A subset of plasma membrane-localized PP2C.D phosphatases negatively regulate SAUR-mediated cell expansion in Arabidopsis

CmCYC2d is a Regulator of Leaf Abaxial Curling in Chrysanthemum morifolium

Leaf morphology is crucial for plant photosynthesis and stress adaptation. While CIN-like TCP transcription factors are well-known for their roles in leaf curling and morphogenesis, the function of CYC-like TCPs in leaf development remains largely unexplored. This study identifies CmCYC2d as a key regulator of abaxial leaf curling in Chrysanthemum morifolium. Phenotypic analysis revealed that the downward curling observed in OX-CmCYC2d transgenic lines was primarily due to the enlargement of adaxial epidermal cells. Furthermore, a reduction in epidermal cell number was identified as a significant contributor to the smaller leaf area in these plants. Transcriptome and WGCNA analyses highlighted CmSAUR55 as a potential downstream target of CmCYC2d. ChIP-qPCR, EMSA, and LUC assays confirmed that CmCYC2d directly bound to the CmSAUR55 promoter. Additionally, transcriptome data revealed that the reduced cell number in OX-CmCYC2d transgenic lines may be mediated by auxin-related pathways and key genes such as CNR7. The CmCYC2d-CmSAUR55 module was also closely linked to the development of enlarged adaxial epidermal cells in the leaf sinus, emphasising its role in this developmental process. This study highlights the regulatory role of CmCYC2d in leaf development and sheds light on the molecular mechanisms underlying leaf curling in chrysanthemum.

Mechanisms of auxin action in plant growth and development

The phytohormone auxin is a major signal coordinating growth and development in plants. The variety of its effects arises from its ability to form local auxin maxima and gradients within tissues, generated through directional cell-to-cell transport and elaborate metabolic control. These auxin distribution patterns instruct cells in a context-dependent manner to undergo predefined developmental transitions. In this Review, we discuss advances in auxin action at the level of homeostasis and signalling. We highlight key insights into the structural basis of PIN-mediated intercellular auxin transport and explore two novel non-transcriptional auxin signalling mechanisms: one involving intracellular Ca2+ transients and another involving cell-surface auxin perception that mediates global, ultrafast phosphorylation. Furthermore, we examine emerging evidence indicating the involvement of cyclic adenosine monophosphate as a second messenger in the transcriptional auxin response. Together, these recent developments in auxin research have profoundly deepened our understanding of the complex and diverse activities of auxin in plant growth and development.

How it all begins: molecular players of the early graviresponse in the non-elongating part of flax stem

Plants have developed two major strategies to adjust their position in response to gravity: differential cell growth on opposing sides of elongating regions and complex processes in non-elongating stem parts, such as the development of reaction wood. Gravistimulation of flax plants induces gravitropic curvature in non-elongating stem parts, largely associated with modifications in phloem and xylem fibers. To gain insight into the key "triggers" and "forward players" that induce negative gravitropic reactions, transcriptome profiling of phloem fibers and xylem tissues from the pulling and opposite stem sides was conducted 1 and 8 h after gravistimulation. The first observed reaction was the activation of processes associated with RNA synthesis and protein folding in both tissues and stem sides, followed by the activation of kinases and transferases. Transcriptomic data revealed rapid and substantial shifts in chloroplast metabolism across all analyzed tissues, including the temporal activation of the branched-chain amino acid pathway, adjustments to light-harvesting complexes, and jasmonic acid biosynthesis. Notably, auxin transporter genes were activated only in the xylem, while other auxin-related genes showed minimal upregulation 1 h after stem inclination in any analyzed sample. Asymmetric changes between stem sides included the sharp activation of ethylene-related genes in the phloem fibers of the opposite stem side, as well as tertiary cell wall deposition in both the phloem and xylem fibers of the pulling stem side during the later stages of the graviresponse. These results provide valuable insights into the mechanisms underlying plant response to gravity.

The apoplastic pH is a key determinant in the hypocotyl growth response to auxin dosage and light

Auxin is a core phytohormone regulating plant elongation growth. While auxin typically promotes hypocotyl elongation, excessive amounts of auxin inhibit elongation. Moreover, auxin usually promotes light-grown, but inhibits dark-grown hypocotyl elongation. How dosage and light condition change the plant's response to auxin, also known as auxin's biphasic effect or dual effect, has long been mysterious. Auxin induces cell expansion primarily through apoplastic acidification and the subsequent 'acid growth' mechanism. Here we show that this pathway operates for both stimulatory and inhibitory auxin doses and under both dark and light conditions. Regardless of the dosage, more auxin induces more transcripts of SAURs (Small Auxin-Up RNAs), leading to a stronger activation of plasma membrane H+-ATPases (AHAs) and progressive acidification of the apoplast in hypocotyl epidermis. Apoplastic acidification promotes growth but only above a certain pH threshold, below which excessive acidification inhibits elongation. Auxin overdosage-triggered hypocotyl inhibition can be alleviated by suppressing the AHA activity or raising the apoplastic pH. Light-grown hypocotyls exhibit a higher apoplastic pH, which impedes cell elongation and counteracts auxin-induced over-acidification. Auxin and light antagonistically regulate the SAUR-PP2C.D-AHA pathway in the hypocotyl and influence plant elongation growth. Our findings suggest that the biphasic effect of auxin results from the biphasic response of hypocotyl cells to decreasing apoplastic pH.

Transcription factor SlSTOP1 regulates Small Auxin-Up RNA Genes for tomato root elongation under aluminum stress

Aluminum (Al) stress, a prevalent constraint in acidic soils, inhibits plant growth by inhibiting root elongation through restricted cell expansion. The molecular mechanisms of Al-induced root inhibition, however, are not fully understood. This study aimed to elucidate the role of Small Auxin-up RNAs (SlSAURs), which function downstream of the key Al stress-responsive transcription factor SENSITIVE TO PROTON RHIZOTOXICITY 1 (SlSTOP1) and its enhancer STOP1-INTERACTING ZINC-FINGER PROTEIN 1 (SlSZP1), in modulating root elongation under Al stress in tomato (Solanum lycopersicum). Our findings demonstrated that tomato lines with knocked-out SlSAURs exhibited shorter root lengths when subjected to Al stress. Further investigation into the underlying mechanisms revealed that SlSAURs interact with Type 2C Protein Phosphatases (SlPP2Cs), specifically D-clade Type 2C Protein Phosphatases (SlPP2C.Ds). This interaction was pivotal as it suppresses the phosphatase activity, leading to the degradation of SlPP2C.D's inhibitory effect on plasma membrane H+-ATPase. Consequently, this promoted cell expansion and root elongation under Al stress. These findings increase our understanding of the molecular mechanisms by which Al ions modulate root elongation. The discovery of the SlSAUR-SlPP2C.D interaction and its impact on H+-ATPase activity also provides a perspective on the adaptive strategies employed by plants to cope with Al toxicity, which may lead to the development of tomato cultivars with enhanced Al stress tolerance, thereby improving crop productivity in acidic soils.

Acidic Stress Induces Cytosolic Free Calcium Oscillation, and an Appropriate Low pH Helps Maintain the Circadian Clock in Arabidopsis

Acidic stress is a formidable environmental factor that exerts adverse effects on plant growth and development, ultimately leading to a potential reduction in agricultural productivity. A low pH triggers Ca2+ influx across the plasma membrane (PM), eliciting distinct responses under various acidic pH levels. However, the underlying mechanisms by which Arabidopsis plant cells generate stimulus-specific Ca2+ signals in response to acidic stress remain largely unexplored. The experimentally induced stimulus may elicit spikes in cytosolic free Ca2+ concentration ([Ca2+]i) spikes or complex [Ca2+]i oscillations that persist for 20 min over a long-term of 24 h or even several days within the plant cytosol and chloroplast. This study investigated the increase in [Ca2+]i under a gradient of low pH stress ranging from pH 3.0 to 6.0. Notably, the peak of [Ca2+]i elevation was lower at pH 4.0 than at pH 3.0 during the initial 8 h, while other pH levels did not significantly increase [Ca2+]i compared to low acidic stress conditions. Lanthanum chloride (LaCl3) can effectively suppress the influx of [Ca2+]i from the apoplastic to the cytoplasm in plants under acid stress, with no discernible difference in intracellular calcium levels observed in Arabidopsis. Following 8 h of acid treatment in the darkness, the intracellular baseline Ca2+ levels in Arabidopsis were significantly elevated when exposed to low pH stress. A moderately low pH, specifically 4.0, may function as a spatial-temporal input into the circadian clock system. These findings suggest that acid stimulation can exert a continuous influence on intracellular calcium levels, as well as plant growth and development.

