Evidence for multiple receptors mediating RALF-triggered Ca2+ signaling and proton pump inhibition

Plasma membrane H+-ATPase activation increases global transcript levels and promotes the shoot growth of light-grown Arabidopsis seedlings

Plant cell growth requires the elongation of cells mediated by cell wall remodelling and turgor pressure changes. The plasma membrane (PM) H+-ATPase facilitates both cell wall loosening and turgor pressure changes by acidifying the apoplast of cells, referred to as acid growth. The acid growth theory is mostly established on the auxin-induced activation of PM H+-ATPase in non-photosynthetic tissues. However, how PM H+-ATPase affects the growth in photosynthetic tissues of Arabidopsis remains unclear. Here, a combination of transcriptomics and cis-regulatory element analysis was conducted to identify the impact of PM H+-ATPase on global transcript levels and the molecular mechanism downstream of the PM H+-ATPase. The PM H+-ATPase activation increased transcript levels globally, especially cell wall modification-related genes. The transcript level changes were in PM H+-ATPase-dependent manner. Involvement of Ca2+ was suggested as CAMTA motif was enriched in the promoter of PM H+-ATPase-induced genes and cytosolic Ca2+ elevated upon PM H+-ATPase activation. PM H+-ATPase activation in photosynthetic tissues promotes the expression of cell wall modification enzymes and shoot growth, adding a novel perspective of photosynthesis-dependent PM H+-ATPase activation in photosynthetic tissues to the acid growth theory that has primarily based on findings from non-photosynthetic tissues.

Peptide hormones in plants

Peptide hormones are defined as small secreted polypeptide-based intercellular communication signal molecules. Such peptide hormones are encoded by nuclear genes, and often go through proteolytic processing of preproproteins and post-translational modifications. Most peptide hormones are secreted out of the cell to interact with membrane-associated receptors in neighboring cells, and subsequently activate signal transductions, leading to changes in gene expression and cellular responses. Since the discovery of the first plant peptide hormone, systemin, in tomato in 1991, putative peptide hormones have continuously been identified in different plant species, showing their importance in both short- and long-range signal transductions. The roles of peptide hormones are implicated in, but not limited to, processes such as self-incompatibility, pollination, fertilization, embryogenesis, endosperm development, stem cell regulation, plant architecture, tissue differentiation, organogenesis, dehiscence, senescence, plant-pathogen and plant-insect interactions, and stress responses. This article, collectively written by researchers in this field, aims to provide a general overview for the discoveries, functions, chemical natures, transcriptional regulations, and post-translational modifications of peptide hormones in plants. We also updated recent discoveries in receptor kinases underlying the peptide hormone sensing and down-stream signal pathways. Future prospective and challenges will also be discussed at the end of the article.

PCP-B peptides and CrRLK1L receptor kinases control pollination via pH gating of aquaporins in Arabidopsis

During pollen-stigma interaction, pollen coat protein B-class peptides (PCP-Bs) compete with stigmatic rapid alkalinization factor (RALF) for interaction with FERONIA/ANJEA receptor kinases (FER/ANJ), stimulating pollen hydration and germination. However, the molecular mechanism underlying PCP-Bs-induced, FER/ANJ-regulated compatible responses remains largely unknown. Through PCP-Bγ-induced phosphoproteomic analysis, we characterized a series of pollination-related signaling pathways regulated by FER/ANJ. Interestingly, on stigmatic papillary cells, pollen PCP-Bγ induced an elevation in cytosolic pH near the plasma membrane (PM), sustained by stigmatic RALF23/33 through regulation of the autoinhibited H+-ATPase 1/2 (AHA1/2) activity. We further found that RALFs/PCP-Bs and FER/ANJ regulated the pH alterations via phosphorylation of AHA1/2 C terminus. Furthermore, RALF23/33-FER/ANJ maintained the protonation of H197 in plasma membrane intrinsic proteins (PIPs), whereas PCP-B relieved the protonation through AHA activity. Altogether, this study reveals that pollen PCP-Bs trigger FER/ANJ-controlled compatible responses, particularly the opening of aquaporins via AHA-mediated pH changes, thereby facilitating pollen hydration in Arabidopsis.

RAPID ALKALINIZATION FACTOR 22 is a key modulator of the root hair growth responses to fungal ethylene emissions in Arabidopsis

