Expression analysis of two gene subfamilies encoding the plasma membrane H+-ATPase in Nicotiana plumbaginifolia reveals the major transport functions of this enzyme

The asymmetric expression of plasma membrane H+-ATPase family genes in response to pulvinus-driven leaf phototropism movement in Vitis vinifera

Phototropism movement is crucial for plants to adapt to various environmental changes. Plant P-type H+-ATPase (HA) plays diverse roles in signal transduction during cell expansion, regulation of cellular osmotic potential and stomatal opening, and circadian movement. Despite numerous studies on the genome-wide analysis of Vitis vinifera, no research has been done on the P-type H+-ATPase family genes, especially concerning pulvinus-driven leaf movement. In this study, 55 VvHAs were identified and classified into nine distinct subgroups (1 to 9). Gene members within the same subgroups exhibit similar features in motif, intron/exon, and protein tertiary structures. Furthermore, four pairs of genes were derived by segmental duplication in grapes. Cis-acting element analysis identified numerous light/circadian-related elements in the promoters of VvHAs. qRT-PCR analysis showed that several genes of subgroup 7 were highly expressed in leaves and pulvinus during leaf movement, especially VvHA14, VvHA15, VvHA16, VvHA19, VvHA51, VvHA52, and VvHA54. Additionally, we also found that the VvHAs genes were asymmetrically expressed on both sides of the extensor and flexor cell of the motor organ, the pulvinus. The expression of VvHAs family genes in extensor cells was significantly higher than that in flexor cells. Overall, this study serves as a foundation for further investigations into the functions of VvHAs and contributes to the complex mechanisms underlying grapevine pulvinus growth and development.

Structural and Functional Diversity of Two ATP-Driven Plant Proton Pumps

Two ATP-dependent proton pumps function in plant cells. Plasma membrane H⁺-ATPase (PM H⁺-ATPase) transfers protons from the cytoplasm to the apoplast, while vacuolar H⁺-ATPase (V-ATPase), located in tonoplasts and other endomembranes, is responsible for proton pumping into the organelle lumen. Both enzymes belong to two different families of proteins and, therefore, differ significantly in their structure and mechanism of action. The plasma membrane H⁺-ATPase is a member of the P-ATPases that undergo conformational changes, associated with two distinct E1 and E2 states, and autophosphorylation during the catalytic cycle. The vacuolar H⁺-ATPase represents rotary enzymes functioning as a molecular motor. The plant V-ATPase consists of thirteen different subunits organized into two subcomplexes, the peripheral V₁ and the membrane-embedded V₀, in which the stator and rotor parts have been distinguished. In contrast, the plant plasma membrane proton pump is a functional single polypeptide chain. However, when the enzyme is active, it transforms into a large twelve-protein complex of six H⁺-ATPase molecules and six 14-3-3 proteins. Despite these differences, both proton pumps can be regulated by the same mechanisms (such as reversible phosphorylation) and, in some processes, such as cytosolic pH regulation, may act in a coordinated way.

Root hair growth from the pH point of view

Root hairs are tubular outgrowths of epidermal cells that increase the root surface area and thereby make the root more efficient at absorbing water and nutrients. Their expansion is limited to the root hair apex, where growth is reported to take place in a pulsating manner. These growth pulses coincide with oscillations of the apoplastic and cytosolic pH in a similar way as has been reported for pollen tubes. Likewise, the concentrations of apoplastic reactive oxygen species (ROS) and cytoplasmic Ca²⁺ oscillate with the same periodicity as growth. Whereas ROS appear to control cell wall extensibility and opening of Ca²⁺ channels, the role of protons as a growth signal in root hairs is less clear and may differ from that in pollen tubes where plasma membrane H⁺-ATPases have been shown to sustain growth. In this review, we outline our current understanding of how pH contributes to root hair development.

Comparative analysis of the P-type ATPase gene family in seven Rosaceae species and an expression analysis in pear (Pyrus bretschneideri Rehd.)

P-type ATPases are integral membrane transporters that play important roles in transmembrane transport in plants. However, a comprehensive analysis of the P-type ATPase gene family has not been conducted in Chinese white pear (Pyrus bretschneideri) or other Rosaceae species. Here, we identified 419 P-type ATPase genes from seven Rosaceae species (Pyrus bretschneideri, Malus domestica, Prunus persica, Fragaria vesca, Prunus mume, Pyrus communis and Pyrus betulifolia). Structural and phylogenetic analyses revealed that P-type ATPase genes can be divided into five subfamilies. Different subfamilies have different conserved motifs and cis-acting elements, which may lead to functional divergence within one gene family. Dispersed duplication and whole-genome duplication may play critical roles in the expansion of the P-type ATPase family. Purifying selection was the primary force driving the evolution of P-type ATPase family genes. Based on the dynamic transcriptome analysis and transient transformation of Chinese white pear fruit, Pbr029767.1 in the P_3A subfamily were found to be associated with malate accumulation during pear fruit development. Using a co-expression network, we identified several transcription factors that may have regulatory relationships with the P-type ATPase gene family. Overall, this study lays a solid foundation for understanding the evolution and functions of P-type ATPase genes in Chinese white pear and six other Rosaceae species.

Activity of the purified plant ABC transporter NtPDR1 is stimulated by diterpenes and sesquiterpenes involved in constitutive and induced defenses

Within the plant ATP-binding cassette transporter family, pleiotropic drug resistance (PDR) transporters play essential functions, such as in hormone transport or defense against biotic and abiotic stresses. NtPDR1 from Nicotiana tabacum has been shown to be involved in the constitutive defense against pathogens through the secretion of toxic cyclic diterpenes, such as the antimicrobial substrates cembrene and sclareol from the leaf hairs (trichomes). However, direct evidence of an interaction between NtPDR1 and terpenes is lacking. Here, we stably expressed NtPDR1 in N. tabacum BY-2 suspension cells. NtPDR1 was purified as an active monomer glycosylated at a single site in the third external loop. NtPDR1 reconstitution in proteoliposomes stimulated its basal ATPase activity from 21 to 38 nmol of Pi·mg-1·min-1, and ATPase activity was further stimulated by the NtPDR1 substrates cembrene and sclareol, providing direct evidence of an interaction between NtPDR1 and its two substrates. Interestingly, NtPDR1 was also stimulated by capsidiol, a sesquiterpene produced by N. tabacum upon pathogen attack. We also monitored the transcriptional activity from the NtPDR1 promoter in situ with a reporter gene and found that, although NtPDR1 expression was limited to trichomes under normal conditions, addition of methyl jasmonate, a biotic stress hormone, induced expression in all leaf tissues. This finding indicated that NtPDR1 is involved not only in constitutive but also in induced plant defenses. In conclusion, we provide direct evidence of an interaction between the NtPDR1 transporter and its substrates and that NtPDR1 transports compounds involved in both constitutive (diterpenes) and induced (sesquiterpenes) plant defenses.