Systematic Investigation of Aluminum Stress-Related Genes and Their Critical Roles in Plants

Aluminum (Al) stress is a dominant obstacle for plant growth in acidic soil, which accounts for approximately 40-50% of the world's potential arable land. The identification and characterization of Al stress response (Al-SR) genes in Arabidopsis, rice, and other plants have deepened our understanding of Al's molecular mechanisms. However, as a crop sensitive to acidic soil, only eight Al-SR genes have been identified and functionally characterized in maize. In this review, we summarize the Al-SR genes in plants, including their classifications, subcellular localizations, expression organs, functions, and primarily molecular regulatory networks. Moreover, we predict 166 putative Al-SR genes in maize based on orthologue analyses, facilitating a comprehensive understanding of the impact of Al stress on maize growth and development. Finally, we highlight the potential applications of alleviating Al toxicity in crop production. This review deepens our understanding of the Al response in plants and provides a blueprint for alleviating Al toxicity in crop production.

PpSAUR5 promotes plant growth by regulating lignin and hormone pathways

Introduction

Peach (Prunus persica) has a high nutritional and economic value. However, its overgrowth can lead to yield loss. Regulating the growth of peach trees is challenging. The small auxin-up RNA (SAUR) gene family is the largest family of auxin-responsive genes, which play important roles in plant growth and development. However, members of this gene family are rarely reported in peach.

Methods

In this study, we measured leaf area, chlorophyll and lignin content to detect the role of PpSAUR5 on growth through transgenic Arabidopsis.

Results

PpSAUR5 responds to auxin and gibberellin, promoting and inhibiting the synthesis of gibberellin and auxin, respectively. The heterologous transformation of PpSAUR5 in Arabidopsis led to enhanced growth of leaves and siliques, lightening of leaf color, decrease in chlorophyll content, increase in lignin content, abnormalities in the floral organs, and distortion of the inflorescence axis. Transcriptome data analysis of PpSAUR5 overexpression and wild-type lines revealed 854 differentially expressed genes (DEGs). GO and KEGG analyses showed that the DEGs were primarily involved in biological processes, such as cellular processes, metabolic processes, response to stimuli, and catalytic activity. These genes were mainly enriched in pathways, such as phenylalanine biosynthesis, phytohormone signaling, and MAPK signaling.

Discussion

In summary, these results suggested that PpSAUR5 might regulate tree vigor by modulating the synthesis of auxin and gibberellin. Future studies can use PpSAUR5 as a candidate gene to elucidate the potential regulatory mechanisms underlying peach tree vigor.

Physiological and transcriptome analysis of changes in endogenous hormone and sugar content during the formation of tender asparagus stems

Asparagus is a nutritionally dense stem vegetable whose growth and development are correlated with its quality and yield. To investigate the dynamic changes and underlying mechanisms during the elongation and growth process of asparagus stems, we documented the growth pattern of asparagus and selected stem segments from four consecutive elongation stages using physiological and transcriptome analyses. Notably, the growth rate of asparagus accelerated at a length of 25 cm. A significant decrease in the concentration of sucrose, fructose, glucose, and additional sugars was observed in the elongation region of tender stems. Conversely, the levels of auxin and gibberellins(GAs) were elevated along with increased activity of enzymes involved in sucrose degradation. A significant positive correlation existed between auxin, GAs, and enzymes involved in sucrose degradation. The ABA content gradually increased with stem elongation. The tissue section showed that cell elongation is an inherent manifestation of stem elongation. The differential genes screened by transcriptome analysis were enriched in pathways such as starch and sucrose metabolism, phytohormone synthesis metabolism, and signal transduction. The expression levels of genes such as ARF, GA20ox, NCED, PIF4, and otherswere upregulated during stem elongation, while DAO, GA2ox, and other genes were downregulated. The gene expression level was consistent with changes in hormone content and influenced the cell length elongation. Additionally, the expression results of RT-qPCR were consistent with RNA-seq. The observed variations in gene expression levels, endogenous hormones and sugar changes during the elongation and growth of asparagus tender stems offer valuable insights for future investigations into the molecular mechanisms of asparagus stem growth and development and provide a theoretical foundation for cultivation and production practices.

Ectopic expression of OsWOX9A alters leaf anatomy and plant architecture in rice

MAIN CONCLUSION

Ectopic expression of OsWOX9A induces narrow adaxially rolled rice leaves with larger bulliform cells and fewer large veins, probably through regulating the expression of auxin-related and expansin genes. The WUSCHEL-related homeobox (WOX) family plays a pivotal role in plant development by regulating genes involved in various aspects of growth and differentiation. OsWOX9A (DWT1) has been linked to tiller growth, uniform plant growth, and flower meristem activity. However, its impact on leaf growth and development in rice has not been studied. In this study, we investigated the biological role of OsWOX9A in rice growth and development using transgenic plants. Overexpression of OsWOX9A conferred narrow adaxially rolled rice leaves and altered plant architecture. These plants exhibited larger bulliform cells and fewer larger veins compared to wild-type plants. OsWOX9A overexpression also reduced plant height, tiller number, and seed-setting rate. Comparative transcriptome analysis revealed several differentially expressed auxin-related and expansin genes in OsWOX9A overexpressing plants, consistent with their roles in leaf and plant development. These results indicate that the ectopic expression of OsWOX9A may have multiple effects on the development and growth of rice, providing a more comprehensive picture of how the WOX9 subfamily contributes to leaf development and plant architecture.

An auxin research odyssey: 1989-2023

The phytohormone auxin is at times called the master regulator of plant processes and has been shown to be a central player in embryo development, the establishment of the polar axis, early aspects of seedling growth, as well as growth and organ formation during later stages of plant development. The Plant Cell has been key, since the inception of the journal, to developing an understanding of auxin biology. Auxin-regulated plant growth control is accomplished by both changes in the levels of active hormones and the sensitivity of plant tissues to these concentration changes. In this historical review, we chart auxin research as it has progressed in key areas and highlight the role The Plant Cell played in these scientific developments. We focus on understanding auxin-responsive genes, transcription factors, reporter constructs, perception, and signal transduction processes. Auxin metabolism is discussed from the development of tryptophan auxotrophic mutants, the molecular biology of conjugate formation and hydrolysis, indole-3-butyric acid metabolism and transport, and key steps in indole-3-acetic acid biosynthesis, catabolism, and transport. This progress leads to an expectation of a more comprehensive understanding of the systems biology of auxin and the spatial and temporal regulation of cellular growth and development.

WAV E3 ubiquitin ligases mediate degradation of IAA32/34 in the TMK1-mediated auxin signaling pathway during apical hook development

Auxin regulates plant growth and development through downstream signaling pathways, including the best-known SCFTIR1/AFB-Aux/IAA-ARF pathway and several other less characterized "noncanonical" pathways. Recently, one SCFTIR1/AFB-independent noncanonical pathway, mediated by Transmembrane Kinase 1 (TMK1), was discovered through the analyses of its functions in Arabidopsis apical hook development. Asymmetric accumulation of auxin on the concave side of the apical hook triggers DAR1-catalyzed release of the C-terminal of TMK1, which migrates into the nucleus, where it phosphorylates and stabilizes IAA32/34 to inhibit cell elongation, which is essential for full apical hook formation. However, the molecular factors mediating IAA32/34 degradation have not been identified. Here, we show that proteins in the CYTOKININ INDUCED ROOT WAVING 1 (CKRW1)/WAVY GROWTH 3 (WAV3) subfamily act as E3 ubiquitin ligases to target IAA32/34 for ubiquitination and degradation, which is inhibited by TMK1c-mediated phosphorylation. This antagonistic interaction between TMK1c and CKRW1/WAV3 subfamily E3 ubiquitin ligases regulates IAA32/34 levels to control differential cell elongation along opposite sides of the apical hook.

