Two widely expressed plasma membrane H(+)-ATPase isoforms of Nicotiana tabacum are differentially regulated by phosphorylation of their penultimate threonine

Plasma membrane H+-ATPases in mineral nutrition and crop improvement

Plasma membrane H+-ATPases (PMAs) pump H+ out of the cytoplasm by consuming ATP to generate a membrane potential and proton motive force for the transmembrane transport of nutrients into and out of plant cells. PMAs are involved in nutrient acquisition by regulating root growth, nutrient uptake, and translocation, as well as the establishment of symbiosis with arbuscular mycorrhizas. Under nutrient stresses, PMAs are activated to pump more H+ and promote organic anion excretion, thus improving nutrient availability in the rhizosphere. Herein we review recent progress in the physiological functions and the underlying molecular mechanisms of PMAs in the efficient acquisition and utilization of various nutrients in plants. We also discuss perspectives for the application of PMAs in improving crop production and quality.

Molecular analysis of the 14-3-3 genes in Panax ginseng and their responses to heat stress

Background

Panax Ginseng is a perennial and semi-shady herb with tremendous medicinal value. Due to its unique botanical characteristics, ginseng is vulnerable to various abiotic factors during its growth and development, especially in high temperatures. Proteins encoded by 14-3-3 genes form a highly conserved protein family that widely exists in eukaryotes. The 14-3-3 family regulates the vital movement of cells and plays an essential role in the response of plants to abiotic stresses, including high temperatures. Currently, there is no relevant research on the 14-3-3 genes of ginseng.

Methods

The identification of the ginseng 14-3-3 gene family was mainly based on ginseng genomic data and Hidden Markov Models (HMM). We used bioinformatics-related databases and tools to analyze the gene structure, physicochemical properties, cis-acting elements, gene ontology (GO), phylogenetic tree, interacting proteins, and transcription factor regulatory networks. We analyzed the transcriptome data of different ginseng tissues to clarify the expression pattern of the 14-3-3 gene family in ginseng. The expression level and modes of 14-3-3 genes under heat stress were analyzed by quantitative real-time PCR (qRT-PCR) technology to determine the genes in the 14-3-3 gene family responding to high-temperature stress.

Results

In this study, 42 14-3-3 genes were identified from the ginseng genome and renamed PgGF14-1 to PgGF14-42. Gene structure and evolutionary relationship research divided PgGF14s into epsilon (ε) and non-epsilon (non-ε) groups, mainly located in four evolutionary branches. The gene structure and motif remained highly consistent within a subgroup. The physicochemical properties and structure of the predicted PgGF14 proteins conformed to the essential characteristics of 14-3-3 proteins. RNA-seq results indicated that the detected PgGF14s existed in different organs and tissues but differed in abundance; their expression was higher in roots, stems, leaves, and fruits but lower in seeds. The analysis of GO, cis-acting elements, interacting proteins, and regulatory networks of transcription factors indicated that PgGF14s might participate in physiological processes, such as response to stress, signal transduction, material synthesis-metabolism, and cell development. The qRT-PCR results indicated PgGF14s had multiple expression patterns under high-temperature stress with different change trends in several treatment times, and 38 of them had an apparent response to high-temperature stress. Furthermore, PgGF14-5 was significantly upregulated, and PgGF14-4 was significantly downregulated in all treatment times. This research lays a foundation for further study on the function of 14-3-3 genes and provides theoretical guidance for investigating abiotic stresses in ginseng.

Calcium-dependent protein kinase 16 phosphorylates and activates the aquaporin PIP2;2 to regulate reversible flower opening in Gentiana scabra

Flower opening is important for successful pollination in many plant species, and some species repeatedly open and close their flowers. This is thought to be due to turgor pressure changes caused by water influx/efflux, which depends on osmotic oscillations in the cells. In some ornamental plants, water-transporting aquaporins, also known as plasma membrane intrinsic proteins (PIPs), may play an important role in flower opening. However, the molecular mechanism(s) involved in corolla movement are largely unknown. Gentian (Gentiana spp.) flowers undergo reversible movement in response to temperature and light stimuli; using gentian as a model, we showed that the Gentiana scabra aquaporins GsPIP2;2 and GsPIP2;7 regulate repeated flower opening. In particular, phosphorylation of a C-terminal serine residue of GsPIP2;2 is important for its transport activity and relates closely to the flower re-opening rate. Furthermore, GsPIP2;2 is phosphorylated and activated by the calcium (Ca2+)-dependent protein kinase GsCPK16, which is activated by elevated cytosolic Ca2+ levels in response to temperature and light stimuli. We propose that GsCPK16-dependent phosphorylation and activation of GsPIP2;2 regulate gentian flower re-opening, with stimulus-induced Ca2+ signals acting as triggers.