The plasma membrane H+-ATPase gene family in Solanum tuberosum L. Role of PHA1 in tuberization

This study presents the characterization of the plasma membrane (PM) H+-ATPases in potato, focusing on their role in stolon and tuber development. Seven PM H+-ATPase genes were identified in the Solanum tuberosum genome, designated PHA1-PHA7. PHA genes show distinct expression patterns in different plant tissues and under different stress treatments. Application of PM H+-ATPase inhibitors arrests stolon growth, promotes tuber induction, and reduces tuber size, indicating that PM H+-ATPases are involved in tuberization, acting at different stages of the process. Transgenic potato plants overexpressing PHA1 were generated (PHA1-OE). At early developmental stages, PHA1-OE stolons elongate faster and show longer epidermal cells than wild-type stolons; this accelerated growth is accompanied by higher cell wall invertase activity, lower starch content, and higher expression of the sucrose-H+ symporter gene StSUT1. PHA1-OE stolons display an increased branching phenotype and develop larger tubers. PHA1-OE plants are taller and also present a highly branched phenotype. These results reveal a prominent role for PHA1 in plant growth and development. Regarding tuberization, PHA1 promotes stolon elongation at early stages, and tuber growth later on. PHA1 is involved in the sucrose-starch metabolism in stolons, possibly providing the driving force for sugar transporters to maintain the apoplastic sucrose transport during elongation.

Expression of a constitutively activated plasma membrane H+-ATPase in Nicotiana tabacum BY-2 cells results in cell expansion

MAIN CONCLUSION

Increased acidification of the external medium by an activated H + -ATPase results in cell expansion, in the absence of upstream activating signaling. The plasma membrane H+-ATPase couples ATP hydrolysis with proton transport outside the cell, and thus creates an electrochemical gradient, which energizes secondary transporters. According to the acid growth theory, this enzyme is also proposed to play a major role in cell expansion, by acidifying the external medium and so activating enzymes that are involved in cell wall-loosening. However, this theory is still debated. To challenge it, we made use of a plasma membrane H+-ATPase isoform from Nicotiana plumbaginifolia truncated from its C-terminal auto-inhibitory domain (ΔCPMA4), and thus constitutively activated. This protein was expressed in Nicotiana tabacum BY-2 suspension cells using a heat shock inducible promoter. The characterization of several independent transgenic lines showed that the expression of activated ΔCPMA4 resulted in a reduced external pH by 0.3-1.2 units, as well as in an increased H+-ATPase activity by 77-155 % (ATP hydrolysis), or 70-306 % (proton pumping) of isolated plasma membranes. In addition, ΔCPMA4-expressing cells were 17-57 % larger than the wild-type cells and displayed abnormal shapes. A proteomic comparison of plasma membranes isolated from ΔCPMA4-expressing and wild-type cells revealed the altered abundance of several proteins involved in cell wall synthesis, transport, and signal transduction. In conclusion, the data obtained in this work showed that H+-ATPase activation is sufficient to induce cell expansion and identified possible actors which intervene in this process.

Hydrophobin fusion of an influenza virus hemagglutinin allows high transient expression in Nicotiana benthamiana, easy purification and immune response with neutralizing activity

The expression of recombinant hemagglutinin in plants is a promising alternative to the current egg-based production system for the influenza vaccines. Protein-stabilizing fusion partners have been developed to overcome the low production yields and the high downstream process costs associated with the plant expression system. In this context, we tested the fusion of hydrophobin I to the hemagglutinin ectodomain of the influenza A (H1N1)pdm09 virus controlled by the hybrid En2PMA4 transcriptional promoter to rapidly produce high levels of recombinant antigen by transient expression in agro-infiltrated Nicotiana benthamiana leaves. The fusion increased the expression level by a factor of ∼ 2.5 compared to the unfused protein allowing a high accumulation level of 8.6% of the total soluble proteins. Hemagglutinin was located in ER-derived protein bodies and was successfully purified by combining an aqueous-two phase partition system and a salting out step. Hydrophobin interactions allowed the formation of high molecular weight hemagglutinin structures, while unfused proteins were produced as monomers. Purified protein was shown to be biologically active and to induce neutralizing antibodies after mice immunization. Hydrophobin fusion to influenza hemagglutinin might therefore be a promising approach for rapid, easy, and low cost production of seasonal or pandemic influenza vaccines in plants.

Histone acetylation associated up-regulation of the cell wall related genes is involved in salt stress induced maize root swelling

BACKGROUND

Salt stress usually causes crop growth inhibition and yield decrease. Epigenetic regulation is involved in plant responses to environmental stimuli. The epigenetic regulation of the cell wall related genes associated with the salt-induced cellular response is still little known. This study aimed to analyze cell morphological alterations in maize roots as a consequence of excess salinity in relation to the transcriptional and epigenetic regulation of the cell wall related protein genes.

RESULTS

In this study, maize seedling roots got shorter and displayed swelling after exposure to 200 mM NaCl for 48 h and 96 h. Cytological observation showed that the growth inhibition of maize roots was due to the reduction in meristematic zone cell division activity and elongation zone cell production. The enlargement of the stele tissue and cortex cells contributed to root swelling in the elongation zone. The cell wall is thought to be the major control point for cell enlargement. Cell wall related proteins include xyloglucan endotransglucosylase (XET), expansins (EXP), and the plasma membrane proton pump (MHA). RT-PCR results displayed an up-regulation of cell wall related ZmEXPA1, ZmEXPA3, ZmEXPA5, ZmEXPB1, ZmEXPB2 and ZmXET1 genes and the down-regulation of cell wall related ZmEXPB4 and ZmMHA genes as the duration of exposure was increased. Histone acetylation is regulated by HATs, which are often correlated with gene activation. The expression of histone acetyltransferase genes ZmHATB and ZmGCN5 was increased after 200 mM NaCl treatment, accompanied by an increase in the global acetylation levels of histones H3K9 and H4K5. ChIP experiment showed that the up-regulation of the ZmEXPB2 and ZmXET1 genes was associated with the elevated H3K9 acetylation levels on the promoter regions and coding regions of these two genes.

CONCLUSIONS

These data suggested that the up-regulation of some cell wall related genes mediated cell enlargement to possibly mitigate the salinity-induced ionic toxicity, and different genes had specific function in response to salt stress. Histone modification as a mediator may contribute to rapid regulation of cell wall related gene expression, which reduces the damage of excess salinity to plants.

