SAUR proteins and PP2C.D phosphatases regulate H+-ATPases and K+ channels to control stomatal movements

Genome-wide identification and expression analysis of SMALL AUXIN UP RNA (SAUR) genes in rice (Oryza sativa)

SMALL AUXIN UP RNAs (SAURs), the largest family of early auxin response genes, plays crucial roles in multiple processes, including cell expansion, leaf growth and senescence, auxin transport, tropic growth and so on. Although the rice SAUR gene family was identified in 2006, it is necessary to identify the rice SAUR gene due to the imperfection of its analysis methods. In this study, a total of 60 OsSAURs (including two pseudogenes) distributed on 10 chromosomes were identified in rice (Oryza sativa). Bioinformatics tools were used to systematically analyze the physicochemical properties, subcellular localization, motif compositions, chromosomal location, gene duplication, evolutionary relationships, auxin-responsive cis-elements of the OsSAURs. In addition, the expression profiles obtained from microarray data analysis showed that OsSAUR genes had different expression patterns in different tissues and responded to auxin treatment, indicating functional differences among members of OsSAUR gene family. In a word, this study provides basic information for SAUR gene family of rice and lays a foundation for further study on the role of SAUR in rice growth and development.

Spermidine augments salt stress resilience in rice roots potentially by enhancing OsbZIP73's RNA binding capacity

BACKGROUND

Rice is a staple crop for over half of the global population, but soil salinization poses a significant threat to its production. As a type of polyamine, spermidine (Spd) has been shown to reduce stress-induced damage in plants, but its specific role and mechanism in protecting rice roots under salt stress require further investigation.

RESULTS

This study suggested spermidine (Spd) mitigates salt stress on rice root growth by enhancing antioxidant enzyme activity and reducing peroxide levels. Transcriptomic analysis showed that salt stress caused 333 genes to be upregulated and 1,765 to be downregulated. However, adding Spd during salt treatment significantly altered this pattern: 2,298 genes were upregulated and 844 were downregulated, which indicated Spd reverses some transcriptional changes caused by salt stress. KEGG pathway analysis suggested that Spd influenced key signaling pathways, including MAPK signaling, plant hormone signal transduction, and phenylalanine metabolism. Additionally, the bZIP transcription factor OsbZIP73 was upregulated after Spd treatment, which is confirmed by Western blot. Further insights into the interaction between OsbZIP73 and Spd were gained through fluorescence polarization experiments, showing that Spd enhances protein OsbZIP73's affinity for RNA. Functional enrichment analyses revealed that OsPYL1, OsSPARK1, and various SAUR family genes involved in Spd-affected pathways. The presence of G/A/C-box elements in these genes suggests they are potential targets for OsbZIP73.

CONCLUSIONS

Our findings suggest a strategy of using spermidine as a chemical alleviator for salt stress and provide insights into the regulatory function of OsbZIP73 in mitigating salt stress in rice roots.

Genome-wide identification and expression analysis of the PP2C gene family in Apocynum venetum and Apocynum hendersonii

BACKGROUND

Protein phosphatase class 2 C (PP2C) is the largest protein phosphatase family in plants. Members of the PP2C gene family are involved in a variety of physiological pathways in plants, including the abscisic acid signalling pathway, the regulation of plant growth and development, etc., and are capable of responding to a wide range of biotic and abiotic stresses, and play an important role in plant growth, development, and response to stress. Apocynum is a perennial persistent herb, divided into Apocynum venetum and Apocynum hendersonii. It mainly grows in saline soil, deserts and other harsh environments, and is widely used in saline soil improvement, ecological restoration, textiles and medicine. A. hendersonii was found to be more tolerant to adverse conditions. The main purpose of this study was to investigate the PP2C gene family and its expression pattern under salt stress and to identify important candidate genes related to salt tolerance.

