Rice phospholipase Dα is involved in salt tolerance by the mediation of H(+)-ATPase activity and transcription

Overexpression of OsNAR2.1 by OsNAR2.1 promoter increases drought resistance by increasing the expression of OsPLDα1 in rice

BACKGROUND

pOsNAR2.1:OsNAR2.1 expression could significantly increase nitrogen uptake efficiency and grain yield of rice.

RESULT

This study reported the effects of overexpression of OsNAR2.1 by OsNAR2.1 promoter on physiological and agronomic traits associated with drought tolerance. In comparison to the wild-type (WT), the pOsNAR2.1:OsNAR2.1 transgenic lines exhibited a significant improvement in survival rate when subjected to drought stress and then irrigation. Under limited water supply conditions, compared with WT, the photosynthesis and water use efficiency (WUE) of transgenic lines were increased by 39.2% and 28.8%, respectively. Finally, the transgenic lines had 25.5% and 66.4% higher grain yield than the WT under full watering and limited water supply conditions, respectively. Compared with the WT, the agronomic nitrogen use efficiency (NUE) of transgenic lines increased by 25.5% and 66.4% under full watering and limited water supply conditions, and the N recovery efficiency of transgenic lines increased by 29.3% and 50.2%, respectively. The interaction between OsNAR2.1 protein and OsPLDα1 protein was verified by yeast hybrids. After drought treatment, PLDα activity on the plasma membrane of the transgenic line increased 85.0% compared with WT.

CONCLUSION

These results indicated that pOsNAR2.1:OsNAR2.1 expression could improve the drought resistance of rice by increasing nitrogen uptake and regulating the expression of OsPLDα1.

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.

Genome-Wide Association Study Reveals a Genetic Mechanism of Salt Tolerance Germinability in Rice (Oryza sativa L.)

Salt stress is one of the factors that limits rice production, and an important task for researchers is to cultivate rice with strong salt tolerance. In this study, 211 rice accessions were used to determine salt tolerance germinability (STG) indices and conduct a genome-wide association study (GWAS) using 36,727 SNPs. The relative germination energy (RGE), relative germination index (RGI), relative vigor index (RVI), relative mean germination time (RMGT), relative shoot length (RSL), and relative root length (RRL) were used to determine the STG indices in rice. A total of 43 QTLs, including 15 for the RGE, 6 for the RGI, 7 for the RVI, 3 for the RMGT, 1 for the RSL, and 11 for the RRL, were identified on nine chromosome regions under 60 and 100 mM NaCl conditions. For these STG-related QTLs, 18 QTLs were co-localized with previous studies, and some characterized salt-tolerance genes, such as OsCOIN, OsHsp17.0, and OsDREB2A, are located in these QTL candidates. Among the 25 novel QTLs, qRGE60-1-2 co-localized with qRGI60-1-1 on chromosome 1, and qRGE60-3-1 and qRVI60-3-1 co-localized on chromosome 3. According to the RNA-seq database, 16 genes, including nine for qRGE60-1-2 (qRGI60-1-1) and seven for qRGE60-3-1 (qRVI60-3-1), were found to show significant differences in their expression levels between the control and salt treatments. Furthermore, the expression patterns of these differentially expressed genes were analyzed, and nine genes (five for qRGE60-1-2 and four for qRGE60-3-1) were highly expressed in embryos at the germination stage. Haplotype analysis of these nine genes showed that the rice varieties with elite haplotypes in the LOC_Os03g13560, LOC_Os03g13840, and LOC_Os03g14180 genes had high STG. GWAS validated the known genes underlying salt tolerance and identified novel loci that could enrich the current gene pool related to salt tolerance. The resources with high STG and significant loci identified in this study are potentially useful in breeding for salt tolerance.

The molecular mechanism of plasma membrane H+-ATPases in plant responses to abiotic stress

Plasma membrane H⁺-ATPases (PM H⁺-ATPases) are critical proton pumps that export protons from the cytoplasm to the apoplast. The resulting proton gradient and difference in electrical potential energize various secondary active transport events. PM H⁺-ATPases play essential roles in plant growth, development, and stress responses. In this review, we focus on recent studies of the mechanism of PM H⁺-ATPases in response to abiotic stresses in plants, such as salt and high pH, temperature, drought, light, macronutrient deficiency, acidic soil and aluminum stress, as well as heavy metal toxicity. Moreover, we discuss remaining outstanding questions about how PM H⁺-ATPases contribute to abiotic stress responses.

