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

Partial cloning, characterization, and analysis of expression and activity of plasma membrane H+-ATPase in Kallar grass [Leptochloa fusca (L.) Kunth] under salt stress

Kallar grass (Leptochloa fusca) is a highly salt-tolerant C4 perennial halophytic forage. The regulation of ion movement across the plasma membrane (PM) to improve salinity tolerance of plant is thought to be accomplished with the aid of the proton electrochemical gradient generated by PM H⁺-ATPase. In this study, we cloned a partial gene sequence of the Lf PM H⁺-ATPase and investigated its expression and activity under salt stress. The amino acid sequence of the isolated region of Lf PM H⁺-ATPase possesses the maximum identity up to 96% to its ortholog in Aeluropus littoralis. The isolated fragment of Lf PM H⁺-ATPase gene is a member of the subfamily Π of plant PM H⁺-ATPase and is most closely related to the Oryza sativa gene OSA7. The transcript level and activity of the PM H⁺-ATPase were increased in roots and shoots in response to NaCl and were peaked at 450 mM NaCl in both tissues. The induction of activity and gene expression of PM H⁺-ATPase in roots and shoots of Kallar grass under salinity indicate the necessity for this pump in these organs during salinity adaptation to establish and maintain the electrochemical gradient across the PM of the cells for adjusting ion homeostasis.

Botanicals and phytochemicals from the bark of Hypericum roeperianum (Hypericaceae) had strong antibacterial activity and showed synergistic effects with antibiotics against multidrug-resistant bacteria expressing active efflux pumps

ETHNOPHARMACOLOGICAL RELEVANCE

Infections due to multidrug-resistant (MDR) bacteria constitute a real problem in the public health worldwide. Hypericum roeperianum Schimp. ex A. Rich (Hypericaceae) is used traditionally for treatment of various ailments such as abdominal pains, constipation, diarrhea, indigestion, nausea, and bacterial diseases.

AIM OF THE STUDY

This study was aimed at investigating the antibacterial and antibiotic-modifying activity of the crude methanol extracts (HRB), ethyl-acetate soluble fraction (HRBa), residual material (HRBb), and 11 compounds from the bark of Hypericum roeperianum against multi-drug resistant (MDR) bacteria expressing active efflux pumps.

MATERIALS AND METHODS

The antibacterial activity, the efflux pump effect using the efflux pump inhibitor (EPI), phenylalanine-arginine-ß-naphthylamide (PAβN), as well as the antibiotic-modifying activity of samples were determined using the broth micro-dilution method. Spectrophotometric methods were used to evaluate the effects of HRB and 8,8-bis(dihydroconiferyl) diferulate (11) on bacterial growth, and bacterial membrane damage, whereas follow-up of the acidification of the bacterial culture was used to study their effects on bacteria proton-ATPase pumps.

RESULTS

The crude extract (HRB), HRBa, and HRBb had selective antibacterial activity with MICs ranging from 16 to 512 μg/mL. Phytochemical 11 displayed the best antibacterial activity (0.5 ≤ MIC ≤ 2 μg/mL). The activity of HRB and 11 in the presence of EPI significantly increased on the tested bacteria strains (up to 32-fold). The activity of cloxacillin (CLO), doxycycline (DOX), and tetracycline (TET), was considerably improved (up to 64-fold) towards the multidrug-resistant Enterobacter aerogenes EA-CM64 strain. The crude extract (HRB) and 11 induced the leakage of bacterial intracellular components and inhibited the proton-ATPase pumps.

CONCLUSIONS

The crude extract (HRB) and 8,8-bis(dihydroconiferyl)diferulate from the bark of Hypericum roeperianum are good antibacterial candidates that deserve further investigations to achieve antibacterial drugs to fight infections involving MDR bacteria.

Deciphering the Role of Ion Channels in Early Defense Signaling against Herbivorous Insects

Plants and insect herbivores are in a relentless battle to outwit each other. Plants have evolved various strategies to detect herbivores and mount an effective defense system against them. These defenses include physical and structural barriers such as spines, trichomes, cuticle, or chemical compounds, including secondary metabolites such as phenolics and terpenes. Plants perceive herbivory by both mechanical and chemical means. Mechanical sensing can occur through the perception of insect biting, piercing, or chewing, while chemical signaling occurs through the perception of various herbivore-derived compounds such as oral secretions (OS) or regurgitant, insect excreta (frass), or oviposition fluids. Interestingly, ion channels or transporters are the first responders for the perception of these mechanical and chemical cues. These transmembrane pore proteins can play an important role in plant defense through the induction of early signaling components such as plasma transmembrane potential (V_m) fluctuation, intracellular calcium (Ca²⁺), and reactive oxygen species (ROS) generation, followed by defense gene expression, and, ultimately, plant defense responses. In recent years, studies on early plant defense signaling in response to herbivory have been gaining momentum with the application of genetically encoded GFP-based sensors for real-time monitoring of early signaling events and genetic tools to manipulate ion channels involved in plant-herbivore interactions. In this review, we provide an update on recent developments and advances on early signaling events in plant-herbivore interactions, with an emphasis on the role of ion channels in early plant defense signaling.

Phosphorus-Induced Adaptation Mechanisms of Rye Grown on Post-Flotation Copper Tailings

Simple Summary

The mining activities for the production of copper, lead, zinc, and others are inevitably associated with the generation of an enormous volume of waste materials, i.e., tailings. The global area covered with tailings is on the order of hundreds of millions of hectares, with this being predicted to rise significantly in the coming decades. Importantly, the physicochemical properties of tailings, such as high content of heavy metals and high pH, pose a serious threat to the surrounding ecological environment. This, combined with low available nutrients, makes revegetation of degraded lands very difficult, with the number of field trials demonstrating successful revitalisation remaining very low. In this study, we investigate in rye, as a model plant, a wide array of physiological processes and their significance in determining survival on the copper tailings. We demonstrate that limitations in plant growth on such wastes is not simply related to high copper content. Rather, we present data that the low availability of phosphorus and activity of the mechanisms involved in phosphorus extraction from the rhizosphere are important determinants of the plant growth and survival rate. With these results, we make a direct and significant contribution towards meeting future demands for effective revitalisation techniques of degraded lands.

Abstract

Although a considerable effort has been made over the last decades to develop cost-effective phytotechnologies as an alternative to conventional techniques for the management of contaminated lands, successful revegetation of the tailings still represents a major challenge. Here, we evaluate the potential of rye (Secale cereale L.) for growth and survival on the tailings after copper (Cu) ore processing. Four rye varieties were cultivated in a pot experiment on the post-flotation sediment with increasing phosphorus (P) doses (22, 44, 66, 88, and 110 mg·kg⁻¹). The resistance of the studied rye genotypes to stress was assessed by observing the growth and development of plants, determining the dry mass accumulation, the Cu and P uptake and content, and a number of physiological parameters related mainly to P mobilisation. Exposure of tested rye varieties to high Cu concentrations in the tailings did not result in any significant plant mortality, with the intracellular Cu concentrations being below the critical toxic level. In contrast, the low availability of P due to alkaline properties of the tailings and the mechanisms involved in the mobilisation of sparingly soluble forms of this element (i.e., H⁺-ATPase-driven proton efflux in roots and organic acid exudation), were identified as main factor determining the level of tolerance. The efficiency of the photosynthetic activity was a key determinant for the P-mobilising capacity of rye. We further showed that rye varieties with more primitive genetic background might be potentially more suitable for growth on the post-flotation copper tailings. The results provide important and novel knowledge that will certainly support future works in developing strategies for successful revitalisation of degraded lands.

Augmenting salt tolerance in rice by regulating uptake and tissue specific accumulation of Na+ - through Ca2+ -induced alteration of biochemical events

The protective effect of Ca²⁺ against NaCl toxicity was investigated in two rice varieties with contrasting for salt tolerance to understand the mechanistic details of the antagonism to address adverse effects of salinity on agriculture. The study primarily examined the influence of Ca²⁺ on expression/activity of the effectors and regulators involved in Na⁺ translocation. Calcium reduced uptake of Na⁺ concomitant with higher tissue K⁺ /Na⁺ in seedlings, comparatively more in salt-tolerant Nona Bokra than in salt-sensitive IR-64, together with a significant increase in root PM H⁺ ATPase in the former, but not in the latter. Increased antagonism in Nona Bokra could be the result of Ca²⁺ signalling-mediated phosphorylation of PM H⁺ ATPase in roots caused by a significant Ca²⁺ -dependent increase in expression of OsCIPK24, which did not occur in IR-64. Furthermore, significant Ca²⁺ -mediated NaCl-induced increase in transcription of 14-3-3 protein in Nona Bokra, but not in IR-64, might also lead to a greater protective effect of Ca²⁺ in the former, as 14-3-3 protein is essential for activating PM H⁺ ATPase. Thus, efficient functioning of PM H⁺ ATPase could be key in determining resistance of plants to salinity, implying that identification of the Ca²⁺ -dependent kinase phosphorylating the PM H⁺ ATPase threonine residue and manipulation of its expression, together with expression of 14-3-3 proteins could be an important strategy to improve salt tolerance of crops and their cultivation in salt-affected lands.

