Salt Tolerance In Maize Research Articles

Maize transcription factor ZmEREB20 enhanced salt tolerance in transgenic Arabidopsis

Soil salinity severely limits agricultural crop production worldwide. As one of the biggest plant specific transcription factor families, AP2/ERF members have been extensively studied to regulate plant growth, development and stress responses. However, the role of AP2/ERF family in maize salt tolerance remains largely unknown. In this study, we identified a maize AP2-ERF family member ZmEREB20 as a positive salinity responsive gene. Overexpression of ZmEREB20in Arabidopsis enhanced ABA sensitivity and resulted in delayed seed germination under salt stress through regulating ABA and GA related genes. ZmEREB20 overexpression lines also showed higher survival rates with elevated ROS scavenging toward high salinity. Furthermore, root hair growth inhibition by salt stress was markedly rescued in ZmEREB20 overexpression lines. Auxin transport inhibitor TIBA drastically enhanced root hair growth in ZmEREB20 overexpression Arabidopsis under salt stress, together with the increased expression of auxin-related genes, ion transporter genes and root hair growth genes by RNA-seq analysis. ZmEREB20 positively regulated salt tolerance through the molecular mechanism associated with hormone signaling, ROS scavenging and root hair plasticity, proving the potential target for crop breeding to improve salt resistance.

H2O2 priming promotes salt tolerance in maize by protecting chloroplasts ultrastructure and primary metabolites modulation

Hydrogen peroxide priming has emerged as a powerful strategy to trigger multiple responses involved in plant acclimation that reinforce tolerance to abiotic stresses, including salt stress. Thus, this study aimed to investigate the impact of foliar H2O2 priming on the physiological, biochemical, and ultrastructural traits related to photosynthesis of salt-stressed plants. Besides, we provided comparative leaf metabolomic profiles of Zea mays plants under such conditions. For this, H2O or H2O2 pretreated plants were grown under saline conditions for 12-days. Salinity drastically affected photosynthetic parameters and structural chloroplasts integrity, also increased reactive oxygen species contents promoting disturbance in the plant metabolism when compared to non-saline conditions. Our results suggest that H2O2-pretreated plants improved photosynthetic performance avoiding salinity-induced energy excess and ultrastructural damage by preserving stacking thylakoids. It displayed modulation of some metabolites, as arabitol, glucose, asparagine, and tyrosine, which may contribute to the maintenance of osmotic balance and reduced oxidative stress. Hence, our study brings new insights into an understanding of plant acclimation to salinity by H2O2 priming based on photosynthesis maintenance and metabolite modulation.

NaCl effects in Zea mays L. x Tripsacum dactyloides (L.) L. hybrid calli and plants

High salt concentrations in soils negatively affect maize growth. Techniques such as remote hybridization and in vitro selection have been extensively used to accelerate breeding processes. In order to determine the usefulness of Tripsacum to improve salt tolerance in maize, the effects of NaCl, in vitro and in vivo , were evaluated in an intergeneric hybrid (MT) obtained from crossing Zea mays with Tripsacum dactyloides . Organogenic calli, induced from immature MT hybrid embryos, were exposed to different NaCl concentrations and the survival and regeneration percentages were calculated. Plants of the MT hybrid, obtained from the organogenic calli, were exposed to NaCl concentrations considered harmful for maize. The shoot dry weights of plants exposed to 250 mM NaCl did not show significant differences respect to the control ones. Although sodium content in shoots was incremented 2,5 fold, it was not toxic for this material. The MT hybrid showed better behavior, in vitro and in vivo , that maize genotypes exposed to similar conditions.

Arbuscular Mycorrhizas Regulate Photosynthetic Capacity and Antioxidant Defense Systems to Mediate Salt Tolerance in Maize.