Integrated Isoform Sequencing and Dynamic Transcriptome Analysis Reveals Diverse Transcripts Responsible for Low Temperature Stress at Anther Meiosis Stage in Rice

Low temperatures stress is one of the important factors limiting rice yield, especially during rice anther development, and can cause pollen sterility and decrease grain yield. In our study, low-temperature stress decreased pollen viability and spikelet fertility by affecting the sugar, nitrogen and amino acid contents of anthers. We performed RNA-seq and ISO-seq experiments to study the genome-wide transcript expression profiles in low-temperature anthers. A total of 4,859 differentially expressed transcripts were detected between the low-temperature and control groups. Gene ontology enrichment analysis revealed significant terms related to cold tolerance. Hexokinase and glutamate decarboxylase participating in starch and sucrose metabolism may play important roles in the response to cold stress. Using weighted gene co-expression network analysis, nine hub transcripts were found that could improve cold tolerance throughout the meiosis period of rice: Os02t0219000-01 (interferon-related developmental regulator protein), Os01t0218350-00 (tetratricopeptide repeat-containing thioredoxin), Os08t0197700-00 (luminal-binding protein 5), Os11t0200000-01 (histone deacetylase 19), Os03t0758700-01 (WD40 repeat domain-containing protein), Os06t0220500-01 (7-deoxyloganetin glucosyltransferase), Pacbio.T01382 (sucrose synthase 1), Os01t0172400-01 (phospholipase D alpha 1), and Os01t0261200-01 (NAC domain-containing protein 74). In the PPI network, the protein minichromosome maintenance 4 (MCM4) may play an important role in DNA replication induced by cold stress.

Interplay between gasotransmitters and potassium is a K+ey factor during plant response to abiotic stress

Carbon monoxide (CO), nitric oxide (NO) and hydrogen sulfide (H₂S) are gasotransmitters known for their roles in plant response to (a)biotic stresses. The crosstalk between these gasotransmitters and potassium ions (K⁺) has received considerable attention in recent years, particularly due to the dual role of K⁺ as an essential mineral nutrient and a promoter of plant tolerance to abiotic stress. This review brings together what it is known about the interplay among NO, CO, H₂S and K⁺ in plants with focus on the response to high salinity. Some findings obtained for plants under water deficit and metal stress are also presented and discussed since both abiotic stresses share similarities with salt stress. The molecular targets of the gasotransmitters NO, CO and H₂S in root and guard cells that drive plant tolerance to salt stress are highlighted as well.

Phospholipids in Salt Stress Response

High salinity threatens crop production by harming plants and interfering with their development. Plant cells respond to salt stress in various ways, all of which involve multiple components such as proteins, peptides, lipids, sugars, and phytohormones. Phospholipids, important components of bio-membranes, are small amphoteric molecular compounds. These have attracted significant attention in recent years due to the regulatory effect they have on cellular activity. Over the past few decades, genetic and biochemical analyses have partly revealed that phospholipids regulate salt stress response by participating in salt stress signal transduction. In this review, we summarize the generation and metabolism of phospholipid phosphatidic acid (PA), phosphoinositides (PIs), phosphatidylserine (PS), phosphatidylcholine (PC), phosphatidylethanolamine (PE) and phosphatidylglycerol (PG), as well as the regulatory role each phospholipid plays in the salt stress response. We also discuss the possible regulatory role based on how they act during other cellular activities.

Tissue-Specific Proteome and Subcellular Microscopic Analyses Reveal the Effect of High Salt Concentration on Actin Cytoskeleton and Vacuolization in Aleurone Cells during Early Germination of Barley

Cereal grain germination provides the basis for crop production and requires a tissue-specific interplay between the embryo and endosperm during heterotrophic germination involving signalling, protein secretion, and nutrient uptake until autotrophic growth is possible. High salt concentrations in soil are one of the most severe constraints limiting the germination of crop plants, affecting the metabolism and redox status within the tissues of germinating seed. However, little is known about the effect of salt on seed storage protein mobilization, the endomembrane system, and protein trafficking within and between these tissues. Here, we used mass spectrometry analyses to investigate the protein dynamics of the embryo and endosperm of barley (Hordeum vulgare, L.) at five different early points during germination (0, 12, 24, 48, and 72 h after imbibition) in germinated grains subjected to salt stress. The expression of proteins in the embryo as well as in the endosperm was temporally regulated. Seed storage proteins (SSPs), peptidases, and starch-digesting enzymes were affected by salt. Additionally, microscopic analyses revealed an altered assembly of actin bundles and morphology of protein storage vacuoles (PSVs) in the aleurone layer. Our results suggest that besides the salt-induced protein expression, intracellular trafficking and actin cytoskeleton assembly are responsible for germination delay under salt stress conditions.