RAF-like protein kinases mediate a deeply conserved, rapid auxin response

The plant-signaling molecule auxin triggers fast and slow cellular responses across land plants and algae. The nuclear auxin pathway mediates gene expression and controls growth and development in land plants, but this pathway is absent from algal sister groups. Several components of rapid responses have been identified in Arabidopsis, but it is unknown if these are part of a conserved mechanism. We recently identified a fast, proteome-wide phosphorylation response to auxin. Here, we show that this response occurs across 5 land plant and algal species and converges on a core group of shared targets. We found conserved rapid physiological responses to auxin in the same species and identified rapidly accelerated fibrosarcoma (RAF)-like protein kinases as central mediators of auxin-triggered phosphorylation across species. Genetic analysis connects this kinase to both auxin-triggered protein phosphorylation and rapid cellular response, thus identifying an ancient mechanism for fast auxin responses in the green lineage.

STOP1-regulated SMALL AUXIN UP RNA55 (SAUR55) is involved in proton/malate co-secretion for Al tolerance in Arabidopsis

Proton (H+) release is linked to aluminum (Al)-enhanced organic acids (OAs) excretion from the roots under Al rhizotoxicity in plants. It is well-reported that the Al-enhanced organic acid excretion mechanism is regulated by SENSITIVE TO PROTON RHIZOTOXICITY1 (STOP1), a zinc-finger TF that regulates major Al tolerance genes. However, the mechanism of H+ release linked to OAs excretion under Al stress has not been fully elucidated. Recent physiological and molecular-genetic studies have implicated the involvement of SMALL AUXIN UP RNAs (SAURs) in the activation of plasma membrane H+-ATPases for stress responses in plants. We hypothesized that STOP1 is involved in the regulation of Al-responsive SAURs, which may contribute to the co-secretion of protons and malate under Al stress conditions. In our transcriptome analysis of the roots of the stop1 (sensitive to proton rhizotoxicity1) mutant, we found that STOP1 regulates the transcription of one of the SAURs, namely SAUR55. Furthermore, we observed that the expression of SAUR55 was induced by Al and repressed in the STOP1 T-DNA insertion knockout (KO) mutant (STOP1-KO). Through in silico analysis, we identified a functional STOP1-binding site in the promoter of SAUR55. Subsequent in vitro and in vivo studies confirmed that STOP1 directly binds to the promoter of SAUR55. This suggests that STOP1 directly regulates the expression of SAUR55 under Al stress. We next examined proton release in the rhizosphere and malate excretion in the T-DNA insertion KO mutant of SAUR55 (saur55), in conjunction with STOP1-KO. Both saur55 and STOP1-KO suppressed rhizosphere acidification and malate release under Al stress. Additionally, the root growth of saur55 was sensitive to Al-containing media. In contrast, the overexpressed line of SAUR55 enhanced rhizosphere acidification and malate release, leading to increased Al tolerance. These associations with Al tolerance were also observed in natural variations of Arabidopsis. These findings demonstrate that transcriptional regulation of SAUR55 by STOP1 positively regulates H+ excretion via PM H+-ATPase 2 which enhances Al tolerance by malate secretion from the roots of Arabidopsis. The activation of PM H+-ATPase 2 by SAUR55 was suggested to be due to PP2C.D2/D5 inhibition by interaction on the plasma membrane with its phosphatase. Furthermore, RNAi-suppression of NtSTOP1 in tobacco shows suppression of rhizosphere acidification under Al stress, which was associated with the suppression of SAUR55 orthologs, which are inducible by Al in tobacco. It suggests that transcriptional regulation of Al-inducible SAURs by STOP1 plays a critical role in OAs excretion in several plant species as an Al tolerance mechanism.

Temperature regulation of auxin-related gene expression and its implications for plant growth

Twenty-five years ago, a seminal paper demonstrated that warm temperatures increase auxin levels to promote hypocotyl growth in Arabidopsis thaliana. Here we highlight recent advances in auxin-mediated thermomorphogenesis and identify unanswered questions. In the warmth, PHYTOCHROME INTERACTING FACTOR 4 (PIF4) and PIF7 bind the YUCCA8 gene promoter and, in concert with histone modifications, enhance its expression to increase auxin synthesis in the cotyledons. Once transported to the hypocotyl, auxin promotes cell elongation. The meta-analysis of expression of auxin-related genes in seedlings exposed to temperatures ranging from cold to hot shows complex patterns of response. Changes in auxin only partially account for these responses. The expression of many SMALL AUXIN UP RNA (SAUR) genes reaches a maximum in the warmth, decreasing towards both temperature extremes in correlation with the rate of hypocotyl growth. Warm temperatures enhance primary root growth, the response requires auxin, and the hormone levels increase in the root tip but the impacts on cell division and cell expansion are not clear. A deeper understanding of auxin-mediated temperature control of plant architecture is necessary to face the challenge of global warming.

Integrated Transcriptomic and Metabolomic Analysis Reveals the Mechanism of Gibberellic acid Regulates the Growth and Flavonoid Synthesis in Phellodendron chinense Schneid Seedlings

The phytohormone gibberellic acids (GAs) play a crucial role in the processes of growth, organ development, and secondary metabolism. However, the mechanism of exogenous GA3 regulating the growth and flavonoid synthesis in Phellodendron chinense Schneid (P. chinense Schneid) seedlings remains unclear. In this study, the physicochemical properties, gene expression level, and secondary metabolite of P. chinense Schneid seedlings under GA3 treatment were investigated. The results showed that GA3 significantly improved the plant height, ground diameter, fresh weight, chlorophyll content, soluble substance content, superoxide dismutase, and peroxidase activities. This was accompanied by elevated relative expression levels of Pc(S)-GA2ox, Pc(S)-DELLA, Pc(S)-SAUR50, Pc(S)-PsaD, Pc(S)-Psb 27, Pc(S)-PGK, Pc(S)-CER3, and Pc(S)-FBA unigenes. Conversely, a notable reduction was observed in the carotenoid content, catalase activity and the relative expression abundances of Pc(S)-KAO, Pc(S)-GID1/2, and Pc(S)-GH 3.6 unigenes in leaves of P. chinense Schneid seedlings (p < 0.05). Furthermore, GA3 evidently decreased the contents of pinocembrin, pinobanksin, isosakuranetin, naringin, naringenin, (-)-epicatechin, tricetin, luteolin, and vitexin belonged to flavonoid in stem bark of P. chinense Schneid seedlings (p < 0.05). These results indicated that exogenous GA3 promoted growth through improving chlorophyll content and gene expression in photosynthesis and phytohormone signal pathway and inhibited flavonoid synthesis in P. chinense Schneid seedlings.

Deciphering transcriptional mechanisms of maize internodal elongation by regulatory network analysis

The lengths of the basal internodes is an important factor for lodging resistance of maize (Zea mays). In this study, foliar application of coronatine (COR) to 10 cultivars at the V8 growth stage had different suppression effects on the length of the eighth internode, with three being categorized as strong-inhibition cultivars (SC), five as moderate (MC), and two as weak (WC). RNA-sequencing of the eighth internode of the cultivars revealed a total of 7895 internode elongation-regulating genes, including 777 transcription factors (TFs). Genes related to the hormones cytokinin, gibberellin, auxin, and ethylene in the SC group were significantly down-regulated compared to WC, and more cell-cycle regulatory factors and cell wall-related genes showed significant changes, which severely inhibited internode elongation. In addition, we used EMSAs to explore the direct regulatory relationship between two important TFs, ZmABI7 and ZmMYB117, which regulate the cell cycle and cell wall modification by directly binding to the promoters of their target genes ZmCYC1, ZmCYC3, ZmCYC7, and ZmCPP1. The transcriptome reported in this study will provide a useful resource for studying maize internode development, with potential use for targeted genetic control of internode length to improve the lodging resistance of maize.