Salt stress inhibits photosynthetic process and triggers excessive formation of reactive oxygen species (ROS). This study examined the role of arbuscular mycorrhizal (AM) association in regulating photosynthetic capacity and antioxidant activity in leaves of two maize genotypes (salt-tolerant JD52 and salt-sensitive FSY1) exposed to salt stress (100 mM NaCl) in soils for 21 days. The leaf water content, chlorophyll content, and photosynthetic capacity in non-mycorrhizal (NM) plants were decreased by salt stress, especially in FSY1, with less reduction in AM plants than NM plants. Salinity increased the activities of antioxidant enzymes (superoxide dismutase (SOD), catalase (CAT), ascorbate peroxidase (APX), and glutathione reductase (GR)) in both genotypes regardless of AM inoculation, but decreased the contents of non-enzymatic antioxidants (reduced glutathione (GSH) and ascorbate (AsA)), especially in FSY1, with less decrease in AM plants than NM plants. The AM plants, especially JD52, maintained higher photosynthetic capacity, CO2 fixation efficiency, and ability to preserve membrane integrity than NM plants under salt stress, as also indicated by the higher antioxidant contents and lower malondialdehyde (MDA)/electrolyte leakage in leaves. To conclude, the higher salt tolerance in AM plants correlates with the alleviation of salinity-induced oxidative stress and membrane damage, and the better performance of photosynthesis could have also contributed to this effect through reduced ROS formation. The greater improvements in photosynthetic processes and antioxidant defense systems by AM fungi in FSY1 than JD52 under salinity demonstrate genotypic variation in antioxidant defenses for mycorrhizal amelioration of salt stress.

Maize genotypes with deep root systems tolerate salt stress better than those with shallow root systems during early growth

AbstractMaize (Zea mays L.) is susceptible to salinity but shows genotypic variation for salt tolerance. How maize genotypes with contrasting root morphological traits respond to salt stress remains unclear. This study assessed genotypic variation in salinity tolerance of 20 maize genotypes with contrasting root systems exposed to NaCl for 10 days (0, 50 mM or 100 mM NaCl, added in four increments every other day from 14 days after transplanting, DAT) in a semi‐hydroponic phenotyping system in a temperature‐controlled greenhouse. Considerable variation was observed for each of the 12 measured shoot and root traits among the 20 genotypes under NaCl treatments. Salt stress significantly decreased biomass production by up to 54% in shoots and 37% in roots compared with the non‐saline control. The 20 genotypes were classified as salt‐tolerant (8 genotypes), moderately tolerant (5) and salt‐sensitive (7) genotypes based on the mean shoot dry weight ratio (the ratio of shoot dry weight at 100 mM NaCl and non‐saline control) ± one standard error. The more salt‐tolerant genotypes (such as Jindan52) had less reductions in growth, and lower shoot Na+ contents and higher shoot K+/Na+ ratios under salt stress. The declared salt tolerance was positively correlated with shoot height, shoot dry weight and primary root depth, and negatively correlated with shoot Na+ content at 100 mM NaCl. Primary root depth is critical for identifying salt responsiveness in maize plants and could be suggested as a selection criterion for screening salt tolerance of maize during early growth. The selected salt‐tolerant genotypes have potentials for cultivation in saline soils and for developing high‐yielding salt‐tolerant maize hybrids in future breeding programmes.

Copalyl Diphosphate Synthase Mutation Improved Salt Tolerance in Maize (Zea mays. L) via Enhancing Vacuolar Na+ Sequestration and Maintaining ROS Homeostasis.