Phospholipase Dα6 and phosphatidic acid regulate gibberellin signaling in rice

Phospholipase D (PLD) hydrolyzes membrane lipids to produce phosphatidic acid (PA), a lipid mediator involved in various cellular and physiological processes. Here, we show that PLDα6 and PA regulate the distribution of GIBBERELLIN (GA)-INSENSITIVE DWARF1 (GID1), a soluble gibberellin receptor in rice. PLDα6-knockout (KO) plants display less sensitivity to GA than WT, and PA restores the mutant to a normal GA response. PA binds to GID1, as documented by liposome binding, fat immunoblotting, and surface plasmon resonance. Arginines 79 and 82 of GID1 are two key amino acid residues required for PA binding and also for GID1's nuclear localization. The loss of PLDα6 impedes GA-induced nuclear localization of GID1. In addition, PLDα6-KO plants attenuated GA-induced degradation of the DELLA protein SLENDER RICE1 (SLR1). These data suggest that PLDα6 and PA positively mediate GA signaling in rice via PA binding to GID1 and promotion of its nuclear translocation.

Stress response of lettuce (Lactuca sativa) to environmental contamination with selected pharmaceuticals: A proteomic study

Pharmaceutical compounds have been found in rivers and treated wastewaters. They often contaminate irrigation waters and consequently accumulate in edible vegetables, causing changes in plants metabolism. The main objective of this work is to understand how lettuce plants cope with the contamination from three selected pharmaceuticals using a label free proteomic analysis. A lettuce hydroponic culture, grown for 36 days, was exposed to metformin, acetaminophen and carbamazepine (at 1 mg/L), during 8 days, after which roots and leaves were sampled and analysed using a liquid chromatography-mass spectrometry proteomics-based approach. In roots, a total of 612 proteins showed differentially accumulation while in leaves 237 proteins were identified with significant differences over controls. Carbamazepine was the contaminant that most affected protein abundance in roots, while in leaves the highest number of differentially accumulated proteins was observed for acetaminophen. In roots under carbamazepine, stress related protein species such as catalase, superoxide dismutase and peroxidases presented higher abundance. Ascorbate peroxidase increased in roots under metformin. Cell respiration protein species were affected by the presence of the three pharmaceuticals suggesting possible dysregulation of the Krebs cycle. Acetaminophen caused the main differences in respiration pathways, with more emphasis in leaves. Lettuce plants revealed different tolerance levels when contaminants were compared, being more tolerant to metformin presence and less tolerant to carbamazepine. SIGNIFICANCE: The significant increase of emerging contaminants in ecosystems makes essential to understand how these compounds may affect the metabolism of different organisms. Our study contributes with a detailed approach of the main interactions that may occur in plant metabolism when subjected to the stress induced by three different pharmaceuticals (acetaminophen, carbamazepine and metformin).

Transcriptome profiling of Puccinellia tenuiflora during seed germination under a long-term saline-alkali stress

BACKGROUND

Puccinellia tenuiflora is the most saline-alkali tolerant plant in the Songnen Plain, one of the three largest soda saline-alkali lands worldwide. Here, we investigated the physicochemical properties of saline-alkali soils from the Songnen Plain and sequenced the transcriptomes of germinated P. tenuiflora seedlings under long-term treatment (from seed soaking) with saline-alkali soil extracts.

RESULTS

We found that the soils from Songnen Plain were reasonably rich in salts and alkali; moreover, the soils were severely deficient in nitrogen [N], phosphorus [P], potassium [K] and several other mineral elements. This finding demonstrated that P. tenuiflora can survive from not only high saline-alkali stress but also a lack of essential mineral elements. To explore the saline-alkali tolerance mechanism, transcriptional analyses of P. tenuiflora plants treated with water extracts from the saline-alkali soils was performed. Interestingly, unigenes involved in the uptake of N, P, K and the micronutrients were found to be significantly upregulated, which indicated the existence of an efficient nutrition-uptake system in P. tenuiflora. Compared with P. tenuiflora, the rice Oryza sativa was hypersensitive to saline-alkali stress. The results obtained using a noninvasive microtest techniques confirmed that the uptake of NO3- and NH4+ and the regulatory flux of Na+ and H+ were significantly higher in the roots of P. tenuiflora than in those of O. sativa. In the corresponding physiological experiments, the application of additional nutrition elements significantly eliminated the sensitive symptoms of rice to saline-alkali soil extracts.

CONCLUSIONS

Our results imply that the survival of P. tenuiflora in saline-alkali soils is due to a combination of at least two regulatory mechanisms and the high nutrient uptake capacity of P. tenuiflora plays a pivotal role in its adaptation to those stress. Taken together, our results highlight the role of nutrition uptake in saline-alkali stress tolerance in plants.

