Effects of ABA, auxin, and gibberellin on the expression of genes for vacuolar H+ -inorganic pyrophosphatase, H+ -ATPase subunit A, and Na+/H+ antiporter in barley

Systematic identification of sugarcane vacuolar H+-translocating pyrophosphatase (VPP) gene family and the role of ScVPP1 in salt resistance

KEY MESSAGE

A total of 24 genes of vacuolar H+-translocating pyrophosphatases H+-PPases (VPP) genes were identified in Saccharum spontaneum AP85-441 and the ScVPP1-overexpressed Arabidopsis plants conferred salt tolerance. The vital role of vacuolar H+-translocating pyrophosphatases H+-PPases (VPP) genes involved in plants in response to abiotic stresses. However, the understanding of VPP functions in sugarcane remained unclear. In this study, a total of 24 VPP genes (SsaVPP1-SsaVPP24) were identified in the Saccharum spontaneum genome of haploid clone AP85-441. These genes were distributed in two phylogenetic groups. The SsaVPPs displayed diverse physio-chemical and gene structure attributes. The SsaVPP family genes have expanded through segmental duplication (20 gene pairs) rather than tandem duplication. A full-length cDNA of ScVPP1 was cloned from the sugarcane cultivar ROC22 and shared 99.48% sequence identity (amino acid) with homologous gene SsaVPP21 from AP85-441. In ROC22, the ScVPP1 gene was considerably upregulated by NaCl and ABA treatments among leaf, root, and stem tissues, while this gene was exclusively upregulated in the root with PEG treatment. Under NaCl and ABA stresses, yeast cells transfected by the ScVPP1 plasmid showed distinct growth rates compared to control yeast cells transfected by the empty vector. In transgenic Arabidopsis lines overexpressing ScVPP1, the seed gemination and survival rate were enhanced under NaCl treatment but not under ABA stress as compared to wild-type plants. These results suggested that the ScVPP1 gene conferred tolerance to slat and may be used as a salt resistance gene source for sugarcane breeding.

Multiomics unravels potential molecular switches in the C3 to CAM transition of Mesembryanthemum crystallinum

Mesembryanthemum crystallinum (common ice plant), a facultative CAM plant, shifts from C3 to CAM photosynthesis under salt stress, enhancing water use efficiency. Here we used transcriptomics, proteomics, and targeted metabolomics to profile molecular changes during the diel cycle of C3 to CAM transition. The results confirmed expected changes associated with CAM photosynthesis, starch biosynthesis and degradation, and glycolysis/gluconeogenesis. Importantly, they yielded new discoveries: 1) Transcripts displayed greater circadian regulation than proteins. 2) Oxidative phosphorylation and inositol methylation may play important roles in initiating the transition. 3) V-type H+-ATPases showed consistent transcriptional regulation, aiding in vacuolar malate uptake. 4) A protein phosphatase 2C, a major component in the ABA signaling pathway, may trigger the C3 to CAM transition. Our work highlights the potential molecular switches in the C3 to CAM transition, including the potential role of ABA signaling. SIGNIFICANCE: The common ice plant is a model facultative CAM plant, and under stress conditions it can shift from C3 to CAM photosynthesis within a three-day period. However, knowledge about the molecular changes during the transition and the molecular switches enabling the transition is lacking. Multi-omic analyses not only revealed the molecular changes during the transition, but also highlighted the importance of ABA signaling, inositol methylation, V-type H+-ATPase in initiating the shift. The findings may explain physiological changes and nocturnal stomatal opening, and inform future synthetic biology effort in improving crop water use efficiency and stress resilience.

Evaluation of high salinity tolerance in Pongamia pinnata (L.) Pierre by a systematic analysis of hormone-metabolic network

Salinity stress results in significant losses in plant productivity and loss of cultivable lands. Although Pongamia pinnata is reported to be a salt-tolerant semiarid biofuel tree, the adaptive mechanisms to saline environments are elusive. Despite a reduction in carbon exchange rate (CER), the unchanged relative water content provides no visible salinity induced symptoms in leaves of hydroponic cultivated Pongamia seedlings for 8 days. Our Na⁺ -specific fluorescence results demonstrated that there was an effective apoplastic sodium sequestration in the roots. Salinity stress significantly increased zeatin (~5.5-fold), and jasmonic acid (~3.8-fold) levels in leaves while zeatin (~2.5-fold) content increased in leaves as well as in roots of salt-treated plants. Metabolite analysis suggested that osmolytes such as myo-inositol and mannitol were enhanced by ~12-fold in leaves and roots of salt-treated plants. Additionally, leaves of Pongamia showed a significant enhancement in carbohydrate content, while fatty acids were accumulated in roots under salt stress condition. At the molecular level, salt stress enhanced the expression of genes related to transporters, including the Salt Overly Sensitive 2 gene (SOS2), SOS3, vacuolar-cation/proton exchanger, and vacuolar-proton/ATPase exclusively in leaves, whereas the Sodium Proton Exchanger1 (NHX1), Cation Calcium Exchanger (CCX), and Cyclic Nucleotide Gated Channel 5 (CNGC5) were up-regulated in roots. Antioxidant gene expression analysis clearly demonstrated that peroxidase levels were significantly enhanced by ~10-fold in leaves, while Catalase and Fe-superoxide Dismutase (Fe-SOD) genes were increased in roots under salt stress. The correlation interaction studies between phytohormones and metabolites revealed new insights into the molecular and metabolic adaptations that confer salinity tolerance to Pongamia.