Salinity stress impairs plant growth and causes crops to yield losses worldwide. Reduction of in vivo gibberellin acid (GA) level is known to repress plant size but is beneficial to plant salt tolerance. However, the mechanisms of in vivo GA deficiency-enhanced salt tolerance in maize are still ambiguous. In this study, we generated two independent maize knockout mutant lines of ent-copalyl diphosphate synthase (one of the key enzymes for early steps of GA biosynthesis), zmcps-1 and zmcps-7, to explore the role of GA in maize salt tolerance. The typical dwarf phenotype with lower GA content and delayed leaf senescence under salinity was observed in the mutant plants. The leaf water potential and cell turgor potential were significantly higher in zmcps-1 and zmcps-7 than in the wild type (WT) under salt stress. The mutant plants exhibited a lower superoxide anion production rate in leaves and also a downregulated relative expression level of NAPDH oxidase ZmRbohA-C than the WT maize under salt stress. Also, the mutant plants had higher enzymatic activities of superoxide dismutase (SOD) and catalase (CAT) and higher content of soluble sugars and proline under salt stress. The Na+/K+ ratio was not significantly different between the mutant maize plants and WT plants under salt stress conditions, but the Na+ and K+ content was increased in zmcps-1 and zmcps-7 leaves and shoots. Na+ fluorescent dye staining showed that the mutant leaves have significantly higher vacuolar Na+ intensity than the WT maize. The expression level of vacuolar Na+/H+ exchanger gene ZmNHX1 and vacuolar proton pump genes ZmVP1-1 and ZmVP2 were upregulated in the zmcps-1 and zmcps-7 plants under salinity, further proving that in vivo GA deficiency enhanced vacuolar Na+ sequestration in zmcps-1 and zmcps-7 leaves cells to avoid Na+ cytotoxicity. Together, our results suggested that maintaining ROS homeostasis and enhancing vacuolar Na+ sequestration could be involved in GA deficiency-improved maize salt tolerance.

Enhancement of salt tolerance in maize (Zea mays L.) using locally isolated Bacillus sp. SR-2-1/1

The current study was aimed at enhancement of salt tolerance of maize by using a halotolerant plant growth promoting rhizobacterium. Maize seeds (FH-1137) were grown under salt stress with and without inoculation of a locally isolated Bacillus sp. SR-2-1/1. Inoculation of Bacillus sp. SR-2-1/1 significantly augmented the salt tolerance of maize plants which was reflected by increased contents of chlorophyll, total phenolics and proline. Moreover, inoculation of strain SR-2-1/1 resulted in less oxidative damage in terms of H2O2 production and lipid peroxidation. This was further supported by higher catalase and peroxidase production in these plants. Bacillus sp. SR-2-1/1 exhibited positive effects on the expression of photosynthesis (RBCL), antioxidants status (CAT1, APX1, APX2) and plant ion homeostasis (NHX1, SOS1, H+-PPase, and HKT1) related genes. To the best of our knowledge, this is the first study reporting enhancement of salt tolerance in maize genotype FH-1137 by using plant growth promoting rhizobacterium. The use of Bacillus sp. SR-2-1/1 as bio-inoculant seems to be a sustainable alternative to alleviate salt-induced damages in maize. This approach can be applied to other plant species to increase salt tolerance.

Seed Biopriming with Salt-Tolerant Endophytic Pseudomonas geniculata-Modulated Biochemical Responses Provide Ecological Fitness in Maize (Zea mays L.) Grown in Saline Sodic Soil.

Under changing climate, soil salinity and sodicity is a limiting factor to crop production and are considered a threat to sustainability in agriculture. A number of attempts are being made to develop microbe-based technologies for alleviation of toxic effects of salts. However, the mechanisms of salt tolerance in agriculturally important crops are not fully understood and still require in-depth study in the backdrop of emerging concepts in biological systems. The present investigation was aimed to decipher the microbe-mediated mechanisms of salt tolerance in maize. Endophytic Pseudomonas geniculate MF-84 was isolated from maize rhizosphere and tagged with green fluorescent protein for localization in the plant system. Confocal microphotographs clearly indicate that MF-84 was localized in the epidermal cells, cortical tissues, endodermis and vascular bundles including proto-xylem, meta-xylem, phloem and bundle sheath. The role of P. geniculate MF-84 in induction and bioaccumulation of soluble sugar, proline and natural antioxidants enzymes in maize plant was investigated which lead not only to growth promotion but also provide protection from salt stress in maize. Results suggested that application of P. geniculate MF-84 reduces the uptake of Na+ and increases uptake of K+ and Ca2+ in maize roots indicative of the role of MF-84 in maintaining ionic balance/homeostasis in the plant roots under higher salt conditions. It not only helps in alleviation of toxic effects of salt but also increases plant growth along with reduction in crop losses due to salinity and sodicity.