Suppression of phospholipase D genes improves chalky grain production by high temperature during the grain-filling stage in rice

High temperature (HT) during the grain developing stage causes deleterious effects on rice quality resulting in mature grains with a chalky appearance. Phospholipase D (PLD) plays an important role in plants, including responses to environmental stresses. OsPLDα1, α3 and β2-knockdown (KD) plants showed decreased production of chalky grains at HT. HT ripening increased H2O2 accumulated in the developing grains. However, the increase was canceled by the knockdown of OsPLDβ2. Expression levels of OsCATA which is one of three rice catalase genes, in developing grains of OsPLDβ2-KD plants at 10 DAF were increased compared with that in vector-controls in HT growth conditions. Overexpression of OsCATA markedly suppressed the production of chalky grains in HT growth conditions. These results suggested that OsPLDβ2 functions as a negative regulator of the induction of OsCATA and is involved in the production of chalky grains in HT growth conditions.

Differential responses of two Egyptian barley (Hordeum vulgare L.) cultivars to salt stress

Although barley (Hordeum vulgare L.) is considered a salt tolerant crop species, productivity of barley is affected differently by ionic, osmotic, and oxidative stresses resulting from a salty rhizosphere. The current study was conducted to elucidate the mechanism of salt tolerance in two barley cultivars, Giza128 and Giza126. The two cultivars were exposed to 200 mM NaCl hydroponically for 12 days. Although both cultivars accumulated a large amount of Na⁺ in their leaves with similar concentrations, the growth of Giza128 was much better than that of Giza126, as measured by maintaining a higher dry weight, relative growth rate, leaf area, and plant height. To ascertain the underlying mechanisms of this differential tolerance, first, the relative expression patterns of the genes encoding Na⁺/H⁺ antiporters (NHX) and the associated proton pumps (V-PPase and V-ATPase) as well as the gene encoding the plasma membrane PM H⁺-ATPase were analyzed in leaf tissues. Salt stress induced higher HvNHX1 expression in Giza128 (3.3-fold) than in Giza126 (1.9-fold), whereas the expression of the other two genes, HvNHX2 and HvNHX3, showed no induction in either cultivar. The expression of HvHVP1 and HvHVA was higher in Giza128 (3.8- and 2.1-fold, respectively) than in Giza126 (1.6- and 1.1-fold, respectively). The expression of the PM H⁺-ATPase (ha1) gene was induced more in Giza128 (8.8-fold) than in Giza126 (1.8-fold). Second, the capacity for ROS detoxification was assessed using the oxidative stress biomarkers electrolyte leakage ratio (ELR) and the concentrations of malondialdehyde (MDA) and hydrogen peroxide (H₂O₂), and these parameters sharply increased in Giza126 leaves by 66.5%, 42.8% and 50.0%, respectively, compared with those in Giza128 leaves. The antioxidant enzyme (CAT, APX, sPOD, GR, and SOD) activities were significantly elevated by salt treatment in Giza128 leaves, whereas in Giza126, these activities were not significantly altered. Overall, the results indicate that the superior salt tolerance of Giza128 is primarily the result of the ability to counter Na⁺-induced oxidative stress by increasing antioxidant enzyme levels and possibly by increasing vacuolar Na⁺ sequestration and prevention of cellular K⁺ leakage.

OsPhyB-Mediating Novel Regulatory Pathway for Drought Tolerance in Rice Root Identified by a Global RNA-Seq Transcriptome Analysis of Rice Genes in Response to Water Deficiencies

Water deficiencies are one of the most serious challenges to crop productivity. To improve our understanding of soil moisture stress, we performed RNA-Seq analysis using roots from 4-week-old rice seedlings grown in soil that had been subjected to drought conditions for 2-3 d. In all, 1,098 genes were up-regulated in response to soil moisture stress for 3 d, which causes severe damage in root development after recovery, unlikely that of 2 d. Comparison with previous transcriptome data produced in drought condition indicated that more than 68% of our candidate genes were not previously identified, emphasizing the novelty of our transcriptome analysis for drought response in soil condition. We then validated the expression patterns of two candidate genes using a promoter-GUS reporter system in planta and monitored the stress response with novel molecular markers. An integrating omics tool, MapMan analysis, indicated that RING box E3 ligases in the ubiquitin-proteasome pathways are significantly stimulated by induced drought. We also analyzed the functions of 66 candidate genes that have been functionally investigated previously, suggesting the primary roles of our candidate genes in resistance or tolerance relating traits including drought tolerance (29 genes) through literature searches besides diverse regulatory roles of our candidate genes for morphological traits (15 genes) or physiological traits (22 genes). Of these, we used a T-DNA insertional mutant of rice phytochrome B (OsPhyB) that negatively regulates a plant's degree of tolerance to water deficiencies through the control of total leaf area and stomatal density based on previous finding. Unlike previous result, we found that OsPhyB represses the activity of ascorbate peroxidase and catalase mediating reactive oxygen species (ROS) processing machinery required for drought tolerance of roots in soil condition, suggesting the potential significance of remaining uncharacterized candidate genes for manipulating drought tolerance in rice.