The AUX1-AFB1-CNGC14 module establishes a longitudinal root surface pH profile

Plant roots navigate in the soil environment following the gravity vector. Cell divisions in the meristem and rapid cell growth in the elongation zone propel the root tips through the soil. Actively elongating cells acidify their apoplast to enable cell wall extension by the activity of plasma membrane AHA H+-ATPases. The phytohormone auxin, central regulator of gravitropic response and root development, inhibits root cell growth, likely by rising the pH of the apoplast. However, the role of auxin in the regulation of the apoplastic pH gradient along the root tip is unclear. Here, we show, by using an improved method for visualization and quantification of root surface pH, that the Arabidopsis thaliana root surface pH shows distinct acidic and alkaline zones, which are not primarily determined by the activity of AHA H+-ATPases. Instead, the distinct domain of alkaline pH in the root transition zone is controlled by a rapid auxin response module, consisting of the AUX1 auxin influx carrier, the AFB1 auxin co-receptor, and the CNCG14 calcium channel. We demonstrate that the rapid auxin response pathway is required for an efficient navigation of the root tip.

PP2C.D phosphatase SAL1 positively regulates aluminum resistance via restriction of aluminum uptake in rice

Aluminum (Al) toxicity represents a primary constraint for crop production in acidic soils. Rice (Oryza sativa) is a highly Al-resistant species; however, the molecular mechanisms underlying its high Al resistance are still not fully understood. Here, we identified SAL1 (SENSITIVE TO ALUMINUM 1), which encodes a plasma membrane (PM)-localized PP2C.D phosphatase, as a crucial regulator of Al resistance using a forward genetic screen. SAL1 was found to interact with and inhibit the activity of PM H+-ATPases, and mutation of SAL1 increased PM H+-ATPase activity and Al uptake, causing hypersensitivity to internal Al toxicity. Furthermore, knockout of NRAT1 (NRAMP ALUMINUM TRANSPORTER 1) encoding an Al uptake transporter in a sal1 background rescued the Al-sensitive phenotype of sal1, revealing that coordination of Al accumulation in the cell, wall and symplasm is critical for Al resistance in rice. By contrast, we found that mutations of PP2C.D phosphatase-encoding genes in Arabidopsis (Arabidopsis thaliana) enhanced Al resistance, which was attributed to increased malate secretion. Our results reveal the importance of PP2C.D phosphatases in Al resistance and the different strategies used by rice and Arabidopsis to defend against Al toxicity.

Save your TIRs - more to auxin than meets the eye

Auxin has long been known as an important regulator of plant growth and development. Classical studies in auxin biology have uncovered a 'canonical' transcriptional auxin-signalling pathway involving the TRANSPORT INHIBITOR RESPONSE1/AUXIN SIGNALING F-BOX (TIR1/AFB) receptors. TIR1/AFB perception of auxin triggers the degradation of repressors and the derepression of auxin-responsive genes. Nevertheless, the canonical pathway cannot account for all aspects of auxin biology, such as physiological responses that are too rapid for transcriptional regulation. This Tansley insight will explore several 'non-canonical' pathways that have been described in recent years mediating fast auxin responses. We focus on the interplay between a nontranscriptional branch of TIR1/AFB signalling and a TRANSMEMBRANE KINASE1 (TMK1)-mediated pathway in root acid growth. Other developmental aspects involving the TMKs and their association with the controversial AUXIN-BINDING PROTEIN 1 (ABP1) will be discussed. Finally, we provide an updated overview of the ETTIN (ETT)-mediated pathway in contexts outside of gynoecium development.

Photosynthetic-Product-Dependent Activation of Plasma Membrane H+-ATPase and Nitrate Uptake in Arabidopsis Leaves

Plasma membrane (PM) proton-translocating adenosine triphosphatase (H+-ATPase) is a pivotal enzyme for plant growth and development that acts as a primary transporter and is activated by phosphorylation of the penultimate residue, threonine, at the C-terminus. Small Auxin-Up RNA family proteins maintain the phosphorylation level via inhibiting dephosphorylation of the residue by protein phosphatase 2C-D clade. Photosynthetically active radiation activates PM H+-ATPase via phosphorylation in mesophyll cells of Arabidopsis thaliana, and phosphorylation of PM H+-ATPase depends on photosynthesis and photosynthesis-related sugar supplementation, such as sucrose, fructose and glucose. However, the molecular mechanism and physiological role of photosynthesis-dependent PM H+-ATPase activation are still unknown. Analysis using sugar analogs, such as palatinose, turanose and 2-deoxy glucose, revealed that sucrose metabolites and products of glycolysis such as pyruvate induce phosphorylation of PM H+-ATPase. Transcriptome analysis showed that the novel isoform of the Small Auxin-Up RNA genes, SAUR30, is upregulated in a light- and sucrose-dependent manner. Time-course analyses of sucrose supplementation showed that the phosphorylation level of PM H+-ATPase increased within 10 min, but the expression level of SAUR30 increased later than 10 min. The results suggest that two temporal regulations may participate in the regulation of PM H+-ATPase. Interestingly, a 15NO3- uptake assay in leaves showed that light increases 15NO3- uptake and that increment of 15NO3- uptake depends on PM H+-ATPase activity. The results opened the possibility of the physiological role of photosynthesis-dependent PM H+-ATPase activation in the uptake of NO3-. We speculate that PM H+-ATPase may connect photosynthesis and nitrogen metabolism in leaves.

Auxin regulation on crop: from mechanisms to opportunities in soybean breeding

Breeding crop varieties with high yield and ideal plant architecture is a desirable goal of agricultural science. The success of "Green Revolution" in cereal crops provides opportunities to incorporate phytohormones in crop breeding. Auxin is a critical phytohormone to determine nearly all the aspects of plant development. Despite the current knowledge regarding auxin biosynthesis, auxin transport and auxin signaling have been well characterized in model Arabidopsis (Arabidopsis thaliana) plants, how auxin regulates crop architecture is far from being understood, and the introduction of auxin biology in crop breeding stays in the theoretical stage. Here, we give an overview on molecular mechanisms of auxin biology in Arabidopsis, and mainly summarize auxin contributions for crop plant development. Furthermore, we propose potential opportunities to integrate auxin biology in soybean (Glycine max) breeding.

To curve for survival: Apical hook development

Apical hook is a simple curved structure formed at the upper part of hypocotyls when dicot seeds germinate in darkness. The hook structure is transient but essential for seedlings' survival during soil emergence due to its efficient protection of the delicate shoot apex from mechanical injury. As a superb model system for studying plant differential growth, apical hook has fascinated botanists as early as the Darwin age, and significant advances have been achieved at both the morphological and molecular levels to understand how apical hook development is regulated. Here, we will mainly summarize the research progress at these two levels. We will also briefly compare the growth dynamics between apical hook and hypocotyl gravitropic bending at early seed germination phase, with the aim to deduce a certain consensus on their connections. Finally, we will outline the remaining questions and future research perspectives for apical hook development.

SALT OVERLY SENSITIVE 1 is inhibited by clade D Protein phosphatase 2C D6 and D7 in Arabidopsis thaliana

The salt overly sensitive (SOS) pathway is essential for maintaining sodium ion homeostasis in plants. This conserved pathway is activated by a calcium signaling-dependent phosphorylation cascade. However, the identity of the phosphatases and their regulatory mechanisms that would deactivate the SOS pathway remain unclear. In this study, we demonstrate that PP2C.D6 and PP2C.D7, which belong to clade D of the protein phosphatase 2C (PP2C) subfamily in Arabidopsis thaliana, directly interact with SOS1 and inhibit its Na+/H+ antiporter activity under non-salt-stress conditions. Upon salt stress, SOS3-LIKE CALCIUM-BINDING PROTEIN8 (SCaBP8), a member of the SOS pathway, interacts with the PP2Cs and suppresses their phosphatase activity; simultaneously, SCaBP8 regulates the subcellular localization of PP2C.D6 by releasing it from the plasma membrane. Thus, we identified two negative regulators of the SOS pathway that repress SOS1 activity under nonstress conditions. These processes set the stage for the activation of SOS1 by the kinase SOS2 to achieve plant salt tolerance. Our results suggest that reversible phosphorylation/dephosphorylation is crucial for the regulation of the SOS pathway, and that calcium sensors play dual roles in activating/deactivating SOS2 and PP2C phosphatases under salt stress.