The influence of salt stress on ABA and auxin concentrations in two maize cultivars differing in salt resistance

The plant hormones abscisic acid (ABA) and auxin (IAA, IBA) play important roles in plant responses to environmental stresses such as salinity. Recent breeding improvements in terms of salt resistance of maize have lead to a genotype with improved growth under saline conditions. By comparing this salt-resistant hybrid with a sensitive hybrid, it was possible to show differences in hormone concentrations in expanding leaves and roots. In response to salinity, the salt-resistant maize significantly increased IBA concentrations in growing leaves and maintained IAA concentration in roots. These hormonal adaptations may help to establish favorable conditions for growth-promoting agents such as β-expansins and maintain growth of resistant maize hybrids under salt stress. Moreover, ABA concentrations significantly increased in resistant maize leaves under salt stress, which may contribute to acidifying the apoplast, which in turn is a prerequisite for growth.

A pleiotropic drug resistance transporter in Nicotiana tabacum is involved in defense against the herbivore Manduca sexta

Pleiotropic drug resistance (PDR) transporters are a group of membrane proteins belonging to the ABCG sub-family of ATP binding cassette (ABC) transporters. There is clear evidence for the involvement of plant ABC transporters in resistance to fungal and bacterial pathogens, but not in the biotic stress response to insect or herbivore attack. Here, we describe a PDR transporter, ABCG5/PDR5, from Nicotiana tabacum. GFP fusion and subcellular fractionation studies revealed that ABCG5/PDR5 is localized to the plasma membrane. Staining of transgenic plants expressing the GUS reporter gene under the control of the ABCG5/PDR5 transcription promoter and immunoblotting of wild-type plants showed that, under standard growth conditions, ABCG5/PDR5 is highly expressed in roots, stems and flowers, but is only expressed at marginal levels in leaves. Interestingly, ABCG5/PDR5 expression is induced in leaves by methyl jasmonate, wounding, pathogen infiltration, or herbivory by Manduca sexta. To address the physiological role of ABCG5/PDR5, N. tabacum plants silenced for the expression of ABCG5/PDR5 were obtained. No phenotypic modification was observed under standard conditions. However, a small increase in susceptibility to the fungus Fusarium oxysporum was observed. A stronger effect was observed in relation to herbivory: silenced plants allowed better growth and faster development of M. sexta larvae than wild-type plants, indicating an involvement of this PDR transporter in resistance to M. sexta herbivory.

Nitrate uptake along the maize primary root: an integrated physiological and molecular approach

The spatial variability of the nitrate (NO₃-) uptake along the maize primary root axis was investigated at physiological and molecular levels. Net NO₃- uptake rate (NNUR) and its kinetic parameters, together with the gene expression of a high-affinity NO₃- transporter (NRT2.1), were evaluated. The activity and the expression of plasma membrane H⁺ -ATPase (PM H⁺ -ATPase), key enzyme in plant nutrition, were also analysed. The NNUR showed a heterogeneous spatial pattern along the root, where the regions closer to the root tip early exhibited higher capacity to absorb NO₃- than the basal regions, because of a higher maximum NNUR and faster induction of the inducible high-affinity transport system (iHATS), the presence of the high-affinity transport system (HATS) also at external NO₃- concentrations >100 µm and an improved NO₃- transport because of lower K(m) values. ZmNRT2.1 transcript abundances were not spatially correlated with NNUR, suggesting that post-translational effects or NAR2 protein co-expression could be involved. By contrast, PM H⁺ -ATPase displayed a similar spatial-temporal pattern as that of nitrate uptake, resulting in higher activity in the root tip than in the basal regions. Increased activities of the enzyme after nitrate supply resulted in enhanced expression of MAH3 and MAH4, PM H⁺ -ATPase subfamily II genes, while MAH1 was not expressed.

Solanaceae XIPs are plasma membrane aquaporins that facilitate the transport of many uncharged substrates

Major intrinsic proteins (MIPs) transport water and uncharged solutes across membranes in all kingdoms of life. Recently, an uncharacterized MIP subfamily was identified in the genomes of plants and fungi and named X Intrinsic Proteins (XIPs). Here, we describe the genetic features, localization, expression, and functions of a group of Solanaceae XIPs. XIP cDNA and gDNA were cloned from tobacco, potato, tomato, and morning glory. A conserved sequence motif in the first intron of Solanaceae XIPs initiates an RNA-processing mechanism that results in two splice variants (α and β). When transiently or stably expressed in tobacco plants, yellow fluorescent protein-tagged NtXIP1;1α and NtXIP1;1β were both localized in the plasma membrane. Transgenic tobacco lines expressing NtXIP1;1-promoter-GUS constructs and RT-PCR studies showed that NtXIP1;1 was expressed in all organs. The NtXIP1;1 promoter was mainly active in cell layers facing the environment in all above-ground tissues. Heterologous expression of Solanaceae XIPs in Xenopus laevis oocytes and various Saccharomyces cerevisiae mutants demonstrated that these isoforms facilitate the transport of bulky solutes, such as glycerol, urea, and boric acid. In contrast, permeability for water was undetectable. These data suggest that XIPs function in the transport of uncharged solutes across the cell plasma membrane in specific plant tissues, including at the interface between the environment and external cell layers.

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.

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

The plasma membrane H(+)-ATPases PMA2 and PMA4 are the most widely expressed in Nicotiana plumbaginifolia, and belong to two different subfamilies. Both are activated by phosphorylation of a Thr at the penultimate position and the subsequent binding of 14-3-3 proteins. Their expression in Saccharomyces cerevisiae revealed functional and regulatory differences. To determine whether different regulatory properties between PMA2 and PMA4 exist in plants, we generated two monoclonal antibodies able to detect phosphorylation of the penultimate Thr of either PMA2 or PMA4 in a total protein extract. We also raised Nicotiana tabacum transgenic plants expressing 6-His-tagged PMA2 or PMA4, enabling their individual purification. Using these tools we showed that phosphorylation of the penultimate Thr of both PMAs was high during the early exponential growth phase of an N. tabacum cell culture, and then progressively declined. This decline correlated with decreased 14-3-3 binding and decreased plasma membrane ATPase activity. However, the rate and extent of the decrease differed between the two isoforms. Cold stress of culture cells or leaf tissues reduced the Thr phosphorylation of PMA2, whereas no significant changes in Thr phosphorylation of PMA4 were seen. These results strongly suggest that PMA2 and PMA4 are differentially regulated by phosphorylation. Analysis of the H(+)-ATPase phosphorylation status in leaf tissues indicated that no more than 44% (PMA2) or 32% (PMA4) was in the activated state under normal growth conditions. Purification of either isoform showed that, when activated, the two isoforms did not form hetero-oligomers, which is further support for these two H(+)-ATPase subfamilies having different properties.