Low ABA concentration promotes root growth and hydrotropism through relief of ABA INSENSITIVE 1-mediated inhibition of plasma membrane H+-ATPase 2

The hab1-1abi1-2abi2-2pp2ca-1 quadruple mutant (Qabi2-2) seedlings lacking key negative regulators of ABA signaling, namely, clade A protein phosphatases type 2C (PP2Cs), show more apoplastic H⁺ efflux in roots and display an enhanced root growth under normal medium or water stress medium compared to the wild type. The presence of low ABA concentration (0.1 micromolar), inhibiting PP2C activity via monomeric ABA receptors, enhances root apoplastic H⁺ efflux and growth of the wild type, resembling the Qabi2-2 phenotype in normal medium. Qabi2-2 seedlings also demonstrate increased hydrotropism compared to the wild type in obliquely-oriented hydrotropic experimental system, and asymmetric H⁺ efflux in root elongation zone is crucial for root hydrotropism. Moreover, we reveal that Arabidopsis ABA-insensitive 1, a key PP2C in ABA signaling, interacts directly with the C terminus of Arabidopsis plasma membrane H⁺-dependent adenosine triphosphatase 2 (AHA2) and dephosphorylates its penultimate threonine residue (Thr⁹⁴⁷), whose dephosphorylation negatively regulates AHA2.

A plant plasma-membrane H+-ATPase promotes yeast TORC1 activation via its carboxy-terminal tail

The Target of Rapamycin Complex 1 (TORC1) involved in coordination of cell growth and metabolism is highly conserved among eukaryotes. Yet the signals and mechanisms controlling its activity differ among taxa, according to their biological specificities. A common feature of fungal and plant cells, distinguishing them from animal cells, is that their plasma membrane contains a highly abundant H+-ATPase which establishes an electrochemical H+ gradient driving active nutrient transport. We have previously reported that in yeast, nutrient-uptake-coupled H+ influx elicits transient TORC1 activation and that the plasma-membrane H+-ATPase Pma1 plays an important role in this activation, involving more than just establishment of the H+ gradient. We show here that the PMA2 H+-ATPase from the plant Nicotiana plumbaginifolia can substitute for Pma1 in yeast, to promote H+-elicited TORC1 activation. This H+-ATPase is highly similar to Pma1 but has a longer carboxy-terminal tail binding 14-3-3 proteins. We report that a C-terminally truncated PMA2, which remains fully active, fails to promote H+-elicited TORC1 activation. Activation is also impaired when binding of PMA2 to 14-3-3 s is hindered. Our results show that at least some plant plasma-membrane H+-ATPases share with yeast Pma1 the ability to promote TORC1 activation in yeast upon H+-coupled nutrient uptake.

Recent insights into the metabolic adaptations of phosphorus-deprived plants

Inorganic phosphate (Pi) is an essential macronutrient required for many fundamental processes in plants, including photosynthesis and respiration, as well as nucleic acid, protein, and membrane phospholipid synthesis. The huge use of Pi-containing fertilizers in agriculture demonstrates that the soluble Pi levels of most soils are suboptimal for crop growth. This review explores recent advances concerning the understanding of adaptive metabolic processes that plants have evolved to alleviate the negative impact of nutritional Pi deficiency. Plant Pi starvation responses arise from complex signaling pathways that integrate altered gene expression with post-transcriptional and post-translational mechanisms. The resultant remodeling of the transcriptome, proteome, and metabolome enhances the efficiency of root Pi acquisition from the soil, as well as the use of assimilated Pi throughout the plant. We emphasize how the up-regulation of high-affinity Pi transporters and intra- and extracellular Pi scavenging and recycling enzymes, organic acid anion efflux, membrane remodeling, and the remarkable flexibility of plant metabolism and bioenergetics contribute to the survival of Pi-deficient plants. This research field is enabling the development of a broad range of innovative and promising strategies for engineering phosphorus-efficient crops. Such cultivars are urgently needed to reduce inputs of unsustainable and non-renewable Pi fertilizers for maximum agronomic benefit and long-term global food security and ecosystem preservation.

Comparison of path-based centrality measures in protein-protein interaction networks revealed proteins with phenotypic relevance during adaptation to changing nitrogen environments

Plants must rapidly adapt to changes in nutrient conditions. Especially adaptations to changing nitrogen environments are very complex involving also major adjustments on the protein level. Here, we used a size-exclusion chromatography-coupled to mass spectrometry approach to study the dynamics of protein-protein interactions induced by transition from full nutrition to nitrogen starvation. Comparison of interaction networks established for each nutrient condition revealed a large overlap of proteins which were part of the protein-protein interaction network, but that same set of proteins underwent different interactions at each treatment. Network topology parameter betweenness centrality (BC) was found to best reflect the relevance of individual proteins in the information flow within each network. Changes in BC for individual proteins may therefore indicate their involvement in the cellular adjustments to the new condition. Based on this analysis, a set of proteins was identified showing high nitrogen-dependent changes in their BC values: The receptor kinase AT5G49770, co-receptor QSK1, and proton-ATPase AHA2. Mutants of those proteins showed a nitrate-dependent root growth phenotype. Individual interactions within the reconstructed network were tested using FRET-FLIM technology. Taken together, we present a systematic strategy comparing dynamic changes in protein-protein interaction networks based on their network parameters to identify regulatory nodes. SIGNIFICANCE: Protein-protein interactions are known to be important in cellular signaling events, but the dynamic changes in interaction networks induced by external stimuli are still rarely studied. We systematically analyzed how changes in the nutrient environment induced a rewiring of protein-protein interactions in roots. We observed small changes in overall protein abundances, but instead a rewiring of pairwise protein-protein interactions. Betweenness centrality was found to be the optimal network topology parameter to identify protein candidates with high relevance to the information flow in the (dynamic) network. Predicted interactions of those relevant nodes were confirmed in FLIM/FRET experiments and in phenotypic analysis. The network approach described here may be a useful application in dynamic network analysis more generally.

Involvement of plasma membrane H+-ATPase in diamide-induced extracellular alkalization by roots from pea seedlings

MAIN CONCLUSION

The plasma membrane H+-ATPase can be considered as a redox-dependent enzyme, because diamide-mediated inhibition of its hydrolytic and transport activities is accompanied by alkalization of the rhizosphere and retardation of root growth. Plasma membranes were isolated from roots of etiolated pea seedlings treated in the presence of an oxidant-diamide and an inhibitor of redox-sensitive protein phosphatase-phenylarsine oxide. Hydrolytic and proton transport activities of H+-ATPase were determined. The effects of diamide appeared in inhibition of both ATP hydrolysis and the proton transport. However, root treatment with phenylarsine oxide only slightly reduced Vmax, but did not affect ATP-dependent proton transport. The thiol groups of cysteines in the proteins can act as molecular targets for both compounds. However, treatment of isolated membranes with diamide or dithiothreitol did not have any effect on the H+ transport. It can be assumed that water-soluble diamide acts indirectly and its effects are not associated with oxidation of H+-ATPase cysteines. Therefore, plasmalemma was subjected to PEGylation-process where reduced cysteines available for PEG maleimide (5 kDa) were alkylated. Detection of such cysteines was carried out by Western blot analysis with anti-ATPase antibodies. It was found that shifts in the apparent molecular weight were detected only for denaturated proteins. These data suggest that available thiols are not localized on the enzyme surfaces. BN-PAGE analysis showed that the molecular weights of the ATPase complexes are almost identical in all samples. Therefore, oligomerization is probably not the reason for the inhibition of ATPase activity. Roots treated with these inhibitors in vivo exhibited stunted growth; however, a strong alkaline zone around the roots was formed only in the presence of diamide. Involvement of H+-ATPase redox regulation in this process is discussed.