RESULTS

In this study, 68 AvPP2C genes and 68 AhPP2C genes were identified from the genomes of A. venetum and A. hendersonii, respectively. They were classified into 13 subgroups based on their phylogenetic relationships and were further analyzed for their subcellular locations, gene structures, conserved structural domains, and cis-acting elements. The results of qRT-PCR analyses of seven AvPP2C genes and seven AhPP2C genes proved that they differed significantly in gene expression under salt stress. It has been observed that the PP2C genes in A. venetum and A. hendersonii exhibit different expression patterns. Specifically, AvPP2C2, 6, 24, 27, 41 and AhPP2C2, 6, 24, 27, 42 have shown significant differences in expression under salt stress. This indicates that these genes may play a crucial role in the salt tolerance mechanism of A. venetum and A. hendersonii.

CONCLUSIONS

In this study, we conducted a genome-wide analysis of the AvPP2C and AhPP2C gene families in Apocynum, which provided a reference for further understanding the functional characteristics of these genes.

Indole-3-acetic acid enhances the co-transport of proton and phenanthrene mediated by TaSAUR80-5A in wheat roots

Polycyclic aromatic hydrocarbons (PAHs) are a type of organic pollution that can accumulate in crops and hazard human health. This study used phenanthrene (PHE) as a model PAH and employed hydroponic experiments to illustrate the role of indole-3-acetic acid (IAA) in the regulation of PHE accumulation in wheat roots. At optimal concentrations, wheat roots treated with PHE + IAA showed a 46.9% increase in PHE concentration, whereas treatment with PHE + P-chlorophenoxyisobutyric acid resulted in a 38.77% reduction. Transcriptome analysis identified TaSAUR80-5A as the crucial gene for IAA-enhancing PHE uptake. IAA increases plasma membrane H+-ATPase activity, promoting active transport of PHE via the PHE/H+ cotransport mechanism. These results provide not only the theoretical basis necessary to better understand the function of IAA in PAHs uptake and transport by staple crops, but also a strategy for controlling PAHs accumulation in staple crops and enhancing phytoremediation of PAH-contaminated environments.

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

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

A novel semi-dominant mutation in brassinosteroid signaling kinase1 increases stomatal density

Stomata play a pivotal role in balancing CO2 uptake for photosynthesis and water loss via transpiration. Thus, appropriate regulation of stomatal movement and its formation are crucial for plant growth and survival. Red and blue light induce phosphorylation of the C-terminal residue of the plasma membrane (PM) H+-ATPase, threonine, in guard cells, generating the driving force for stomatal opening. While significant progress has been made in understanding the regulatory mechanism of PM H+-ATPase in guard cells, the regulatory components for the phosphorylation of PM H+-ATPase have not been fully elucidated. Recently, we established a new immunohistochemical technique for detecting guard-cell PM H+-ATPase phosphorylation using leaves, which was expected to facilitate investigations with a single leaf. In this study, we applied the technique to genetic screening experiment to explore novel regulators for the phosphorylation of PM H+-ATPase in guard cells, as well as stomatal development. We successfully performed phenotyping using a single leaf. During the experiment, we identified a mutant exhibiting high stomatal density, jozetsu (jzt), named after a Japanese word meaning 'talkative'. We found that a novel semi-dominant mutation in BRASSINOSTEROID SIGNALING KINASE1 (BSK1) is responsible for the phenotype in jzt mutant. The present results demonstrate that the new immunohistochemical technique has a wide range of applications, and the novel mutation would provide genetic tool to expand our understanding of plant development mediated by brassinosteroid signaling.

Light-induced stomatal opening requires phosphorylation of the C-terminal autoinhibitory domain of plasma membrane H+-ATPase

Plasma membrane H+-ATPase provides the driving force for light-induced stomatal opening. However, the mechanisms underlying the regulation of its activity remain unclear. Here, we show that the phosphorylation of two Thr residues in the C-terminal autoinhibitory domain is crucial for H+-ATPase activation and stomatal opening in Arabidopsis thaliana. Using phosphoproteome analysis, we show that blue light induces the phosphorylation of Thr-881 within the C-terminal region I, in addition to penultimate Thr-948 in AUTOINHIBITED H+-ATPASE 1 (AHA1). Based on site-directed mutagenesis experiments, phosphorylation of both Thr residues is essential for H+ pumping and stomatal opening in response to blue light. Thr-948 phosphorylation is a prerequisite for Thr-881 phosphorylation by blue light. Additionally, red light-driven guard cell photosynthesis induces Thr-881 phosphorylation, possibly contributing to red light-dependent stomatal opening. Our findings provide mechanistic insights into H+-ATPase activation that exploits the ion transport across the plasma membrane and light signalling network in guard cells.