In Arabidopsis (Arabidopsis thaliana (L.) Heynh), exposure to volatile compounds (VCs) emitted by Penicillium aurantiogriseum promotes root hair (RH) proliferation and hyper-elongation through mechanisms involving ethylene, auxin, and photosynthesis signaling. In addition, this treatment enhances the levels of the small signaling peptide RAPID ALKALINIZATION FACTOR 22 (RALF22). Here, we used genetics to address the role of RALF22 in fungal VC-promoted RH growth and to identify the bioactive fungal VC. We found that RHs of ralf22 and feronia (fer-4) plants impaired in the expression of RALF22 and its receptor FERONIA, respectively, responded weakly to fungal VCs. Unlike in wild-type roots, fungal VC exposure did not enhance RALF22 transcript levels in roots of fer-4 and ethylene- and auxin-insensitive mutants. In ralf22 and fer-4 roots, this treatment did not enhance the levels of ERS2 transcripts encoding one member of the ethylene receptor family and those of some RH-related genes. RHs of ers2-1 and the rsl2rsl4 double mutants impaired in the expression of ERS2 and the ethylene- and auxin-responsive ROOT HAIR DEFECTIVE 6-LIKE 2 and 4 transcription factors, respectively, weakly responded to fungal VCs. Moreover, roots of plants defective in photosynthetic responsiveness to VCs exhibited weak RALF22 expression and RH growth responses to fungal VCs. VCs of ΔefeA strains of P. aurantiogriseum cultures impaired in ethylene synthesis weakly promoted RH proliferation and elongation in exposed plants. We conclude that RALF22 simultaneously functions as a transcriptionally regulated signaling molecule that participates in the ethylene, auxin, and photosynthesis signaling-mediated RH growth response to fungal ethylene emissions and regulation of ethylene perception in RHs.

Small but powerful: RALF peptides in plant adaptive and developmental responses

Plants live in a highly dynamic environment and require to rapidly respond to a plethora of environmental stimuli, so that to maintain their optimal growth and development. A small plant peptide, rapid alkalization factor (RALF), can rapidly increase the pH value of the extracellular matrix in plant cells. RALFs always function with its corresponding receptors. Mechanistically, effective amount of RALF is induced and released at the critical period of plant growth and development or under different external environmental factors. Recent studies also highlighted the role of RALF peptides as important regulators in plant intercellular communications, as well as their operation in signal perception and as ligands for different receptor kinases on the surface of the plasma membrane, to integrate various environmental cues. In this context, understanding the fine-print of above processes may be essential to solve the problems of crop adaptation to various harsh environments under current climate trends scenarios, by genetic means. This paper summarizes the current knowledge about the structure and diversity of RALF peptides and their roles in plant development and response to stresses, highlighting unanswered questions and problems to be solved.

The mechanism behind tenuazonic acid-mediated inhibition of plant plasma membrane H+-ATPase and plant growth

The increasing prevalence of herbicide-resistant weeds has led to a search for new herbicides that target plant growth processes differing from those targeted by current herbicides. In recent years, some studies have explored the use of natural compounds from microorganisms as potential new herbicides. We previously demonstrated that tenuazonic acid (TeA) from the phytopathogenic fungus Stemphylium loti inhibits the plant plasma membrane (PM) H+-ATPase, representing a new target for herbicides. In this study, we further investigated the mechanism by which TeA inhibits PM H+-ATPase and the effect of the toxin on plant growth using Arabidopsis thaliana. We also studied the biochemical effects of TeA on the PM H+-ATPases from spinach (Spinacia oleracea) and A. thaliana (AHA2) by examining PM H+-ATPase activity under different conditions and in different mutants. Treatment with 200 μM TeA-induced cell necrosis in larger plants and treatment with 10 μM TeA almost completely inhibited cell elongation and root growth in seedlings. We show that the isoleucine backbone of TeA is essential for inhibiting the ATPase activity of the PM H+-ATPase. Additionally, this inhibition depends on the C-terminal domain of AHA2, and TeA binding to PM H+-ATPase requires the Regulatory Region I of the C-terminal domain in AHA2. TeA likely has a higher binding affinity toward PM H+-ATPase than the phytotoxin fusicoccin. Finally, our findings show that TeA retains the H+-ATPase in an inhibited state, suggesting that it could act as a lead compound for creating new herbicides targeting the PM H+-ATPase.

Salt-Induced Early Changes in Photosynthesis Activity Caused by Root-to-Shoot Signaling in Potato

Salinity is one of the most dangerous types of stress in agriculture. Acting on the root, salinity causes changes in physiological processes in the shoot, especially photosynthesis, which is crucial for plant productivity. In our study, we used potato plants, the most important crop, to investigate the role of salt-induced signals in changes in photosynthesis activity. We found a salt-induced polyphasic decrease in photosynthesis activity, and the earliest phase started several minutes after salt addition. We found that salt addition triggered rapid hydraulic and calcium waves from root to shoot, which occurred earlier than the first phase of the photosynthesis response. The inhibition of calcium signals by lanthanum decreased with the formation of rapid changes in photosynthesis. In addition to this, a comparison of the characteristic times of signal propagation and the formation of a response revealed the role of calcium waves in the modulation of rapid changes in photosynthesis. Calcium waves are activated by the ionic component of salinity. The salt-induced decrease in transpiration corresponds in time to the second phase of the photosynthetic response, and it can be the cause of this change. The accumulation of sodium in the leaves occurs a few hours after salt addition, and it can be the cause of the long-term suppression of photosynthesis. Thus, salinity modulates photosynthetic activity in plants in different ways: both through the activation of rapid distant signals and by reducing the water input and sodium accumulation.