Production of Recombinant Glycoproteins in Nicotiana tabacum BY-2 Suspension Cells

This protocol describes a robust method to obtain transgenic Nicotiana tabacum BY-2 cells that produce glycoproteins of interest via Agrobacterium tumefaciens transformation. Compared to biolistics-based transformation, this procedure requires only standard laboratory equipment.

Promotion and Upregulation of a Plasma Membrane Proton-ATPase Strategy: Principles and Applications

Plasma membrane proton-ATPase (PM H+-ATPase) is a primary H+ transporter that consumes ATP in vivo and is a limiting factor in the blue light-induced stomatal opening signaling pathway. It was recently reported that manipulation of PM H+-ATPase in stomatal guard cells and other tissues greatly improved leaf photosynthesis and plant growth. In this report, we review and discuss the function of PM H+-ATPase in the context of the promotion and upregulation H+-ATPase strategy, including associated principles pertaining to enhanced stomatal opening, environmental plasticity, and potential applications in crops and nanotechnology. We highlight the great potential of the promotion and upregulation H+-ATPase strategy, and explain why it may be applied in many crops in the future.

Augmented CO2 tolerance by expressing a single H+-pump enables microalgal valorization of industrial flue gas

Microalgae can accumulate various carbon-neutral products, but their real-world applications are hindered by their CO2 susceptibility. Herein, the transcriptomic changes in a model microalga, Chlamydomonas reinhardtii, in a high-CO2 milieu (20%) are evaluated. The primary toxicity mechanism consists of aberrantly low expression of plasma membrane H+-ATPases (PMAs) accompanied by intracellular acidification. Our results demonstrate that the expression of a universally expressible PMA in wild-type strains makes them capable of not only thriving in acidity levels that they usually cannot survive but also exhibiting 3.2-fold increased photoautotrophic production against high CO2 via maintenance of a higher cytoplasmic pH. A proof-of-concept experiment involving cultivation with toxic flue gas (13 vol% CO2, 20 ppm NOX, and 32 ppm SOX) shows that the production of CO2-based bioproducts by the strain is doubled compared with that by the wild-type, implying that this strategy potentially enables the microalgal valorization of CO2 in industrial exhaust.

Plasma Membrane H+-ATPase SmPHA4 Negatively Regulates the Biosynthesis of Tanshinones in Salvia miltiorrhiza

Salvia miltiorrhiza Bunge has been widely used in the treatment of cardiovascular and cerebrovascular diseases, due to the pharmacological action of its active components such as the tanshinones. Plasma membrane (PM) H⁺-ATPase plays key roles in numerous physiological processes in plants. However, little is known about the PM H⁺-ATPase gene family in S. miltiorrhiza (Sm). Here, nine PM H⁺-ATPase isoforms were identified and named SmPHA1-SmPHA9. Phylogenetic tree analysis showed that the genetic distance of SmPHAs was relatively far in the S. miltiorrhiza PM H⁺-ATPase family. Moreover, the transmembrane structures were rich in SmPHA protein. In addition, SmPHA4 was found to be highly expressed in roots and flowers. HPLC revealed that accumulation of dihydrotanshinone (DT), cryptotanshinone (CT), and tanshinone I (TI) was significantly reduced in the SmPHA4-OE lines but was increased in the SmPHA4-RNAi lines, ranging from 2.54 to 3.52, 3.77 to 6.33, and 0.35 to 0.74 mg/g, respectively, suggesting that SmPHA4 is a candidate regulator of tanshinone metabolites. Moreover, qRT-PCR confirmed that the expression of tanshinone biosynthetic-related key enzymes was also upregulated in the SmPHA4-RNAi lines. In summary, this study highlighted PM H⁺-ATPase function and provided new insights into regulatory candidate genes for modulating secondary metabolism biosynthesis in S. miltiorrhiza.