Phosphorylation of plasma membrane H+-ATPase Thr881 participates in light-induced stomatal opening

Plasma membrane (PM) H+-ATPase is crucial for light-induced stomatal opening and phosphorylation of a penultimate residue, Thr948 (pen-Thr, numbering according to Arabidopsis AHA1) is required for enzyme activation. In this study, a comprehensive phosphoproteomic analysis using guard cell protoplasts from Vicia faba shows that both red and blue light increase the phosphorylation of Thr881, of PM H+-ATPase. Light-induced stomatal opening and the blue light-induced increase in stomatal conductance are reduced in transgenic Arabidopsis plants expressing mutant AHA1-T881A in aha1-9, whereas the blue light-induced phosphorylation of pen-Thr is unaffected. Auxin and photosynthetically active radiation induce the phosphorylation of both Thr881 and pen-Thr in etiolated seedlings and leaves, respectively. The dephosphorylation of phosphorylated Thr881 and pen-Thr are mediated by type 2 C protein phosphatase clade D isoforms. Taken together, Thr881 phosphorylation, in addition of the pen-Thr phosphorylation, are important for PM H+-ATPase function during physiological responses, such as light-induced stomatal opening in Arabidopsis thaliana.

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.

Transcriptome analysis reveals the mechanism of different fruit appearance between apricot (Armeniaca vulgaris Lam.) and its seedling

BACKGROUND

Apricot fruit has great economic value. In the process of apricot breeding using traditional breeding methods, we obtained a larger seedling (named Us) from the original variety (named U). And Us fruit is larger than U, taste better. Therefore, revealing its mechanism is very important for Apricot breeding.

METHODS

In this study, de novo assembly and transcriptome sequencing (RNA-Seq) was used to screen the differently expressed genes (DEGs) between U and Us at three development stages, including young fruits stage, mid-ripening stage and mature fruit stage.

RESULTS

The results showed that there were 6,753 DEGs at different sampling time. "Cellulose synthase (UDP-forming) activity" and "cellulose synthase activity" were the key GO terms enriched in GO, of which CESA and CSL family played a key role. "Photosynthesis-antenna proteins" and "Plant hormone signal transduction" were the candidate pathways and lhca, lhcb, Aux/IAA and SAUR were the main regulators.

CONCLUSION

The auxin signaling pathway was active in Us, of which Aux/IAAs and SAUR were the key fruit size regulators. The low level of lhca and lhcb in Us could reveal the low demand for exogenous carbon, but they increased at mature stage, which might be due to the role of aux, who was keeping the fruit growing. Aux and photosynthesis maight be the main causes of appearance formation of Us fruits. Interestingly, the higher expression of CESA and CSL proved that Us entered the hardening process earlier than U. The advanced developmental progress might also be due to the role of Aux.

Unraveling the Mechanism of StWRKY6 in Potato (Solanum tuberosum)'s Cadmium Tolerance for Ensuring Food Safety

The WRKY transcription factor plays a crucial role in plant stress adaptation. Our research has found that WRKY6 in Solanum tuberosum (potatoes) is closely related to cadmium (Cd) tolerance. Therefore, investigating the mechanism of StWRKY6 in plant resistance to Cd toxicity is of great scientific importance for food safety. This research further analyzed the gene structure and functional regions of the nuclear transcription factor WRKY6 in potatoes, discovering that StWRKY6 contains W box, GB/box, ABRE, and other elements that can act as a nuclear transcription regulatory factor to execute multiple functional regulations. The results of the heterologous expression of StWRKY6 in Arabidopsis under Cd stress showed that the overexpression line (StWRKY6-OE) had significantly higher SAPD values and content of reactive oxygen species scavenging enzymes than the wild type, indicating that StWRKY6 plays a crucial role in protecting the photosynthetic system and promoting carbohydrate synthesis. Transcriptome analysis also revealed that the Cd-induced expression of StWRKY6 up-regulated many potential gene targets, including APR2, DFRA, ABCG1, VSP2, ERF013, SAUR64/67, and BBX20, which are involved in Cd chelation (APR2, DFRA), plant defense (VSP2, PDF1.4), toxic substance efflux (ABCG1), light morphology development (BBX20), and auxin signal (SAUR64/67). These genes coordinate the regulation of Cd tolerance in the StWRKY6 overexpression line. In summary, this study identified a potential gene set of the co-expression module of StWRKY6, providing useful evidence for the remediation of Cd-contaminated soil and the genetic breeding of low Cd-accumulating crops, thereby ensuring food safety.