Assaying the Effect of Peptide Treatment on H+-Pumping Activity in Plasma Membranes from Arabidopsis Seedlings

Extracellular acidification or alkalization is a common response to many plant-signaling peptides and microbial elicitors. This may be a result of peptide-mediated regulation of plasma membrane-localized ion transporters, such as the plasma membrane H+-ATPase. Early responses to some signaling peptides can therefore be analyzed by assaying H+-pumping across the plasma membrane.We describe a set-up suited for the purification of plasma membranes by aqueous two-phase partitioning from a small sample of Arabidopsis seedlings. Seedlings are grown in a liquid culture, suited for the analysis of in vivo peptide treatment. Additionally, we describe how to measure the H+-pumping activity of the plasma membrane H+-ATPase using the fluorescent probe ACMA.

Protein Phosphorylation Orchestrates Acclimations of Arabidopsis Plants to Environmental pH

Environment pH (pHe) is a key parameter dictating a surfeit of conditions critical to plant survival and fitness. To elucidate the mechanisms that recalibrate cytoplasmic and apoplastic pH homeostasis, we conducted a comprehensive proteomic/phosphoproteomic inventory of plants subjected to transient exposure to acidic or alkaline pH, an approach that covered the majority of protein-coding genes of the reference plant Arabidopsis thaliana. Our survey revealed a large set-of so far undocumented pHe-dependent phospho-sites, indicative of extensive post-translational regulation of proteins involved in the acclimation to pHe. Changes in pHe altered both electrogenic H+ pumping via P-type ATPases and H+/anion co-transport processes, putatively leading to altered net trans-plasma membrane translocation of H+ ions. In pH 7.5 plants, the transport (but not the assimilation) of nitrogen via NRT2-type nitrate and AMT1-type ammonium transporters was induced, conceivably to increase the cytosolic H+ concentration. Exposure to both acidic and alkaline pH resulted in a marked repression of primary root elongation. No such cessation was observed in nrt2.1 mutants. Alkaline pH decreased the number of root hairs in the wild type but not in nrt2.1 plants, supporting a role of NRT2.1 in developmental signaling. Sequestration of iron into the vacuole via alterations in protein abundance of the vacuolar iron transporter VTL5 was inversely regulated in response to high and low pHe, presumptively in anticipation of associated changes in iron availability. A pH-dependent phospho-switch was also observed for the ABC transporter PDR7, suggesting changes in activity and, possibly, substrate specificity. Unexpectedly, the effect of pHe was not restricted to roots and provoked pronounced changes in the shoot proteome. In both roots and shoots, the plant-specific TPLATE complex components AtEH1 and AtEH2-essential for clathrin-mediated endocytosis-were differentially phosphorylated at multiple sites in response to pHe, indicating that the endocytic cargo protein trafficking is orchestrated by pHe.

pH regulates peptide-receptor perception

Diverse plant small peptides are perceived by their corresponding receptors to mediate local or long-distance intercellular communications in various developmental and adaptive programs; notably, the mechanisms of peptide-receptor perception remain largely unrevealed. Two reports (Liu et al.; Diaz-Ardila et al.) shed light on how pH regulates peptide-receptor perception.

Alkalinity modulates a unique suite of genes to recalibrate growth and pH homeostasis

Alkaline soils pose a conglomerate of constraints to plants, restricting the growth and fitness of non-adapted species in habitats with low active proton concentrations. To thrive under such conditions, plants have to compensate for a potential increase in cytosolic pH and restricted softening of the cell wall to invigorate cell elongation in a proton-depleted environment. To discern mechanisms that aid in the adaptation to external pH, we grew plants on media with pH values ranging from 5.5 to 8.5. Growth was severely restricted above pH 6.5 and associated with decreasing chlorophyll levels at alkaline pH. Bicarbonate treatment worsened plant performance, suggesting effects that differ from those exerted by pH as such. Transcriptional profiling of roots subjected to short-term transfer from optimal (pH 5.5) to alkaline (pH 7.5) media unveiled a large set of differentially expressed genes that were partially congruent with genes affected by low pH, bicarbonate, and nitrate, but showed only a very small overlap with genes responsive to the availability of iron. Further analysis of selected genes disclosed pronounced responsiveness of their expression over a wide range of external pH values. Alkalinity altered the expression of various proton/anion co-transporters, possibly to recalibrate cellular proton homeostasis. Co-expression analysis of pH-responsive genes identified a module of genes encoding proteins with putative functions in the regulation of root growth, which appears to be conserved in plants subjected to low pH or bicarbonate. Our analysis provides an inventory of pH-sensitive genes and allows comprehensive insights into processes that are orchestrated by external pH.

Rapid alkalinization factor: function, regulation, and potential applications in agriculture

Rapid alkalinization factor (RALF) is widespread throughout the plant kingdom and controls many aspects of plant life. Current studies on the regulatory mechanism underlying RALF function mainly focus on Arabidopsis, but little is known about the role of RALF in crop plants. Here, we systematically and comprehensively analyzed the relation between RALF family genes from five important crops and those in the model plant Arabidopsis thaliana. Simultaneously, we summarized the functions of RALFs in controlling growth and developmental behavior using conservative motifs as cues and predicted the regulatory role of RALFs in cereal crops. In conclusion, RALF has considerable application potential in improving crop yields and increasing economic benefits. Using gene editing technology or taking advantage of RALF as a hormone additive are effective way to amplify the role of RALF in crop plants.

