaline stress. Alkali stress treatment caused cell membrane damage, increased electrolyte leakage, and induced higher production of reactive oxygen species (ROS), which caused growth inhibition and oxidative stress in N.tangutorum seedlings. Exogenous application of ABA (15μm) and Sodium nitroprusside (50μm) significantly increased the plant height, fresh weight, relative water content, and degree of succulency in N.tangutorum seedlings under alkali stress. Meanwhile, the contents of ABA and NO in plant leaves were significantly increased. ABA and SNP can promote stomatal closure, decrease the water loss rate, increase leaf surface temperature and the contents of osmotic regulator proline, soluble protein, and betaine under alkali stress. Meanwhile, SNP more significantly promoted the accumulation of chlorophyll a/b and carotenoids, increased quantum yield of photosystem II (φPSII) and electron transport rate (ETRII) than ABA, and decreased photochemical quenching (qP), which improved photosynthetic efficiency and accelerated the accumulation of soluble sugar, glucose, fructose, sucrose, starch, and total sugar. However, compared with exogenous application of SNP in the alkaline stress, ABA significantly promoted the transcription of NtFLS/NtF3H/NtF3H/NtANR genes and the accumulation of naringin, quercetin, isorhamnetin, kaempferol, and catechin in the synthesis pathway of flavonoid metabolites, and isorhamnetin content was the highest. These results indicate that both ABA and SNP can reduce the growth inhibition and physiological damage caused by alkali stress. Among them, SNP has a better effect on the improvement of photosynthetic efficiency and the regulation of carbohydrate accumulation than ABA, while ABA has a more significant effect on the regulation of flavonoid and anthocyanin secondary metabolite accumulation. Exogenous application of ABA and SNP also improved the antioxidant capacity and the ability to maintain Na⁺/K⁺ balance of N. tangutorum seedlings under alkali stress. These results demonstrate the beneficial effects of ABA and NO as stress hormones and signaling molecules that positively regulate the defensive response of N. tangutorum to alkaline stress.

Endophytic Fungi Regulate HbNHX1 Expression and Ion Balance in Hordeum bogdanii under Alkaline Stress

Plants cope with abiotic stress in several ways, including by collaborating with microorganisms. Epichloë, an endophytic fungus, has been shown to improve plant tolerance to extreme external environments. Hordeum bogdanii is a known salt-tolerant plant with the potential to improve alkaline lands. NHX1 plays a key role in the transport of ions in the cell and is overexpressed in plants with increased salt tolerance. However, the expression levels of HbNHX1 in Epichloë endophytic fungal symbionts in H. bogdanii have not been elucidated. We used Hordeum bogdanii (E+) with the endophytic fungi Epichloë bromicola and H. bogdanii (E-) without the endophytic fungi and compared the differences in the ion content and HbNHX1 expression between the shoots and roots of E+ and E- plants under alkaline stress. The absorption capacity of both K⁺ and Na⁺ of H. bogdanii with endophytic fungi was higher than that without endophytic fungi. In the absence of alkaline stress, endophytic fungi significantly reduced the Cl^- content in the host H. bogdanii. Alkaline stress reduced SO₄^2- content in H. bogdanii; however, compared with E-, endophytic fungi increased the content of SO₄^2- in E+ plants. With an increase in the alkaline concentration, the expression of HbNHX1 in the roots of H. bogdanii with endophytic fungus exhibited an upward trend, whereas the expression in the shoots exhibited a downward trend first and then an upward trend. Under 100 mmol·L^-1 mixed alkaline stress, the expression of HbNHX1 in E+ was significantly higher than that in E-, indicating that endophytic fungi could increase the Na⁺ region in vacuoles. The external environment affects the regulation of endophytic fungi in H. bogdanii and that endophytic fungi can play a key role in soil salinization. Therefore, the findings of this study will provide technical support and a theoretical basis for better utilization of endophytic fungi from H. bogdanii in saline land improvement.

Effects of Alkali Stress on the Growth and Menaquinone-7 Metabolism of Bacillus subtilis natto