Silencing the Oryza sativa plasma membrane H+-ATPase isoform OsA2 affects grain yield and shoot growth and decreases nitrogen concentration

The plasma membrane (PM) H⁺-ATPase (EC 3.6.1.3.) is a key component involved in nutrient uptake. There are 10 PM H⁺-ATPase isoforms in the rice genome (OsA1-OsA10), and OsA2 is highly responsive to nitrate (NO₃^-). We investigated the role that the OsA2 isoform plays in the total N and growth of rice (Oryza sativa). By the use of artificial microRNA, mutant osa2 rice lines presented ∼70 % downregulated levels of OsA2. Three osa2 lines and control plants (transformed with an empty IRS154 vector and named IRS) were cultivated in the greenhouse to evaluate grain and shoot production. For hydroponic experiments, the same lines were grown in Hoagland solution under two different NO₃^- levels for 30 days - 0.2 mM NO₃^--N (low N) or 2.0 mM NO₃^--N (sufficient N) - or were grown for three days without NO₃^- (starvation) after 27 days under 2.0 mM NO₃^--N. In the greenhouse experiments, compared with the IRS plants, the osa2 lines had lower shoot fresh weights, grain yields and SPAD values. Moreover, compared with the IRS plants, the three osa2 lines grown hydroponically under low NO₃^- levels had lower N concentration and net flux of NO₃^-. PM H⁺-ATPase activity was lower in the osa2 mutants than in the IRS plants. The relatively low N concentration in the osa2 lines was not due to lower expression of OsNRT2.1, OsNRT2.2, or OsNAR2.1. These results indicate that the specific PM H⁺-ATPase isoform OsA2 affects the net flux of NO₃^-, N concentration, and grain yield.

The pH of the Apoplast: Dynamic Factor with Functional Impact Under Stress

The apoplast is an interconnected compartment with a thin water-film that alkalinizes under stress. This systemic pH increase may be a secondary effect without functional implications, arising from ion movements or proton-pump regulations. On the other hand, there are increasing indications that it is part of a mechanism to withstand stress. Regardless of this controversy, alkalinization of the apoplast has received little attention. The apoplastic pH (pH_apo) increases not only during plant-pathogen interactions but also in response to salinity or drought. Not much is known about the mechanisms that cause the leaf apoplast to alkalinize, nor whether, and if so, how functional impact is conveyed. Controversial explanations have been given, and the unusual complexity of pH_apo regulation is considered as the primary reason behind this lack of knowledge. A gathering of scattered information revealed that changes in pH_apo convey functionality by regulating stomatal aperture via the effects exerted on abscisic acid. Moreover, apoplastic alkalinization may regulate growth under stress, whereas this needs to be verified. In this review, a comprehensive survey about several physiological mechanisms that alkalize the apoplast under stress is given, and the suitability of apoplastic alkalinization as transducing element for the transmission of sensory information is discussed.

Significant and unique changes in phosphorylation levels of four phosphoproteins in two apple rootstock genotypes under drought stress

Drought stress is a major problem around the world and there is still little molecular mechanism about how fruit crops deal with moderate drought stress. Here, the physiological and phosphoproteomic responses of drought-sensitive genotype (M26) and drought-tolerant genotype (MBB) under moderate drought stress were investigated. Our results of the physiology analysis indicated that the MBB genotype could produce more osmosis-regulating substances. Furthermore, phosphoproteins from leaves of both genotypes under moderate drought stress were analyzed using the isobaric tags for relative and absolute quantification technology. A total of 595 unique phosphopeptides, 682 phosphorylated sites, and 446 phosphoproteins were quantitatively analyzed in the two genotypes. Five and thirty-five phosphoproteins with the phosphorylation levels significantly changed (PLSC) were identified in M26 and MBB, respectively. Among these, four PLSC phosphoproteins were common to both genotypes, perhaps indicating a partial overlap of the mechanisms to moderate drought stress. Gene ontology analyses revealed that the PLSC phosphoproteins represent a unique combination of metabolism, transcription, translation, and protein processing, suggesting that the response in apple to moderate drought stress encompasses a new and unique homeostasis of major cellular processes. The basic trend was an increase in protein and organic molecules abundance related to drought. These increases were higher in MBB than in M26. Our study is the first to address the phosphoproteome of apple rootstocks in response to moderate drought stress, and provide insights into the molecular regulation mechanisms of apple rootstock under moderate drought stress.