Phosphoproteomic Analysis of Plant Membranes

Mass spectrometry (MS) is a powerful tool to investigate plant phosphorylation dynamics on a system-wide scale (phosphoproteomics). Plant membrane phosphoproteomics enables elucidating regulatory patterns in membranes, such as kinase-target relationships in different signaling pathways. Here, we present "ShortPhos," an efficient and simple phosphoproteomics protocol for research on plant membrane proteins, which allows fast and efficient identification and quantification of phosphopeptides from small amounts of starting plant material and/or membrane proteins. This method improves upon the efficiency of plant membrane phosphoproteomics profiling and can be applied to the study of membrane-based signaling networks.

Biochemical and molecular characterisations of salt tolerance components in rice varieties tolerant and sensitive to NaCl: the relevance of Na+ exclusion in salt tolerance in the species

Soil salinisation is a major abiotic stress in agriculture, and is especially a concern for rice production because among cereal crops, rice is the most salt-sensitive. However, the production of rice must be increased substantially by the year 2050 to meet the demand of the ever growing population. Hence, understanding the biochemical events determining salt tolerance in rice is highly desirable so that the trait can be introduced in cultivars of interest through biotechnological intervention. In this context, an initial study on NaCl response in four Indica rice varieties showed a lower uptake of Na+ in the salt-tolerant Nona Bokra and Pokkali than in the salt-sensitive IR64 and IR29, indicating Na+ exclusion as a primary requirement of salt tolerance in the species. This was also supported by the following features in the salt-tolerant, but not in the -sensitive varieties: (1) highly significant NaCl-induced increase in the activity of PM-H+ATPase, (2) a high constitutive level and NaCl-induced threonine phosphorylation of PM-H+ATPase, necessary to promote its activity, (3) a high constitutive expression of 14-3-3 protein that makes PM-H+ATPase active by binding with the phosphorylated threonine at the C-terminal end, (4) a high constitutive and NaCl-induced expression of SOS1 in roots, and (5) significant NaCl-induced expression of OsCIPK 24, a SOS2 that phosphorylates SOS1. The vacuolar sequestration of Na+ in seedlings was not reflected from the expression pattern of NHX1/NHX1 in response to NaCl. NaCl-induced downregulation of expression of HKTs in roots of Nona Bokra, but upregulation in Pokkali also indicates that their role in salt tolerance in rice could be cultivar specific. The study indicates that consideration of increasing exclusion of Na+ by enhancing the efficiency of SOS1/PM-H+ATPase Na+ exclusion module could be an important aspect in attempting to increase salt tolerance in the rice varieties or cultivars of interest.

When everything changes at once: finding a new normal after genome duplication

Whole-genome duplication (WGD), which leads to polyploidy, is implicated in adaptation and speciation. But what are the immediate effects of WGD and how do newly polyploid lineages adapt to them? With many studies of new and evolved polyploids now available, along with studies of genes under selection in polyploids, we are in an increasingly good position to understand how polyploidy generates novelty. Here, I will review consistent effects of WGD on the biology of plants, such as an increase in cell size, increased stress tolerance and more. I will discuss how a change in something as fundamental as cell size can challenge the function of some cell types in particular. I will also discuss what we have learned about the short- to medium-term evolutionary response to WGD. It is now clear that some of this evolutionary response may 'lock in' traits that happen to be beneficial, while in other cases, it might be more of an 'emergency response' to work around physiological changes that are either deleterious, or cannot be undone in the polyploid context. Yet, other traits may return rapidly to a diploid-like state. Polyploids may, by re-jigging many inter-related processes, find a new, conditionally adaptive, normal.

Proteomic and transcriptomic profiling of aerial organ development in Arabidopsis

Plant growth and development are regulated by a tightly controlled interplay between cell division, cell expansion and cell differentiation during the entire plant life cycle from seed germination to maturity and seed propagation. To explore some of the underlying molecular mechanisms in more detail, we selected different aerial tissue types of the model plant Arabidopsis thaliana, namely rosette leaf, flower and silique/seed and performed proteomic, phosphoproteomic and transcriptomic analyses of sequential growth stages using tandem mass tag-based mass spectrometry and RNA sequencing. With this exploratory multi-omics dataset, development dynamics of photosynthetic tissues can be investigated from different angles. As expected, we found progressive global expression changes between growth stages for all three omics types and often but not always corresponding expression patterns for individual genes on transcript, protein and phosphorylation site level. The biggest difference between proteomic- and transcriptomic-based expression information could be observed for seed samples. Proteomic and transcriptomic data is available via ProteomeXchange and ArrayExpress with the respective identifiers PXD018814 and E-MTAB-7978.

Bacillus amyloliquefaciens PDR1 from root of karst adaptive plant enhances Arabidopsis thaliana resistance to alkaline stress through modulation of plasma membrane H+-ATPase activity

Exploration of native microbes is a feasible way to develop microbial agents for ecological restoration. This study was aimed to explore the impact of Bacillus amyloliquefaciens PDR1 from karst adaptive plant on the activity of root plasma membrane H⁺-ATPase in Arabidopsis thaliana. A. thaliana was cultured in presence or absence of B. amyloliquefaciens PDR1 and its effects on the growth were evaluated by measuring the taproot length and dry weight. The rhizosphere acidification capacity was detected by a pH indicator, a pH meter and non-invasive micro-test techniques (NMT). The nutrient uptake was performed using appropriate methods. A combination of transcriptome sequencing and real-time quantitative polymerase chain reaction (qRT-PCR) was used to measure the expression of functional genes that regulate the plasma membrane H⁺-ATPase activity in A. thaliana roots. Functional analysis was performed to understand how B. amyloliquefaciens regulates biological processes and metabolic pathways to strengthen A. thaliana resistance to alkaline stress. Here, we show that volatile organic compounds (VOCs) from B. amyloliquefaciens PDR1 promoted the growth and development of A. thaliana, enhanced the plasma membrane H⁺-ATPase activity, and affected ion absorption in Arabidopsis roots. Moreover, B. amyloliquefaciens PDR1 VOCs did not affect the expression of the gene coding for plasma membrane H⁺-ATPase, but affected the expression of genes regulating the activity of plasma membrane H⁺-ATPase. Our findings illuminate the mechanism by which B. amyloliquefaciens regulates the growth and alkaline stress resistance of A. thaliana, and lay a foundation for wide and efficient application for agricultural production and ecological protection.

Serine phosphorylation regulates the P-type potassium pump KdpFABC

KdpFABC is an ATP-dependent K⁺ pump that ensures bacterial survival in K⁺-deficient environments. Whereas transcriptional activation of kdpFABC expression is well studied, a mechanism for down-regulation when K⁺ levels are restored has not been described. Here, we show that KdpFABC is inhibited when cells return to a K⁺-rich environment. The mechanism of inhibition involves phosphorylation of Ser162 on KdpB, which can be reversed in vitro by treatment with serine phosphatase. Mutating Ser162 to Alanine produces constitutive activity, whereas the phosphomimetic Ser162Asp mutation inactivates the pump. Analyses of the transport cycle show that serine phosphorylation abolishes the K⁺-dependence of ATP hydrolysis and blocks the catalytic cycle after formation of the aspartyl phosphate intermediate (E1~P). This regulatory mechanism is unique amongst P-type pumps and this study furthers our understanding of how bacteria control potassium homeostasis to maintain cell volume and osmotic potential.

Dual-Reporting Transcriptionally Linked Genetically Encoded Fluorescent Indicators Resolve the Spatiotemporal Coordination of Cytosolic Abscisic Acid and Second Messenger Dynamics in Arabidopsis

Deciphering signal transduction processes is crucial for understanding how plants sense and respond to environmental changes. Various chemical compounds function as central messengers within deeply intertwined signaling networks. How such compounds act in concert remains to be elucidated. We have developed dual-reporting transcriptionally linked genetically encoded fluorescent indicators (2-in-1-GEFIs) for multiparametric in vivo analyses of the phytohormone abscisic acid (ABA), Ca2+, protons (H+), chloride (anions), the glutathione redox potential, and H2O2 Simultaneous analyses of two signaling compounds in Arabidopsis (Arabidopsis thaliana) roots revealed that ABA treatment and uptake did not trigger rapid cytosolic Ca2+ or H+ dynamics. Glutamate, ATP, Arabidopsis PLANT ELICITOR PEPTIDE, and glutathione disulfide (GSSG) treatments induced rapid spatiotemporally overlapping cytosolic Ca2+, H+, and anion dynamics, but except for GSSG, only weakly affected the cytosolic redox state. Overall, 2-in-1-GEFIs enable complementary, high-resolution in vivo analyses of signaling compound dynamics and facilitate an advanced understanding of the spatiotemporal coordination of signal transduction processes in Arabidopsis.