Lateral Root Initiation in Cucumber (Cucumis sativus): What Does the Expression Pattern of Rapid Alkalinization Factor 34 (RALF34) Tell Us?

In Arabidopsis, the small signaling peptide (peptide hormone) RALF34 is involved in the gene regulatory network of lateral root initiation. In this study, we aimed to understand the nature of the signals induced by RALF34 in the non-model plant cucumber (Cucumis sativus), where lateral root primordia are induced in the apical meristem of the parental root. The RALF family members of cucumber were identified using phylogenetic analysis. The sequence of events involved in the initiation and development of lateral root primordia in cucumber was examined in detail. To elucidate the role of the small signaling peptide CsRALF34 and its receptor CsTHESEUS1 in the initial stages of lateral root formation in the parental root meristem in cucumber, we studied the expression patterns of both genes, as well as the localization and transport of the CsRALF34 peptide. CsRALF34 is expressed in all plant organs. CsRALF34 seems to differ from AtRALF34 in that its expression is not regulated by auxin. The expression of AtRALF34, as well as CsRALF34, is regulated in part by ethylene. CsTHESEUS1 is expressed constitutively in cucumber root tissues. Our data suggest that CsRALF34 acts in a non-cell-autonomous manner and is not involved in lateral root initiation in cucumber.

DEFECTIVE KERNEL1 regulates cellulose synthesis and affects primary cell wall mechanics

The cell wall is one of the defining features of plants, controlling cell shape, regulating growth dynamics and hydraulic conductivity, as well as mediating plants interactions with both the external and internal environments. Here we report that a putative mechanosensitive Cys-protease DEFECTIVE KERNEL1 (DEK1) influences the mechanical properties of primary cell walls and regulation of cellulose synthesis. Our results indicate that DEK1 is an important regulator of cellulose synthesis in epidermal tissue of Arabidopsis thaliana cotyledons during early post-embryonic development. DEK1 is involved in regulation of cellulose synthase complexes (CSCs) by modifying their biosynthetic properties, possibly through interactions with various cellulose synthase regulatory proteins. Mechanical properties of the primary cell wall are altered in DEK1 modulated lines with DEK1 affecting both cell wall stiffness and the thickness of the cellulose microfibril bundles in epidermal cell walls of cotyledons.

Cell Wall Integrity Signaling in Fruit Ripening

Plant cell walls are essential structures for plant growth and development as well as plant adaptation to environmental stresses. Thus, plants have evolved signaling mechanisms to monitor the changes in the cell wall structure, triggering compensatory changes to sustain cell wall integrity (CWI). CWI signaling can be initiated in response to environmental and developmental signals. However, while environmental stress-associated CWI signaling has been extensively studied and reviewed, less attention has been paid to CWI signaling in relation to plant growth and development under normal conditions. Fleshy fruit development and ripening is a unique process in which dramatic alternations occur in cell wall architecture. Emerging evidence suggests that CWI signaling plays a pivotal role in fruit ripening. In this review, we summarize and discuss the CWI signaling in relation to fruit ripening, which will include cell wall fragment signaling, calcium signaling, and NO signaling, as well as Receptor-Like Protein Kinase (RLKs) signaling with an emphasis on the signaling of FERONIA and THESEUS, two members of RLKs that may act as potential CWI sensors in the modulation of hormonal signal origination and transduction in fruit development and ripening.

Salt-Induced Changes in Cytosolic pH and Photosynthesis in Tobacco and Potato Leaves

Salinity is one of the most common factors limiting the productivity of crops. The damaging effect of salt stress on many vital plant processes is mediated, on the one hand, by the osmotic stress caused by large concentrations of Na⁺ and Cl^- outside the root and, on the other hand, by the toxic effect of these ions loaded in the cell. In our work, the influence of salinity on the changes in photosynthesis, transpiration, water content and cytosolic pH in the leaves of two important crops of the Solanaceae family-tobacco and potato-was investigated. Salinity caused a decrease in photosynthesis activity, which manifested as a decrease in the quantum yield of photosystem II and an increase in non-photochemical quenching. Along with photosynthesis limitation, there was a slight reduction in the relative water content in the leaves and a decrease in transpiration, determined by the crop water stress index. Furthermore, a decrease in cytosolic pH was detected in tobacco and potato plants transformed by the gene of pH-sensitive protein Pt-GFP. The potential mechanisms of the salinity influence on the activity of photosynthesis were analyzed with the comparison of the parameters' dynamics, as well as the salt content in the leaves.