A HAK family Na+ transporter confers natural variation of salt tolerance in maize.

Excessive sodium ion (Na+) concentrations in cultivated land alter crop yield and quality worldwide. Previous studies have shown that shoot Na+ exclusion is essential in most crops for salt tolerance. Here, we show by a genome-wide association study that Zea may L. Na+ content 2 (ZmNC2), encoding the HAK family ion transporter ZmHAK4, confers the natural variation of shoot Na+ exclusion and salt tolerance in maize. The ZmHAK4 locus accounts for ~11% of the shoot Na+ variation, and a natural ZmHAK4-deficient allele displays a decreased ZmHAK4 expression level and an increased shoot Na+ content. ZmHAK4 is preferentially expressed in the root stele and encodes a novel membrane-localized Na+-selective transporter that mediates shoot Na+ exclusion, probably by retrieving Na+ from xylem sap. ZmHAK4 orthologues were identified in other plant species, and the orthologues of ZmHAK4 in rice and wheat show identical expression patterns and ion transport properties, suggesting that ZmHAK4 orthologues mediate an evolutionarily conserved salt-tolerance mechanism. Finally, we show that ZmHAK4 and ZmHKT1 (a HKT1 family Na+-selective transporter) confer distinct roles in promoting shoot Na+ exclusion and salt tolerance, indicating that the combination of the favourable alleles of ZmHKT1 and ZmHAK4 can facilitate the development of salt-tolerant maize varieties.

Comparative Proteomics of Salt-Tolerant and Salt-Sensitive Maize Inbred Lines to Reveal the Molecular Mechanism of Salt Tolerance.

Salt stress is one of the key abiotic stresses that causes great loss of yield and serious decrease in quality in maize (Zea mays L.). Therefore, it is very important to reveal the molecular mechanism of salt tolerance in maize. To acknowledge the molecular mechanisms underlying maize salt tolerance, two maize inbred lines, including salt-tolerant 8723 and salt-sensitive P138, were used in this study. Comparative proteomics of seedling roots from two maize inbred lines under 180 mM salt stress for 10 days were performed by the isobaric tags for relative and absolute quantitation (iTRAQ) approach. A total of 1056 differentially expressed proteins (DEPs) were identified. In total, 626 DEPs were identified in line 8723 under salt stress, among them, 378 up-regulated and 248 down-regulated. There were 473 DEPs identified in P138, of which 212 were up-regulated and 261 were down-regulated. Venn diagram analysis showed that 17 DEPs were up-regulated and 12 DEPs were down-regulated in the two inbred lines. In addition, 8 DEPs were up-regulated in line 8723 but down-regulated in P138, 6 DEPs were down-regulated in line 8723 but up-regulated in P138. In salt-stressed 8723, the DEPs were primarily associated with phenylpropanoid biosynthesis, starch and sucrose metabolism, and the mitogen-activated protein kinase (MAPK) signaling pathway. Intriguingly, the DEPs were only associated with the nitrogen metabolism pathway in P138. Compared to P138, the root response to salt stress in 8723 could maintain stronger water retention capacity, osmotic regulation ability, synergistic effects of antioxidant enzymes, energy supply capacity, signal transduction, ammonia detoxification ability, lipid metabolism, and nucleic acid synthesis. Based on the proteome sequencing information, changes of 8 DEPs abundance were related to the corresponding mRNA levels by quantitative real-time PCR (qRT-PCR). Our results from this study may elucidate some details of salt tolerance mechanisms and salt tolerance breeding of maize.