Menaquinone-7 (MK-7) is an important vitamin K₂, synthesized from the menaquinone parent ring and seven isoprene side chains. Presently, the synthesis of MK-7 stimulated by environmental stress primarily focuses on oxygen stress, while the effect of alkali stress is rarely studied. Therefore, this study researched the effects of alkali stress on the fermentation performance and gene expression of Bacillus subtilis natto. The organism’s growth characteristics, biomass, sporogenesis, MK-7 biosynthesis, and gene expression were analyzed. After a pH 8.5 stress adaptation treatment for 0.5 h and subsequent fermentation at pH 8.5, which promoted the growth of the strain and inhibited the spore formation rate. In addition, biomass was significantly increased (P < 0.05). The conversion rate of glycerol to MK-7 was 1.68 times higher than that of the control group, and the yield of MK-7 increased to 2.10 times. Transcriptomic analysis showed that the MK-7 high-yielding strain had enhanced carbon source utilization, increased glycerol and pyruvate metabolism, enhanced the Embden-Meyerhof pathway (EMP), tricarboxylic acid (TCA) circulation flux, and terpenoid biosynthesis pathway, and promoted the accumulation of acetyl-CoA, the side-chain precursor of isoprene. At the same time, the up-regulation of transketolase increased the metabolic flux of the pentose phosphate (HMP) pathway, which was conducive to the accumulation of D-erythrose 4-phosphate, the precursor of the menadione parent ring. This study’s results contribute to a better understanding of the effects of environmental stress on MK-7 fermentation by Bacillus subtilis natto and the molecular regulatory mechanism of MK-7 biosynthesis.

Genome-Wide Investigation of the Cysteine Synthase Gene Family Shows That Overexpression of CSase Confers Alkali Tolerance to Alfalfa (Medicago sativa L.)

Alfalfa is widely grown worldwide as a perennial high-quality legume forage and as a good ecological landcover. The cysteine synthase (CSase) gene family is actively involved in plant growth and development and abiotic stress resistance but has not been systematically investigated in alfalfa. We identified 39 MsCSase genes on 4 chromosomes of the alfalfa genome. Phylogenetic analysis demonstrated that these genes were clustered into six subfamilies, and members of the same subfamily had similar physicochemical properties and sequence structures. Overexpression of the CSase gene in alfalfa increased alkali tolerance. Compared with control plants, the overexpression lines presented higher proline, soluble sugars, and cysteine and reduced glutathione contents and superoxide dismutase and peroxidase activities as well as lower hydrogen peroxide and superoxide anion contents after alkali stress. The relative expression of γ-glutamyl cysteine synthetase gene (a downstream gene of CSase) in the overexpression lines was much higher than that in the control line. The CSase gene enhanced alkalinity tolerance by regulating osmoregulatory substances and improving antioxidant capacity. These results provide a reference for studying the CSase gene family in alfalfa and expanding the alkali tolerance gene resources of forage plants.

Proteomic Responses to Alkali Stress in Oats and the Alleviatory Effects of Exogenous Spermine Application

Alkali stress limits plant growth and yield more strongly than salt stress and can lead to the appearance of yellow leaves; however, the reasons remain unclear. In this study, we found that (1) the down-regulation of coproporphyrinogen III oxidase, protoporphyrinogen oxidase, and Pheophorbide a oxygenase in oats under alkali stress contributes to the appearance of yellow leaves (as assessed by proteome and western blot analyses). (2) Some oat proteins that are involved in the antioxidant system, root growth, and jasmonic acid (JA) and indole-3-acetic acid (IAA) synthesis are up-regulated in response to alkalinity and help increase alkali tolerance. (3) We added exogenous spermine to oat plants to improve their alkali tolerance, which resulted in higher chlorophyll contents and plant dry weights than in plants subjected to alkaline stress alone. This was due to up-regulation of chitinase and proteins related to chloroplast structure, root growth, and the antioxidant system. Spermine addition increased sucrose utilization efficiency, and promoted carbohydrate export from leaves to roots to increase energy storage in roots. Spermine addition also increased the IAA and JA contents required for root growth.

Exogenous Abscisic Acid Alleviates Harmful Effect of Salt and Alkali Stresses on Wheat Seedlings

Exogenous hormones play an important role in plant growth regulation and stress tolerance. However, little is known about the effect of exogenous abscisic acid (ABA) on wheat seedlings under salt and alkali stresses. Here, a pot experiment of saline and alkaline stresses (0 and 100 mmol/L) in which ABA water solution (0, 50 and 100 μmol/L) was sprayed on wheat seedlings was conducted to study the alleviative effectiveness of ABA on salt and alkali stresses. After spraying ABA (50 μmol·L-1), shoot biomass increased 19.0% and 26.7%, respectively. The Na+ content in shoots reduced from 15-fold and 61.5-fold to 10-fold and 37.3-fold in salt and alkali stresses, compared to controls. In addition, proline and organic acid synthesis in shoots also reduced significantly, but the soluble sugar content increased under alkali stress. A high concentration of ABA (100 μmol·L-1) had no significant effects on biomass and ion content in wheat seedlings under both stresses. In conclusion, foliar application of ABA with moderate concentration could effectively accelerate shoot growth of salt-induced wheat seedlings by adjusting the levels of ions and organic solutes.