How a single receptor-like kinase exerts diverse roles: lessons from FERONIA

FERONIA (FER) is a member of the Catharanthus roseus receptor-like kinase 1-like (CrRLK1L) protein subfamily, which participates in reproduction, abiotic stress, biotic stress, cell growth, hormone response, and other molecular mechanisms of plants. However, the mechanism by which a single RLK is capable of mediating multiple signals and activating multiple cellular responses remains unclear. Here, we summarize research progress revealing the spatial-temporal expression of FER, along with its co-receptors and ligands determined the function of FER signaling pathway in multiple organs. The specificity of the FER signaling pathway is proposed to operate under a four-layered mechanism: (1) Spatial-temporal expression of FER, co-receptors, and ligands specify diverse functions, (2) Specific ligands or ligand combinations trigger variable FER signaling pathways, (3) Diverse co-receptors confer diverse FER perception and response modes, and (4) Unique downstream components that modify FER signaling and responses. Moreover, the regulation mechanism of the signaling pathway- appears to depend on the interaction among the ligands, RLK receptors, co-receptors, and downstream components, which may be a general mechanism of RLKs to maintain signal specificity. This review will provide a insight into understanding the specificity determination of RLKs signaling in both model and horticultural crops.

Genetically encoded fluorescent sensors adapted to acidic pH highlight subdomains within the plant cell apoplast

Monitoring pH is one of the challenges in understanding diverse physiological regulations as well as ionic balance, especially in highly acidic environments such as the apoplast and the vacuole. To circumvent the poor efficiency of pH measurements below pH 5, we designed three genetically encoded sensors composed of two fluorescent proteins in tandem. We selected fluorescent protein pairs of low but sufficiently different pKa so that each protein could differentially sense the imposed pH. The generated tandems, named Acidin2, Acidin3, and Acidin4, were produced in Escherichia coli and extensively characterized. Altogether, these generated tandems cover a pH range of 3-8. The Acidins were targeted either for release in the apoplast (Apo) or for anchoring at the outer face of the plasma membrane (PM-Apo), with the fluorescent part exposed in the apoplast. Apoplastic Acidins in stably transformed Arabidopsis thaliana primary roots responded immediately and reversibly to pH changes, directly reporting physiological conditions related to cell elongation. In addition, membrane-anchored Acidins reveal a gradual acidification from the surface through the anticlinal wall of pavement cells, a process controlled at least partially by H+-ATPase activity.

The root apoplastic pH as an integrator of plant signaling

Plant nutrition, growth, and response to environmental stresses are pH-dependent processes that are regulated at the apoplastic and subcellular levels. The root apoplastic pH is especially sensitive to external cues and can also be modified by intracellular inputs, such as hormonal signaling. Optimal crosstalk of the mechanisms involved in the extent and span of the apoplast pH fluctuations promotes plant resilience to detrimental biotic and abiotic factors. The fact that variations in local pHs are a standard mechanism in different signaling pathways indicates that the pH itself can be the pivotal element to provide a physiological context to plant cell regions, allowing a proportional reaction to different situations. This review brings a collective vision of the causes that initiate root apoplastic pHs variations, their interaction, and how they influence root response outcomes.

RALF1 peptide triggers biphasic root growth inhibition upstream of auxin biosynthesis

Plant cell growth responds rapidly to various stimuli, adapting architecture to environmental changes. Two major endogenous signals regulating growth are the phytohormone auxin and the secreted peptides rapid alkalinization factors (RALFs). Both trigger very rapid cellular responses and also exert long-term effects [Du et al., Annu. Rev. Plant Biol. 71, 379-402 (2020); Blackburn et al., Plant Physiol. 182, 1657-1666 (2020)]. However, the way, in which these distinct signaling pathways converge to regulate growth, remains unknown. Here, using vertical confocal microscopy combined with a microfluidic chip, we addressed the mechanism of RALF action on growth. We observed correlation between RALF1-induced rapid Arabidopsis thaliana root growth inhibition and apoplast alkalinization during the initial phase of the response, and revealed that RALF1 reversibly inhibits primary root growth through apoplast alkalinization faster than within 1 min. This rapid apoplast alkalinization was the result of RALF1-induced net H⁺ influx and was mediated by the receptor FERONIA (FER). Furthermore, we investigated the cross-talk between RALF1 and the auxin signaling pathways during root growth regulation. The results showed that RALF-FER signaling triggered auxin signaling with a delay of approximately 1 h by up-regulating auxin biosynthesis, thus contributing to sustained RALF1-induced growth inhibition. This biphasic RALF1 action on growth allows plants to respond rapidly to environmental stimuli and also reprogram growth and development in the long term.

Tripartite hormonal regulation of plasma membrane H+-ATPase activity

The enzyme activity of the plasma membrane (PM) proton pump, well known as arabidopsis PM H⁺-ATPase (AHA) in the model plant arabidopsis (Arabidopsis thaliana), is controlled by phosphorylation. Three different classes of phytohormones, brassinosteroids (BRs), abscisic acid (ABA), and auxin regulate plant growth and responses to environmental stimuli, at least in part by modulating the activity of the pump through phosphorylation of the penultimate Thr residue in its carboxyl terminus. Here, we review the current knowledge regarding this tripartite hormonal AHA regulation and highlight mechanisms of activation and deactivation, as well as the significance of hormonal crosstalk. Understanding the complexity of PM H⁺-ATPase regulation in plants might provide new strategies for sustainable agriculture.