Overexpression of Cucumber Phospholipase D alpha Gene (CsPLDα) in Tobacco Enhanced Salinity Stress Tolerance by Regulating Na+-K+ Balance and Lipid Peroxidation

Plant phospholipase D (PLD), which can hydrolyze membrane phospholipids to produce phosphatidic acid (PA), a secondary signaling molecule, has been proposed to function in diverse plant stress responses. In this research, we characterized the roles of the cucumber phospholipase D alpha gene (PLDα, GenBank accession number EF363796) in growth and tolerance to short- and long-term salt stress in transgenic tobacco (Nicotiana tabacum). Fresh and dry weights of roots, PLD activity and content, mitogen activated protein kinase (MAPK) gene expression, Na⁺-K⁺ homeostasis, expression of genes encoding ion exchange, reactive oxygen species (ROS) metabolism and osmotic adjustment substances were investigated in wild type (WT) and CsPLDα-overexpression tobacco lines grown under short- and long-term high salt (250 mM) stress. Under short-term stress (5 h), in both overexpression lines, the PA content, and the expression levels of MAPK and several genes related to ion exchange (NtNHX1, NtNKT1, NtHAK1, NtNHA1, NtVAG1), were promoted by high PLD activity. Meanwhile, the Na⁺/K⁺ ratio decreased. Under long-term stress (16 days), ROS scavenging systems (superoxide dismutase, peroxidase, catalase, ascorbate peroxidase activities) in leaves of transgenic lines were more active than those in WT plants. Meanwhile, the contents of proline, soluble sugar, and soluble protein significantly increased. In contrast, the contents of O₂^•- and H₂O₂, the electrolytic leakage and the accumulation of malondialdehyde in leaves significantly decreased. The root fresh and dry weights of the overexpression lines increased significantly. Na⁺-K⁺ homeostasis had the same trend as under the short-term treatment. These findings suggested that CsPLDα-produced PA can activate the downstream signals' adaptive response to alleviate the damage of salt stress, and the main strategies for adaptation to salt stress are the accumulation of osmoprotective compounds, maintaining Na⁺-K⁺ homeostasis and the scavenging of ROS, which function in the osmotic balancing and structural stabilization of membranes.

Membrane proteins involved in transport, vesicle traffic and Ca(2+) signaling increase in beetroots grown in saline soils

By separating plasma membrane proteins according to their hydropathy from beetroots grown in saline soils, several proteins probably involved in salt tolerance were identified by mass spectrometry. Beetroots, as a salt-tolerant crop, have developed mechanisms to cope with stresses associated with saline soils. To observe which plasma membrane (PM) proteins were more abundant in beet roots grown in saline soils, beet root plants were irrigated with water or 0.2 M NaCl. PM-enriched membrane preparations were obtained from these plants, and their proteins were separated according to their hydropathy by serial phase partitioning with Triton X-114. Some proteins whose abundance increased visibly in membranes from salt-grown beetroots were identified by mass spectrometry. Among them, there was a V-type H(+)-ATPase (probably from contaminating vacuolar membranes), which increased with salt at all stages of beetroots' development. Proteins involved in solute transport (an H(+)-transporting PPase and annexins), vesicle traffic (clathrin and synaptotagmins), signal perception and transduction (protein kinases and phospholipases, mostly involved in calcium signaling) and metabolism, appeared to increase in salt-grown beetroot PM-enriched membranes. These results suggest that PM and vacuolar proteins involved in transport, metabolism and signal transduction increase in beet roots adapted to saline soils. In addition, these results show that serial phase partitioning with Triton X-114 is a useful method to separate membrane proteins for their identification by mass spectrometry.