Ca2+-dependent TaCCD1 cooperates with TaSAUR215 to enhance plasma membrane H+-ATPase activity and alkali stress tolerance by inhibiting PP2C-mediated dephosphorylation of TaHA2 in wheat

Alkali stress is a major constraint for crop production in many regions of saline-alkali land. However, little is known about the mechanisms through which wheat responds to alkali stress. In this study, we identified a calcium ion-binding protein from wheat, TaCCD1, which is critical for regulating the plasma membrane (PM) H⁺-ATPase-mediated alkali stress response. PM H⁺-ATPase activity is closely related to alkali tolerance in the wheat variety Shanrong 4 (SR4). We found that two D-clade type 2C protein phosphatases, TaPP2C.D1 and TaPP2C.D8 (TaPP2C.D1/8), negatively modulate alkali stress tolerance by dephosphorylating the penultimate threonine residue (Thr926) of TaHA2 and thereby inhibiting PM H⁺-ATPase activity. Alkali stress induces the expression of TaCCD1 in SR4, and TaCCD1 interacts with TaSAUR215, an early auxin-responsive protein. These responses are both dependent on calcium signaling triggered by alkali stress. TaCCD1 enhances the inhibitory effect of TaSAUR215 on TaPP2C.D1/8 activity, thereby promoting the activity of the PM H⁺-ATPase TaHA2 and alkali stress tolerance in wheat. Functional and genetic analyses verified the effects of these genes in response to alkali stress, indicating that TaPP2C.D1/8 function downstream of TaSAUR215 and TaCCD1. Collectively, this study uncovers a new signaling pathway that regulates wheat responses to alkali stress, in which Ca²⁺-dependent TaCCD1 cooperates with TaSAUR215 to enhance PM H⁺-ATPase activity and alkali stress tolerance by inhibiting TaPP2C.D1/8-mediated dephosphorylation of PM H⁺-ATPase TaHA2 in wheat.

Pre treatment of melatonin rescues cotton seedlings from cadmium toxicity by regulating key physio-biochemical and molecular pathways

Melatonin, a plant/animal origin hormone, regulates plant response to abiotic stresses by protecting them from oxidative damage. This study identified physiochemical and molecular mechanism of melatonin-induced cadmium (Cd) stress tolerance and detoxification in cotton seedlings. Cotton seedlings, with or without melatonin (15 µM) pretreatment, were subjected to Cd (100 µM) stress in a hydroponic medium for eight days. We found that higher cellular Cd accumulation in leaf tissues significantly inhibited the growth and physiology of cotton seedlings. In contrast, melatonin-treated seedlings maintained leaf photosynthetic capacity, producing relatively higher fresh (17.4%) and dry (19.3%) weights than non-melatonin-treated plants under Cd-contaminated environments. The improved growth and leaf functioning were strongly linked with the melatonin-induced repression of Cd transporter genes (LOC107894197, LOC107955631, LOC107899273) in roots. Thus, melatonin induced downregulation of the Cd transporter genes further inhibited Cd ion transport towards leaf tissues. This suggests that the differentially expressed transporter genes (DEG) are key drivers of the melatonin-mediated regulation of Cd transportation and sequestration in cotton. Melatonin also protected cotton seedlings from Cd-induced oxidative injury by reducing tissues malondialdehyde (MDA) and hydrogen peroxide (H₂O₂) levels and increasing the activities of superoxide dismutase (SOD), peroxidase (POD), catalase (CAT), and ascorbate peroxidase (APX) enzymes. Transcriptomic analysis revealed that melatonin activated mitogen-activated protein kinase (MAPK) signaling pathways to simulate stomatal adjustment and photosynthesis in Cd-stressed leaves. Further, melatonin protects intercellular organs, particularly ribosomes, from Cd-induced oxidative damage by promoting ribosomal biosynthesis and improving translational efficiency. The findings elucidated the molecular basis of melatonin-mediated Cd stress tolerance in plants and provided a key for the effective strategy of Cd accumulation in cotton.