H+-ATPases in Plant Growth and Stress Responses

H⁺-ATPases, including the phosphorylated intermediate-type (P-type) and vacuolar-type (V-type) H⁺-ATPases, are important ATP-driven proton pumps that generate membrane potential and provide proton motive force for secondary active transport. P- and V-type H⁺-ATPases have distinct structures and subcellular localizations and play various roles in growth and stress responses. A P-type H⁺-ATPase is mainly regulated at the posttranslational level by phosphorylation and dephosphorylation of residues in its autoinhibitory C terminus. The expression and activity of both P- and V-type H⁺-ATPases are highly regulated by hormones and environmental cues. In this review, we summarize the recent advances in understanding of the evolution, regulation, and physiological roles of P- and V-type H⁺-ATPases, which coordinate and are involved in plant growth and stress adaptation. Understanding the different roles and the regulatory mechanisms of P- and V-type H⁺-ATPases provides a new perspective for improving plant growth and stress tolerance by modulating the activity of H⁺-ATPases, which will mitigate the increasing environmental stress conditions associated with ongoing global climate change.

Genome-Wide Identification of Rapid Alkalinization Factor Family in Brassica napus and Functional Analysis of BnRALF10 in Immunity to Sclerotinia sclerotiorum

Rapid alkalinization factors (RALFs) were recently reported to be important players in plant immunity. Nevertheless, the signaling underlying RALF-triggered immunity in crop species against necrotrophic pathogens remains largely unknown. In this study, RALF family in the important oil crop oilseed rape (Brassica napus) was identified and functions of BnRALF10 in immunity against the devastating necrotrophic pathogen Sclerotinia sclerotiorum as well as the signaling underlying this immunity were revealed. The oilseed rape genome carried 61 RALFs, half of them were atypical, containing a less conserved YISY motif and lacking a RRXL motif or a pair of cysteines. Family-wide gene expression analyses demonstrated that patterns of expression in response to S. sclerotiorum infection and DAMP and PAMP treatments were generally RALF- and stimulus-specific. Most significantly responsive BnRALF genes were expressionally up-regulated by S. sclerotiorum, while in contrast, more BnRALF genes were down-regulated by BnPep5 and SsNLP1. These results indicate that members of BnRALF family are likely differentially involved in plant immunity. Functional analyses revealed that BnRALF10 provoked diverse immune responses in oilseed rape and stimulated resistance to S. sclerotiorum. These data support BnRALF10 to function as a DAMP to play a positive role in plant immunity. BnRALF10 interacted with BnFER. Silencing of BnFER decreased BnRALF10-induced reactive oxygen species (ROS) production and compromised rape resistance to S. sclerotiorum. These results back BnFER to be a receptor of BnRALF10. Furthermore, quantitative proteomic analysis identified dozens of BnRALF10-elicited defense (RED) proteins, which respond to BnRALF10 in protein abundance and play a role in defense. Our results revealed that BnRALF10 modulated the abundance of RED proteins to fine tune plant immunity. Collectively, our results provided some insights into the functions of oilseed rape RALFs and the signaling underlying BnRALF-triggered immunity.

Apple receptor-like kinase FERONIA regulates salt tolerance and ABA sensitivity in Malus domestica

FERONIA (FER) is a membrane-localized receptor-like kinase that plays pivotal roles in male and female gametophyte recognition, hormone signaling crosstalk, and biotic and abiotic responses. Most reports focus on the functions of FER in model plant Arabidopsis thaliana. However, the functions of FER homologs have not been deeply investigated in apple (Malus domestica), an important economic fruit crop distributed worldwide, especially in China. In this study, we identified an apple homolog of Arabidopsis FER, named MdFER (MDP0000390677). The two proteins encoded by AtFER and MdFER share similar domains: an extracellular malectin-like domain, a transmembrane domain, and an intracellular kinase domain. MdFER was further proven to localize to the plasma membrane in the epidermal cells of Nicotiana benthamiana. MdFER was widely expressed in different apple tissues, but the highest expression was found in roots. In addition, expression of MdFER was significantly induced by treatment with abscisic acid (ABA) and salt (NaCl). Overexpressing MdFER dramatically improved the resistance to salt stress and reduced the sensitivity to ABA in apple callus, while suppressing MdFER expression showed contrary effects. Furthermore, ectopic expression of MdFER in Arabidopsis significantly increased the salt tolerance and reduced the sensitivity to ABA. In addition, under salt stress and ABA treatment, Arabidopsis with highly expressed MdFER accumulated less reactive oxygen species (ROS), and the enzymatic activity of two ROS scavengers, superoxide dismutase and catalase, was higher compared with that of wild type (WT). Our work proves that MdFER positively regulates salt tolerance and negatively regulates ABA sensitivity in apple, which enriched the functions of FER in different plant species.