Root-Apex Proton Fluxes at the Centre of Soil-Stress Acclimation

Proton (H⁺) fluxes in plant roots play critical roles in maintaining root growth and facilitating plant responses to multiple soil stresses, including fluctuations in nutrient supply, salt infiltration, and water stress. Soil mining for nutrients and water, rates of nutrient uptake, and the modulation of cell expansion all depend on the regulation of root H⁺ fluxes, particularly at the root apex, mediated primarily by the activity of plasma membrane (PM) H⁺-ATPases. Here, we summarize recent findings on the regulatory mechanisms of H⁺ fluxes at the root apex under three abiotic stress conditions - phosphate deficiency, salinity stress, and water deficiency - and present an integrated physiomolecular view of the functions of H⁺ fluxes in maintaining root growth in the acclimation to soil stress.

Intermolecular and Intramolecular Interactions of the Arabidopsis Plasma Membrane Proton Pump Revealed Using a Mass Spectrometry Cleavable Cross-Linker

In plants and fungi, the plasma membrane proton pump (H⁺-ATPase) establishes an electrochemical gradient across the plasma membrane, which serves as the driving force for the secondary transport of ions and nutrients across the cell membrane. This is an essential enzyme that functions in many important processes including stomatal movement, cell elongation, and cellular responses to stimuli from hormones, light, and other environmental conditions. Therefore, understanding how the activity of the H⁺-ATPase is regulated is important to understand how plants adapt to different growth conditions. The autoinhibitory effect of the C-terminal regulatory domain of H⁺-ATPase is well-established and is thought to be mediated by interactions with the catalytic domains. Here, using the lysine reactive mass spectrometry cleavable cross-linker DSSO, we found that the C-terminal domain of the Arabidopsis H⁺-ATPase 2 (AHA2) cross-linked extensively with the actuator, nucleotide-binding, and phosphorylation domains, suggesting that the C-terminal domain regulates the catalytic cycle by modulating the relative positions of these domains. Interestingly, several C-terminal cross-links occurred near a predicted proton binding site (Asp-684 in TM6), suggesting that the C-terminal domain may regulate proton efflux. Additionally, cross-links between the C-terminal domain and other domains of AHA2 were detected in a monomeric protein resolved on SDS-PAGE, suggesting that intramolecular interactions may also be involved in the regulation of enzyme activity. Finally, we observed mixed-isotope cross-linking between the C-terminal domain and other domains of ¹⁴N-AHA2 (unlabeled) and ¹⁵N-AHA2 (labeled), supporting our model that oligomeric H⁺-ATPase may autoinhibit the neighboring monomer in a "head-to-tail" configuration.

Engineering plants to secrete affinity-tagged pathogen elicitors for deciphering immune receptor complex or inducing enhanced immunity

Plant cells mount plenty of pattern-recognition receptors (PRRs) to detect the microbe-associated molecular patterns (MAMPs) from potential microbial pathogens. MAMPs are overrepresented by proteinaneous patterns, such as the flg22 peptide from bacterial flagellin. Identification of PRR receptor complex components by forward or reverse genetics can be time/labor-consuming, and be confounded by functional redundancies. Here, we present a strategy for identifying PRR complex components by engineering plants to inducibly secrete affinity-tagged proteinaneous MAMPs to the apoplast. The PRR protein complexes bound to self-secreted MAMPs are enriched through affinity purification and dissected by mass spectrometry. As a proof of principle, we could capture the flg22 receptor FLS2 and co-receptor BAK1 using Arabidopsis plants secreting FLAG-tagged flg22 under estradiol induction. Moreover, we identified receptor-like kinases LIK1 and PEPR1/PEPR2 as potential components in the FLS2 receptor complex, which were further validated by protein-protein interaction assays and the reverse genetics approach. Our study showcases a simple way to biochemically identify endogenous PRR complex components without overexpressing the PRR or using chemical cross-linkers, and suggests a possible crosstalk between different immune receptors in plants. A modest dose of estradiol can also be applied to inducing enhanced immunity in engineered plants to both bacterial and fungal pathogens.

Salt Stress Signals on Demand: Cellular Events in the Right Context

Plant stress is a real dilemma; it puzzles plant biologists and is a global problem that negatively affects people’s daily lives. Of particular interest is salinity, because it represents one of the major water-related stress types. We aimed to determine the signals that guide the cellular-related events where various adaptation mechanisms cross-talk to cope with salinity-related water stress in plants. In an attempt to unravel these mechanisms and introduce cellular events in the right context, we expansively discussed how salt-related signals are sensed, with particular emphasis on aquaporins, nonselective cation channels (NSCCs), and glycosyl inositol phosphorylceramide (GIPC). We also elaborated on the critical role Ca2⁺, H⁺, and ROS in mediating signal transduction pathways associated with the response and tolerance to salt stress. In addition, the fragmentary results from the literature were compiled to develop a harmonized, informational, and contemplative model that is intended to improve our perception of these adaptative mechanisms and set a common platform for plant biologists to identify intriguing research questions in this area.

Tenuazonic acid from Stemphylium loti inhibits the plant plasma membrane H+ -ATPase by a mechanism involving the C-terminal regulatory domain

Pathogenic fungi often target the plant plasma membrane (PM) H+ -ATPase during infection. To identify pathogenic compounds targeting plant H+ -ATPases, we screened extracts from 10 Stemphylium species for their effect on H+ -ATPase activity. We identified Stemphylium loti extracts as potential H+ -ATPase inhibitors, and through chemical separation and analysis, tenuazonic acid (TeA) as a potent H+ -ATPase inhibitor. By assaying ATP hydrolysis and H+ pumping, we confirmed TeA as a H+ -ATPase inhibitor both in vitro and in vivo. To visualize in planta inhibition of the H+ -ATPase, we treated pH-sensing Arabidopsis thaliana seedlings with TeA and quantified apoplastic alkalization. TeA affected both ATPase hydrolysis and H+ pumping, supporting a direct effect on the H+ -ATPase. We demonstrated apoplastic alkalization of A. thaliana seedlings after short-term TeA treatment, indicating that TeA effectively inhibits plant PM H+ -ATPase in planta. TeA-induced inhibition was highly dependent on the regulatory C-terminal domain of the plant H+ -ATPase. Stemphylium loti is a phytopathogenic fungus. Inhibiting the plant PM H+ -ATPase results in membrane potential depolarization and eventually necrosis. The corresponding fungal H+ -ATPase, PMA1, is less affected by TeA when comparing native preparations. Fungi are thus able to target an essential plant enzyme without causing self-toxicity.

Rapid Auxin-Mediated Cell Expansion

The promotive effect of auxin on shoot cell expansion provided the bioassay used to isolate this central plant hormone nearly a century ago. While the mechanisms underlying auxin perception and signaling to regulate transcription have largely been elucidated, how auxin controls cell expansion is only now attaining molecular-level definition. The good news is that the decades-old acid growth theory invoking plasma membrane H⁺-ATPase activation is still useful. The better news is that a mechanistic framework has emerged, wherein Small Auxin Up RNA (SAUR) proteins regulate protein phosphatases to control H⁺-ATPase activity. In this review, we focus on rapid auxin effects, their relationship to H⁺-ATPase activation and other transporters, and dependence on TIR1/AFB signaling. We also discuss how some observations, such as near-instantaneous effects on ion transport and root growth, do not fit into a single, comprehensive explanation of how auxin controls cell expansion, and where more research is warranted.

The SAUR41 subfamily of SMALL AUXIN UP RNA genes is abscisic acid inducible to modulate cell expansion and salt tolerance in Arabidopsis thaliana seedlings

BACKGROUND AND AIMS

Most primary auxin response genes are classified into three families: AUX/IAA, GH3 and SAUR genes. Few studies have been conducted on Arabidopsis thaliana SAUR genes, possibly due to genetic redundancy among different subfamily members. Data mining on arabidopsis transcriptional profiles indicates that the SAUR41 subfamily members of SMALL AUXIN UP RNA genes are, strikingly, induced by an inhibitory phytohormone, abscisic acid (ABA). We aimed to reveal the physiological roles of arabidopsis SAUR41 subfamily genes containing SAUR40, SAUR41, SAUR71 and SAUR72.

METHODS

Transcriptional responses of arabidopsis SAUR41 genes to phytohormones were determined by quantitative real-time PCR. Knock out of SAUR41 genes was carried out with the CRISPR/Cas9 (clustered regulatory interspaced short palindromic repeats/CRISPR-associated protein 9) genome editing technique. The saur41/40/71/72 quadruple mutants, SAUR41 overexpression lines and the wild type were subjected to ultrastructural observation, transcriptome analysis and physiological characterization.