Fate and metabolism of the brominated flame retardant tetrabromobisphenol A (TBBPA) in rice cell suspension culture

Tetrabromobisphenol A (TBBPA) is the brominated flame retardant with the highest production volume and its bioaccumulation in environment has caused both human health and environmental concerns, however the fate and metabolism of TBBPA in plants is unknown. We studied the fate, metabolites, and transformation of (14)C-labeled TBBPA in rice cell suspension culture. During the incubation for 14 days, TBBPA degradation occurred continuously in the culture, accompanied by formation of one anisolic metabolite [2,6-dibromo-4-(2-(2-hydroxy)-propyl)-anisole] (DBHPA) (50% of the degraded TBBPA) and cellular debris-bound residues (46.4%) as well as mineralization (3.6%). The cells continuously accumulated TBBPA in the cytoplasm, while a small amount of DBHPA (2.1% of the initially applied TBBPA) was detectable inside the cells only at the end of incubation. The majority of the accumulated residues in the cells was attributed to the cellular debris-bound residues, accounting for 70-79% of the accumulation after the first incubation day. About 5.4% of the accumulation was associated with cell organelles, which contributed 7.5% to the cellular debris-bound residues. Based on the fate and metabolism of TBBPA in the rice cell suspension culture, a type II ipso-substitution pathway was proposed to describe the initial step for TBBPA degradation in the culture and balance the fate of TBBPA in the cells. To the best of our knowledge, our study provides for the first time the insights into the fate and metabolism of TBBPA in plants and points out the potential role of type II ipso-hydroxylation substitution in degradation of alkylphenols in plants. Further studies are required to reveal the mechanisms for the bound-residue formation (e.g., binding of residues to specific cell wall components), nature of the binding, and toxicological effects of the bound residues and DBHPA.

The functions of a cucumber phospholipase D alpha gene (CsPLDα) in growth and tolerance to hyperosmotic stress

Plant phospholipase D (PLD), which can hydrolyze membrane phospholipids to produce phosphatidic acid (PA), a secondary signaling molecule, has been proposed to function in diverse plant stress responses. In this research, a qRT-PCR analysis indicated that the expression of a cucumber phospholipase D alpha gene (CsPLDα) was induced by salt and drought stresses in the roots and leaves. To further study the roles of CsPLDα in regulating plant tolerance to salt, polyethylene glycol (PEG) and abscisic acid (ABA) stresses, transgenic tobacco plants constitutively overexpressing CsPLDα were produced. A qRT-PCR analysis showed that the CsPLDα transcript levels were high in transgenic tobacco lines, whereas no expression was found in wild type (WT) tobacco, indicating that CsPLDα was successfully transferred into the tobacco genome and overexpressed. Under normal conditions for 30 d, seeds of transgenic lines germinated neatly, and the seedlings were robust and bigger than WT plants. When treated with different concentrations of NaCl, PEG and ABA, germination rates and seedling sizes of the transgenic lines were significantly greater than WT. In addition, the germination times for transgenic lines were also remarkably shorter. Further studies indicated that transgenic lines had longer primary roots and more biomass accumulation than WT plants. The water loss in transgenic lines was also much lower than in WT. These findings suggest that the CsPLDα overexpression positively regulates plant tolerance to hyperosmotic stresses, and that CsPLDα is involved in the ABA regulation of stomatal closure and the alleviation of ABA inhibition on seed germination and seedling growth.

Overexpression of a phospholipase Dα gene from Ammopiptanthus nanus enhances salt tolerance of phospholipase Dα1-deficient Arabidopsis mutant

A phospholipase Dα gene ( AnPLDα ) was cloned from xerophytic desert plant Ammopiptanthus nanus and its overexpression enhanced salt tolerance of a PLDα1 deficient Arabidopsis mutant. Phospholipase Dα (PLDα) hydrolyzes phosphatidylcholine to produce phosphatidic acid, and plays crucial role in plant tolerance to abiotic stress. In this study, a phospholipase Dα gene (AnPLDα) was cloned from xerophyte Ammopiptanthus nanus by the methods of homologous cloning and rapid amplification of cDNA ends, and evaluated for its function in stress tolerance. The full-length cDNA was 2832 bp long, containing an open reading frame of 2427 bp that encodes 808 amino acids. The putative protein was predicted to be localized to the cytoplasm and this was confirmed by transient expression of a fluorescent fusion protein. The endogenous expression of the AnPLDα gene was induced by high salt, dehydration, cold and abscisic acid. The heterologous expression of the AnPLDα gene improved salt tolerance of an Arabidopsis pldα1 knocked out mutant, and positively regulated the expression of the AtABI, AtNCED, AtRD29A, AtRD29B and AtADH genes. Therefore, the AnPLDα gene was concluded to be involved in response to abiotic stress. The AnPLDα gene is a hopeful candidate for transgenic application to improve stress tolerance of commercial crops.