Characterization of natural genetic variation identifies multiple genes involved in salt tolerance in maize

Progressive decline in irrigation water is forcing farmers to use brackish water which increases soil salinity and exposes the crop plants to salinity. Maize, one of the most important crops, is sensitive to salinity. Salt tolerance is a complex trait controlled by a number of physiological and biochemical processes. Scant information is available on the genetic architecture of salt tolerance in maize. We evaluated 399 inbred lines for six early vigor shoot and root traits upon exposure of 18-day seedlings to salinity (ECiw = 16 dS m-1) stress. Contrasting response of shoot and root growth to salinity indicated a meticulous reprogramming of resource partitioning by the plants to cope with the stress. The genomic analysis identified 57 single nucleotide polymorphisms (SNP) associated with early vigor traits. Candidate genes systematically associated with each SNP include both previously known and novel genes. Important candidates include a late embryogenesis protein, a divalent ion symporter, a proton extrusion protein, an RNA-binding protein, a casein kinase 1, and an AP2/EREBP transcription factor. The late embryogenesis protein is associated with both shoot and root length, indicating a coordinated change in resource allocation upon salt stress. Identification of a casein kinase 1 indicates an important role for Ser/Thr kinases in salt tolerance. Validation of eight candidates based on expression in a salt-tolerant and a salt-sensitive inbred line supported their role in salt tolerance. The candidate genes identified in this investigation provide a foundation for dissecting genetic and molecular regulation of salt tolerance in maize and related grasses.

Phosphorus Limitation Improved Salt Tolerance in Maize Through Tissue Mass Density Increase, Osmolytes Accumulation, and Na+ Uptake Inhibition.

Low phosphorus (P) availability and salt stress are two major constraints for maize (Zea mays L.) growth in north China. A combination of salinity and high P rather than low P is more detrimental to the growth of maize. However, little is known about the mechanisms by which P nutrition modifies the salt tolerance and P uptake of maize. The present study aimed to investigate the combined effects of salinity and P on maize growth and P uptake, and to address the physiological mechanisms of salt tolerance influenced by P availability in maize. Seedlings of a local maize cultivar XY335 were grown hydroponically for 35 days under low (5 μM) or sufficient P supply (200 μM) with or without 100 mM NaCl. Root morphological traits, tissue mass density, leaf osmolytes (sugars and proline) accumulation, and Na+/K+ ratio were measured to allow evaluation of the combined effects of salinity and P on maize growth and P uptake. Both P deficiency and salinity markedly reduced the growth of maize. However, P deficiency had a more pronounced effect on shoot growth while salinity affected root growth more prominently. Combined effects of P deficiency and salinity on total root length, root surface area, and average root diameter were similar to that of plants grown under salt stress. The combination of P deficiency and salinity treatments had a more pronounced effect on tissue mass density, leaf proline and soluble sugars compared to individual treatment of either low P or NaCl. When exposed to salt stress, maize plants of sufficient P accumulated greater amount of Na+ than those under P deficit, but similar amounts of K+ were observed between the two P treatments. Salt stress significantly increased shoot P concentration of maize with sufficient P (P < 0.01), but not for P-deficient plants. In sum, shoots and roots of maize exhibited different responses to P deficiency and salinity, with more marked effect of P deficiency on shoots and of salinity on roots. P deficiency improved salt tolerance of maize plants, which was associated with the increase of tissue mass density, accumulation of osmolytes, reduction of Na+ accumulation, and selective absorption of K+ over Na+.

Genome-wide association analysis of salt tolerance QTLs with SNP markers in maize (Zea mays L.).