Analysis of tomato plasma membrane H(+)-ATPase gene family suggests a mycorrhiza-mediated regulatory mechanism conserved in diverse plant species

In plants, the plasma membrane H(+)-ATPase (HA) is considered to play a crucial role in regulating plant growth and respoding to environment stresses. Multiple paralogous genes encoding different isozymes of HA have been identified and characterized in several model plants, while limited information of the HA gene family is available to date for tomato. Here, we describe the molecular and expression features of eight HA-encoding genes (SlHA1-8) from tomato. All these genes are interrupted by multiple introns with conserved positions. SlHA1, 2, and 4 were widely expressed in all tissues, while SlHA5, 6, and 7 were almost only expressed in flowers. SlHA8, the transcripts of which were barely detectable under normal or nutrient-/salt-stress growth conditions, was strongly activated in arbuscular mycorrhizal (AM) fungal-colonized roots. Extreme lack of SlHA8 expression in M161, a mutant defective to AM fungal colonization, provided genetic evidence towards the dependence of its expression on AM symbiosis. A 1521-bp SlHA8 promoter could direct the GUS reporter expression specifically in colonized cells of transgenic tobacco, soybean, and rice mycorrhizal roots. Promoter deletion assay revealed a 223-bp promoter fragment of SlHA8 containing a variant of AM-specific cis-element MYCS (vMYCS) sufficient to confer the AM-induced activity. Targeted deletion of this motif in the corresponding promoter region causes complete abolishment of GUS staining in mycorrhizal roots. Together, these results lend cogent evidence towards the evolutionary conservation of a potential regulatory mechanism mediating the activation of AM-responsive HA genes in diverse mycorrhizal plant species.

Plasma Membrane H(+)-ATPase Regulation in the Center of Plant Physiology

The plasma membrane (PM) H(+)-ATPase is an important ion pump in the plant cell membrane. By extruding protons from the cell and generating a membrane potential, this pump energizes the PM, which is a prerequisite for growth. Modification of the autoinhibitory terminal domains activates PM H(+)-ATPase activity, and on this basis it has been hypothesized that these regulatory termini are targets for physiological factors that activate or inhibit proton pumping. In this review, we focus on the posttranslational regulation of the PM H(+)-ATPase and place regulation of the pump in an evolutionary and physiological context. The emerging picture is that multiple signals regulating plant growth interfere with the posttranslational regulation of the PM H(+)-ATPase.

Regulation of the plasma membrane proton pump (H(+)-ATPase) by phosphorylation

In plants and fungi, energetics at the plasma membrane is provided by a large protonmotive force (PMF) generated by the family of P-type ATPases specialized for proton transport (commonly called PM H(+)-ATPases or, in Arabidopsis, AHAs for Arabidopsis H(+)-ATPases). Studies have demonstrated that this 100-kDa protein is essential for plant growth and development. Posttranslational modifications of the H(+)-ATPase play crucial roles in its regulation. Phosphorylation of several Thr and Ser residues within the carboxy terminal regulatory domain composed of ∼100 amino acids change in response to environmental stimuli, endogenous hormones, and nutrient conditions. Recently developed mass spectrometric technologies provide a means to carefully quantify these changes in H(+)-ATPase phosphorylation at the different sites. These chemical modifications can then be genetically tested in planta by complementing the loss-of-function aha mutants with phosphomimetic mutations. Interestingly, recent data suggest that phosphatase-mediated changes in PM H(+)-ATPase phosphorylation are important in mediating auxin-regulated growth. Thus, as with another hormone (abscisic acid), dephosphorylation by phosphatases, rather than kinase mediated phosphorylation, may be an important focal point for regulation during plant signal transduction. Although interactions with other proteins have also been implicated in ATPase regulation, the very hydrophobic nature and high concentration of this polytopic protein presents special challenges in evaluating the biological significance of these interactions. Only by combining biochemical and genetic experiments can we attempt to meet these challenges to understand the essential molecular details by which this protein functions in planta.

Potential regulatory phosphorylation sites in a Medicago truncatula plasma membrane proton pump implicated during early symbiotic signaling in roots

In plants and fungi the plasma membrane proton pump generates a large proton-motive force that performs essential functions in many processes, including solute transport and the control of cell elongation. Previous studies in yeast and higher plants have indicated that phosphorylation of an auto-inhibitory domain is involved in regulating pump activity. In this report we examine the Medicago truncatula plasma membrane proton pump gene family, and in particular MtAHA5. Yeast complementation assays with phosphomimetic mutations at six candidate sites support a phosphoregulatory role for two residues, suggesting a molecular model to explain early Nod factor-induced changes in the plasma membrane proton-motive force of legume root cells.