Salinity Stress: Toward Sustainable Plant Strategies and Using Plant Growth-Promoting Rhizobacteria Encapsulation for Reducing It

Salinity is one of the most important abiotic stresses that influences plant growth and productivity worldwide. Salinity affects plant growth by ionic toxicity, osmotic stress, hormonal imbalance, nutrient mobilization reduction, and reactive oxygen species (ROS). To survive in saline soils, plants have developed various physiological and biochemical strategies such as ion exchange, activation of antioxidant enzymes, and hormonal stimulation. In addition to plant adaption mechanisms, plant growth-promoting rhizobacteria (PGPR) can enhance salt tolerance in plants via ion homeostasis, production of antioxidants, ACC deaminase, phytohormones, extracellular polymeric substance (EPS), volatile organic compounds, accumulation of osmolytes, activation of plant antioxidative enzymes, and improvement of nutrients uptake. One of the important issues in microbial biotechnology is establishing a link between the beneficial strains screened in the laboratory with industry and the consumer. Therefore, in the development of biocontrol agents, it is necessary to study the optimization of conditions for mass reproduction and the selection of a suitable carrier for their final formulation. Toward sustainable agriculture, the use of appropriate formulations of bacterial agents as high-performance biofertilizers, including microbial biocapsules, is necessary to improve salt tolerance and crop productivity.

Salinity tolerance mechanisms and their breeding implications

BACKGROUND

The era of first green revolution brought about by the application of chemical fertilizers surely led to the explosion of food grains, but left behind the notable problem of salinity. Continuous application of these fertilizers coupled with fertilizer-responsive crops make the country self-reliant, but continuous deposition of these led to altered the water potential and thus negatively affecting the proper plant functioning from germination to seed setting.

MAIN BODY

Increased concentration of anion and cations and their accumulation and distribution cause cellular toxicity and ionic imbalance. Plants respond to salinity stress by any one of two mechanisms, viz., escape or tolerate, by either limiting their entry via root system or controlling their distribution and storage. However, the understanding of tolerance mechanism at the physiological, biochemical, and molecular levels will provide an insight for the identification of related genes and their introgression to make the crop more resilient against salinity stress.

SHORT CONCLUSION

Novel emerging approaches of plant breeding and biotechnologies such as genome-wide association studies, mutational breeding, marker-assisted breeding, double haploid production, hyperspectral imaging, and CRISPR/Cas serve as engineering tools for dissecting the in-depth physiological mechanisms. These techniques have well-established implications to understand plants' adaptions to develop more tolerant varieties and lower the energy expenditure in response to stress and, constitutively fulfill the void that would have led to growth resistance and yield penalty.

Type I H+-pyrophosphatase regulates the vacuolar storage of sucrose in citrus fruit

The aim of this work was to evaluate the general role of the vacuolar pyrophosphatase proton pump (V-PPase) in sucrose accumulation in citrus species. First, three citrus V-PPase genes, designated CsVPP-1, CsVPP-2, and CsVPP-4, were identified in the citrus genome. CsVPP-1 and CsVPP-2 belonging to citrus type I V-PPase genes are targeted to the tonoplast, and CsVPP-4 belonging to citrus type II V-PPase genes is located in the Golgi bodies. Moreover, there was a significantly positive correlation between transcript levels of type I V-PPase genes and sucrose, rather than hexose, content in fruits of seven citrus cultivars. Drought and abscisic acid treatments significantly induced the CsVPP-1 and CsVPP-2 transcript levels, as well as the sucrose content. The overexpression of type I V-PPase genes significantly increased PPase activity, decreased pyrophosphate contents, and increased sucrose contents, whereas V-PPase inhibition produced the opposite effect in both citrus fruits and leaves. Furthermore, altering the expression levels of type I V-PPase genes significantly influenced the transcript levels of sucrose transporter genes. Taken together, this study demonstrated that CsVPP-1 and CsVPP-2 play key roles in sucrose storage in the vacuole by regulating pyrophosphate homeostasis, ultimately the sucrose biosynthesis and transcript levels of sucrose transport genes, providing a novel lead for engineering or breeding modified taste in citrus and other fruits.