Brassinosteroids promote etiolated apical structures in darkness by amplifying the ethylene response via the EBF-EIN3/PIF3 circuit

Germinated plants grow in darkness until they emerge above the soil. To help the seedling penetrate the soil, most dicot seedlings develop an etiolated apical structure consisting of an apical hook and folded, unexpanded cotyledons atop a rapidly elongating hypocotyl. Brassinosteroids (BRs) are necessary for etiolated apical development, but their precise role and mechanisms remain unclear. Arabidopsis thaliana SMALL AUXIN UP RNA17 (SAUR17) is an apical-organ-specific regulator that promotes production of an apical hook and closed cotyledons. In darkness, ethylene and BRs stimulate SAUR17 expression by transcription factor complexes containing PHYTOCHROME-INTERACTING FACTORs (PIFs), ETHYLENE INSENSITIVE 3 (EIN3), and its homolog EIN3-LIKE 1 (EIL1), and BRASSINAZOLE RESISTANT1 (BZR1). BZR1 requires EIN3 and PIFs for enhanced DNA-binding and transcriptional activation of the SAUR17 promoter; while EIN3, PIF3, and PIF4 stability depends on BR signaling. BZR1 transcriptionally downregulates EIN3-BINDING F-BOX 1 and 2 (EBF1 and EBF2), which encode ubiquitin ligases mediating EIN3 and PIF3 protein degradation. By modulating the EBF-EIN3/PIF protein-stability circuit, BRs induce EIN3 and PIF3 accumulation, which underlies BR-responsive expression of SAUR17 and HOOKLESS1 and ultimately apical hook development. We suggest that in the etiolated development of apical structures, BRs primarily modulate plant sensitivity to darkness and ethylene.

Protein phosphatases and their targets: Comprehending the interactions in plant signaling pathways

Protein phosphorylation is a vital reversible post-translational modification. This process is established by two classes of enzymes: protein kinases and protein phosphatases. Protein kinases phosphorylate proteins while protein phosphatases dephosphorylate phosphorylated proteins, thus, functioning as 'critical regulators' in signaling pathways. The eukaryotic protein phosphatases are classified as phosphoprotein phosphatases (PPP), metallo-dependent protein phosphatases (PPM), protein tyrosine (Tyr) phosphatases (PTP), and aspartate (Asp)-dependent phosphatases. The PPP and PPM families are serine (Ser)/threonine (Thr) specific phosphatases (STPs) that dephosphorylate Ser and Thr residues. The PTP family dephosphorylates Tyr residues while dual-specificity phosphatases (DsPTPs/DSPs) dephosphorylate Ser, Thr, and Tyr residues. The composition of these enzymes as well as their substrate specificity are important determinants of their functional significance in a number of cellular processes and stress responses. Their role in animal systems is well-understood and characterized. The functional characterization of protein phosphatases has been extensively covered in plants, although the comprehension of their mechanistic basis is an ongoing pursuit. The nature of their interactions with other key players in the signaling process is vital to our understanding. The substrates or targets determine their potential as well as magnitude of the impact they have on signaling pathways. In this article, we exclusively overview the various substrates of protein phosphatases in plant signaling pathways, which are a critical determinant of the outcome of various developmental and stress stimuli.

The evolution of plant proton pump regulation via the R domain may have facilitated plant terrestrialization

Plasma membrane (PM) H⁺-ATPases are the electrogenic proton pumps that export H⁺ from plant and fungal cells to acidify the surroundings and generate a membrane potential. Plant PM H⁺-ATPases are equipped with a C‑terminal autoinhibitory regulatory (R) domain of about 100 amino acid residues, which could not be identified in the PM H⁺-ATPases of green algae but appeared fully developed in immediate streptophyte algal predecessors of land plants. To explore the physiological significance of this domain, we created in vivo C-terminal truncations of autoinhibited PM H⁺‑ATPase2 (AHA2), one of the two major isoforms in the land plant Arabidopsis thaliana. As more residues were deleted, the mutant plants became progressively more efficient in proton extrusion, concomitant with increased expansion growth and nutrient uptake. However, as the hyperactivated AHA2 also contributed to stomatal pore opening, which provides an exit pathway for water and an entrance pathway for pests, the mutant plants were more susceptible to biotic and abiotic stresses, pathogen invasion and water loss, respectively. Taken together, our results demonstrate that pump regulation through the R domain is crucial for land plant fitness and by controlling growth and nutrient uptake might have been necessary already for the successful water-to-land transition of plants.

MdPP2C24/37, Protein Phosphatase Type 2Cs from Apple, Interact with MdPYL2/12 to Negatively Regulate ABA Signaling in Transgenic Arabidopsis

The phytohormone abscisic acid (ABA) plays an important role in the ability of plants to cope with drought stress. As core members of the ABA signaling pathway, protein phosphatase type 2Cs (PP2Cs) have been reported in many species. However, the functions of MdPP2Cs in apple (Malus domestica) are unclear. In this study, we identified two PP2C-encoding genes, MdPP2C24/37, with conserved PP2C catalytic domains, using sequence alignment. The nucleus-located MdPP2C24/37 genes were induced by ABA or mannitol in apple. Genetic analysis revealed that overexpression of MdPP2C24/37 in Arabidopsis thaliana led to plant insensitivity to ABA or mannitol treatment, in terms of inhibiting seed germination and overall seedling establishment. The expression of stress marker genes was upregulated in MdPP2C24/37 transgenic lines. At the same time, MdPP2C24/37 transgenic lines displayed inhibited ABA-mediated stomatal closure, which led to higher water loss rates. Moreover, when exposed to drought stress, chlorophyll levels decreased and MDA and H₂O₂ levels accumulated in the MdPP2C24/37 transgenic lines. Further, MdPP2C24/37 interacted with MdPYL2/12 in vitro and vivo. The results indicate that MdPP2C24/37 act as negative regulators in response to ABA-mediated drought resistance.

Overexpression of a Plasma Membrane H+-ATPase Gene OSA1 Stimulates the Uptake of Primary Macronutrients in Rice Roots

Plasma membrane (PM) H⁺-ATPase is a master enzyme involved in various plant physiological processes, such as stomatal movements in leaves and nutrient uptake and transport in roots. Overexpression of Oryza sativa PM H⁺-ATPase 1 (OSA1) has been known to increase NH₄⁺ uptake in rice roots. Although electrophysiological and pharmacological experiments have shown that the transport of many substances is dependent on the proton motive force provided by PM H⁺-ATPase, the exact role of PM H⁺-ATPase on the uptake of nutrients in plant roots, especially for the primary macronutrients N, P, and K, is still largely unknown. Here, we used OSA1 overexpression lines (OSA1-oxs) and gene-knockout osa1 mutants to investigate the effect of modulation of PM H⁺-ATPase on the absorption of N, P, and K nutrients through the use of a nutrient-exhaustive method and noninvasive microtest technology (NMT) in rice roots. Our results showed that under different concentrations of P and K, the uptake rates of P and K were enhanced in OSA1-oxs; by contrast, the uptake rates of P and K were significantly reduced in roots of osa1 mutants when compared with wild-type. In addition, the net influx rates of NH₄⁺ and K⁺, as well as the efflux rate of H⁺, were enhanced in OSA1-oxs and suppressed in osa1 mutants under low concentration conditions. In summary, this study indicated that overexpression of OSA1 stimulated the uptake rate of N, P, and K and promoted flux rates of cations (i.e., H⁺, NH₄⁺, and K⁺) in rice roots. These results may provide a novel insight into improving the coordinated utilization of macronutrients in crop plants.