Cytoplasmic pH-Stat during Phenanthrene Uptake by Wheat Roots: A Mechanistic Consideration

Dietary intake of plant-based foods is a major contribution to the total exposure of polycyclic aromatic hydrocarbons (PAHs). However, the mechanisms underlying PAH uptake by roots remain poorly understood. This is the first study, to our knowledge, to reveal cytoplasmic pH change and regulation in response to PAH uptake by wheat roots. An initial drop of cytoplasmic pH, which is concentration-dependent upon exposure to phenanthrene (a model PAH), was followed by a slow recovery, indicating the operation of a powerful cytoplasmic pH regulating system. Intracellular buffers are prevalent and act in the first few minutes of acidification. Phenanthrene activates plasmalemma and tonoplast H(+) pump. Cytolasmic acidification is also accompanied by vacuolar acidification. In addition, phenanthrene decreases the activity of phosphoenolpyruvate carboxylase and malate concentration. Moreover, phenanthrene stimulates nitrate reductase. Therefore, it is concluded that phenanthrene uptake induces cytoplasmic acidification, and cytoplasmic pH recovery is achieved via physicochemical buffering, proton transport outside cytoplasm into apoplast and vacuole, and malate decarboxylation along with nitrate reduction. Our results provide a novel insight into PAH uptake by wheat roots, which is relevant to strategies for reducing PAH accumulation in wheat for food safety and improving phytoremediation of PAH-contaminated soils or water by agronomic practices.

Phosphoproteomic analysis of the response of maize leaves to drought, heat and their combination stress

Drought and heat stress, especially their combination, greatly affect crop production. Many studies have described transcriptome, proteome and phosphoproteome changes in response of plants to drought or heat stress. However, the study about the phosphoproteomic changes in response of crops to the combination stress is scare. To understand the mechanism of maize responses to the drought and heat combination stress, phosphoproteomic analysis was performed on maize leaves by using multiplex iTRAQ-based quantitative proteomic and LC-MS/MS methods. Five-leaf-stage maize was subjected to drought, heat or their combination, and the leaves were collected. Globally, heat, drought and the combined stress significantly changed the phosphorylation levels of 172, 149, and 144 phosphopeptides, respectively. These phosphopeptides corresponded to 282 proteins. Among them, 23 only responded to the combined stress and could not be predicted from their responses to single stressors; 30 and 75 only responded to drought and heat, respectively. Notably, 19 proteins were phosphorylated on different sites in response to the single and combination stresses. Of the seven significantly enriched phosphorylation motifs identified, two were common for all stresses, two were common for heat and the combined stress, and one was specific to the combined stress. The signaling pathways in which the phosphoproteins were involved clearly differed among the three stresses. Functional characterization of the phosphoproteins and the pathways identified here could lead to new targets for the enhancement of crop stress tolerance, which will be particularly important in the face of climate change and the increasing prevalence of abiotic stressors.

Phosphoproteomic analysis of the response of maize leaves to drought, heat and their combination stress

Drought and heat stress, especially their combination, greatly affect crop production. Many studies have described transcriptome, proteome and phosphoproteome changes in response of plants to drought or heat stress. However, the study about the phosphoproteomic changes in response of crops to the combination stress is scare. To understand the mechanism of maize responses to the drought and heat combination stress, phosphoproteomic analysis was performed on maize leaves by using multiplex iTRAQ-based quantitative proteomic and LC-MS/MS methods. Five-leaf-stage maize was subjected to drought, heat or their combination, and the leaves were collected. Globally, heat, drought and the combined stress significantly changed the phosphorylation levels of 172, 149, and 144 phosphopeptides, respectively. These phosphopeptides corresponded to 282 proteins. Among them, 23 only responded to the combined stress and could not be predicted from their responses to single stressors; 30 and 75 only responded to drought and heat, respectively. Notably, 19 proteins were phosphorylated on different sites in response to the single and combination stresses. Of the seven significantly enriched phosphorylation motifs identified, two were common for all stresses, two were common for heat and the combined stress, and one was specific to the combined stress. The signaling pathways in which the phosphoproteins were involved clearly differed among the three stresses. Functional characterization of the phosphoproteins and the pathways identified here could lead to new targets for the enhancement of crop stress tolerance, which will be particularly important in the face of climate change and the increasing prevalence of abiotic stressors.