The plasma membrane transport systems and adaptation to salinity

Salt stress represents one of the environmental challenges that drastically affect plant growth and yield. Evidence suggests that glycophytes and halophytes have a salt tolerance mechanisms working at the cellular level, and the plasma membrane (PM) is believed to be one facet of the cellular mechanisms. The responses of the PM transport proteins to salinity in contrasting species/cultivars were discussed. The review provides a comprehensive overview of the recent advances describing the crucial roles that the PM transport systems have in plant adaptation to salt. Several lines of evidence were presented to demonstrate the correlation between the PM transport proteins and adaptation of plants to high salinity. How alterations in these transport systems of the PM allow plants to cope with the salt stress was also addressed. Although inconsistencies exist in some of the information related to the responses of the PM transport proteins to salinity in different species/cultivars, their key roles in adaptation of plants to high salinity is obvious and evident, and cannot be precluded. Despite the promising results, detailed investigations at the cellular/molecular level are needed in some issues of the PM transport systems in response to salinity to further evaluate their implication in salt tolerance.

Involvement of phospholipase D-related signal transduction in chemical-induced programmed cell death in tomato cell cultures

Phospholipase D (PLD) and its product phosphatidic acid (PA) are incorporated in a complex metabolic network in which the individual PLD isoforms are suggested to regulate specific developmental and stress responses, including plant programmed cell death (PCD). Despite the accumulating knowledge, the mechanisms through which PLD/PA operate during PCD are still poorly understood. In this work, the role of PLDα1 in PCD and the associated caspase-like proteolysis, ethylene and hydrogen peroxide (H(2)O(2)) synthesis in tomato suspension cells was studied. Wild-type (WT) and PLDα1-silenced cell lines were exposed to the cell death-inducing chemicals camptothecin (CPT), fumonisin B1 (FB1) and CdSO(4). A range of caspase inhibitors effectively suppressed CPT-induced PCD in WT cells, but failed to alleviate cell death in PLDα1-deficient cells. Compared to WT, in CPT-treated PLDα1 mutant cells, reduced cell death and decreased production of H(2)O(2) were observed. Application of ethylene significantly enhanced CPT-induced cell death both in WT and PLDα1 mutants. Treatments with the PA derivative lyso-phosphatidic acid and mastoparan (agonist of PLD/PLC signalling downstream of G proteins) caused severe cell death. Inhibitors, specific to PLD and PLC, remarkably decreased the chemical-induced cell death. Taken together with our previous findings, the results suggest that PLDα1 contributes to caspase-like-dependent cell death possibly communicated through PA, reactive oxygen species and ethylene. The dead cells expressed morphological features of PCD such as protoplast shrinkage and nucleus compaction. The presented findings reveal novel elements of PLD/PA-mediated cell death response and suggest that PLDα1 is an important factor in chemical-induced PCD signal transduction.

Modification of plasma membrane proton pumps in cucumber roots as an adaptation mechanism to salt stress

The effect of salt stress (50mM NaCl) on modification of plasma membrane (PM) H(+)-ATPase (EC 3.6.3.14) activity in cucumber roots was studied. Plants were grown under salt stress for 1, 3 or 6 days. In salt-stressed plants, weak stimulation of ATP hydrolytic activity of PM H(+)-ATPase and significant stimulation of proton transport through the plasma membrane were observed. The H(+)/ATP coupling ratio in the plasma membrane of plants subjected to salt stress significantly increased. The greatest stimulation of PM H(+)-ATPase was in 6-day stressed plants. Increased H2O2 accumulation under salt stress conditions in cucumber roots was also observed, with the greatest accumulation observed in 6-day stressed plants. Additionally, during the sixth day of salinity, there appeared heat shock proteins (HSPs) 17.7 and 101, suggesting that repair processes and adaptation to stress occurred in plants. Under salt stress conditions, fast post-translational modifications took place. Protein blot analysis with antibody against phosphothreonine and 14-3-3 proteins showed that, under salinity, the level of those elements increased. Additionally, under salt stress, activity changes of PM H(+)-ATPase can partly result from changes in the pattern of expression of PM H(+)-ATPase genes. In cucumber seedlings, there was increased expression of CsHA10 under salt stress and the transcript of a new PM H(+)-ATPase gene isoform, CsHA1, also appeared. Accumulation of the CsHA1 transcript was induced by NaCl exposure, and was not expressed at detectable levels in roots of control plants. The appearance of a new PM H(+)-ATPase transcript, in addition to the increase in enzyme activity, indicates the important role of the enzyme in maintaining ion homeostasis in plants under salt stress.

Characterization and mapping of a salt-sensitive mutant in rice (Oryza sativa L.)