The unexpected concentration-dependent response of periphytic biofilm during indole acetic acid removal

Due to its extensive application in agriculture as a germinating agent and growth promoter, indole acetic acid (IAA) is present in a variety of aquatic ecosystems. To explore the response of microbial aggregates to exogenous IAA in aquatic ecosystems, periphytic biofilm, a typical microbial aggregate, was exposed to IAA at different concentrations. Results reveal an unexpected concentration-dependent effect of IAA on periphytic biofilm. Concentrations of IAA less than 10 mg/L inhibit periphytic growth, but stimulate growth when the IAA concentration exceeds 50 mg/L. Periphytic biofilm adapts to different IAA concentrations by antioxidant enzyme activation, community structure optimization and carbon-metabolism pattern change, and promotes bioremediation of IAA contaminated water in the process. The removal rates of IAA reached up to 95%-100%. This study reveals the capacity of periphytic biofilm for IAA removal in practice.

iTRAQ-Based Proteomic Analysis Reveals Several Strategies to Cope with Drought Stress in Maize Seedlings

Drought stress, especially during the seedling stage, seriously limits the growth of maize and reduces production in the northeast of China. To investigate the molecular mechanisms of drought response in maize seedlings, proteome changes were analyzed. Using an isotopic tagging relative quantitation (iTRAQ) based method, a total of 207 differentially accumulated protein species (DAPS) were identified under drought stress in maize seedlings. The DAPS were classified into ten essential groups and analyzed thoroughly, which involved in signaling, osmotic regulation, protein synthesis and turnover, reactive oxygen species (ROS) scavenging, membrane trafficking, transcription related, cell structure and cell cycle, fatty acid metabolism, carbohydrate and energy metabolism, as well as photosynthesis and photorespiration. The enhancements of ROS scavenging, osmotic regulation, protein turnover, membrane trafficking, and photosynthesis may play important roles in improving drought tolerance of maize seedlings. Besides, the inhibitions of some protein synthesis and slowdown of cell division could reduce the growth rate and avoid excessive water loss, which is possible to be the main reasons for enhancing drought avoidance of maize seedlings. The incongruence between protein and transcript levels was expectedly observed in the process of confirming iTRAQ data by quantitative real-time polymerase chain reaction (qRT-PCR) analysis, which further indicated that the multiplex post-transcriptional regulation and post-translational modification occurred in drought-stressed maize seedlings. Finally, a hypothetical strategy was proposed that maize seedlings coped with drought stress by improving drought tolerance (via. promoting osmotic adjustment and antioxidant capacity) and enhancing drought avoidance (via. reducing water loss). Our study provides valuable insight to mechanisms underlying drought response in maize seedlings.

Molecular cloning and functional characterisation of an H+-pyrophosphatase from Iris lactea

Tonoplast H+-pyrophosphatases (VPs) mediate vacuolar Na+ sequestration, a process important for salt tolerance of plants. The function of VP in the highly drought- and salt-tolerant perennial Iris lactea under salt stress is unclear. Here, we isolated IlVP from I. lactea and investigated its function in transgenic tobacco. IlVP was found to comprise 771 amino acid residues and showed 88% similarity with Arabidopsis AtVP1. IlVP was mainly expressed in shoots and was up-regulated by salt stress. Overexpression of IlVP enhanced growth of transgenic tobacco plants compared with wild-type (WT) plants exposed to salt stress. Transgenic plants accumulated higher quantities of Na+ and K+ in leaves, stems, and roots under salt stress, which caused higher leaf relative water content and decreased cell membrane damage compared with WT plants. Overall, IlVP encoding a tonoplast H+-pyrophosphatase can reduce Na+ toxicity in plant cells through increased sequestration of ions into vacuoles by enhanced H+-pyrophosphatase activity.

Proteomic analysis on roots of Oenothera glazioviana under copper-stress conditions

Proteomic studies were performed to identify proteins involved in the response of Oenothera glazioviana seedlings under Cu stress. Exposure of 28-d-old seedlings to 50 μM CuSO4 for 3 d led to inhibition of shoot and root growth as well as a considerable increase in the level of lipid peroxidation in the roots. Cu absorbed by O. glazioviana accumulated more easily in the root than in the shoot. Label-free proteomic analysis indicated 58 differentially abundant proteins (DAPs) of the total 3,149 proteins in the roots of O. glazioviana seedlings, of which 36 were upregulated and 22 were downregulated under Cu stress conditions. Gene Ontology analysis showed that most of the identified proteins could be annotated to signal transduction, detoxification, stress defence, carbohydrate, energy, and protein metabolism, development, and oxidoreduction. We also retrieved 13 proteins from the enriched Kyoto Encyclopaedia of Genes and Genomes and the protein-protein interaction databases related to various pathways, including the citric acid (CA) cycle. Application of exogenous CA to O. glazioviana seedlings exposed to Cu alleviated the stress symptoms. Overall, this study provided new insights into the molecular mechanisms of plant response to Cu at the protein level in relation to soil properties.