Nitric oxide mediates humic acids-induced root development and plasma membrane H+-ATPase activation

It is widely reported that some humic substances behave as exogenous auxins influencing root growth by mechanisms that are not yet completely understood. This study explores the hypothesis that the humic acids' effects on root development involve a nitric oxide signaling. Maize seedlings were treated with HA 20 mg C L(-1), IAA 0.1 nM, and NO donors (SNP or GSNO), in combination with either the auxin-signaling inhibitor PCIB, the auxin efflux inhibitor TIBA, or the NO scavenger PTIO. H(+)-transport-competent plasma membrane vesicles were isolated from roots to investigate a possible link between NO-induced H(+)-pump and HA bioactivity. Plants treated with either HA or SNP stimulated similarly the lateral roots emergence even in the presence of the auxin inhibitors, whereas NO scavenger diminished this effect. These treatments induced H(+)-ATPase stimulation by threefold, which was abolished by PTIO and decreased by auxin inhibitors. HA-induced NO synthesis was also detected in the sites of lateral roots emergence. These data depict a new scenario where the root development stimulation and the H(+)-ATPase activation elicited by either HA or exogenous IAA depend essentially on mechanisms that use NO as a messenger induced site-specifically in the early stages of lateral root development.

A novel mechanism of P-type ATPase autoinhibition involving both termini of the protein

The activity of many P-type ATPases is found to be regulated by interacting proteins or autoinhibitory elements located in N- or C-terminal extensions. An extended C terminus of fungal and plant P-type plasma membrane H(+)-ATPases has long been recognized to be part of a regulatory apparatus involving an autoinhibitory domain. Here we demonstrate that both the N and the C termini of the plant plasma membrane H(+)-ATPase are directly involved in controlling the pump activity state and that N-terminal displacements are coupled to secondary modifications taking place at the C-terminal end. This identifies the first group of P-type ATPases for which both ends of the polypeptide chain constitute regulatory domains, which together contribute to the autoinhibitory apparatus. This suggests an intricate mechanism of cis-regulation with both termini of the protein communicating to obtain the necessary control of the enzyme activity state.

Chemical composition and bioactivity properties of size-fractions separated from a vermicompost humic acid

Preparative high performance size-exclusion chromatography (HPSEC) was applied to humic acids (HA) extracted from vermicompost in order to separate humic matter of different molecular dimension and evaluate the relationship between chemical properties of size-fractions (SF) and their effects on plant root growth. Molecular dimensions of components in humic SF was further achieved by diffusion-ordered nuclear magnetic resonance spectroscopy (DOSY-NMR) based on diffusion coefficients (D), while carbon distribution was evaluated by solid state (CP/MAS) (13)C NMR. Seedlings of maize and Arabidopsis were treated with different concentrations of SF to evaluate root growth. Six different SF were obtained and their carbohydrate-like content and alkyl chain length decreased with decreasing molecular size. Progressive reduction of aromatic carbon was also observed with decreasing molecular size of separated fractions. Diffusion-ordered spectroscopy (DOSY) spectra showed that SF were composed of complex mixtures of aliphatic, aromatic and carbohydrates constituents that could be separated on the basis of their diffusion. All SF promoted root growth in Arabidopsis and maize seedlings but the effects differed according to molecular size and plant species. In Arabidopsis seedlings, the bulk HA and its SF revealed a classical large auxin-like exogenous response, i.e.: shortened the principal root axis and induced lateral roots, while the effects in maize corresponded to low auxin-like levels, as suggested by enhanced principal axis length and induction of lateral roots. The reduction of humic heterogeneity obtained in HPSEC separated size-fractions suggested that their physiological influence on root growth and architecture was less an effect of their size than their content of specific bioactive molecules. However, these molecules may be dynamically released from humic superstructures and exert their bioactivity when weaker is the humic conformational stability as that obtained in the separated size-fractions.

Adaptation of plasma membrane H(+)-ATPase of rice roots to low pH as related to ammonium nutrition

The preference of paddy rice for NH(4)(+) rather than NO(3)(-) is associated with its tolerance to low pH since a rhizosphere acidification occurs during NH(4)(+) absorption. However, the adaptation of rice root to low pH has not been fully elucidated. This study investigated the acclimation of plasma membrane H(+)-ATPase of rice root to low pH. Rice seedlings were grown either with NH(4)(+) or NO(3)(-). For both nitrogen forms, the pH value of nutrient solutions was gradually adjusted to pH 6.5 or 3.0. After 4 d cultivation, hydrolytic H(+)-ATPase activity, V(max), K(m), H(+)-pumping activity, H(+) permeability and pH gradient across the plasma membrane were significantly higher in rice roots grown at pH 3.0 than at 6.5, irrespective of the nitrogen forms supplied. The higher activity of plasma membrane H(+)-ATPase of adapted rice roots was attributed to the increase in expression of OSA1, OSA3, OSA7, OSA8 and OSA9 genes, which resulted in an increase of H(+)-ATPase protein concentration. In conclusion, a high regulation of various plasma membrane H(+)-ATPase genes is responsible for the adaptation of rice roots to low pH. This mechanism may be partly responsible for the preference of rice plants to NH(4)(+) nutrition.

Different subcellular localization and glycosylation for a functional antibody expressed in Nicotiana tabacum plants and suspension cells

Genes encoding the heavy and light chains of LO-BM2, a therapeutic IgG antibody, were assembled in the tandem or inverted convergent orientation and expressed in Nicotiana tabacum plants and BY-2 suspension cells. The tandem construct allowed higher expression in both expression systems. A similar degradation pattern was observed for the secreted antibody recovered from the leaf intercellular fluid and BY-2 culture medium. Degradation increased with leaf age or culture time. Antibodies purified from leaf tissues and BY-2 cells were both functional. However, MS analysis of the N-glycosylation showed complex plant-type glycans to be the major type in the antibody purified from plants, whereas, oligomannosidic was the major glycosylation type in that purified from BY-2 cells. LO-BM2 was observed mainly in the endoplasmic reticulum of BY-2 cells while, in leaf cells, it was localized mostly to vesicles resembling prevacuolar compartments. These results and those from endoglycosidase H studies suggest that LO-BM2 is secreted from BY-2 cells more readily than from leaf cells where it accumulates in a post-Golgi compartment.

Establishment of a protein reference map for soybean root hair cells

Root hairs are single tubular cells formed from the differentiation of epidermal cells on roots. They are involved in water and nutrient uptake and represent the infection site on leguminous roots by rhizobia, soil bacteria that establish a nitrogen-fixing symbiosis. Root hairs develop by polar cell expansion or tip growth, a unique mode of plant growth shared only with pollen tubes. A more complete characterization of root hair cell biology will lead to a better understanding of tip growth, the rhizobial infection process, and also lead to improvements in plant water and nutrient uptake. We analyzed the proteome of isolated soybean (Glycine max) root hair cells using two-dimensional polyacrylamide gel electrophoresis (2D-PAGE) and shotgun proteomics (1D-PAGE-liquid chromatography and multidimensional protein identification technology) approaches. Soybean was selected for this study due to its agronomic importance and its root size. The resulting soybean root hair proteome reference map identified 1,492 different proteins. 2D-PAGE followed by mass spectrometry identified 527 proteins from total cell contents. A complementary shotgun analysis identified 1,134 total proteins, including 443 proteins that were specific to the microsomal fraction. Only 169 proteins were identified by the 2D-PAGE and shotgun methods, which highlights the advantage of using both methods. The proteins identified are involved not only in basic cell metabolism but also in functions more specific to the single root hair cell, including water and nutrient uptake, vesicle trafficking, and hormone and secondary metabolism. The data presented provide useful insight into the metabolic activities of a single, differentiated plant cell type.