KEY RESULTS

Transcription of arabidopsis SAUR41 subfamily genes is activated by ABA but not by gibberellic acids and brassinosteroids. Quadruple mutations in saur41/40/71/72 led to reduced cell expansion/elongation in cotyledons and hypocotyls, opposite to the overexpression of SAUR41; however, an irregular arrangement of cell size and shape was observed in both cases. The quadruple mutants had increased transcription of calcium homeostasis/signalling genes in seedling shoots, and the SAUR41 overexpression lines had decreased transcription of iron homeostasis genes in roots and increased ABA biosynthesis in shoots. Notably, both the quadruple mutants and the SAUR41 overexpression lines were hypersensitive to salt stress during seedling establishment, whereas specific expression of SAUR41 under the ABA-responsive RD29A (Responsive to Desiccation 29A) promoter in the quadruple mutants rescued the inhibitory effect of salt stress.

CONCLUSIONS

The SAUR41 subfamily genes of arabidopsis are ABA inducible to modulate cell expansion, ion homeostasis and salt tolerance. Our work may provide new candidate genes for improvement of plant abiotic stress tolerance.

Gain-of-function mutations: key tools for modifying or designing novel proteins in plant molecular engineering

This article comments on: Deng WJ, Li RQ, Xu YW, Mao RY, Chen SF, Chen LB, Chen LT, Liu YG, Chen YL. 2020. A lipid transfer protein variant with a mutant eight-cysteine motif causes photoperiod- and temperature-sensitive dwarfism in rice. Journal of Experimental Botany 71, 1294–1305.

Auxin-Abscisic Acid Interactions in Plant Growth and Development

Plant hormones regulate many aspects of plant growth, development, and response to biotic and abiotic stress. Much research has gone into our understanding of individual plant hormones, focusing primarily on their mechanisms of action and the processes that they regulate. However, recent research has begun to focus on a more complex problem; how various plant hormones work together to regulate growth and developmental processes. In this review, we focus on two phytohormones, abscisic acid (ABA) and auxin. We begin with brief overviews of the hormones individually, followed by in depth analyses of interactions between auxin and ABA, focusing on interactions in individual tissues and how these interactions are occurring where possible. Finally, we end with a brief discussion and future prospects for the field.

Sphingolipid Effects on the Plasma Membrane Produced by Addition of Fumonisin B1 to Maize Embryos

Fumonisin B1 is a mycotoxin produced by Fusarium verticillioides that modifies the membrane properties from animal cells and inhibits complex sphingolipids synthesis through the inhibition of ceramide synthase. The aim of this work was to determine the effect of Fumonisin B1 on the plant plasma membrane when the mycotoxin was added to germinating maize embryos. Fumonisin B1 addition to the embryos diminished plasma membrane fluidity, increased electrolyte leakage, caused a 7-fold increase of sphinganine and a small decrease in glucosylceramide in the plasma membrane, without affecting phytosphingosine levels or fatty acid composition. A 20%-30% inhibition of the plasma membrane H+-ATPase activity was observed when embryos were germinated in the presence of the mycotoxin. Such inhibition was only associated to the decrease in glucosylceramide and the addition of exogenous ceramide to the embryos relieved the inhibition of Fumonisin B1. These results indicate that exposure of the maize embryos for 24 h to Fumonisin B1 allowed the mycotoxin to target ceramide synthase at the endoplasmic reticulum, eliciting an imbalance of endogenous sphingolipids. The latter disrupted membrane properties and inhibited the plasma membrane H+-ATPase activity. Altogether, these results illustrate the mode of action of the pathogen and a plant defense strategy.

Detection of Phosphorylation on Immunoprecipitates from Total Protein Extracts of Arabidopsis thaliana Seedlings

Phosphorylation is a versatile posttranslational modification that can regulate the localization, stability, and conformation of proteins; protein-protein interactions; and enzyme activities. Phosphorylation of plasma membrane proteins, for example, can serve as recognition signals for ubiquitin ligases and hence can trigger its endocytic degradation. Key determinants of protein phosphorylation are kinases and phosphatases that are spatiotemporally regulated to phosphorylate or dephosphorylate specific target proteins. To understand the dynamics and regulatory mechanisms of protein phosphorylation, it is essential to analyze the phosphorylation status of the proteins and identify phosphorylation sites as well as the modifying enzymes. In this chapter, we describe methods that can be used for the detection of phosphoproteins that are immunoprecipitated from Arabidopsis total extracts.

Pathogen-induced pH changes regulate the growth-defense balance in plants

Environmental adaptation of organisms relies on fast perception and response to external signals, which lead to developmental changes. Plant cell growth is strongly dependent on cell wall remodeling. However, little is known about cell wall-related sensing of biotic stimuli and the downstream mechanisms that coordinate growth and defense responses. We generated genetically encoded pH sensors to determine absolute pH changes across the plasma membrane in response to biotic stress. A rapid apoplastic acidification by phosphorylation-based proton pump activation in response to the fungus Fusarium oxysporum immediately reduced cellulose synthesis and cell growth and, furthermore, had a direct influence on the pathogenicity of the fungus. In addition, pH seems to influence cellulose structure. All these effects were dependent on the COMPANION OF CELLULOSE SYNTHASE proteins that are thus at the nexus of plant growth and defense. Hence, our discoveries show a remarkable connection between plant biomass production, immunity, and pH control, and advance our ability to investigate the plant growth-defense balance.

P-type transport ATPases in Leishmania and Trypanosoma

P-type ATPases are critical to the maintenance and regulation of cellular ion homeostasis and membrane lipid asymmetry due to their ability to move ions and phospholipids against a concentration gradient by utilizing the energy of ATP hydrolysis. P-type ATPases are particularly relevant in human pathogenic trypanosomatids which are exposed to abrupt and dramatic changes in their external environment during their life cycles. This review describes the complete inventory of ion-motive, P-type ATPase genes in the human pathogenic Trypanosomatidae; eight Leishmania species (L. aethiopica, L. braziliensis, L. donovani, L. infantum, L. major, L. mexicana, L. panamensis, L. tropica), Trypanosoma cruzi and three Trypanosoma brucei subspecies (Trypanosoma brucei brucei TREU927, Trypanosoma brucei Lister strain 427, Trypanosoma brucei gambiense DAL972). The P-type ATPase complement in these trypanosomatids includes the P_1B (metal pumps), P_2A (SERCA, sarcoplasmic-endoplasmic reticulum calcium ATPases), P_2B (PMCA, plasma membrane calcium ATPases), P_2D (Na⁺ pumps), P_3A (H⁺ pumps), P₄ (aminophospholipid translocators), and P_5B (no assigned specificity) subfamilies. These subfamilies represent the P-type ATPase transport functions necessary for survival in the Trypanosomatidae as P-type ATPases for each of these seven subfamilies are found in all Leishmania and Trypanosoma species included in this analysis. These P-type ATPase subfamilies are correlated with current molecular and biochemical knowledge of their function in trypanosomatid growth, adaptation, infectivity, and survival.

Adaptation of Plants to Salt Stress: Characterization of Na+ and K+ Transporters and Role of CBL Gene Family in Regulating Salt Stress Response

Salinity is one of the most serious factors limiting the productivity of agricultural crops, with adverse effects on germination, plant vigor, and crop yield. This salinity may be natural or induced by agricultural activities such as irrigation or the use of certain types of fertilizer. The most detrimental effect of salinity stress is the accumulation of Na+ and Cl− ions in tissues of plants exposed to soils with high NaCl concentrations. The entry of both Na+ and Cl− into the cells causes severe ion imbalance, and excess uptake might cause significant physiological disorder(s). High Na+ concentration inhibits the uptake of K+, which is an element for plant growth and development that results in lower productivity and may even lead to death. The genetic analyses revealed K+ and Na+ transport systems such as SOS1, which belong to the CBL gene family and play a key role in the transport of Na+ from the roots to the aerial parts in the Arabidopsis plant. In this review, we mainly discuss the roles of alkaline cations K+ and Na+, Ion homeostasis-transport determinants, and their regulation. Moreover, we tried to give a synthetic overview of soil salinity, its effects on plants, and tolerance mechanisms to withstand stress.