Molecular responses of genetically modified maize to abiotic stresses as determined through proteomic and metabolomic analyses

Some genetically modified (GM) plants have transgenes that confer tolerance to abiotic stressors. Meanwhile, other transgenes may interact with abiotic stressors, causing pleiotropic effects that will affect the plant physiology. Thus, physiological alteration might have an impact on the product safety. However, routine risk assessment (RA) analyses do not evaluate the response of GM plants exposed to different environmental conditions. Therefore, we here present a proteome profile of herbicide-tolerant maize, including the levels of phytohormones and related compounds, compared to its near-isogenic non-GM variety under drought and herbicide stresses. Twenty differentially abundant proteins were detected between GM and non-GM hybrids under different water deficiency conditions and herbicide sprays. Pathway enrichment analysis showed that most of these proteins are assigned to energetic/carbohydrate metabolic processes. Among phytohormones and related compounds, different levels of ABA, CA, JA, MeJA and SA were detected in the maize varieties and stress conditions analysed. In pathway and proteome analyses, environment was found to be the major source of variation followed by the genetic transformation factor. Nonetheless, differences were detected in the levels of JA, MeJA and CA and in the abundance of 11 proteins when comparing the GM plant and its non-GM near-isogenic variety under the same environmental conditions. Thus, these findings do support molecular studies in GM plants Risk Assessment analyses.

Drought-Responsive Mechanisms in Plant Leaves Revealed by Proteomics

Plant drought tolerance is a complex trait that requires a global view to understand its underlying mechanism. The proteomic aspects of plant drought response have been extensively investigated in model plants, crops and wood plants. In this review, we summarize recent proteomic studies on drought response in leaves to reveal the common and specialized drought-responsive mechanisms in different plants. Although drought-responsive proteins exhibit various patterns depending on plant species, genotypes and stress intensity, proteomic analyses show that dominant changes occurred in sensing and signal transduction, reactive oxygen species scavenging, osmotic regulation, gene expression, protein synthesis/turnover, cell structure modulation, as well as carbohydrate and energy metabolism. In combination with physiological and molecular results, proteomic studies in leaves have helped to discover some potential proteins and/or metabolic pathways for drought tolerance. These findings provide new clues for understanding the molecular basis of plant drought tolerance.

Increasing abscisic acid levels by immunomodulation in barley grains induces precocious maturation without changing grain composition

Abscisic acid (ABA) accumulates in seeds during the transition to the seed filling phase. ABA triggers seed maturation, storage activity, and stress signalling and tolerance. Immunomodulation was used to alter the ABA status in barley grains, with the resulting transgenic caryopses responding to the anti-ABA antibody gene expression with increased accumulation of ABA. Calculation of free versus antibody-bound ABA reveals large excess of free ABA, increasing signficantly in caryopses from 10 days after fertilization. Metabolite and transcript profiling in anti-ABA grains expose triggered and enhanced ABA-functions such as transcriptional up-regulation of sucrose-to-starch metabolism, storage protein synthesis and ABA-related signal transduction. Thus, enhanced ABA during transition phases induces precocious maturation but negatively interferes with growth and development. Anti-ABA grains display broad constitutive gene induction related to biotic and abiotic stresses. Most of these genes are ABA- and/or stress-inducible, including alcohol and aldehyde dehydrogenases, peroxidases, chaperones, glutathione-S-transferase, drought- and salt-inducible proteins. Conclusively, ABA immunomodulation results in precocious ABA accumulation that generates an integrated response of stress and maturation. Repression of ABA signalling, occurring in anti-ABA grains, potentially antagonizes effects caused by overshooting production. Finally, mature grain weight and composition are unchanged in anti-ABA plants, although germination is somewhat delayed. This indicates that anti-ABA caryopses induce specific mechanisms to desensitize ABA signalling efficiently, which finally yields mature grains with nearly unchanged dry weight and composition. Such compensation implicates the enormous physiological and metabolic flexibilities of barley grains to adjust effects of unnaturally high ABA amounts in order to ensure and maintain proper grain development.