Small signaling peptides mediate plant adaptions to abiotic environmental stress

Peptide-receptor complexes activate distinct downstream regulatory networks to mediate plant adaptions to abiotic environmental stress. Plants are constantly exposed to various adverse environmental factors; thus they must adjust their growth accordingly. Plants recruit small secretory peptides to adapt to these detrimental environments. These small peptides, which are perceived by their corresponding receptors and/or co-receptors, act as local- or long-distance mobile signaling molecules to establish cell-to-cell regulatory networks, resulting in optimal cellular and physiological outputs. In this review, we highlight recent advances on the regulatory role of small peptides in plant abiotic responses and nutrients signaling.

Proton and calcium pumping P-type ATPases and their regulation of plant responses to the environment

Plant plasma membrane H+-ATPases and Ca2+-ATPases maintain low cytoplasmic concentrations of H+ and Ca2+, respectively, and are essential for plant growth and development. These low concentrations allow plasma membrane H+-ATPases to function as electrogenic voltage stats, and Ca2+-ATPases as "off" mechanisms in Ca2+-based signal transduction. Although these pumps are autoregulated by cytoplasmic concentrations of H+ and Ca2+, respectively, they are also subject to exquisite regulation in response to biotic and abiotic events in the environment. A common paradigm for both types of pumps is the presence of terminal regulatory (R) domains that function as autoinhibitors that can be neutralized by multiple means, including phosphorylation. A picture is emerging in which some of the phosphosites in these R domains appear to be highly, nearly constantly phosphorylated, whereas others seem to be subject to dynamic phosphorylation. Thus, some sites might function as major switches, whereas others might simply reduce activity. Here, we provide an overview of the relevant transport systems and discuss recent advances that address their relation to external stimuli and physiological adaptations.

Knockout of FER decreases cadmium concentration in roots of Arabidopsis thaliana by inhibiting the pathway related to iron uptake

Identifying the genes that affect cadmium (Cd) accumulation in plants is a prerequisite for minimizing dietary Cd uptake from contaminated edible parts of plants by genetic engineering. This study showed that Cd stress inhibited the expression of FERONIA (FER) gene in the roots of wild-type Arabidopsis. Knockout of FER in fer-4 mutants downregulated the Cd-induced expression of several genes related to iron (Fe) uptake, including IRT1, bHLH38, NRAMP1, NRAMP3, FRO2 andFIT. In addition, the Cd concentration in fer-4 mutant roots reduced to approximately half of that in the wild-type seedlings. As a result, the Cd tolerance of fer-4 was higher. Furthermore, increased Fe supplementation had little effect on the Cd tolerance of fer-4 mutants, but clearly improved the Cd tolerance of wild-type seedlings, showing that the alleviation of Cd toxicity by Fe depends on the action of FER. Taken together, the findings demonstrate that the knockout of FER might provide a strategy to reduce Cd contamination and improve the Cd tolerance in plants by regulating the pathways related to Fe uptake.

New paradigms in cell adaptation: decades of discoveries on the CrRLK1L receptor kinase signalling network

Receptor-like kinases (RLKs), which constitute the largest receptor family in plants, are essential for perceiving and relaying information about various environmental stimuli. Tremendous progress has been made in the past few decades towards elucidating the mechanisms of action of several RLKs, with emerging paradigms pointing to their roles in cell adaptations. Among these paradigms, Catharanthus roseus receptor-like kinase 1-like (CrRLK1L) proteins and their rapid alkalinization factor (RALF) peptide ligands have attracted much interest. In particular, FERONIA (FER) is a CrRLK1L protein that participates in a wide array of physiological processes associated with RALF signalling, including cell growth and monitoring cell wall integrity, RNA and energy metabolism, and phytohormone and stress responses. Here, we analyse FER in the context of CrRLK1L members and their ligands in multiple species. The FER working model raises many questions about the role of CrRLK1L signalling networks during cell adaptation. For example, how do CrRLK1Ls recognize various RALF peptides from different organisms to initiate specific phosphorylation signal cascades? How do RALF-FER complexes achieve their specific, sometimes opposite, functions in different cell types? Here, we summarize recent major findings and highlight future perspectives in the field of CrRLK1L signalling networks.

pH biosensing in the plant apoplast-a focus on root cell elongation

The pH parameter of soil plays a key role for plant nutrition as it is affecting the availability of minerals and consequently determines plant growth. Although the mechanisms by which root perceive the external pH is still unknown, the impact of external pH on tissue growth has been widely studied especially in hypocotyl and root. Thanks to technological development of cell imaging and fluorescent sensors, we can now monitor pH in real time with at subcellular definition. In this focus, fluorescent dye-based, as well as genetically-encoded pH indicators are discussed especially with respect to their ability to monitor acidic pH in the context of primary root. The notion of apoplastic subdomains is discussed and suggestions are made to develop fluorescent indicators for pH values below 5.0.