SAUR63 stimulates cell growth at the plasma membrane

In plants, regulated cell expansion determines organ size and shape. Several members of the family of redundantly acting Small Auxin Up RNA (SAUR) proteins can stimulate plasma membrane (PM) H⁺-ATPase proton pumping activity by inhibiting PM-associated PP2C.D phosphatases, thereby increasing the PM electrochemical potential, acidifying the apoplast, and stimulating cell expansion. Similarly, Arabidopsis thaliana SAUR63 was able to increase growth of various organs, antagonize PP2C.D5 phosphatase, and increase H⁺-ATPase activity. Using a gain-of-function approach to bypass genetic redundancy, we dissected structural requirements for SAUR63 growth-promoting activity. The divergent N-terminal domain of SAUR63 has a predicted basic amphipathic α-helix and was able to drive partial PM association. Deletion of the N-terminal domain decreased PM association of a SAUR63 fusion protein, as well as decreasing protein level and eliminating growth-promoting activity. Conversely, forced PM association restored ability to promote H⁺-ATPase activity and cell expansion, indicating that SAUR63 is active when PM-associated. Lipid binding assays and perturbations of PM lipid composition indicate that the N-terminal domain can interact with PM anionic lipids. Mutations in the conserved SAUR domain also reduced PM association in root cells. Thus, both the N-terminal domain and the SAUR domain may cooperatively mediate the SAUR63 PM association required to promote growth.

Plant organs reach their final shape and size after substantial cell expansion. Proton pumps at the plasma membrane promote cell expansion by acidifying the cell wall to loosen it, and by increasing electrochemical potential across the plasma membrane for solute uptake that maintains intracellular turgor. Plasma-membrane-associated proteins tightly regulate proton pump activity, in order for organs to grow to an appropriate extent. We have studied requirements for activity of one such regulatory protein in the model plant Arabidopsis called SAUR63. This protein is made rapidly in response to plant growth hormones, and it increases proton pump activity to promote organ growth. These activities depend on its binding to anionic lipids in the plasma membrane, and forced plasma membrane association of SAUR63 can increase growth. Many proteins in the same family are found within Arabidopsis and in all land plants, and likely differ in their affinity for the plasma membrane or in other properties. Further studies of other family members may show how such proteins regulate growth under diverse physiological contexts.

The Emerging Role of PP2C Phosphatases in Tomato Immunity

The antagonistic effect of plant immunity on growth likely drove evolution of molecular mechanisms that prevent accidental initiation and prolonged activation of plant immune responses. Signaling networks of pattern-triggered and effector-triggered immunity, the two main layers of plant immunity, are tightly regulated by the activity of protein phosphatases that dephosphorylate their protein substrates and reverse the action of protein kinases. Members of the PP2C class of protein phosphatases have emerged as key negative regulators of plant immunity, primarily from research in the model plant Arabidopsis thaliana, revealing the potential to employ PP2C proteins to enhance plant disease resistance. As a first step towards focusing on the PP2C family for both basic and translational research, we analyzed the tomato genome sequence to ascertain the complement of the tomato PP2C family, identify conserved protein domains and signals in PP2C amino acid sequences, and examine domain combinations in individual proteins. We then identified tomato PP2Cs that are candidate regulators of single or multiple layers of the immune signaling network by in-depth analysis of publicly available RNA-seq datasets. These included expression profiles of plants treated with fungal or bacterial pathogen-associated molecular patterns, with pathogenic, nonpathogenic, and disarmed bacteria, as well as pathogenic fungi and oomycetes. Finally, we discuss the possible use of immunity-associated PP2Cs to better understand the signaling networks of plant immunity and to engineer durable and broad disease resistance in crop plants. [Formula: see text] Copyright © 2022 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

PIF7 is a master regulator of thermomorphogenesis in shade

The size of plant organs is highly responsive to environmental conditions. The plant’s embryonic stem, or hypocotyl, displays phenotypic plasticity, in response to light and temperature. The hypocotyl of shade avoiding species elongates to outcompete neighboring plants and secure access to sunlight. Similar elongation occurs in high temperature. However, it is poorly understood how environmental light and temperature cues interact to effect plant growth. We found that shade combined with warm temperature produces a synergistic hypocotyl growth response that dependent on PHYTOCHROME-INTERACTING FACTOR 7 (PIF7) and auxin. This unique but agriculturally relevant scenario was almost totally independent on PIF4 activity. We show that warm temperature is sufficient to promote PIF7 DNA binding but not transcriptional activation and we demonstrate that additional, unknown factor/s must be working downstream of the phyB-PIF-auxin module. Our findings will improve the predictions of how plants will respond to increased ambient temperatures when grown at high density.

Plant hypocotyl elongation response to light and temperature. Here the authors show that shade combined with warm temperature synergistically enhances the hypocotyl growth response via the PIF7 transcription factor, auxin, and as yet unknown factor.

SAUR15 interaction with BRI1 activates plasma membrane H+-ATPase to promote organ development of Arabidopsis

Brassinosteroids (BRs) are an important group of plant steroid hormones that regulate growth and development. Several members of the SMALL AUXIN UP RNA (SAUR) family have roles in BR-regulated hypocotyl elongation and root growth. However, the mechanisms are unclear. Here, we show in Arabidopsis (Arabidopsis thaliana) that SAUR15 interacts with cell surface receptor-like kinase BRASSINOSTEROID-INSENSITIVE 1 (BRI1) in BR-treated plants, resulting in enhanced BRI1 phosphorylation status and recruitment of the co-receptor BRI1-ASSOCIATED RECEPTOR KINASE 1. Genetic and phenotypic assays indicated that the SAUR15 effect on BRI1 can be uncoupled from BRASSINOSTEROID INSENSITIVE 2 activity. Instead, we show that SAUR15 promotes BRI1 direct activation of plasma membrane H+-ATPase (PM H+-ATPase) via phosphorylation. Consequently, SAUR15-BRI1-PM H+-ATPase acts as a direct, PM-based mode of BR signaling that drives cell expansion to promote the growth and development of various organs. These data define an alternate mode of BR signaling in plants.

Plant-specific small peptide AtZSP1 interacts with ROCK1 to regulate organ size in Arabidopsis

Organ size is an important agronomic trait. Small peptides function in various stages of plant growth, but their regulatory mechanisms in organ growth remain poorly understood. Here, we characterize a novel small peptide, AtZSP1, which positively regulates organ size in Arabidopsis. Loss-of-function mutant atzsp1-1 exhibited small organs, whereas AtZSP1 overexpression plants (p35S:AtZSP1#1) produced larger organs. Differentially expressed genes in the shoots of atzsp1-1 and p35S:AtZSP1#1 were enriched in the cytokinin pathway. Further analysis on shoots of atzsp1-1 showed that endogenous cytokinin levels were significantly reduced, consistent with reduced expression of the cytokinin response genes ARR5/6/7 and a decrease in pARR5:GUS activity. By contrast, cytokinin levels were elevated in p35S:AtZSP1#1. These results indicate that AtZSP1 affects shoot size via changes in cytokinin levels. AtZSP1 is ubiquitously expressed and encodes a 57-amino acid endomembrane-associated protein that is highly conserved among plant species. AtZSP1 interacts with ROCK1 at the endomembrane. Genetic analysis confirmed that the small organs and low cytokinin levels in atzsp1-1 shoots are partially suppressed by the rock1-4 mutation, suggesting that AtZSP1 may function in a common pathway with ROCK1 to antagonistically regulate organ growth. Our study identified an unknown small peptide, AtZSP1, and defined its function in regulating organ size in Arabidopsis.

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.

The molecular mechanism of plasma membrane H+-ATPases in plant responses to abiotic stress

Plasma membrane H⁺-ATPases (PM H⁺-ATPases) are critical proton pumps that export protons from the cytoplasm to the apoplast. The resulting proton gradient and difference in electrical potential energize various secondary active transport events. PM H⁺-ATPases play essential roles in plant growth, development, and stress responses. In this review, we focus on recent studies of the mechanism of PM H⁺-ATPases in response to abiotic stresses in plants, such as salt and high pH, temperature, drought, light, macronutrient deficiency, acidic soil and aluminum stress, as well as heavy metal toxicity. Moreover, we discuss remaining outstanding questions about how PM H⁺-ATPases contribute to abiotic stress responses.