Potential role of phytohormones and plant growth-promoting rhizobacteria in abiotic stresses: consequences for changing environment

Plants are sessile beings, so the need of mechanisms to flee from unfavorable circumstances has provided the development of unique and sophisticated responses to environmental stresses. Depending on the degree of plasticity, many morphological, cellular, anatomical, and physiological changes occur in plants in response to abiotic stress. Phytohormones are small molecules that play critical roles in regulating plant growth and development, as well as stress tolerance to promote survival and acclimatize to varying environments. To congregate the challenges of salinity, temperature extremes, and osmotic stress, plants use their genetic mechanism and different adaptive and biological approaches for survival and high production. In the present attempt, we review the potential role of different phytohormones and plant growth-promoting rhizobacteria in abiotic stresses and summarize the research progress in plant responses to abiotic stresses at physiological and molecular levels. We emphasized the regulatory circuits of abscisic acid, indole acetic acid, cytokinins, gibberellic acid, salicylic acid, brassinosteroids, jasmonates, ethylene, and triazole on exposure to abiotic stresses. Current progress is exemplified by the identification and validation of several significant genes that enhanced crop tolerance to stress in the field. These findings will make the modification of hormone biosynthetic pathways for the transgenic plant generation with augmented abiotic stress tolerance and boosting crop productivity in the coming decades possible.

Proteomics of methyl jasmonate induced defense response in maize leaves against Asian corn borer

BACKGROUND

Jasmonic acid (JA) and methyl jasmonate (MeJA) regulate plant development, resistance to stress, and insect attack by inducing specific gene expression. However, little is known about the mechanism of plant defense against herbivore attack at a protein level. Using a high-resolution 2-D gel, we identified 62 MeJA-responsive proteins and measured protein expression level changes.

RESULTS

Among these 62 proteins, 43 proteins levels were increased while 11 proteins were decreased. We also found eight proteins uniquely expressed in response to MeJA treatment. Data are available via ProteomeXchange with identifier PXD001793. The proteins identified in this study have important biological functions including photosynthesis and energy related proteins (38.4%), protein folding, degradation and regulated proteins (15.0%), stress and defense regulated proteins (11.7%), and redox-responsive proteins (8.3%). The expression levels of four important genes were determined by qRT-PCR analysis. The expression levels of these proteins did not correlate well with their translation levels. To test the defense functions of the differentially expressed proteins, expression vectors of four protein coding genes were constructed to express in-fusion proteins in E. coli. The expressed proteins were used to feed Ostrinia furnacalis, the Asian corn borer (ACB). Our results demonstrated that the recombinant proteins of pathogenesis-related protein 1 (PR1) and thioredoxin M-type, chloroplastic precursor (TRXM) showed the significant inhibition on the development of larvae and pupae.

CONCLUSIONS

We found MeJA could not only induce plant defense mechanisms to insects, it also enhanced toxic protein production that potentially can be used for bio-control of ACB.

Mechanism of salinity tolerance in plants: physiological, biochemical, and molecular characterization

Salinity is a major abiotic stress limiting growth and productivity of plants in many areas of the world due to increasing use of poor quality of water for irrigation and soil salinization. Plant adaptation or tolerance to salinity stress involves complex physiological traits, metabolic pathways, and molecular or gene networks. A comprehensive understanding on how plants respond to salinity stress at different levels and an integrated approach of combining molecular tools with physiological and biochemical techniques are imperative for the development of salt-tolerant varieties of plants in salt-affected areas. Recent research has identified various adaptive responses to salinity stress at molecular, cellular, metabolic, and physiological levels, although mechanisms underlying salinity tolerance are far from being completely understood. This paper provides a comprehensive review of major research advances on biochemical, physiological, and molecular mechanisms regulating plant adaptation and tolerance to salinity stress.

Drought and salt tolerance enhancement of transgenic Arabidopsis by overexpression of the vacuolar pyrophosphatase 1 (EVP1) gene from Eucalyptus globulus

Vacuolar solute accumulation has been shown to be a mechanism by which plants are capable of increasing drought and salt tolerance. The exposure of plants to NaCl induces H+ transport into the vacuole by specialized pumps. One of them corresponds to the vacuolar H+-pyrophosphatase, which generates a H+ gradient across the vacuolar membrane. In our laboratory we isolated the first cDNA sequence of a vacuolar pyrophosphatase type I (EVP1) from Eucalyptus globulus. Using real-time PCR we confirmed that EVP1 participates in Eucalyptus plants' response to drought and salt stress through an ABA independent pathway. Additionally, the overexpression of EVP1 in transgenic Arabidopsis resulted in an enhancement of drought and salt tolerance. Interestingly we established that the transgenic plants had a higher number of root hairs, which may have a positive effect on the plant's response to drought and salt stress. These results suggest that EVP1 plays an active role in abiotic stress tolerance in E. globulus, and that it may be potentially used to enhance drought and stress tolerance of plants.