The plant plasma membrane proton pump ATPase: a highly regulated P-type ATPase with multiple physiological roles

Around 40 P-type ATPases have been identified in Arabidopsis and rice, for which the genomes are known. None seems to exchange sodium and potassium, as does the animal Na(+)/K(+)-ATPase. Instead, plants, together with fungi, possess a proton pumping ATPase (H(+)-ATPase), which couples ATP hydrolysis to proton transport out of the cell, and so establishes an electrochemical gradient across the plasma membrane, which is dissipated by secondary transporters using protons in symport or antiport, as sodium is used in animal cells. Additional functions, such as stomata opening, cell growth, and intracellular pH homeostasis, have been proposed. Crystallographic data and homology modeling suggest that the H(+)-ATPase has a broadly similar structure to the other P-type ATPases but has an extended C-terminal region, which is involved in enzyme regulation. Phosphorylation of the penultimate residue, a Thr, and the subsequent binding of regulatory 14-3-3 proteins result in the formation of a dodecamer (six H(+)-ATPase and six 14-3-3 molecules) and enzyme activation. This type of regulation is unique to the P-type ATPase family. However, the recent identification of additional phosphorylated residues suggests further regulatory features.

NaRALF, a peptide signal essential for the regulation of root hair tip apoplastic pH in Nicotiana attenuata, is required for root hair development and plant growth in native soils

Rapid alkalinization factor (RALF) is a 49-amino-acid peptide that rapidly alkalinizes cultivated tobacco cell cultures. In the native tobacco Nicotiana attenuata, NaRALF occurs as a single-copy gene and is highly expressed in roots and petioles. Silencing the NaRALF transcript by transforming N. attenuata with an inverted-repeat construct generated plants (irRALF) with normal wild-type (WT) above-ground parts, but with roots that grew longer and produced trichoblasts that developed into abnormal root hairs. Most trichoblasts produced a localized 'bulge' without commencing root hair tip growth; fewer trichoblasts grew, but were only 10% as long as those of WT plants. The root hair phenotype was associated with slowed apoplastic pH oscillations, increased pH at the tips of trichoblasts and decreased accumulation of reactive oxygen species in the root hair initiation zone. The root hair growth phenotype was partially restored when irRALF lines were grown in a low-pH-buffered medium, and reproduced in WT plants grown in a high-pH-buffered medium. When irRALF plants were grown in pH 5.6, 6.7 and 8.1 soils together with WT plants in glasshouse experiments, they were out-competed by WT plants in basic, but not acidic, soils. When WT and irRALF lines were planted into the basic soils of the native habitat of N. attenuata in the Great Basin Desert, irRALF plants had smaller leaves, shorter stalks, and produced fewer flowers and seed capsules than did WT plants. We conclude that NaRALF is required for regulating root hair extracellular pH, the transition from root hair initiation to tip growth and plant growth in basic soils.

Involvement of hydrogen peroxide and nitric oxide in salt resistance in the calluses from Populus euphratica

Nitric oxide (NO) and hydrogen peroxide (H2O2) function as signalling molecules in plants under abiotic and biotic stresses. Calluses from Populus euphratica, which show salt tolerance, were used to study the interaction of NO and H2O2 in plant adaptation to salt resistance. The nitric oxide synthase (NOS) activity was identified in the calluses, and this activity was induced under 150 mM NaCl treatment. Under 150 mM NaCl treatment, the sodium (Na) percentage decreased, but the potassium (K) percentage and the K/Na ratio increased in P. euphratica calluses. Application of glucose/glucose oxidase (G/GO, a H2O2 donor) and sodium nitroprusside (SNP, a NO donor) revealed that both H2O2 and NO resulted in increased K/Na ratio in a concentration-dependent manner. Diphenylene iodonium (DPI, an NADPH oxidase inhibitor) counteracted H2O2 and NO effect by increasing the Na percentage, decreasing the K percentage and K/Na ratio. NG-monomethyl-L-Arg monoacetate (NMMA, an NO synthase inhibitor) and 2-phenyl-4,4,5,5-tetramethyl-imidazoline-1-oxyl-3-oxyde (PTIO, a specific NO scavenger) only reversed NO effect, but did not block H2O2 effect. The increased activity of plasma membrane (PM) H+ -ATPase caused by salt stress was reversed by treatment with DPI and NMMA. Exogenous H2O2 increased the activity of PM H+ -ATPase, but the effect could not be diminished by NMMA and PTIO. The NO-induced increase of PM H+ -ATPase can be reversed by NMMA and PTIO, but not by DPI. Western blot analysis demonstrated that NO and H2O2 stimulated the expression of PM H+ -ATPase in P. euphratica calluses. These results indicate that NO and H2O2 served as intermediate molecules in inducing salt resistance in the calluses from P. euphratica under slat stress by increasing the K/Na ratio, which was dependent on the increased PM H+ -ATPase activity.

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.

Functional biology of plant phosphate uptake at root and mycorrhiza interfaces

Phosphorus (P) is an essential plant nutrient and one of the most limiting in natural habitats as well as in agricultural production world-wide. The control of P acquisition efficiency and its subsequent uptake and translocation in vascular plants is complex. The physiological role of key cellular structures in plant P uptake and underlying molecular mechanisms are discussed in this review, with emphasis on phosphate transport across the cellular membrane at the root and arbuscular-mycorrhizal (AM) interfaces. The tools of molecular genetics have facilitated novel approaches and provided one of the major driving forces in the investigation of the basic transport mechanisms underlying plant P nutrition. Genetic engineering holds the potential to modify the system in a targeted way at the root-soil or AM symbiotic interface. Such approaches should assist in the breeding of crop plants that exhibit improved P acquisition efficiency and thus require lower inputs of P fertilizer for optimal growth. Whether engineering of P transport systems can contribute to enhanced P uptake will be discussed.