Ammonium and nitrate regulate NH4+ uptake activity of Arabidopsis ammonium transporter AtAMT1;3 via phosphorylation at multiple C-terminal sites

In plants, nutrient transporters require tight regulation to ensure optimal uptake in complex environments. The activities of many nutrient transporters are post-translationally regulated by reversible phosphorylation, allowing rapid adaptation to variable environmental conditions. Here, we show that the Arabidopsis root epidermis-expressed ammonium transporter AtAMT1;3 was dynamically (de-)phosphorylated at multiple sites in the cytosolic C-terminal region (CTR) responding to ammonium and nitrate signals. Under ammonium resupply rapid phosphorylation of a Thr residue (T464) in the conserved part of the CTR (CTRC) effectively inhibited AtAMT1;3-dependent NH4+ uptake. Moreover, phosphorylation of Thr (T494), one of three phosphorylation sites in the non-conserved part of the CTR (CRTNC), moderately decreased the NH4+ transport activity of AtAMT1;3, as deduced from functional analysis of phospho-mimic mutants in yeast, oocytes, and transgenic Arabidopsis. Double phospho-mutants indicated a role of T494 in fine-tuning the NH4+ transport activity when T464 was non-phosphorylated. Transient dephosphorylation of T494 with nitrate resupply closely paralleled a transient increase in ammonium uptake. These results suggest that T464 phosphorylation at the CTRC acts as a prime switch to prevent excess ammonium influx, while T494 phosphorylation at the CTRNC fine tunes ammonium uptake in response to nitrate. This provides a sophisticated regulatory mechanism for plant ammonium transporters to achieve optimal ammonium uptake in response to various nitrogen forms.

Arabidopsis H+-ATPase AHA1 controls slow wave potential duration and wound-response jasmonate pathway activation

Electrogenic proton pumps have been implicated in the generation of slow wave potentials (SWPs), damage-induced membrane depolarizations that activate the jasmonate (JA) defense pathway in leaves distal to wounds. However, no defined H⁺-ATPases have been shown to modulate these electrical signals. Pilot experiments revealed that the proton pump activator fusicoccin attenuated SWP duration in Arabidopsis Using mutant analyses, we identified Arabidopsis H⁺-ATPase 1 (AHA1) as a SWP regulator. The duration of the repolarization phase was strongly extended in reduced function aha1 mutants. Moreover, the duration of SWP repolarization was shortened in the presence of a gain-of-function AHA1 allele. We employed aphid electrodes to probe the effects of the aha1 mutation on wound-stimulated electrical activity in the phloem. Relative to the wild type, the aha1-7 mutant increased the duration and reduced the amplitudes of electrical signals in sieve tube cells. In addition to affecting electrical signaling, expression of the JA pathway marker gene JAZ10 in leaves distal to wounds was enhanced in aha1-7 Consistent with this, levels of wound-response jasmonoyl-isoleucine were enhanced in the mutant, as was defense against a lepidopteran herbivore. The work identifies a discrete member of the P-type ATPase superfamily with a role in leaf-to-leaf electrical signaling and plant defense.

Mutation of a Conserved Motif of PP2C.D Phosphatases Confers SAUR Immunity and Constitutive Activity

The phytohormone auxin promotes the growth of plant shoots by stimulating cell expansion via plasma membrane (PM) H⁺-ATPase activation, which facilitates cell wall loosening and solute uptake. Mechanistic insight was recently obtained by demonstrating that auxin-induced SMALL AUXIN UP RNA (SAUR) proteins inhibit D-CLADE TYPE 2C PROTEIN PHOSPHATASE (PP2C.D) activity, thereby trapping PM H⁺-ATPases in the phosphorylated, activated state, but how SAURs bind PP2C.D proteins and inhibit their activity is unknown. Here, we identified a highly conserved motif near the C-terminal region of the PP2C.D catalytic domain that is required for SAUR binding in Arabidopsis (Arabidopsis thaliana). Missense mutations in this motif abolished SAUR binding but had no apparent effect on catalytic activity. Consequently, mutant PP2C.D proteins that could not bind SAURs exhibited constitutive activity, as they were immune to SAUR inhibition. In planta expression of SAUR-immune pp2c.d2 or pp2c.d5 derivatives conferred severe cell expansion defects and corresponding constitutively low levels of PM H⁺-ATPase phosphorylation. These growth defects were not alleviated by either auxin treatment or 35S:StrepII-SAUR19 coexpression. In contrast, a PM H⁺-ATPase gain-of-function mutation that results in a constitutively active H⁺ pump partially suppressed SAUR-immune pp2c.d5 phenotypes, demonstrating that impaired PM H⁺-ATPase function is largely responsible for the reduced growth of the SAUR-immune pp2c.d5 mutant. Together, these findings provide crucial genetic support for SAUR-PP2C.D regulation of cell expansion via modulation of PM H⁺-ATPase activity. Furthermore, SAUR-immune pp2c.d derivatives provide new genetic tools for elucidating SAUR and PP2C.D functions and manipulating plant organ growth.

Dark septate endophytic fungi increase the activity of proton pumps, efficiency of 15N recovery from ammonium sulphate, N content, and micronutrient levels in rice plants

Plants colonised by dark septate endophytic (DSE) fungi show increased uptake of nutrients available in the environment. The objective of the present study was to evaluate the impact of DSE fungi on the activity of proton pumps, nitrogen (N) recovery from ammonium sulphate, and nutrient accumulation in rice plants. Treatments consisted of non-inoculated plants and plants inoculated with two isolates of DSE fungi, A101 and A103. To determine N recovery from the soil, ammonium sulphate enriched with 15N was added to a non-sterile substrate while parameters associated with the activity of proton pumps and with NO3- uptake were determined in a sterile environment. The A101 and A103 fungal isolates colonised the roots of rice plants, promoting 15N uptake, growth, and accumulation of nutrients as compared with the mock control. A103 induced the expression of the plasma membrane H+-ATPase (PM H+-ATPase) isoforms OsA5 and OsA8, the activity of the PM H+-ATPase and H+-pyrophosphatase. Our results suggest that the inoculation of rice plants with DSE fungi represents a strategy to improve the N recovery from ammonium sulphate and rice plant growth through the induction of OsA5 and OsA8 isoforms and stimulation of the PM H+-ATPase and H+-pyrophosphatase.

Diurnal changes in concerted plant protein phosphorylation and acetylation in Arabidopsis organs and seedlings

Protein phosphorylation and acetylation are the two most abundant post-translational modifications (PTMs) that regulate protein functions in eukaryotes. In plants, these PTMs have been investigated individually; however, their co-occurrence and dynamics on proteins is currently unknown. Using Arabidopsis thaliana, we quantified changes in protein phosphorylation, acetylation and protein abundance in leaf rosettes, roots, flowers, siliques and seedlings at the end of day (ED) and at the end of night (EN). This identified 2549 phosphorylated and 909 acetylated proteins, of which 1724 phosphorylated and 536 acetylated proteins were also quantified for changes in PTM abundance between ED and EN. Using a sequential dual-PTM workflow, we identified significant PTM changes and intersections in these organs and plant developmental stages. In particular, cellular process-, pathway- and protein-level analyses reveal that the phosphoproteome and acetylome predominantly intersect at the pathway- and cellular process-level at ED versus EN. We found 134 proteins involved in core plant cell processes, such as light harvesting and photosynthesis, translation, metabolism and cellular transport, that were both phosphorylated and acetylated. Our results establish connections between PTM motifs, PTM catalyzing enzymes and putative substrate networks. We also identified PTM motifs for further characterization of the regulatory mechanisms that control cellular processes during the diurnal cycle in different Arabidopsis organs and seedlings. The sequential dual-PTM analysis expands our understanding of diurnal plant cell regulation by PTMs and provides a useful resource for future analyses, while emphasizing the importance of analyzing multiple PTMs simultaneously to elucidate when, where and how they are involved in plant cell regulation.

Unusual Roles of Secretory SNARE SYP132 in Plasma Membrane H+-ATPase Traffic and Vegetative Plant Growth

The plasma membrane proton (H⁺)-ATPases of plants generate steep electrochemical gradients and activate osmotic solute uptake. H⁺-ATPase-mediated proton pumping orchestrates cellular homeostasis and is a prerequisite for plastic cell expansion and plant growth. All evidence suggests that the population of H⁺-ATPase proteins at the plasma membrane reflects a balance of their roles in exocytosis, endocytosis, and recycling. Auxin governs both traffic and activation of the plasma membrane H⁺-ATPase proteins already present at the membrane. As in other eukaryotes, in plants, SNARE-mediated membrane traffic influences the density of several proteins at the plasma membrane. Even so, H⁺-ATPase traffic, its relationship with SNAREs, and its regulation by auxin have remained enigmatic. Here, we identify the Arabidopsis (Arabidopsis thaliana) Qa-SNARE SYP132 (Syntaxin of Plants132) as a key factor in H⁺-ATPase traffic and demonstrate its association with endocytosis. SYP132 is a low-abundant, secretory SNARE that primarily localizes to the plasma membrane. We find that SYP132 expression is tightly regulated by auxin and that augmented SYP132 expression reduces the amount of H⁺-ATPase proteins at the plasma membrane. The physiological consequences of SYP132 overexpression include reduced apoplast acidification and suppressed vegetative growth. Thus, SYP132 plays unexpected and vital roles in auxin-regulated H⁺-ATPase traffic and associated functions at the plasma membrane.