Phosphoproteome analysis reveals new drought response and defense mechanisms of seedling leaves in bread wheat (Triticum aestivum L.)

Drought is a major form of abiotic stress that significantly affects plant growth and development. In this study, we performed the first phosphoproteome analysis of seedling leaves from two bread wheat cultivars (Hanxuan 10 and Ningchun 47) subjected to drought stress. As a result, a total of 191 and 251 unique phosphopeptides, representing 173 and 227 phosphoproteins in two cultivars, respectively, were identified as being significant changes in phosphorylation level (SCPL) under drought stress. Through the comparison of SCPL phosphoproteins between two cultivars, 31 common SCPL phosphoproteins were found in both cultivars. Function analysis showed that the SCPL phosphoproteins in the two cultivars are mainly involved in three biological processes: RNA transcription/processing, stress/detoxification/defense, and signal transduction. Further analyses revealed that some SCPL phosphoproteins may play key roles in signal transduction and the signaling cascade under drought stress. Furthermore, some phosphoproteins related to drought tolerance and osmotic regulation exhibited significant phosphorylation changes. This study used a series of bioinformatics tools to profile the phosphorylation status of wheat seedling leaves under drought stress with greater accuracy.

BIOLOGICAL SIGNIFICANCE

Drought is of the most studied abiotic stresses, because it severely restricts the development and yield of plants. In this study, large numbers of stress-related phosphoproteins are identified from the two bread wheat cultivars. These phosphoproteins contribute to signal transduction, osmotic regulation and ROS scavenging under water stress. This work provides a detailed insight into the mechanisms of drought response and defense in bread wheat from the perspective of phosphoproteomics, and identifies some important drought-tolerant candidates for further transgenosis study and incorporation into the breeding of resistant cultivars.

Pump up the volume - a central role for the plasma membrane H(+) pump in pollen germination and tube growth

The plasma membrane H(+) ATPase is a member of the P-ATPase family transporting H(+) from the cytosol to the extracellular space and thus energizing the plasma membrane for the uptake of ions and nutrients. As a housekeeping gene, this protein can be detected in almost every plant cell including the exclusive expression of specific isoforms in pollen grains and tubes where its activity is a prerequisite for successful germination and growth of pollen tubes. This review summarizes the current knowledge on pollen PM H(+) ATPases and hypothesizes a central role for pollen-specific isoforms of this protein in tube growth. External as well as cytosolic signals from signal transduction and metabolic pathways are integrated by the PM H(+) ATPase and directly translated to tube growth rates, allocating the PM H(+) ATPase to an essential node in the signalling network of pollen tubes in their race to the ovule.

Expression of a translationally fused TAP-tagged plasma membrane proton pump in Arabidopsis thaliana

The Arabidopsis thaliana plasma membrane proton ATPase genes, AHA1 and AHA2, are the two most highly expressed isoforms of an 11 gene family and are collectively essential for embryo development. We report the translational fusion of a tandem affinity-purification tag to the 5′ end of the AHA1 open reading frame in a genomic clone. Stable expression of TAP-tagged AHA1 in Arabidopsis rescues the embryonic lethal phenotype of endogenous double aha1/aha2 knockdowns. Western blots of SDS-PAGE and Blue Native gels show enrichment of AHA1 in plasma membrane fractions and indicate a hexameric quaternary structure. TAP-tagged AHA1 rescue lines exhibited reduced vertical root growth. Analysis of the plasma membrane and soluble proteomes identified several plasma membrane-localized proteins with alterred abundance in TAP-tagged AHA1 rescue lines compared to wild type. Using affinity-purification mass spectrometry, we uniquely identified two additional AHA isoforms, AHA9 and AHA11, which copurified with TAP-tagged AHA1. In conclusion, we have generated transgenic Arabidopsis lines in which a TAP-tagged AHA1 transgene has complemented all essential endogenous AHA1 and AHA2 functions and have shown that these plants can be used to purify AHA1 protein and to identify in planta interacting proteins by mass spectrometry.