MdVHA-A encodes an apple subunit A of vacuolar H(+)-ATPase and enhances drought tolerance in transgenic tobacco seedlings

Vacuole H(+)-ATPases (VHAs) are plant proton pumps, which play a crucial role in plant growth and stress tolerance. In the present study, we demonstrated that the apple vacuolar H(+)-ATPase subunit A (MdVHA-A) is highly conserved with subunit A of VHA (VHA-A) proteins from other plant species. MdVHA-A was expressed in vegetative and reproductive organs. In apple in vitro shoot cultures, expression was induced by polyethylene glycol (PEG)-mediated osmotic stress. We further verified that over-expression of MdVHA-A conferred transgenic tobacco seedlings with enhanced vacuole H+-ATPase (VHA) activity and improved drought tolerance. The enhanced PEG-mimic drought response of transgenic tobacco seedlings was related to an extended lateral root system (dependent on auxin translocation) and more efficient osmotic adjustment. Our results indicate that MdVHA-A is a candidate gene for improving drought tolerance in plants.

Role of Vacuolar H+-inorganic pyrophosphatase in tomato fruit development

cDNA corresponding to two type-I vacuolar H(+)-inorganic pyrophosphatases (V-PPases) (SlVP1, SlVP2) and one type-II V-PPase (SlVP3) was isolated from tomato fruit to investigate their role in fruit development. Southern analysis revealed that type-I V-PPase genes form a multigene family, whereas there is only one type-II V-PPase gene in the tomato genome. Although SlVP1 and SlVP2 were differentially expressed in leaves and mature fruit, the highest levels of both SlVP1 and SlVP2 mRNA were observed in fruit at 2-4 days after anthesis. The expression pattern of type-II SlVP3 was similar to that of SlVP2, and the highest levels of SlVP3 mRNA were also observed in fruit at 2-4 days after anthesis, thus suggesting that SlVP3 plays a role in early fruit development. Because SlVP1 and SlVP2 mRNA was more abundant than SlVP3 mRNA, expression of type-I V-PPases was analysed further. Type-I V-PPase mRNA was localized in ovules and their vicinities and in vascular tissue at an early stage of fruit development. Tomato RNAi lines in which the expression of type-I V-PPase genes was repressed using the fruit-specific promoter TPRP-F1 exhibited fruit growth retardation at an early stage of development. Although the major function of V-PPases in fruit has been believed to be the accumulation of materials such as sugars and organic acids in the vacuole during cell expansion and ripening, these results show that specific localization of V-PPase mRNA induced by pollination has a novel role in the cell division stage.

Ion Transporters and Abiotic Stress Tolerance in Plants

Adaptation of plants to salt stress requires cellular ion homeostasis involving net intracellular Na⁺ and Cl⁻ uptake and subsequent vacuolar compartmentalization without toxic ion accumulation in the cytosol. Sodium ions can enter the cell through several low- and high-affinity K⁺ carriers. Some members of the HKT family function as sodium transporter and contribute to Na⁺ removal from the ascending xylem sap and recirculation from the leaves to the roots via the phloem vasculature. Na⁺ sequestration into the vacuole depends on expression and activity of Na⁺/H⁺ antiporter that is driven by electrochemical gradient of protons generated by the vacuolar H⁺-ATPase and the H⁺-pyrophosphatase. Sodium extrusion at the root-soil interface is presumed to be of critical importance for the salt tolerance. Thus, a very rapid efflux of Na⁺ from roots must occur to control net rates of influx. The Na⁺/H⁺ antiporter SOS1 localized to the plasma membrane is the only Na⁺ efflux protein from plants characterized so far. In this paper, we analyze available data related to ion transporters and plant abiotic stress responses in order to enhance our understanding about how salinity and other abiotic stresses affect the most fundamental processes of cellular function which have a substantial impact on plant growth development.

Growth promotion of Yunnan Pine early seedlings in response to foliar application of IAA and IBA

A field experiment was conducted using a 3 × 3 orthogonal regression design to explore the growth promotion of one-year-old Yunnan pine seedlings (Pinus yunnanensis Franch.) in response to foliar application of IAA (indole-3-acetic acid) at rates of 0, 200 and 400 mg·L⁻¹ and IBA (indole-3-butyric acid) at rates of 0, 200 and 400 mg·L⁻¹ in order to promote the growth during the seedlings’ early stage. The experiment was conducted at the Lufeng Village Forest Farm of Yiliang County in Kunming, Yunnan, China. The results showed that IAA and IBA were effective in growth promotion of Yunnan pine seedlings. The response of both growth increment and biomass accumulation to the concentration of IAA and IBA can be modeled using a bivariate surface response, and each growth index had a peak value. Growth indexes increased with the increase of the dosage of photohormones before reaching a peak value, and then decreased. The different growth indexes had various responses to the concentrations and ratio of IAA and IBA. The foliar application of IAA in combination with IBA showed the largest improvement on the biomass of the needles, followed by stems and roots. The higher ratio of IAA promoted stem diameter growth, root system development and biomass accumulation in the needles, while a higher ratio of IBA contributed to height growth and biomass accumulation in the stem. Based on the auxin effect equations on the different growth indexes and surface response, the optimum concentrations and the (IAA:IBA) ratios can be obtained. The optimum concentrations of IAA and IBA were 167 and 186, 310 and 217, 193 and 159, 191 and 221, and 206 and 186 mg·L⁻¹, with corresponding ratios of 1:1.11, 1:0.70, 1:0.82, 1:1.15 and 1:0.90, respectively, at the maximum seedling height and collar diameter growth as well as biomass accumulation at the root, stem and needle. The above growth indexes were 22.00%, 79.80%, 48.65%, 82.20% and 107.00% higher than the control treatment.