Salt-tolerant breeding of maize has great significance to the development and utilization of saline-alkaline soil and the maintenance of grain security. Genome-wide association study (GWAS) has been widely used in maize genetics and breeding. To discover new salt-tolerant genes in maize by association analysis, which can provide technical supports for the innovation and genetic improvement of salt-tolerant germplasm resources in maize. Totally 150 maize inbred lines were genotyped with a high-density chip. GWAS was carried out to identify the significant single nucleotide polymorphisms (SNPs) which were associated with maize salt tolerance. Totally 34,972 SNPs with high quality and diversity were selected from 56,110 SNP markers, which were distributed on 10 chromosomes of maize. The GLM algorithm in TASSEL5.2 was used to analyze the five traits related to salt tolerance. Using a strict LOD threshold of 4.5, totally 7 SNP loci were identified, which were significantly correlated with plant height change rate and fresh weight change rate. The high density fingerprints of 150 inbred lines were clustered by TASSEL5.2 software to construct genetic clustering map to estimate the genetic distance and the subgroups. The 150 maize inbred lines were divided into two groups: SS group and NSS group, and the SNP loci of the salt-tolerant index showed difference in chromosome distribution. Based on previous studies, we screened 8 candidate genes for salt tolerance in maize and four of them were further validated by real-time quantitative PCR. Totally 7 SNP loci and 8 candidate genes related to salt tolerance in maize were identified, which will be of special value in molecular breeding of salt-tolerant maize.

Salt stress induces genotype-specific DNA hypomethylation in ZmEXPB2 and ZmXET1 genes in maize

Maize, a moderately salt sensitive crop, first experiences osmotic stress that cause reduction in plant growth under salt stress. Fluctuation in cell wall elongation is one of the reasons of this reduction. Along with others, two important proteins expansins and xyloglucan endotransglucosylase are involved in regulation of cell wall elasticity, but the role of epigenetic mechanisms in regulating the cell wall related genes is still elusive. The present study was conducted with the aim of understanding the role of DNA methylation in regulating ZmEXPB2 and ZmXET1 genes. One salt sensitive and one salt tolerant maize cultivar was grown under hydroponic conditions at different levels of salt stress: T1 = 1 mM (control), T2 = 100 mM and T3 = 200 mM in three replicates. DNA and RNA were extracted from roots. After bisulfite treatment, Methyl Sensitive PCR was used for the DNA methylation analysis. It was revealed that fragment in promoter of ZmEXPB2 gene showed high level of DNA methylation under T1 in both varieties. Comparison of different stress treatments revealed decrease in DNA methylation with the increase in salt stress, significantly lower methylation appearing in T3. Similarly, the fragment in promoter of ZmXET1 gene also showed high levels of DNA methylation in T1. When different treatments were analysed, this gene significantly hypomethylated at T2 which continued to decrease in T3 in sensitive variety but remain stable in tolerant variety. Although, further in-depth analysis is required, our results demonstrate region-specific and genotype-specific methylation shift in the promoter of the ZmEXPB2 and ZmXET1 genes when subjected to the salt stress confirming the epigenetic regulation of these genes under stress conditions.

Pre-breeding: the role of antioxidant enzymes on maize in salt stress tolerance

Maize is a crop that is moderately sensitive to salt stress. Salinization of soil is a severe threat to maize production worldwide. Understanding the response and tolerance mechanism of maize to salt stress may be conducive to formulate strategies to improve maize performance under saline environments. In this study, salt-tolerant, salt-sensitive and moderate salt-tolerant maize plants were investigated, respectively, under salt stress conditions in three aspects: growth status, enzyme activity and gene expression level. After 30 days of planting and salt stress treatment, the plant height of USTB-297 (salt-tolerant maize) was 49.40% higher than that of USTB-265 (salt-sensitive maize) and 25.10% higher than that of USTB-109 (moderate salt-tolerant maize). Analysis of antioxidant enzymes superoxide dismutase (EC1.15.1.1), ascorbate peroxidase (EC1.11.1.11) and catalase (EC1.11.1.6) revealed that there are distinctions between these different breeds. Salt-tolerant breed with a higher plant height also had higher antioxidant enzyme activity and related genes expression compared to salt-sensitive or moderate salt-tolerant breed. The detection of gene expression in superoxide dismutase, catalase and ascorbate peroxidase using real-time PCR and the data of enzyme activity indicate that we can build a method of breeding for maize.