Membrane transport, sensing and signaling in plant adaptation to environmental stress

Plants are generally well adapted to a wide range of environmental conditions. Even though they have notably prospered in our planet, stressful conditions such as salinity, drought and cold or heat, which are increasingly being observed worldwide in the context of the ongoing climate changes, limit their growth and productivity. Behind the remarkable ability of plants to cope with these stresses and still thrive, sophisticated and efficient mechanisms to re-establish and maintain ion and cellular homeostasis are involved. Among the plant arsenal to maintain homeostasis are efficient stress sensing and signaling mechanisms, plant cell detoxification systems, compatible solute and osmoprotectant accumulation and a vital rearrangement of solute transport and compartmentation. The key role of solute transport systems and signaling proteins in cellular homeostasis is addressed in the present work. The full understanding of the plant cell complex defense mechanisms under stress may allow for the engineering of more tolerant plants or the optimization of cultivation practices to improve yield and productivity, which is crucial at the present time as food resources are progressively scarce.

A phosphorylation in the c-terminal auto-inhibitory domain of the plant plasma membrane H+-ATPase activates the enzyme with no requirement for regulatory 14-3-3 proteins

The plant plasma membrane H(+)-ATPase is regulated by an auto-inhibitory C-terminal domain that can be displaced by phosphorylation of the penultimate residue, a Thr, and the subsequent binding of 14-3-3 proteins. By mass spectrometric analysis of plasma membrane H(+)-ATPase isoform 2 (PMA2) isolated from Nicotiana tabacum plants and suspension cells, we identified a new phosphorylation site, Thr-889, in a region of the C-terminal domain upstream of the 14-3-3 protein binding site. This residue was mutated into aspartate or alanine, and the mutated H(+)-ATPases expressed in the yeast Saccharomyces cerevisiae. Unlike wild-type PMA2, which could replace the yeast H(+)-ATPases, the PMA2-Thr889Ala mutant did not allow yeast growth, whereas the PMA2-Thr889Asp mutant resulted in improved growth and increased H(+)-ATPase activity despite reduced phosphorylation of the PMA2 penultimate residue and reduced 14-3-3 protein binding. To determine whether the regulation taking place at Thr-889 was independent of phosphorylation of the penultimate residue and 14-3-3 protein binding, we examined the effect of combining the PMA2-Thr889Asp mutation with mutations of other residues that impair phosphorylation of the penultimate residue and/or binding of 14-3-3 proteins. The results showed that in yeast, PMA2 Thr-889 phosphorylation could activate H(+)-ATPase if PMA2 was also phosphorylated at its penultimate residue. However, binding of 14-3-3 proteins was not required, although 14-3-3 binding resulted in further activation. These results were confirmed in N. tabacum suspension cells. These data define a new H(+)-ATPase activation mechanism that can take place without 14-3-3 proteins.

The proteome complement of Nicotiana tabacum Bright-Yellow-2 culture cells

The Nicotiana tabacum Bright-Yellow-2 (BY2) cell line is one of most commonly used plant suspension cell lines and offers interesting properties, such as fast growth, amenability to genetic transformation, and synchronization of cell division. To build a proteome reference map of BY2 cell proteins, we isolated the soluble proteins from N. tabacum BY2 cells at the end of the exponential growth phase and analyzed them by 2-DE and MALDI TOF-TOF. Of the 1422 spots isolated, 795 were identified with a significant score, corresponding to 532 distinct proteins.

Activation of the plasma membrane H (+) -ATPase by acid stress: antibodies as a tool to follow the phosphorylation status of the penultimate activating Thr

Tight regulation of the plasma membrane proton pump ATPase (H (+) -ATPase) is necessary for controlling the membrane potential that energizes secondary transporters. This regulation relies on the phosphorylation of the H (+) -ATPase penultimate residue, a theonine, and the subsequent binding of regulatory 14-3-3 proteins, which results in enzyme activation. Using phospho-specific antibodies directed against the phosphorylable Thr of either PMA2 (Plasma membrane H (+) -ATPase from N. plumbaginifolia) or PMA4, we showed that the kinetics and extent of phosphorylation differ between both isoforms according to the growth or environmental conditions like cold stress. (1) Here, we used phospho-specific antibodies to follow PMA2 Thr phosphorylation upon acidification of the cytosol by incubating N. tabacum BY2 cells with four different weak organic acids. Increased PMA2 phosphorylation was observed for three of them, thus highlighting the role of the H (+) -ATPase in cell pH homeostasis.