Expression and secretion of recombinant outer-surface protein A from the Lyme disease agent, Borrelia burgdorferi, in Nicotiana tabacum suspension cells

The ospA gene of Borrelia burgdorferi codes for an outer membrane lipoprotein, which is a major antigen of the Lyme disease agent. Recombinant OspA vaccines tested so far were expressed in Escherichia coli. In this study, we investigated the expression of a soluble OspA protein in Nicotiana tabacum suspension cells and evaluated the secretion of OspA driven by either its own bacterial signal peptide or a plant signal peptide fused to the amino-terminal cysteine of the mature form. In both cases, the signal peptide was cleaved off and OspA secreted. During secretion, OspA was N-glycosylated. Addition of a C-terminal KDEL sequence led to retention of OspA in the endoplasmic reticulum.

NpPDR1, a pleiotropic drug resistance-type ATP-binding cassette transporter from Nicotiana plumbaginifolia, plays a major role in plant pathogen defense

Nicotiana plumbaginifolia NpPDR1, a plasma membrane pleiotropic drug resistance-type ATP-binding cassette transporter formerly named NpABC1, has been suggested to transport the diterpene sclareol, an antifungal compound. However, direct evidence for a role of pleiotropic drug resistance transporters in the plant defense is still lacking. In situ immunolocalization and histochemical analysis using the gusA reporter gene showed that NpPDR1 was constitutively expressed in the whole root, in the leaf glandular trichomes, and in the flower petals. However, NpPDR1 expression was induced in the whole leaf following infection with the fungus Botrytis cinerea, and the bacteria Pseudomonas syringae pv tabaci, Pseudomonas fluorescens, and Pseudomonas marginalis pv marginalis, which do not induce a hypersensitive response in N. plumbaginifolia, whereas a weaker response was observed using P. syringae pv syringae, which does induce a hypersensitive response. Induced NpPDR1 expression was more associated with the jasmonic acid than the salicylic acid signaling pathway. These data suggest that NpPDR1 is involved in both constitutive and jasmonic acid-dependent induced defense. Transgenic plants in which NpPDR1 expression was prevented by RNA interference showed increased sensitivity to sclareol and reduced resistance to B. cinerea. These data show that NpPDR1 is involved in pathogen resistance and thus demonstrate a new role for the ATP-binding cassette transporter family.

Identification of a Nicotiana plumbaginifolia plasma membrane H(+)-ATPase gene expressed in the pollen tube

In Nicotiana plumbaginifolia, plasma membrane H(+)-ATPases (PMAs) are encoded by a gene family of nine members. Here, we report on the characterization of a new isogene, NpPMA5 (belonging to subfamily IV), and the determination of its expression pattern using the beta-glucuronidase (gusA) reporter gene. pNpPMA5-gusA was expressed in cotyledons, in vascular tissues of the stem (mainly in nodal zones), and in the flower and fruit. In the flower, high expression was found in the pollen tube after in vitro or in vivo germination. Northern blotting analysis confirmed that NpPMA5 was expressed in the pollen tube contrary to NpPMA2 (subfamily I) or NpPMA4 (subfamily II), two genes highly expressed in other tissues. The subcellular localization of PM H(+)-ATPase in the pollen tube was analyzed by immunocytodecoration. As expected, this enzyme was localized to the plasma membrane. However, neither the tip nor the base of the pollen tube was labeled, showing an asymmetrical distribution of this enzyme. This observation supports the hypothesis that the PM H(+)-ATPase is involved in creating the pH gradient that is observed along the pollen tube and is implicated in cell elongation. Compared to other plant PM H(+)-ATPases, the C-terminal region of NpPMA5 is shorter by 26 amino acid residues and is modified in the last 6 residues, due to a sequence rearrangement, which was also found in the orthologous gene of Nicotiana glutinosa, a Nicotiana species distant from N. plumbaginifolia and Petunia hybrida and Lycopersicon esculentum, other Solanacae species. This modification alters part of the PM H(+)-ATPase regulatory domain and raises the question whether this isoform is still regulated.

Two plasma membrane H(+)-ATPase genes are differentially expressed in iron-deficient cucumber plants

Aim of the present work was to investigate the involvement of plasma membrane (PM) H(+)-ATPase (E.C. 3.6.3.6) isoforms of cucumber (Cucumis sativus L.) in the response to Fe deficiency. Two PM H(+)-ATPase cDNAs (CsHA1 and CsHA2) were isolated from cucumber and their expression analysed as a function of Fe nutritional status. Semi-quantitative reverse transcriptase (RT)-PCR and quantitative real-time RT-PCR revealed in Fe-deficient roots an enhanced accumulation of CsHA1 gene transcripts, which were hardly detectable in leaves. Supply of iron to deficient plants caused a decrease in the transcript level of CsHA1. In contrast, CsHA2 transcripts, detected both in roots and leaves, appeared to be unaffected by Fe. This work shows for the first time that a transcriptional regulation of PM H(+)-ATPase involving a specific isoform occurs in the response to Fe deficiency.

Cell expansion in roots

Cell expansion in roots is crucial for the exploration and exploitation of the soil substrate and the plethora of activities that roots engage in. Expansion requires the coordinated activities of many cell processes. Central to this is the control of ion transport during vacuolar growth, which mediates the increase in cell size and the concomitant production of new wall and membrane at the surface of growing cells. The cytoskeleton plays an important role in growth and the control of growth direction. Evidence is accumulating to show that plant hormones also coordinate cell expansion throughout the plant by controlling the activities of growth-regulating DELLA proteins.

Adaptation of H+-pumping and plasma membrane H+ ATPase activity in proteoid roots of white lupin under phosphate deficiency

White lupin (Lupinus albus) is able to adapt to phosphorus deficiency by producing proteoid roots that release a huge amount of organic acids, resulting in mobilization of sparingly soluble soil phosphate in rhizosphere. The mechanisms responsible for the release of organic acids by proteoid root cells, especially the trans-membrane transport processes, have not been elucidated. Because of high cytosolic pH, the release of undissociated organic acids is not probable. In the present study, we focused on H+ export by plasma membrane H+ ATPase in active proteoid roots. In vivo, rhizosphere acidification of active proteoid roots was vanadate sensitive. Plasma membranes were isolated from proteoid roots and lateral roots from P-deficient and -sufficient plants. In vitro, in comparison with two types of lateral roots and proteoid roots of P-sufficient plants, the following increase of the various parameters was induced in active proteoid roots of P-deficient plants: (a) hydrolytic ATPase activity, (b) Vmax and Km, (c) H+ ATPase enzyme concentration of plasma membrane, (d) H+-pumping activity, (e) pH gradient across the membrane of plasmalemma vesicles, and (f) passive H+ permeability of plasma membrane. In addition, lower vanadate sensitivity and more acidic pH optimum were determined for plasma membrane ATPase of active proteoid roots. Our data support the hypothesis that in active proteoid root cells, H+ and organic anions are exported separately, and that modification of plasma membrane H+ ATPase is essential for enhanced rhizosphere acidification by active proteoid roots.