Genome-wide association study dissects the genetic bases of salt tolerance in maize seedlings.

Excess salinity is a natural stress that causes crop yield losses worldwide. The genetic bases of maize salt tolerance remain largely unknown. Here we investigated the survival rates of 445 maize natural accessions after salt treatments. A skewed distribution of the salt-tolerant phenotypes was observed in this population. Genome-wide association studies (GWAS) revealed 57 loci significantly associated with salt tolerance. Forty-nine candidate genes were detected from these loci. About 10% of these genes were co-localized with loci from QTL mapping. Forty four percent of the candidate genes were involved in stress responses, ABA signaling, stomata division, DNA binding/transcription regulation and auxin signaling, suggesting that they are key genetic mechanisms of maize salt tolerance. Transgenic studies showed that two genes, the salt-tolerance-associated-gene 4 (SAG4, GRMZM2G077295) and SAG6 (GRMZM2G106056), which encode a protein transport protein and the double-strand break repair protein MRE11, respectively, had positive roles in plant salt tolerance, and their salt-tolerant haplotypes were revealed. The genes we identified in this study provide a list of candidate targets for further study of maize salt tolerance, and of genetic markers and materials that may be used for breeding salt-tolerance in maize.

Mapping of quantitative trait loci for seedling salt tolerance in maize

Improved salt tolerance is a major goal for maize breeders, but the control of salt tolerance-related traits in maize is poorly understood. A set of 209 doubled haploid (DH) lines derived from the maize hybrid Xianyu335 was genotyped using 1335 single-nucleotide polymorphism (SNP) markers. Biomass-related traits under salt stress were used as salt tolerance indicators, and a quantitative trait loci (QTL) analysis was conducted in hydroponic culture under salt-stress and normal conditions. Fourteen traits showed significant phenotypic differences between parents, and 16 traits showed transgressive segregation in the DH population. We detected 65 QTL: 41 associated with salt tolerance and 24 related to normal growth. Salt tolerance-related QTL clustered on chromosomes 1, 3, 7, and 9, among which 13 major effect QTL on chromosome 1 individually explained more than 21% of the phenotypic variation. Of these large-effect QTL, QTL controlling seven traits upon salt treatment and two salt tolerance indexes were co-located; they extensively overlapped with the remaining four co-localized QTL related to four other salt tolerance indexes. The QTL identified here shed new light on the molecular mechanism of salt tolerance in maize and may provide robust functional markers for marker-assisted selection of salt tolerant maize varieties.

Phosphoproteomic Analysis of Two Contrasting Maize Inbred Lines Provides Insights into the Mechanism of Salt-Stress Tolerance.

Salinity is a major abiotic stress that limits maize yield and quality throughout the world. We investigated phosphoproteomics differences between a salt-tolerant inbred line (Zheng58) and a salt-sensitive inbred line (Chang7-2) in response to short-term salt stress using label-free quantitation. A total of 9448 unique phosphorylation sites from 4116 phosphoproteins in roots and shoots of Zheng58 and Chang7-2 were identified. A total of 209 and 243 differentially regulated phosphoproteins (DRPPs) in response to NaCl treatment were detected in roots and shoots, respectively. Functional analysis of these DRPPs showed that they were involved in carbon metabolism, glutathione metabolism, transport, and signal transduction. Among these phosphoproteins, the expression of 6-phosphogluconate dehydrogenase 2, pyruvate dehydrogenase, phosphoenolpyruvate carboxykinase, glutamate decarboxylase, glutamate synthase, l-gulonolactone oxidase-like, potassium channel AKT1, high-affinity potassium transporter, sodium/hydrogen exchanger, and calcium/proton exchanger CAX1-like protein were significantly regulated in roots, while phosphoenolpyruvate carboxylase 1, phosphoenolpyruvate carboxykinase, sodium/hydrogen exchanger, plasma membrane intrinsic protein 2, glutathione transferases, and abscisic acid-insensitive 5-like protein were significantly regulated in shoots. Zheng58 may activate carbon metabolism, glutathione and ascorbic acid metabolism, potassium and sodium transportation, and the accumulation of glutamate to enhance its salt tolerance. Our results help to elucidate the mechanisms of salt response in maize seedlings. They also provide a basis for further study of the mechanism underlying salt response and tolerance in maize and other crops.