Brassinosteroid Induces Phosphorylation of the Plasma Membrane H+-ATPase during Hypocotyl Elongation in Arabidopsis thaliana

Brassinosteroids (BRs) are steroid phytohormones that regulate plant growth and development, and promote cell elongation at least in part via the acid-growth process. BRs have been suggested to induce cell elongation by the activating plasma membrane (PM) H+-ATPase. However, the mechanism by which BRs activate PM H+-ATPase has not been clarified. In this study, we investigated the effects of BR on hypocotyl elongation and the phosphorylation status of a penultimate residue, threonine, of PM H+-ATPase, which affects the activation, in the etiolated seedlings of Arabidopsis thaliana. Brassinolide (BL), an active endogenous BR, induced hypocotyl elongation, phosphorylation of the penultimate, threonine residue of PM H+-ATPase, and binding of the 14-3-3 protein to PM H+-ATPase in the endogenous BR-depleted seedlings. Changes in both BL-induced elongation and phosphorylation of PM H+-ATPase showed similar concentration dependency. BL did not induce phosphorylation of PM H+-ATPase in the BR receptor mutant bri1-6. In contrast, bikinin, a specific inhibitor of BIN2 that acts as a negative regulator of BR signaling, induced its phosphorylation. Furthermore, BL accumulated the transcripts of SMALL AUXIN UP RNA 9 (SAUR9) and SAUR19, which suppress dephosphorylation of the PM H+-ATPase penultimate residue by inhibiting D-clade type 2C protein phosphatase in the hypocotyls of etiolated seedlings. From these results, we conclude that BL-induced phosphorylation of PM H+-ATPase penultimate residue is mediated via the BRI1-BIN2 signaling pathway, together with the accumulation of SAURs during hypocotyl elongation.

The Scope, Functions, and Dynamics of Posttranslational Protein Modifications

Assessing posttranslational modification (PTM) patterns within protein molecules and reading their functional implications present grand challenges for plant biology. We combine four perspectives on PTMs and their roles by considering five classes of PTMs as examples of the broader context of PTMs. These include modifications of the N terminus, glycosylation, phosphorylation, oxidation, and N-terminal and protein modifiers linked to protein degradation. We consider the spatial distribution of PTMs, the subcellular distribution of modifying enzymes, and their targets throughout the cell, and we outline the complexity of compartmentation in understanding of PTM function. We also consider PTMs temporally in the context of the lifetime of a protein molecule and the need for different PTMs for assembly, localization, function, and degradation. Finally, we consider the combined action of PTMs on the same proteins, their interactions, and the challenge ahead of integrating PTMs into an understanding of protein function in plants.

The plasma membrane H+ -ATPase, a simple polypeptide with a long history

The plasma membrane H+ -ATPase of fungi and plants is a single polypeptide of fewer than 1,000 residues that extrudes protons from the cell against a large electric and concentration gradient. The minimalist structure of this nanomachine is in stark contrast to that of the large multi-subunit FO F1 ATPase of mitochondria, which is also a proton pump, but under physiological conditions runs in the reverse direction to act as an ATP synthase. The plasma membrane H+ -ATPase is a P-type ATPase, defined by having an obligatory phosphorylated reaction cycle intermediate, like cation pumps of animal membranes, and thus, this pump has a completely different mechanism to that of FO F1 ATPases, which operates by rotary catalysis. The work that led to these insights in plasma membrane H+ -ATPases of fungi and plants has a long history, which is briefly summarized in this review.

Transgenerational Response to Heat Stress in the Form of Differential Expression of Noncoding RNA Fragments in Brassica rapa Plants

Epigenetic regulations in the form of changes in differential expression of noncoding RNAs (ncRNAs) are an essential mechanism of stress response in plants. Previously we showed that heat treatment in L. results in the differential processing and accumulation of ncRNA fragments (ncRFs) stemming from transfer RNAs (tRNAs), ribosomal RNAs (rRNAs), small nuclear RNAs (snRNAs), and small nucleolar RNAs (snoRNAs). In this work, we analyzed whether ncRFs are differentially expressed in the progeny of heat-stressed plants. We found significant changes in the size of tRF reads and a significant decrease in the percentage of tRFs mapping to tRNA-Ala, tRNA-Arg, and tRNA-Tyr and an increase in tRFs mapping to tRNA-Asp. The enrichment analysis showed significant differences in processing of tRFs from tRNA, tRNA, tRNA, tRNA, tRNA, and tRNA isoacceptors. Analysis of potential targets of tRFs showed that they regulate brassinosteroid metabolism, the proton pump ATPase activity, the antiporter activity, the mRNA decay activity as well as nucleosome positioning and the epigenetic regulation of transgenerational response. Gene ontology term analysis of potential targets demonstrated a significant enrichment in tRFs that potentially targeted a cellular component endoplasmic reticulum (ER) and in small nucleolar RNA fragments (snoRFs), the molecular function protein binding. To summarize, our work demonstrated that the progeny of heat-stressed plants exhibit changes in the expression of tRFs and snoRFs but not of small nuclear RNA fragments (snRFs) or ribosomal RNA fragments (rRFs) and these changes likely better prepare the progeny of stressed plants to future stress encounters.

Systems, variation, individuality and plant hormones

Inter-individual variation in plants and particularly in hormone content, figures strongly in evolution and behaviour. Homo sapiens and Arabidopsis exhibit similar and substantial phenotypic and molecular variation. Whereas there is a very substantial degree of hormone variation in mankind, reports of inter-individual variation in plant hormone content are virtually absent but are likely to be as large if not larger than that in mankind. Reasons for this absence are discussed. Using an example of inter-individual variation in ethylene content in ripening, the article shows how biological time is compressed by hormones. It further resolves an old issue of very wide hormone dose response that result directly from negative regulation in hormone (and light) transduction. Negative regulation is used because of inter-individual variability in hormone synthesis, receptors and ancillary proteins, a consequence of substantial genomic and environmental variation. Somatic mosaics have been reported for several plant tissues and these too contribute to tissue variation and wide variation in hormone response. The article concludes by examining what variation exists in gravitropic responses. There are multiple sensing systems of gravity vectors and multiple routes towards curvature. These are an aspect of the need for reliability in both inter-individual variation and unpredictable environments. Plant hormone inter-individuality is a new area for research and is likely to change appreciation of the mechanisms that underpin individual behaviour.

The Nicotiana tabacum ABC transporter NtPDR3 secretes O-methylated coumarins in response to iron deficiency

Although iron is present in large amounts in the soil, its poor solubility means that plants have to use various strategies to facilitate its uptake. In this study, we show that expression of NtPDR3/NtABCG3, a Nicotiana tabacum plasma-membrane ABC transporter in the pleiotropic drug resistance (PDR) subfamily, is strongly induced in the root epidermis under iron deficiency conditions. Prevention of NtPDR3 expression resulted in N. tabacum plants that were less tolerant to iron-deficient conditions, displaying stronger chlorosis and slower growth than those of the wild-type when not supplied with iron. Metabolic profiling of roots and root exudates revealed that, upon iron deficiency, secretion of catechol-bearing O-methylated coumarins such as fraxetin, hydroxyfraxetin, and methoxyfraxetin to the rhizosphere was compromised in NtPDR3-silenced plants. However, exudation of flavins such as riboflavin was not markedly affected by NtPDR3-silencing. Expression of NtPDR3 in N. tabacum Bright Yellow-2 (BY-2) cells resulted in altered intra- and extracellular coumarin pools, supporting coumarin transport by this transporter. The results demonstrate that N. tabacum secretes both coumarins and flavins in response to iron deficiency and that NtPDR3 plays an essential role in the plant response to iron deficiency by mediating secretion of O-methylated coumarins to the rhizosphere.