First patch, then catch: measuring the activity and the mRNA transcripts of a proton pump in individual Lilium pollen protoplasts

Combining the patch-clamp method with single-cell reverse transcription polymerase chain reaction (scRT-PCR) a fusicoccin-induced current reflecting the activity of the plasma membrane H(+) ATPase of lily pollen protoplasts was measured and subsequently, the ATPase-encoding mRNAs were collected and amplified. Southern blot signals were observed in all 'patch-catch' experiments and could be detected even in 2560-fold dilutions of the pollen contents. H(+) ATPase mRNAs were detectable only in the vegetative but not in the generative cell of pollen as confirmed by immunolocalisation. In 15% of the scRT-PCR experiments, a random non-reproducibility of the PCR was observed, probably caused by varying amounts of ATPase mRNAs in the protoplasts.

Calcium uptake by plant cells--channels and pumps acting in concert

How do plant cells accomplish a net uptake of Ca(2+) but keep the membrane potential under control? Consideration of the voltage dependence of the depolarization-activated calcium channel and hyperpolarization-activated calcium channel types, and two other major transporters in the plasma membrane, the H(+)-ATPase and I(K,out), suggests that one channel is well suited for both nutritive and signalling Ca(2+) uptake whereas the other could be limited to a signalling function.

Evidence for a role in growth and salt resistance of a plasma membrane H+-ATPase in the root endodermis

The plasma membrane of plant cells is energized by an electrochemical gradient produced by P-type H+-ATPases (proton pumps). These pumps are encoded by at least 12 genes in Arabidopsis. Here we provide evidence that isoform AHA4 contributes to solute transport through the root endodermis. AHA4 is expressed most strongly in the root endodermis and flowers, as suggested by promoter-GUS reporter assays. A disruption of this pump (aha4-1) was identified as a T-DNA insertion in the middle of the gene (after VFP(574)). Truncated aha4-1 transcripts accumulate to approximately 50% of the level observed for AHA4 mRNA in wild-type plants. Plants homozygous for aha4-1 (-/-) show a subtle reduction in root and shoot growth compared with wild-type plants when grown under normal conditions. However, a mutant phenotype is very clear in plants grown under salt stress (e.g., 75 or 110 mM NaCl). In leaves of mutant plants subjected to Na stress, the ratio of Na to K increased 4-5-fold. Interestingly, the aha4-1 mutation appears to be semidominant and was only partially complemented by the introduction of additional wild-type copies of AHA4. These results are consistent with the hypothesis that aha4-1 may produce a dominant negative protein or RNA that partially disrupts the activity of other pumps or functions in the root endodermal tissue, thereby compromising the function of this cell layer in controlling ion homeostasis and nutrient transport.

Inventory of the superfamily of P-type ion pumps in Arabidopsis

A total of 45 genes encoding for P-type ATPases have been identified in the complete genome sequence of Arabidopsis. Thus, this plant harbors a primary transport capability not seen in any other eukaryotic organism sequenced so far. The sequences group in all five subfamilies of P-type ATPases. The most prominent subfamilies are P(1B) ATPases (heavy metal pumps; seven members), P(2A) and P(2B) ATPases (Ca(2+) pumps; 14 in total), P(3A) ATPases (plasma membrane H(+) pumps; 12 members including a truncated pump, which might represent a pseudogene or an ATPase-like protein with an alternative function), and P(4) ATPases (12 members). P(4) ATPases have been implicated in aminophosholipid flipping but it is not known whether this is a direct or an indirect effect of pump activity. Despite this apparent plethora of pumps, Arabidopsis appears to be lacking Na(+) pumps and secretory pathway (PMR1-like) Ca(2+)-ATPases. A cluster of Arabidopsis heavy metal pumps resembles bacterial Zn(2+)/Co(2+)/Cd(2+)/Pb(2+) transporters. Two members of the cluster have extended C termini containing putative heavy metal binding motifs. The complete inventory of P-type ATPases in Arabidopsis is an important starting point for reverse genetic and physiological approaches aiming at elucidating the biological significance of these pumps.

PLANT PLASMA MEMBRANE H+-ATPases: Powerhouses for Nutrient Uptake

Most transport proteins in plant cells are energized by electrochemical gradients of protons across the plasma membrane. The formation of these gradients is due to the action of plasma membrane H+ pumps fuelled by ATP. The plasma membrane H+-ATPases share a membrane topography and general mechanism of action with other P-type ATPases, but differ in regulatory properties. Recent advances in the field include the identification of the complete H+-ATPase gene family in Arabidopsis, analysis of H+-ATPase function by the methods of reverse genetics, an improved understanding of the posttranslational regulation of pump activity by 14-3-3 proteins, novel insights into the H+ transport mechanism, and progress in structural biology. Furthermore, the elucidation of the three-dimensional structure of a related Ca2+ pump has implications for understanding of structure-function relationships for the plant plasma membrane H+-ATPase.

Analysis of the phosphorylation level in guard-cell plasma membrane H+-ATPase in response to fusicoccin

A fungal phytotoxin fusicoccin (FC) causes irreversible opening of stomata by activation of the plasma membrane H+-ATPase in guard cells. However, the mechanism by which FC activates the H+-ATPase is not fully understood with respect to the event of phosphorylation. In this study, we provide quantitative evidence that FC-dependent activation of H+-ATPase requires the phosphorylation of the C-terminus, and that FC maintains the activated state by preventing the dephosphorylation. The plasma membrane H+-ATPase in guard cells was phosphorylated on serine and threonine residues in the C-termini of both VHA1 and VHA2 by FC, and the phosphorylation level paralleled the rates of H+-pumping and ATP hydrolysis. An endogenous 14-3-3 protein was co-precipitated with the H+-ATPase, and the amount of 14-3-3 protein was proportional to the phosphorylation level of H+-ATPASE: The recombinant 14-3-3 protein bound to the C-terminus only when it was phosphorylated, even in the presence of FC. The phosphorylated C-terminus was dephosphorylated by alkaline phosphatase, and the dephosphorylation was completely prevented when the C-terminus had been incubated with both FC and 14-3-3 protein. The results suggest that FC activates the H+-ATPase by accumulating the complex of phosphorylated H+-ATPase and 14-3-3 protein through inhibition of the dephosphorylation in guard cells.