Metabolomics Analysis Reveals the Alkali Tolerance Mechanism in Puccinellia tenuiflora Plants Inoculated with Arbuscular Mycorrhizal Fungi

Soil alkalization is a major environmental threat that affects plant distribution and yield in northeastern China. Puccinellia tenuiflora is an alkali-tolerant grass species that is used for salt-alkali grassland restoration. However, little is known about the molecular mechanisms by which arbuscular mycorrhizal fungi (AMF) enhance P. tenuiflora responses to alkali stress. Here, metabolite profiling in P. tenuiflora seedlings with or without arbuscular mycorrhizal fungi (AMF) under alkali stress was conducted using liquid chromatography combined with time-of-flight mass spectrometry (LC/TOF-MS). The results showed that AMF colonization increased seedling biomass under alkali stress. In addition, principal component analysis (PCA) and orthogonal projections to latent structures discriminant analysis (OPLS-DA) demonstrated that non-AM and AM seedlings showed different responses under alkali stress. A heat map analysis showed that the levels of 88 metabolites were significantly changed in non-AM seedlings, but those of only 31 metabolites were significantly changed in AM seedlings. Moreover, the levels of a total of 62 metabolites were significantly changed in P. tenuiflora seedlings after AMF inoculation. The results suggested that AMF inoculation significantly increased amino acid, organic acid, flavonoid and sterol contents to improve osmotic adjustment and maintain cell membrane stability under alkali stress. P. tenuiflora seedlings after AMF inoculation produced more plant hormones (salicylic acid and abscisic acid) than the non-AM seedlings, probably to enhance the antioxidant system and facilitate ion balance under stress conditions. In conclusion, these findings provide new insights into the metabolic mechanisms of P. tenuiflora seedlings with arbuscular mycorrhizal fungi under alkali conditions and clarify the role of AM in the molecular regulation of this species under alkali stress.

Exogenous Dopamine Application Promotes Alkali Tolerance of Apple Seedlings

Arid and semiarid apple producing areas suffer from severe alkalinity of soil, which strongly affects the yield and quality of apples. Dopamine (DA) is involved in metabolic activities in response to abiotic stress in plants. To detect the effects of exogenous DA application on the adaption of apple (Malus hupehensis) seedlings to alkali stress and as a protection from oxidative stress, 0.1 mM DA was identified as the most suitable concentration by hydroponic culture. Further experimentation showed that the growth and photosynthesis of apple seedlings were significantly inhibited under alkali stress, and more reactive oxygen species accumulated, compared with control. However, exogenous DA application suppressed the loss of the plant height, root length, chlorophyll levels, and photosynthetic capacity of apple seedlings that were caused by alkali stress. In the leaves of alkali stressed seedlings, the catalase, superoxide dismutase, and peroxidase activities were lower and hydrogen peroxide and malondialdehyde levels were higher than in the untreated plants. The presence of DA significantly alleviated such effects of alkali stress. In addition, exogenous DA application increased the antioxidant capacity of apple seedlings under alkali stress by increasing the level of chlorogenic acid. These results are significant for improving the alkali tolerance of apple in apple-producing areas with alkalized soil.

Genome-Wide Identification of microRNAs in Response to Salt/Alkali Stress in Medicago truncatula through High-Throughput Sequencing

Saline-alkaline stress is a universal abiotic stress that adversely affects plant growth and productivity. Saline-alkaline conditions results in plant abnormal transcriptome expression finally manifesting as defective phenotypes. Considerable research has revealed the active role of microRNA in various stress conditions. This study was aimed to identify novel miRNAs and the miRNA expression patterns in the leguminous model plant R108 (Medicago truncatula). The miRNA contained in the total RNA extracted from Medicago truncatula seedlings (72 h) that had been treated with solutions mimicking saline and alkaline soils was subjected to miRNA deep sequencing. The Illumina HiSeq sequencing platform was used to analyze nine small RNA libraries of three treatment groups: distilled water, 20 mM NaCl + Na₂SO₄ and 5 mM Na₂CO₃ + NaHCO₃. Sequencing revealed that 876 miRNAs including 664 known miRNAs and 212 potential novel miRNAs were present in all the libraries. The miR159 family, miR156 family, miR2086-3p, miR396, miR166, miR319, miR167, miR5213-5p, miR1510 and miR2643 were among the most expressed miRNAs in all libraries. The results of miRNAs expression under treatments were validated by reverse-transcription quantitative PCR (RT-qPCR). Target gene prediction through computational analysis and pathway annotation analysis revealed that the primary pathways affected by stress were related to plant development, including metabolic processes, single-organism processes and response to the stimulus. Our results provide valuable information towards elucidating the molecular mechanisms of salt/alkali tolerance in Medicago truncatula and provide insight into the putative role of miRNAs in plant stress resistance.