Identification and Characterization of Compounds that Affect Stomatal Movements

Regulation of stomatal aperture is essential for plant growth and survival in response to environmental stimuli. Opening of stomata induces uptake of CO2 for photosynthesis and transpiration, which enhances uptake of nutrients from roots. Light is the most important stimulus for stomatal opening. Under drought stress, the plant hormone ABA induces stomatal closure to prevent water loss. However, the molecular mechanisms of stomatal movements are not fully understood. In this study, we screened chemical libraries to identify compounds that affect stomatal movements in Commelina benghalensis and characterize the underlying molecular mechanisms. We identified nine stomatal closing compounds (SCL1-SCL9) that suppress light-induced stomatal opening by >50%, and two compounds (temsirolimus and CP-100356) that induce stomatal opening in the dark. Further investigations revealed that SCL1 and SCL2 had no effect on autophosphorylation of phototropin or the activity of the inward-rectifying plasma membrane (PM) K+ channel, KAT1, but suppressed blue light-induced phosphorylation of the penultimate residue, threonine, in PM H+-ATPase, which is a key enzyme for stomatal opening. SCL1 and SCL2 had no effect on ABA-dependent responses, including seed germination and expression of ABA-induced genes. These results suggest that SCL1 and SCL2 suppress light-induced stomatal opening at least in part by inhibiting blue light-induced activation of PM H+-ATPase, but not by the ABA signaling pathway. Interestingly, spraying leaves onto dicot and monocot plants with SCL1 suppressed wilting of leaves, indicating that inhibition of stomatal opening by these compounds confers tolerance to drought stress in plants.

Acidic cell elongation drives cell differentiation in the Arabidopsis root

In multicellular systems, the control of cell size is fundamental in regulating the development and growth of the different organs and of the whole organism. In most systems, major changes in cell size can be observed during differentiation processes where cells change their volume to adapt their shape to their final function. How relevant changes in cell volume are in driving the differentiation program is a long-standing fundamental question in developmental biology. In the Arabidopsis root meristem, characteristic changes in the size of the distal meristematic cells identify cells that initiated the differentiation program. Here, we show that changes in cell size are essential for the initial steps of cell differentiation and that these changes depend on the concomitant activation by the plant hormone cytokinin of the EXPAs proteins and the AHA1 and AHA2 proton pumps. These findings identify a growth module that builds on a synergy between cytokinin-dependent pH modification and wall remodeling to drive differentiation through the mechanical control of cell walls.

Mechanism of freeze-thaw injury and recovery: A cool retrospective and warming up to new ideas

Understanding cellular mechanism(s) of freeze-thaw injury (FTI) is key to the efforts for improving plant freeze-tolerance by cultural methods or molecular/genetic approaches. However, not much work has been done in the last 25+ years to advance our understanding of the nature and cellular loci of FTI. Currently, two FTI lesions are predominantly implicated: 1) structural and functional perturbations in plasma membrane; 2) ROS-induced oxidative damage. While both have stood the test of time, many questions remain unresolved and other potentially significant lesions need to be investigated. Additionally, molecular mechanism of post-thaw recovery (PTR), a critical component of frost-survival, has not been well investigated. Mechanistic understanding of repair after reversible injury could expand the options for strategies to improve frost-hardiness. In this review, without claiming to be exhaustive, I have attempted to synthesize major discoveries from last several decades on the mechanisms of FTI and the relatively little research conducted thus far on PTR mechanisms. It is followed by proposing of hypotheses for mechanism(s) for irreversible FTI or PTR involving cytosolic calcium and ROS signaling. Perspective is presented on some unresolved questions and research on new ideas to fill the knowledge gaps and advance the field.

HERV Envelope Proteins: Physiological Role and Pathogenic Potential in Cancer and Autoimmunity

Human endogenous retroviruses (HERVs) are relics of ancient infections accounting for about the 8% of our genome. Despite their persistence in human DNA led to the accumulation of mutations, HERVs are still contributing to the human transcriptome, and a growing number of findings suggests that their expression products may have a role in various diseases. Among HERV products, the envelope proteins (Env) are currently highly investigated for their pathogenic properties, which could likely be participating to several disorders with complex etiology, particularly in the contexts of autoimmunity and cancer. In fact, HERV Env proteins have been shown, on the one side, to trigger both innate and adaptive immunity, prompting inflammatory, cytotoxic and apoptotic reactions; and, on the other side, to prevent the immune response activation, presenting immunosuppressive properties and acting as immune downregulators. In addition, HERV Env proteins have been shown to induce abnormal cell-cell fusion, possibly contributing to tumor development and metastasizing processes. Remarkably, even highly defective HERV env genes and alternative env splicing variants can provide further mechanisms of pathogenesis. A well-known example is the HERV-K(HML2) env gene that, depending on the presence or the absence of a 292-bp deletion, can originate two proteins of different length (Np9 and Rec) proposed to have oncogenic properties. The understanding of their involvement in complex pathological disorders made HERV Env proteins potential targets for therapeutic interventions. Of note, a monoclonal antibody directed against a HERV-W Env is currently under clinical trial as therapeutic approach for multiple sclerosis, representing the first HERV-based treatment. The present review will focus on the current knowledge of the HERV Env expression, summarizing its role in human physiology and its possible pathogenic effects in various cancer and autoimmune disorders. It moreover analyzes HERV Env possible exploitation for the development of innovative therapeutic strategies.

Coumarin enhances nitrate uptake in maize roots through modulation of plasma membrane H+ -ATPase activity

Coumarin is one of the simplest plant secondary metabolites, widely distributed in the plant kingdom, affecting root form and function, including anatomy, morphology and nutrient uptake. Although, some plant responses to coumarin have been described, comprehensive knowledge of the physiological and molecular mechanisms is lacking. Maize seedlings exposed to different coumarin concentrations, alone or in combination with 200 μm nitrate (NO₃^- ), were analysed, through a physiological and molecular approach, to elucidate action of coumarin on net NO₃^- uptake rate (NNUR). In detail, the time course of NNUR, plasma membrane (PM) H⁺ -ATPase activity, proton pumping and related gene expression (ZmNPF6.3, ZmNRT2.1, ZmNAR2.1, ZmHA3 and ZmHA4) were evaluated. Coumarin alone did not affect nitrate uptake, PM H⁺ -ATPase activity or transcript levels of ZmNRT2.1 and ZmHA3. In contrast, coumarin alone increased ZmNPF6.3, ZmNAR2.1 and ZmHA4 expression in response to abiotic stress. When coumarin and NO₃^- were concurrently added to the nutrient solution, a significant increase in the NNUR, PM H⁺ -ATPase activity, together with ZmNAR2.1:ZmNRT2.1 and ZmHA4 expression was observed, suggesting that coumarin affected the inducible component of the high affinity transport system (iHATS), and this effect appeared to be mediated by nitrate. Moreover, results with vanadate, an inhibitor of the PM H⁺ -ATPase, suggested that this enzyme could be the main target of coumarin. Surprisingly, coumarin did not affect PM H⁺ -ATPase activity by direct contact with plasma membrane vesicles isolated from maize roots, indicating its possible elicitor role in gene transcription.

Environmental and Genetic Factors Regulating Localization of the Plant Plasma Membrane H+-ATPase

A P-type H⁺-ATPase is the primary transporter that converts ATP to electrochemical energy at the plasma membrane of higher plants. Its product, the proton-motive force, is composed of an electrical potential and a pH gradient. Many studies have demonstrated that this proton-motive force not only drives the secondary transporters required for nutrient uptake, but also plays a direct role in regulating cell expansion. Here, we have generated a transgenic Arabidopsis (Arabidopsis thaliana) plant expressing H⁺-ATPase isoform 2 (AHA2) that is translationally fused with a fluorescent protein and examined its cellular localization by live-cell microscopy. Using a 3D imaging approach with seedlings grown for various times under a variety of light intensities, we demonstrate that AHA2 localization at the plasma membrane of root cells requires light. In dim light conditions, AHA2 is found in intracellular compartments, in addition to the plasma membrane. This localization profile was age-dependent and specific to cell types found in the transition zone located between the meristem and elongation zones. The accumulation of AHA2 in intracellular compartments is consistent with reduced H⁺ secretion near the transition zone and the suppression of root growth. By examining AHA2 localization in a knockout mutant of a receptor protein kinase, FERONIA, we found that the intracellular accumulation of AHA2 in the transition zone is dependent on a functional FERONIA-dependent inhibitory response in root elongation. Overall, this study provides a molecular underpinning for understanding the genetic, environmental, and developmental factors influencing root growth via localization of the plasma membrane H⁺-ATPase.