Overexpression of the halophyte Kalidium foliatum H⁺-pyrophosphatase gene confers salt and drought tolerance in Arabidopsis thaliana

According to sequences of H(+)-pyrophosphatase genes from GenBank, a new H(+)-pyrophosphatase gene (KfVP1) from the halophyte Kalidium foliatum, a very salt-tolerant shrub that is highly succulent, was obtained by using reverse transcription PCR and rapid amplification of cDNA ends methods. The obtained KfVP1 cDNA contained a 2295 bp ORF and a 242 bp 3'-untranslated region. It encoded 764 amino acids with a calculated molecular mass of 79.78 kDa. The deduced amino acid sequence showed high identity to those of H(+)-PPase of some Chenopodiaceae plant species. Semi-quantitative PCR results revealed that transcription of KfVP1 in K. foliatum was induced by NaCl, ABA and PEG stress. Transgenic lines of A. thaliana with 35S::KfVP1 were generated. Three transgenic lines grew more vigorous than the wild type (ecotype Col-0) under salt and drought stress. Moreover, the transgenic plants accumulated more Na(+) in the leaves compared to wild type plants. These results demonstrated that KfVP1 from K. foliatum may be a functional tonoplast H(+)-pyrophosphatase in contributing to salt and drought tolerance.

Identification and characterization of interactions between abscisic acid and mitochondrial adenine nucleotide translocators

ABA (abscisic acid) is a plant hormone involved in important processes including development and stress responses. Recent reports have identified a number of plant ABA receptors and transporters, highlighting novel mechanisms of ABA action. In the present paper we describe application of a chemical proteomics approach leading to the identification of mitochondrial ANTs (adenine nucleotide translocators) as ABA-interacting proteins. Initial in vitro studies confirmed inhibition of ANT-dependent ATP translocation by ABA. Further analysis demonstrated ANT-dependent uptake of ABA into both recombinant Arabidopsis thaliana ANT2-containing proteoliposomes and native isolated spinach mitochondria; the latter with a Km of 3.5 μM and a Vmax of 2.5 nmol/min per g of protein. ATP was found to inhibit ANT-dependent ABA translocation. Specificity profiles highlight the possibility of mechanistic differences in translocation of ABA and ATP. Finally, ABA was shown to stimulate ATPase activity in spinach mitochondrial extracts. ABA concentrations in plant cells are estimated to reach the low micromolar range during stress responses, supporting potential physiological relevance of these in vitro findings. Overall, the present in vitro work suggests the possibility of as yet uncharacterized mechanisms of ABA action in planta related to inhibition of mitochondrial ATP translocation and functional localization of ABA in the mitochondrial matrix.

Na+-translocating membrane pyrophosphatases are widespread in the microbial world and evolutionarily precede H+-translocating pyrophosphatases

Membrane pyrophosphatases (PPases), divided into K(+)-dependent and K(+)-independent subfamilies, were believed to pump H(+) across cell membranes until a recent demonstration that some K(+)-dependent PPases function as Na(+) pumps. Here, we have expressed seven evolutionarily important putative PPases in Escherichia coli and estimated their hydrolytic, Na(+) transport, and H(+) transport activities as well as their K(+) and Na(+) requirements in inner membrane vesicles. Four of these enzymes (from Anaerostipes caccae, Chlorobium limicola, Clostridium tetani, and Desulfuromonas acetoxidans) were identified as K(+)-dependent Na(+) transporters. Phylogenetic analysis led to the identification of a monophyletic clade comprising characterized and predicted Na(+)-transporting PPases (Na(+)-PPases) within the K(+)-dependent subfamily. H(+)-transporting PPases (H(+)-PPases) are more heterogeneous and form at least three independent clades in both subfamilies. These results suggest that rather than being a curious rarity, Na(+)-PPases predominantly constitute the K(+)-dependent subfamily. Furthermore, Na(+)-PPases possibly preceded H(+)-PPases in evolution, and transition from Na(+) to H(+) transport may have occurred in several independent enzyme lineages. Site-directed mutagenesis studies facilitated the identification of a specific Glu residue that appears to be central in the transport mechanism. This residue is located in the cytoplasm-membrane interface of transmembrane helix 6 in Na(+)-PPases but shifted to within the membrane or helix 5 in H(+)-PPases. These results contribute to the prediction of the transport specificity and K(+) dependence for a particular membrane PPase sequence based on its position in the phylogenetic tree, identity of residues in the K(+) dependence signature, and position of the membrane-located Glu residue.