Family-wide evaluation of RAPID ALKALINIZATION FACTOR peptides

Plant peptide hormones are important players that control various aspects of the lives of plants. RAPID ALKALINIZATION FACTOR (RALF) peptides have recently emerged as important players in multiple physiological processes. Numerous studies have increased our understanding of the evolutionary processes that shaped the RALF family of peptides. Nevertheless, to date, there is no comprehensive, family-wide functional study on RALF peptides. Here, we analyzed the phylogeny of the proposed multigenic RALF peptide family in the model plant Arabidopsis (Arabidopsis thaliana), ecotype Col-0, and tested a variety of physiological responses triggered by RALFs. Our phylogenetic analysis reveals that two of the previously proposed RALF peptides are not genuine RALF peptides, which leads us to propose a revision to the consensus AtRALF peptide family annotation. We show that the majority of AtRALF peptides, when applied exogenously as synthetic peptides, induce seedling or root growth inhibition and modulate reactive oxygen species (ROS) production in Arabidopsis. Moreover, our findings suggest that alkalinization and growth inhibition are, generally, coupled characteristics of RALF peptides. Additionally, we show that for the majority of the peptides, these responses are genetically dependent on FERONIA, suggesting a pivotal role for this receptor kinase in the perception of multiple RALF peptides.

With an Ear Up against the Wall: An Update on Mechanoperception in Arabidopsis

Cells interpret mechanical signals and adjust their physiology or development appropriately. In plants, the interface with the outside world is the cell wall, a structure that forms a continuum with the plasma membrane and the cytoskeleton. Mechanical stress from cell wall damage or deformation is interpreted to elicit compensatory responses, hormone signalling, or immune responses. Our understanding of how this is achieved is still evolving; however, we can refer to examples from animals and yeast where more of the details have been worked out. Here, we provide an update on this changing story with a focus on candidate mechanosensitive channels and plasma membrane-localized receptors.

Carbon/nitrogen metabolism and stress response networks - calcium-dependent protein kinases as the missing link?

Calcium-dependent protein kinases (CDPKs) play essential roles in plant development and stress responses. CDPKs have a conserved kinase domain, followed by an auto-inhibitory junction connected to the calmodulin-like domain that binds Ca2+. These structural features allow CDPKs to decode the dynamic changes in cytoplasmic Ca2+ concentrations triggered by hormones and by biotic and abiotic stresses. In response to these signals, CDPKs phosphorylate downstream protein targets to regulate growth and stress responses according to the environmental and developmental circumstances. The latest advances in our understanding of the metabolic, transcriptional, and protein-protein interaction networks involving CDPKs suggest that they have a direct influence on plant carbon/nitrogen (C/N) balance. In this review, we discuss how CDPKs could be key signaling nodes connecting stress responses with metabolic homeostasis, and acting together with the sugar and nutrient signaling hubs SnRK1, HXK1, and TOR to improve plant fitness.

The enigma of environmental pH sensing in plants

Environmental pH is a critical parameter for innumerable chemical reactions, myriad biological processes and all forms of life. The mechanisms that underlie the perception of external pH (pH_e) have been elucidated in detail for bacteria, fungi and mammalian cells; however, little information is available on whether and, if so, how pH_e is perceived by plants. This is particularly surprising since hydrogen ion activity of the substrate is of paramount significance for plants, governing the availability of mineral nutrients, the structure of the soil microbiome and the composition of natural plant communities. Rapid changes in soil pH require constant readjustment of nutrient acquisition strategies, which is associated with dynamic alterations in gene expression. Referring to observations made in diverse experimental set-ups that unambiguously show that pH_e per se affects gene expression, we hypothesize that sensing of pH_e in plants is mandatory to prioritize responses to various simultaneously received environmental cues.

Modulation of Ion Transport Across Plant Membranes by Polyamines: Understanding Specific Modes of Action Under Stress

This work critically discusses the direct and indirect effects of natural polyamines and their catabolites such as reactive oxygen species and γ-aminobutyric acid on the activity of key plant ion-transporting proteins such as plasma membrane H+ and Ca2+ ATPases and K+-selective and cation channels in the plasma membrane and tonoplast, in the context of their involvement in stress responses. Docking analysis predicts a distinct binding for putrescine and longer polyamines within the pore of the vacuolar TPC1/SV channel, one of the key determinants of the cell ionic homeostasis and signaling under stress conditions, and an additional site for spermine, which overlaps with the cytosolic regulatory Ca2+-binding site. Several unresolved problems are summarized, including the correct estimates of the subcellular levels of polyamines and their catabolites, their unexplored effects on nucleotide-gated and glutamate receptor channels of cell membranes and Ca2+-permeable and K+-selective channels in the membranes of plant mitochondria and chloroplasts, and pleiotropic mechanisms of polyamines' action on H+ and Ca2+ pumps.

Versatile Roles of the Receptor-Like Kinase Feronia in Plant Growth, Development and Host-Pathogen Interaction

As a member of the Catharanthus roseus receptor-like kinase 1-like (CrRLK1L) protein kinase subfamily, FERONIA (FER) has emerged as a versatile player regulating multifaceted functions in growth and development, as well as responses to environmental factors and pathogens. With the concerted efforts of researchers, the molecular mechanism underlying FER-dependent signaling has been gradually elucidated. A number of cellular processes regulated by FER-ligand interactions have been extensively reported, implying cell type-specific mechanisms for FER. Here, we provide a review on the roles of FER in male-female gametophyte recognition, cell elongation, hormonal signaling, stress responses, responses to fungi and bacteria, and present a brief outlook for future efforts.