Fourteen Stations of Auxin

Auxin has always been at the forefront of research in plant physiology and development. Since the earliest contemplations by Julius von Sachs and Charles Darwin, more than a century-long struggle has been waged to understand its function. This largely reflects the failures, successes, and inevitable progress in the entire field of plant signaling and development. Here I present 14 stations on our long and sometimes mystical journey to understand auxin. These highlights were selected to give a flavor of the field and to show the scope and limits of our current knowledge. A special focus is put on features that make auxin unique among phytohormones, such as its dynamic, directional transport network, which integrates external and internal signals, including self-organizing feedback. Accented are persistent mysteries and controversies. The unexpected discoveries related to rapid auxin responses and growth regulation recently disturbed our contentment regarding understanding of the auxin signaling mechanism. These new revelations, along with advances in technology, usher us into a new, exciting era in auxin research.

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.

Phylogenetic analysis of PP2C proteins and interactive proteins analyze of BjuPP2C52 in Brassica juncea

Brassica juncea var. tumida Tsen et Lee (Tumorous stem mustard) is an unique vegetable in China. Its enlarged tumorous stem was used as main raw material to produce pickle (Zhacai). In practice, early-bolting happens around 15% of planting area all year and inhibits its production. Here, about 209 PP2C proteins were identified through HMMER software and divided into 13 sub-families in B. juncea. BjuPP2C52 belongs to E sub-family, was up-regulated at reproductive growth stages and interacts with BjuFKF1, a key protein in regulating plant photoperiod flowering, in vitro and in vivo. To explore interactive proteins, BjuPP2C52 was used as bait, 12 potential interactive proteins were screened from yeast library, and they are BjuCOL3, BjuCOL5, BjuAP2, BjuAP2-1, BjuSVP-1, BjuFLC-2, BjuSKP1f, BjuA014572, BjuA008686, BjuO002119, BjuB036787 and BjuA019268. Further study verified that 10 out of the 12 screened proteins interacted with BjuPP2C52 in vivo. qRT-PCR was conducted to understand the expression pattern of those 10 interactive proteins in different tissues and development stages in B. juncea. The results showed that BjuCOL3, BjuCOL5, BjuB036787 and BjuA019268 were significantly up-regulated, while BjuA008686 and BjuO002119 were down-regulated in flowers compared with other four tissues. In developmental stages, BjuCOL5, BjuAP2, BjuAP2-1, BjuA014572, BjuB036787 and BjuA019268 were significantly up-regulated, while BjuSVP-1, BjuA008686 and BjuO002119 were down-regulated at reproductive stages. Based on the results, BjuCOL5, BjuAP2, BjuAP2-1, BjuSVP-1, BjuA014572, BjuB036787 and BjuA019268 may function in regulating flowering time in B. juncea.

Genome-wide characterization and expression analysis of SAUR gene family in Melon (Cucumis melo L.)

We identified 66 melon SAUR genes by bioinformatic analyses. CmSAUR19, 38, 58, 62 genes are specifically expressed in different stages of fruit growth, suggesting their participation in regulating fruit development. Auxin plays a crucial role in plant growth by regulating the multiple auxin response genes. However, in melon (Cucumis melo L.), the functions of the auxin early response gene family SAUR (Small auxin up RNA) genes in fruit development are still poorly understood. Through genome-wide characterization of CmSAUR family in melon, we identified a total of 66 CmSAUR genes. The open reading frames of the CmSAUR genes ranged from 234 to 525 bp, containing only one exon and lacking introns. Chromosomal position and phylogenetic tree analyses found that the two gene clusters in the melon chromosome are highly homologous in the Cucurbitaceae plants. Among the four conserved motifs in CmSAUR proteins, motif 1, motif 2, and motif 3 located in most of the family protein sequences, and motif 4 showed a close correlation with the two gene clusters. The CmSAUR28 and CmSAUR58 genes have auxin response elements located in the promoters, suggesting they may be involved in the auxin signaling pathway to regulate fruit development. Through transcriptomic profiling in the four developmental stages of fruit and different lateral organs, we selected 16 differentially-expressed SAUR genes for performing further expression analyses. qRT-PCR results showed that five SAUR genes are specifically expressed in flower organs and ovaries. CmSAUR19 and CmSAUR58 were significantly accumulated in the early developmental stage of the fruit. CmSAUR38 and CmAUR62 showed high expression in the climacteric and post-climacteric stages, suggesting their specific role in controlling fruit ripening. This work provides a foundation for further exploring the function of the SAUR gene in fruit development.

Bending to auxin: fast acid growth for tropisms

The phytohormone auxin is the major growth regulator governing tropic responses including gravitropism. Auxin build-up at the lower side of stimulated shoots promotes cell expansion, whereas in roots it inhibits growth, leading to upward shoot bending and downward root bending, respectively. Yet it remains an enigma how the same signal can trigger such opposite cellular responses. In this review, we discuss several recent unexpected insights into the mechanisms underlying auxin regulation of growth, challenging several existing models. We focus on the divergent mechanisms of apoplastic pH regulation in shoots and roots revisiting the classical Acid Growth Theory and discuss coordinated involvement of multiple auxin signaling pathways. From this emerges a more comprehensive, updated picture how auxin regulates growth.

Type 2C protein phosphatase clade D family members dephosphorylate guard cell plasma membrane H+-ATPase

Plasma membrane (PM) H+-ATPase in guard cells is activated by phosphorylation of the penultimate residue, threonine (Thr), in response to blue and red light, promoting stomatal opening. Previous in vitro biochemical investigation suggested that Mg2+- and Mn2+-dependent membrane-localized type 2C protein phosphatase (PP2C)-like activity mediates the dephosphorylation of PM H+-ATPase in guard cells. PP2C clade D (PP2C.D) was later demonstrated to be involved in PM H+-ATPase dephosphorylation during auxin-induced cell expansion in Arabidopsis (Arabidopsis thaliana). However, it is unclear whether PP2C.D phosphatases are involved in PM H+-ATPase dephosphorylation in guard cells. Transient expression experiments using Arabidopsis mesophyll cell protoplasts revealed that all PP2C.D isoforms dephosphorylate the endogenous PM H+-ATPase. We further analyzed PP2C.D6/8/9, which display higher expression levels than other isoforms in guard cells, observing that pp2c.d6, pp2c.d8, and pp2c.d9 single mutants showed similar light-induced stomatal opening and phosphorylation status of PM H+-ATPase in guard cells as Col-0. In contrast, the pp2c.d6/9 double mutant displayed wider stomatal apertures and greater PM H+-ATPase phosphorylation in response to blue light, but delayed dephosphorylation of PM H+-ATPase in guard cells; the pp2c.d6/8/9 triple mutant showed similar phenotypes to those of the pp2c.d6/9 double mutant. Taken together, these results indicate that PP2C.D6 and PP2C.D9 redundantly mediate PM H+-ATPase dephosphorylation in guard cells. Curiously, unlike auxin-induced cell expansion in seedlings, auxin had no effect on the phosphorylation status of PM H+-ATPase in guard cells.

Mapping of CaPP2C35 involved in the formation of light-green immature pepper (Capsicum annuum L.) fruits via GWAS and BSA

Genome-wide association study, bulked segregant analysis, and genetic analysis delimited the LG locus controlling light-green immature pepper fruits into a 35.07 kbp region on chromosome 10. A strong candidate gene, CaPP2C35, was identified in this region. In pepper (Capsicum annuum L.), the common colors of immature fruits are yellowish white, milky yellow, green, purple, and purplish black. Genes related to dark green, white, and purple immature fruits have been cloned; however, only a few studies have investigated light-green immature fruits. Here, we performed a genetic study using light-green (17C827) and green (17C658) immature fruits. The light-green color of immature fruits was controlled by a single locus-dominant genetic trait compared with the green color of immature fruits. We also performed a genome-wide association study and bulked segregant analysis of immature-fruit color and mapped the LG locus to a 35.07 kbp region on chromosome 10. Only one gene, Capana10g001710, was found in this region. A G-A substitution occurred at the 313th base of the Capana10g001710 coding sequence in 17C827, resulting in the conversion of the α-helix of its encoded PP2C35 protein into a β-fold. The expression of Capana10g001710 (termed CaPP2C35) in 17C827 was significantly higher than in 17C658. Silencing CaPP2C35 in 17C827 resulted in an increase in chlorophyll content in the exocarp and the appearance of green stripes on the surface of the fruit. These results indicate that CaPP2C35 may be involved in the formation of light-green immature fruits by regulating the accumulation of chlorophyll content in the exocarp. Thus, these findings lay the foundation for further studies and genetic improvement of immature-fruit color in pepper.