A salt-sensitive mutant designated rice salt sensitive 2 (rss2) was isolated from the M2 generation of the rice cultivar Nipponbare mutagenized with ethyl methanesulfonate (EMS). This mutant exhibited a greater decrease in salt tolerance with a significant increase in Na(+) content in its shoots. Genetic analysis indicated that the increase in Na(+) in rss2 was controlled by a single recessive gene. Further genome-wide analysis of the linkage map constructed from the F2 population of rss2/Zhaiyeqing 8 (ZYQ8) showed that two quantitative trait loci (QTLs) on chromosomes 1 and 6 were responsible for the Na(+) concentration in shoots, which explained 14.5% and 53.3%, respectively, of the phenotypic variance. The locus on chromosome 1, but not that on chromosome 6, was also detected in the F2 population of Nipponbare/ZYQ8, suggesting that the QTL on chromosome 6 was responsible for the salt sensitivity in rss2. By analyzing the recombination events in 220 mutant individuals of an enlarged mapping population of rss2/ZYQ8, the rss2 locus was precisely mapped to an interval of 605.3 kb between insertion/deletion (InDel) markers IM21962 and IM22567. This finding will facilitate the cloning of the rss2 locus and provide insight into the physiological mechanisms of salt sensitivity in rice.

Physiological and molecular features of Puccinellia tenuiflora tolerating salt and alkaline-salt stress

Saline-alkali soil seriously threatens agriculture productivity; therefore, understanding the mechanism of plant tolerance to alkaline-salt stress has become a major challenge. Halophytic Puccinellia tenuiflora can tolerate salt and alkaline-salt stress, and is thus an ideal plant for studying this tolerance mechanism. In this study, we examined the salt and alkaline-salt stress tolerance of P. tenuiflora, and analyzed gene expression profiles under these stresses. Physiological experiments revealed that P. tenuiflora can grow normally with maximum stress under 600 mmol/L NaCl and 150 mmol/L Na2 CO3 (pH 11.0) for 6 d. We identified 4,982 unigenes closely homologous to rice and barley. Furthermore, 1,105 genes showed differentially expressed profiles under salt and alkaline-salt treatments. Differentially expressed genes were overrepresented in functions of photosynthesis, oxidation reduction, signal transduction, and transcription regulation. Almost all genes downregulated under salt and alkaline-salt stress were related to cell structure, photosynthesis, and protein synthesis. Comparing with salt stress, alkaline-salt stress triggered more differentially expressed genes and significantly upregulated genes related to H(+) transport and citric acid synthesis. These data indicate common and diverse features of salt and alkaline-salt stress tolerance, and give novel insights into the molecular and physiological mechanisms of plant salt and alkaline-salt tolerance.

Genomic analysis of phospholipase D family and characterization of GmPLDαs in soybean (Glycine max)

Phospholipase D (PLD) and its product phosphatidic acid play important roles in the regulation of plant growth, development, and stress responses. The genome database analysis has revealed PLD family in Arabidopsis, rice, poplar and grape. In this study, we report a genomic analysis of 18 putative soybean (Glycine max) PLD genes (GmPLDs), which exist in the 14 of 20 chromosomes. GmPLDs were grouped into six types, α(3), β(4), γ, δ(5), ε(2), and ζ(3), based on gene architectures, protein domains, evolutionary relationship, and sequence identity. These GmPLDs contained two HKD domains, PX/PH domains (for GmPLDζs), and C2 domain (for the other GmPLDs). The expression patterns analyzed by quantitative reverse transcription PCR demonstrated that GmPLDs were expressed differentially in various tissues. GmPLDα1, α2, and β2 were highly expressed in most tissues, whereas GmPLDδ5 was only expressed in flowers and GmPLDζ1 was predominantly expressed in flowers and early pods. The expression of GmPLDα1 and α2 was increased and that of GmPLDγ was decreased by salt stress. GmPLDα1 protein expressed in E. coli was active under the reaction conditions for both PLDα and PLDδ, hydrolyzing the common membrane phospholipids phosphatidylcholine, phosphatidylethanolamine, and phosphatidylglycerol. The genomic analysis for soybean PLD family provides valuable data for further identity and characterization of their functions.

The rice diacylglycerol kinase family: functional analysis using transient RNA interference

Diacylglycerol kinase (DGK) is a pivotal enzyme that phosphorylates diacylglycerol (DAG) to form phosphatidic acid (PA). The production of PA from phospholipase D (PLD) and the coupled phospholipase C/DGK route is an important signaling process in animal and plant cells. In this study, we report a genomic analysis of eight putative rice DGKs encoded by a gene family (OsDGKs) grouped into three clusters. To further investigate the functions of the OsDGKs, a double-stranded RNA (dsRNA)-induced RNA silencing method was established. Introduction of in vitro-synthesized dsRNAs corresponding to a unique or conserved region of OsDGKs into rice protoplasts abolished or diminished the expression of individual or multiple OsDGK genes. Suppressing the expression of OsDGKs resulted in a distinct depletion of the transcripts of the defense gene OsNPR1 and the salt-responsive gene OsCIPK15. Our primary results suggest that OsDGKs are involved in the signaling of stress responses.