Abscisic acid deficiency of developing pea embryos achieved by immunomodulation attenuates developmental phase transition and storage metabolism

The transition of pea embryos from pre-storage to maturation is partially controlled by abscisic acid (ABA). Immunomodulation in pea embryos specifically reduces free ABA levels during transition stages. Such seeds are, therefore, suitable models for studying ABA deficiency by global transcript and metabolite analysis. Compared with the wild type, anti-ABA seeds are smaller, contain fewer globulins and show lower dry matter accumulation and delayed differentiation. Free sugars are decreased, indicating lower uptake and/or elevated mobilisation. Lower levels of trans-zeatins suggest that ABA reduction influences rates of cytokinin synthesis and/or its level of accumulation. Abscisic acid deficiency leads to a general downregulation of gene expression related to transcription and translation. At the transcriptional level, anti-ABA embryos reveal a wide-range repression of carbohydrate oxidation, downregulated sucrose mobilisation, glycolysis and the tricarboxylic acid cycle/Krebs cycle (TCA cycle). Genes related to starch, amino acid and storage protein biosynthesis are downregulated, indicating a general decrease in metabolic fluxes. We conclude that during embryo differentiation ABA triggers broad upregulation of gene activity and genetic reprogramming, involving regulated protein degradation via the ubiquitin/proteasome system. Abscisic acid deficiency affects gene expression associated with transport processes and stimulation of membrane energisation. Our study identified mediators and downstream signalling elements of ABA during embryo differentiation, such as the transcription factor FUSCA3, SnRK1 kinase and Ca(2+) signalling processes. This suggests that ABA interacts with SnRK1 complexes, thus connecting SnRK1, sugar and stress signalling with ABA. Certain protein kinases/phosphatases known to negatively respond to ABA are upregulated in the modulated line, whilst those which respond positively are downregulated, pointing to a highly coordinated response of the gene network to ABA levels.

Isolation and functional characterization of the Arabidopsis salt-tolerance 32 (AtSAT32) gene associated with salt tolerance and ABA signaling

Recently, we have isolated salt-tolerance genes (SATs) on the basis of the overexpression screening of yeast with a maize cDNA library from kernels. One of the selected genes [salt-tolerance 32 (SAT32)] appears to be a key determinant for salt stress tolerance in yeast cells. Maize SAT32 cDNA encodes for a 49-kDa protein, which is 41% identity with the Arabidopsis salt-tolerance 32 (AtSAT32) unknown gene. Arabidopsis Transfer-DNA (T-DNA) knockout AtSAT32 (atsat32) altered root elongation, including reduced silique length and reduced seed number. In an effort to further assess salinity tolerance in Arabidopsis, we have functionally characterized the AtSAT32 gene and determined that salinity and the plant hormone ABA induced the expression of AtSAT32. The atsat32 mutant was more sensitive to salinity than the wild-type plant. On the contrary, Arabidopsis overexpressing AtSAT32 (35S::AtSAT32) showed enhanced salt tolerance and increased activity of vacuolar H(+)-pyrophosphatase (V-PPase, EC 3.6.1.1) under high-salt conditions. Consistent with these observations, 35S::AtSAT32 plants exhibited increased expression of salt-responsive and ABA-responsive genes, including the Rd29A, Erd15, Rd29B, Rd22 and RAB18 genes. Therefore, our results indicate that AtSAT32 is involved in both salinity tolerance and ABA signaling as a positive regulator in Arabidopsis.

Plant proton pumps

Chemiosmotic circuits of plant cells are driven by proton (H(+)) gradients that mediate secondary active transport of compounds across plasma and endosomal membranes. Furthermore, regulation of endosomal acidification is critical for endocytic and secretory pathways. For plants to react to their constantly changing environments and at the same time maintain optimal metabolic conditions, the expression, activity and interplay of the pumps generating these H(+) gradients have to be tightly regulated. In this review, we will highlight results on the regulation, localization and physiological roles of these H(+)- pumps, namely the plasma membrane H(+)-ATPase, the vacuolar H(+)-ATPase and the vacuolar H(+)-PPase.