A plastidial retrograde signal potentiates biosynthesis of systemic stress response activators

Plants employ an array of intricate and hierarchical signaling cascades to perceive and transduce informational cues to synchronize and tailor adaptive responses. Systemic stress response (SSR) is a recognized complex signaling and response network quintessential to plant's local and distal responses to environmental triggers; however, the identity of the initiating signals has remained fragmented. Here, we show that both biotic (aphids and viral pathogens) and abiotic (high light and wounding) stresses induce accumulation of the plastidial-retrograde-signaling metabolite methylerythritol cyclodiphosphate (MEcPP), leading to reduction of the phytohormone auxin and the subsequent decreased expression of the phosphatase PP2C.D1. This enables phosphorylation of mitogen-activated protein kinases 3/6 and the consequential induction of the downstream events ultimately, resulting in biosynthesis of the two SSR priming metabolites pipecolic acid and N-hydroxy-pipecolic acid. This work identifies plastids as a major initiation site, and the plastidial retrograde signal MEcPP as an initiator of a multicomponent signaling cascade potentiating the biosynthesis of SSR activators, in response to biotic and abiotic triggers.

Biphasic control of cell expansion by auxin coordinates etiolated seedling development

Seedling emergence is critical for food security. It requires rapid hypocotyl elongation and apical hook formation, both of which are mediated by regulated cell expansion. How these events are coordinated in etiolated seedlings is unclear. Here, we show that biphasic control of cell expansion by the phytohormone auxin underlies this process. Shortly after germination, high auxin levels restrain elongation. This provides a temporal window for apical hook formation, involving a gravity-induced auxin maximum on the eventual concave side of the hook. This auxin maximum induces PP2C.D1 expression, leading to asymmetrical H⁺-ATPase activity across the hypocotyl that contributes to the differential cell elongation underlying hook development. Subsequently, auxin concentrations decline acropetally and switch from restraining to promoting elongation, thereby driving hypocotyl elongation. Our findings demonstrate how differential auxin concentrations throughout the hypocotyl coordinate etiolated development, leading to successful soil emergence.

Abscisic acid employs NRP-dependent PIN2 vacuolar degradation to suppress auxin-mediated primary root elongation in Arabidopsis

How plants balance growth and stress adaptation is a long-standing topic in plant biology. Abscisic acid (ABA) induces the expression of the stress-responsive Asparagine Rich Protein (NRP), which promotes the vacuolar degradation of PP6 phosphatase FyPP3, releasing ABI5 transcription factor to initiate transcription. Whether NRP is required for growth remains unknown. We generated an nrp1 nrp2 double mutant, which had a dwarf phenotype that can be rescued by inhibiting auxin transport. Insufficient auxin in the transition zone and over-accumulation of auxin at the root tip was responsible for the short elongation zone and short-root phenotype of nrp1 nrp2. The auxin efflux carrier PIN2 over-accumulated in nrp1 nrp2 and became de-polarized at the plasma membrane, leading to slower root basipetal auxin transport. Knock-out of PIN2 suppressed the dwarf phenotype of nrp1 nrp2. Furthermore, ABA can induce NRP-dependent vacuolar degradation of PIN2 to inhibit primary root elongation. FyPP3 also is required for NRP-mediated PIN2 turnover. In summary, in growth condition, NRP promotes PIN2 vacuolar degradation to help maintain PIN2 protein concentration and polarity, facilitating the establishment of the elongation zone and primary root elongation. When stressed, ABA employs this pathway to inhibit root elongation for stress adaptation.

Cell surface and intracellular auxin signalling for H+ fluxes in root growth

Growth regulation tailors development in plants to their environment. A prominent example of this is the response to gravity, in which shoots bend up and roots bend down1. This paradox is based on opposite effects of the phytohormone auxin, which promotes cell expansion in shoots while inhibiting it in roots via a yet unknown cellular mechanism2. Here, by combining microfluidics, live imaging, genetic engineering and phosphoproteomics in Arabidopsis thaliana, we advance understanding of how auxin inhibits root growth. We show that auxin activates two distinct, antagonistically acting signalling pathways that converge on rapid regulation of apoplastic pH, a causative determinant of growth. Cell surface-based TRANSMEMBRANE KINASE1 (TMK1) interacts with and mediates phosphorylation and activation of plasma membrane H+-ATPases for apoplast acidification, while intracellular canonical auxin signalling promotes net cellular H+ influx, causing apoplast alkalinization. Simultaneous activation of these two counteracting mechanisms poises roots for rapid, fine-tuned growth modulation in navigating complex soil environments.

TMK-based cell-surface auxin signalling activates cell-wall acidification

The phytohormone auxin controls many processes in plants, at least in part through its regulation of cell expansion1. The acid growth hypothesis has been proposed to explain auxin-stimulated cell expansion for five decades, but the mechanism that underlies auxin-induced cell-wall acidification is poorly characterized. Auxin induces the phosphorylation and activation of the plasma membrane H+-ATPase that pumps protons into the apoplast2, yet how auxin activates its phosphorylation remains unclear. Here we show that the transmembrane kinase (TMK) auxin-signalling proteins interact with plasma membrane H+-ATPases, inducing their phosphorylation, and thereby promoting cell-wall acidification and hypocotyl cell elongation in Arabidopsis. Auxin induced interactions between TMKs and H+-ATPases in the plasma membrane within seconds, as well as TMK-dependent phosphorylation of the penultimate threonine residue on the H+-ATPases. Our genetic, biochemical and molecular evidence demonstrates that TMKs directly phosphorylate plasma membrane H+-ATPase and are required for auxin-induced H+-ATPase activation, apoplastic acidification and cell expansion. Thus, our findings reveal a crucial connection between auxin and plasma membrane H+-ATPase activation in regulating apoplastic pH changes and cell expansion through TMK-based cell surface auxin signalling.

Casting the Net-Connecting Auxin Signaling to the Plant Genome

Auxin represents one of the most potent and most versatile hormonal signals in the plant kingdom. Built on a simple core of only a few dedicated components, the auxin signaling system plays important roles for diverse aspects of plant development, physiology, and defense. Key to the diversity of context-dependent functional outputs generated by cells in response to this small molecule are gene duplication events and sub-functionalization of signaling components on the one hand, and a deep embedding of the auxin signaling system into complex regulatory networks on the other hand. Together, these evolutionary innovations provide the mechanisms to allow each cell to display a highly specific auxin response that suits its individual requirements. In this review, we discuss the regulatory networks connecting auxin with a large number of diverse pathways at all relevant levels of the signaling system ranging from biosynthesis to transcriptional response.

Molecular basis of plasma membrane H+-ATPase function and potential application in the agricultural production

Increase of crop yield is always the desired goal, manipulation of genes in relation to plant growth is a shortcut to promote crop yield. The plasma membrane (PM) H⁺-ATPase is the plant master enzyme; the energy yielded by ATP hydrolysis pumps H⁺ out of cells, establishes the membrane potential, maintains pH homeostasis and provides the proton-motive force required for transmembrane transport of many materials. PM H⁺-ATPase is involved in root nutrient uptake, epidermal stomatal opening, phloem sucrose loading and unloading, and hypocotyl cell elongation. In this review, we summarize the recent progresses in roles of PM H⁺-ATPase in nutrient uptake and light-induced stomatal opening and discuss the pivotal role of PM H⁺-ATPase in crop yield improvement and its potential application in agricultural production by modulating the expression of PM H⁺-ATPase in crops.