Effects of potassium‐solubulizing and photosynthetic bacteria on tolerance to salt stress in maize

The purpose of this study was to determine the positive effects of potassium-solubilizing bacteria and photosynthetic bacteria on the salt tolerance of maize. We selected the maize inbred lines USTB-265 (salt-sensitive), USTB-109 (moderately salt-tolerant) and USTB-297 (salt-tolerant) to investigate their growth characteristics, enzyme activity and gene expression in response to inoculation with photosynthetic bacteria and potassium-solubilizing bacteria under salt-stress conditions. Photosynthetic bacteria and potassium-solubilizing bacteria inoculation significantly enhanced the expression of antioxidant enzyme-related genes and increased the activities of the antioxidant enzymes superoxide dismutase, catalase and ascorbate peroxidase. In addition, inoculation with photosynthetic bacteria more efficiently improved maize salt tolerance than inoculation with potassium-solubilizing bacteria. While the effects of these bacteria differed among the three maize lines, both photosynthetic bacteria and potassium-solubilizing bacteria can enhance salt tolerance in maize. Soil salinization is one of the most critical factors affecting maize growth. These two types of bacteria (e.g. Bacillus mojavensis JK07 and Rhodopseudomonas palustris) have proven useful in salinized agricultural lands as bio-inoculants to increase crop productivity.

Comparative transcriptome analysis of salt-sensitive and salt-tolerant maize reveals potential mechanisms to enhance salt resistance.

Salt stress is a devastating environmental stress that causes plant growth inhibition and yield reduction. The identification of salt-tolerant genes brings hope for the generation of salinity-tolerant crop plants through molecular breeding. In this study, one salt-sensitive and one salt-tolerant maize inbred line were screened from 242 maize inbred lines. Reactive oxygen species (ROS)-related enzyme activities were detected and salt-responsive comparative transcriptome analysis was performed for control and 220 mM NaCl treated maize leaves. Salt-tolerant maize inbred line (L87) showed higher ROS-related enzyme (SOD, POD, APX and CAT) activities and accumulated relatively lower levels of ROS under salt stress. Of the total DEGs, 1856 upregulated DEGs were specific to L87, including stress tolerance-related members of the 70kDa family of heat shock proteins (Hsp70s) and aquaporins. The DEGs involved in the abscisic acid (ABA), ethylene, jasmonic acid (JA) and salicylic acid (SA) signal transduction pathways may determine the difference in salt tolerance between the two varieties, especially one central component SnRK2, that positively regulates ABA signaling and was only upregulated in L87. Analysis of DEGs related to ROS scavenging showed that some peroxidase (POD), glutathione S-transferase (GST), catalase (CAT) and superoxide dismutase (SOD) genes specific to L87 probably enhanced its salt tolerance. The analysis of differentially expressed transcription factors (TFs) suggested that WRKY TFs could contribute to the difference in salt tolerance between the two maize lines. Compared with Salt-sensitive maize inbred line (L29), L87 exhibits specific regulatory mechanisms related to salt tolerance, including plant hormone interactions, ROS scavenging and the regulation of TFs. Our study identifies new candidate genes that may regulate maize tolerance to salt stress and provides useful information for breeding maize with high salt resistance.