Genome-Wide Analysis of Glycine soja Response Regulator GsRR Genes Under Alkali and Salt Stresses

Soil salt-alkalization is a dramatic challenging factor for plant growth. Wild soybean (Glycine soja) exhibits a favorable trait of superior tolerance to salt-alkali stress, and recent discoveries show that response regulator family genes are involved in diverse abiotic stresses. Genomic and transcriptomic analyses of all response regulator genes in wild soybean will provide insight into their function in plant stress response. In this study, we identified and characterized a total of 56 Glycine soja response regulator (GsRR) genes. Phylogenetic analysis suggested that GsRR genes could be classified into five subclasses (A1, A2, B1, B2, and C). We further investigated the chromosome locations, gene duplications and conserved domains of the GsRRs. Furthermore, the clustering analysis of GsRR transcript profiles revealed five different expression patterns under alkali stress. The A1 and A2 subclasses display significantly higher transcriptional levels than the B subclass. In addition, quantitative real-time PCR results verified that the GsRR genes were also significantly influenced by salt stress. Notably, GsRR2a in the A1 subclass showed opposite expression patterns under salt stress comparing with alkali stress. Moreover, overexpression of GsRR2a in Arabidopsis significantly improved the tolerance to alkali stress, but not salt stress. These results suggest the important roles of GsRR genes in response to salt and alkaline stresses, and also provide valuable clues for further functional characterization of GsRR family genes.

Metabolomic Analysis of Alfalfa (Medicago sativa L.) Root-Symbiotic Rhizobia Responses under Alkali Stress

Alkaline salts (e.g., NaHCO₃ and Na₂CO₃) causes more severe morphological and physiological damage to plants than neutral salts (e.g., NaCl and Na₂SO₄) due to differences in pH. The mechanism by which plants respond to alkali stress is not fully understood, especially in plants having symbotic relationships such as alfalfa (Medicago sativa L.). Therefore, a study was designed to evaluate the metabolic response of the root-nodule symbiosis in alfalfa under alkali stress using comparative metabolomics. Rhizobium-nodulized (RI group) and non-nodulized (NI group) alfalfa roots were treated with 200 mmol/L NaHCO₃ and, roots samples were analyzed for malondialdehydyde (MDA), proline, glutathione (GSH), superoxide dismutase (SOD), and peroxidase (POD) content. Additionally, metabolite profiling was conducted using gas chromatography combined with time-of-flight mass spectrometry (GC/TOF-MS). Phenotypically, the RI alfalfa exhibited a greater resistance to alkali stress than the NI plants examined. Physiological analysis and metabolic profiling revealed that RI plants accumulated more antioxidants (SOD, POD, GSH), osmolytes (sugar, glycols, proline), organic acids (succinic acid, fumaric acid, and alpha-ketoglutaric acid), and metabolites that are involved in nitrogen fixation. Our pairwise metabolomics comparisons revealed that RI alfalfa plants exhibited a distinct metabolic profile associated with alkali putative tolerance relative to NI alfalfa plants. Data provide new information about the relationship between non-nodulized, rhizobium-nodulized alfalfa and alkali resistance.

Comparison of Ionomic and Metabolites Response under Alkali Stress in Old and Young Leaves of Cotton (Gossypium hirsutum L.) Seedlings

Soil salinization is an important agriculture-related environmental problem. Alkali stress and salt stress strongly influence the metabolic balance in plants. Salt and alkali stresses exert varied effects on old and young tissues, which display different adaptive strategies. In this study, we used cotton (Gossypium hirsutum L.) plants as experimental material to investigate whether alkali stress induces ionic and metabolism changes in old and young leaves of cotton plants exposed to alkali stress. Results showed that alkali stress exerted a considerably stronger growth inhibition on old leaves than on young leaves. Under alkali stress, young leaves can maintain low Na and high K contents and retain relatively stable tricarboxylic acid cycle, resulting in greater accumulation of photosynthetic metabolites. In terms of metabolic response, the young and old leaves clearly displayed different mechanisms of osmotic regulation. The amounts of inositol and mannose significantly increased in both old and young leaves of cotton exposed to alkali stress, and the extent of increase was higher in young leaves than in old leaves. In old leaves, synthesis of amino acids, such as GABA, valine, and serine, was dramatically enhanced, and this phenomenon is favorable for osmotic adjustment and membrane stability. Organs at different developmental stages possibly display different mechanisms of metabolic regulation under stress condition. Thus, we propose that future investigations on alkali stress should use more organs obtained at different developmental stages.