The two major plant plasma membrane H+-ATPases display different regulatory properties

The major plant plasma membrane H(+)-ATPases fall into two gene categories, subfamilies I and II. However, in many plant tissues, expression of the two subfamilies overlaps, thus precluding individual characterization. Yeast expression of PMA2 and PMA4, representatives of the two plasma membrane H(+)-ATPase subfamilies in Nicotiana plumbaginifolia, has previously shown that (i) the isoforms have distinct enzymatic properties and that (ii) PMA2 is regulated by phosphorylation of its penultimate residue (Thr) and binds regulatory 14-3-3 proteins, resulting in the displacement of the autoinhibitory C-terminal domain. To obtain insights into regulatory differences between the two subfamilies, we have constructed various chimeric proteins in which the 110-residue C-terminal-encoding region of PMA2 was progressively substituted by the corresponding sequence from PMA4. The PMA2 autoinhibitory domain was localized to a region between residues 851 and 915 and could not be substituted by the corresponding region of PMA4. In contrast to PMA2, PMA4 was poorly phosphorylated at its penultimate residue (Thr) and bound 14-3-3 proteins weakly. The only sequence difference around the phosphorylation site is located two residues upstream of the phosphorylated Thr. It is Ser in PMA2 (as in most members of subfamily I) and His in PMA4 (as in most members of subfamily II). Substitution of His by Ser in PMA4 resulted in an enzyme showing increased phosphorylation status, 14-13-3 binding, and ATPase activity, as well as improved yeast growth. The reverse substitution of Ser by His in PMA2 resulted in the failure of this enzyme to complement the absence of yeast H(+)-ATPases. These results show that the two plant H(+)-ATPase subfamilies differ functionally in their regulatory properties.

Specific binding of vf14-3-3a isoform to the plasma membrane H+-ATPase in response to blue light and fusicoccin in guard cells of broad bean

The plasma membrane H(+)-ATPase is activated by blue light with concomitant binding of the 14-3-3 protein to the C terminus in guard cells. Because several isoforms of the 14-3-3 protein are expressed in plants, we determined which isoform(s) bound to the H(+)-ATPase in vivo. Four cDNA clones (vf14-3-3a, vf14-3-3b, vf14-3-3c, and vf14-3-3d) encoding 14-3-3 proteins were isolated from broad bean (Vicia faba) guard cells. Northern analysis revealed that mRNAs encoding vf14-3-3a and vf14-3-3b proteins were expressed predominantly in guard cells. The 14-3-3 protein that bound to the H(+)-ATPase in guard cells had the same molecular mass as the recombinant vf14-3-3a protein. The H(+)-ATPase immunoprecipitated from mesophyll cell protoplasts, which had been stimulated by fusicoccin, coprecipitated with the 32.5-kD 14-3-3 protein, although three 14-3-3 isoproteins were found in mesophyll cell protoplasts. Digestions of the bound 14-3-3 protein and recombinant vf14-3-3a with cyanogen bromide gave the identical migration profiles on sodium dodecyl sulfate-polyacrylamide gel electrophoresis, but that of vf14-3-3b gave a different profile. Mass profiling of trypsin-digested 14-3-3 protein bound to the H(+)-ATPase gave the predicted peptide masses of vf14-3-3a. Far western analysis revealed that the H(+)-ATPase had a higher affinity for vf14-3-3a than for vf14-3-3b. These results suggest that the 14-3-3 protein that bound to the plasma membrane H(+)-ATPase in vivo is vf14-3-3a and that it may play a key role in the activation of H(+)-ATPase in guard cells.

A plant plasma membrane H+-ATPase expressed in yeast is activated by phosphorylation at its penultimate residue and binding of 14-3-3 regulatory proteins in the absence of fusicoccin

The Nicotiana plumbaginifolia plasma membrane H(+)-ATPase isoform PMA2, equipped with a His(6) tag, was expressed in Saccharomyces cerevisiae and purified. Unexpectedly, a fraction of the purified tagged PMA2 associated with the two yeast 14-3-3 regulatory proteins, BMH1 and BMH2. This complex was formed in vivo without treatment with fusicoccin, a fungal toxin known to stabilize the equivalent complex in plants. When gel filtration chromatography was used to separate the free ATPase from the 14-3-3.H(+)-ATPase complex, the complexed ATPase was twice as active as the free form. Trypsin treatment of the complex released a smaller complex, composed of a 14-3-3 dimer and a fragment from the PMA2 C-terminal region. The latter was identified by Edman degradation and mass spectrometry as the PMA2 C-terminal 57 residues, whose penultimate residue (Thr-955) was phosphorylated. In vitro dephosphorylation of this C-terminal fragment prevented binding of 14-3-3 proteins, even in the presence of fusicoccin. Mutation of Thr-955 to alanine, aspartate, or a stop codon prevented PMA2 from complementing the yeast H(+)-ATPase. These mutations were also introduced in an activated PMA2 mutant (Gln-14 --> Asp) characterized by a higher H(+) pumping activity. Each mutation directly modifying Thr-955 prevented 14-3-3 binding, decreased ATPase specific activity, and reduced yeast growth. We conclude that the phosphorylation of Thr-955 is required for 14-3-3 binding and that formation of the complex activates the enzyme.

The plant plasma membrane H(+)-ATPase: structure, function and regulation

The proton-pumping ATPase (H(+)-ATPase) of the plant plasma membrane generates the proton motive force across the plasma membrane that is necessary to activate most of the ion and metabolite transport. In recent years, important progress has been made concerning the identification and organization of H(+)-ATPase genes, their expression, and also the kinetics and regulation of individual H(+)-ATPase isoforms. At the gene level, it is now clear that H(+)-ATPase is encoded by a family of approximately 10 genes. Expression, monitored by in situ techniques, has revealed a specific distribution pattern for each gene; however, this seems to differ between species. In the near future, we can expect regulatory aspects of gene expression to be elucidated. Already the expression of individual plant H(+)-ATPases in yeast has shown them to have distinct enzymatic properties. It has also allowed regulatory aspects of this enzyme to be studied through random and site-directed mutagenesis, notably its carboxy-terminal region. Studies performed with both plant and yeast material have converged towards deciphering the way phosphorylation and binding of regulatory 14-3-3 proteins intervene in the modification of H(+)-ATPase activity. The production of high quantities of individual functional H(+)-ATPases in yeast constitutes an important step towards crystallization studies to derive structural information. Understanding the specific roles of H(+)-ATPase isoforms in whole plant physiology is another challenge that has been approached recently through the phenotypic analysis of the first transgenic plants in which the expression of single H(+)-ATPases has been up- or down-regulated. In conclusion, the progress made recently concerning the H(+)-ATPase family, at both the gene and protein level, has come to a point where we can now expect a more integrated investigation of the expression, function and regulation of individual H(+)-ATPases in the whole plant context.

Through form to function: root hair development and nutrient uptake

Root hairs project from the surface of the root to aid nutrient and water uptake and to anchor the plant in the soil. Their formation involves the precise control of cell fate and localized cell growth. We are now beginning to unravel the complexities of the molecular interactions that underlie this developmental regulation. In addition, after years of speculation, nutrient transport by root hairs has been demonstrated clearly at the physiological and molecular level, with evidence for root hairs being intense sites of H(+)-ATPase activity and involved in the uptake of Ca(2+), K(+), NH(4)(+), NO(3)(-), Mn(2+), Zn(2+), Cl(-) and H(2)PO(4)(-).