Comparative metabolic responses and adaptive strategies of wheat (Triticum aestivum) to salt and alkali stress

BACKGROUND

It is well known that salinization (high-pH) has been considered as a major environmental threat to agricultural systems. The aim of this study was to investigate the differences between salt stress and alkali stress in metabolic profiles and nutrient accumulation of wheat; these parameters were also evaluated to determine the physiological adaptive mechanisms by which wheat tolerates alkali stress.

RESULTS

The harmful effect of alkali stress on the growth and photosynthesis of wheat were stronger than those of salt stress. High-pH of alkali stress induced the most of phosphate and metal ions to precipitate; as a result, the availability of nutrients significantly declined. Under alkali stress, Ca sharply increased in roots, however, it decreased under salt stress. In addition, we detected the 75 metabolites that were different among the treatments according to GC-MS analysis, including organic acids, amino acids, sugars/polyols and others. The metabolic data showed salt stress and alkali stress caused different metabolic shifts; alkali stress has a stronger injurious effect on the distribution and accumulation of metabolites than salt stress. These outcomes correspond to specific detrimental effects of a highly pH environment.

CONCLUSIONS

Ca had a significant positive correlation with alkali tolerates, and increasing Ca concentration can immediately trigger SOS Na exclusion system and reduce the Na injury. Salt stress caused metabolic shifts toward gluconeogenesis with increased sugars to avoid osmotic stress; energy in roots and active synthesis in leaves were needed by wheat to develop salt tolerance. Alkali stress (at high pH) significantly inhibited photosynthetic rate; thus, sugar production was reduced, N metabolism was limited, amino acid production was reduced, and glycolysis was inhibited.

The regulators of yeast PHO system participate in the transcriptional regulation of G1 cyclin under alkaline stress conditions

The yeast Pho85 kinase oversees whether environmental conditions are favourable for cell growth and enables yeast cells to express only genes that are appropriate for the conditions. Alkaline stress perturbs transport of molecules across the plasma membrane that is vital for cell survival. Progression through the cell cycle is halted until the cells can adapt to the stress conditions. I found that Pho85 is required for CLN2 expression and that overproduction of the transcription factors Pho4, Rim101 and Crz1, all targets of Pho85, inhibited CLN2 expression. CLN2 expression in the absence of Pho85 could be recovered only when all the three transcription factors were deleted. Whi5, a functional homologue of the mammalian Rb protein, represses CLN2 expression and is inactivated when phosphorylated by either of the CDK-cyclin complexes, Cdc28-Cln3 or Pho85-Pcl9. Under alkaline conditions, the absence of Whi5 caused an increase in CLN2 expression but failed to do so when Pho85 was also absent, or when Pho4 was overproduced. The expression level of CLN2 in a Δpho85 Δpho4 Δrim101 Δcrz1 quadruple mutant was stimulated when the Whi5 activity was repressed by overproduction of Pho85-Pcl9. These results indicate that Whi5 is also under control of alkaline stress. The inhibitory function of Whi5 on CLN2 is dependent on Rpd3 HDAC, and the absence of Rpd3 could also suppress the inhibitory effect of Pho4 overproduction. Based on these findings, a model is presented in which Pho85 and Pho4 functions in CLN2 regulation under alkaline conditions.

Overexpression of S-adenosyl-L-methionine synthetase increased tomato tolerance to alkali stress through polyamine metabolism

S-adenosyl-L-methionine (SAM) synthetase is the key enzyme involved in the biosynthesis of SAM, which serves as a common precursor for polyamines (PAs) and ethylene. A SAM synthetase cDNA (SlSAMS1) was introduced into the tomato genome using the Agrobacterium tumefaciens transformation method. Transgenic plants overexpressing SlSAMS1 exhibited a significant increase in tolerance to alkali stress and maintained nutrient balance, higher photosynthetic capacity and lower oxidative stress compared with WT lines. Both in vivo and in vitro experiments indicated that the function of SlSAMS1 mainly depended on the accumulation of Spd and Spm in the transgenic lines. A grafting experiment showed that rootstocks from SlSAMS1-overexpressing plants provided a stronger root system, increased PAs accumulation, essential elements absorption, and decreased Na(+) absorption in the scions under alkali stress. As a result, fruit set and yield were significantly enhanced. To our knowledge, this is the first report to provide evidence that SlSAMS1 positively regulates tomato tolerance to alkali stress and plays a major role in modulating polyamine metabolism, resulting in maintainability of nutrient and ROS balance.