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.

Elevated CO2 induces rapid dephosphorylation of plasma membrane H+ -ATPase in guard cells

Light induces stomatal opening, which is driven by plasma membrane (PM) H⁺ -ATPase in guard cells. The activation of guard-cell PM H⁺ -ATPase is mediated by phosphorylation of the penultimate C-terminal residue, threonine. The phosphorylation is induced by photosynthesis as well as blue light photoreceptor phototropin. Here, we investigated the effects of cessation of photosynthesis on the phosphorylation level of guard-cell PM H⁺ -ATPase in Arabidopsis thaliana. Immunodetection of guard-cell PM H⁺ -ATPase, time-resolved leaf gas-exchange analyses and stomatal aperture measurements were carried out. We found that light-dark transition of leaves induced dephosphorylation of the penultimate residue at 1 min post-transition. Gas-exchange analyses confirmed that the dephosphorylation is accompanied by an increase in the intercellular CO₂ concentration, caused by the cessation of photosynthetic CO₂ fixation. We discovered that CO₂ induces guard-cell PM H⁺ -ATPase dephosphorylation as well as stomatal closure. Interestingly, reverse-genetic analyses using guard-cell CO₂ signal transduction mutants suggested that the dephosphorylation is mediated by a mechanism distinct from the established CO₂ signalling pathway. Moreover, type 2C protein phosphatases D6 and D9 were required for the dephosphorylation and promoted stomatal closure upon the light-dark transition. Our results indicate that CO₂ -mediated dephosphorylation of guard-cell PM H⁺ -ATPase underlies stomatal closure.

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.

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.

OsPP65 Negatively Regulates Osmotic and Salt Stress Responses Through Regulating Phytohormone and Raffinose Family Oligosaccharide Metabolic Pathways in Rice

Although type 2C protein phosphatases (PP2Cs) have been demonstrated to play important roles in regulating plant development and various stress responses, their specific roles in rice abiotic stress tolerance are still largely unknown. In this study, the functions of OsPP65 in rice osmotic and salt stress tolerance were investigated. Here, we report that OsPP65 is responsive to multiple stresses and is remarkably induced by osmotic and salt stress treatments. OsPP65 was highly expressed in rice seedlings and leaves and localized in the nucleus and cytoplasm. OsPP65 knockout rice plants showed enhanced tolerance to osmotic and salt stresses. Significantly higher induction of genes involved in jasmonic acid (JA) and abscisic acid (ABA) biosynthesis or signaling, as well as higher contents of endogenous JA and ABA, were observed in the OsPP65 knockout plants compared with the wild-type plants after osmotic stress treatment. Further analysis indicated that JA and ABA function independently in osmotic stress tolerance conferred by loss of OsPP65. Moreover, metabolomics analysis revealed higher endogenous levels of galactose and galactinol but a lower content of raffinose in the OsPP65 knockout plants than in the wild-type plants after osmotic stress treatment. These results together suggest that OsPP65 negatively regulates osmotic and salt stress tolerance through regulation of the JA and ABA signaling pathways and modulation of the raffinose family oligosaccharide metabolism pathway in rice. OsPP65 is a promising target for improvement of rice stress tolerance using gene editing.

Root-specific activation of plasma membrane H+-ATPase 1 enhances plant growth and shoot accumulation of nutrient elements under nutrient-poor conditions in Arabidopsis thaliana

Plasma membrane (PM) H⁺-ATPase contributes to nutrient uptake and stomatal opening by creating proton gradient across the membrane. Previous studies report that a dominant mutation in the OPEN STOMATA2 locus (OST2-2D) constitutively activates Arabidopsis PM H⁺-ATPase 1 (AHA1), which enlarges proton motive force for root nutrient uptake. However, the stomatal opening is also constitutively enhanced in the ost2-2D, causing drought hypersensitivity. To develop plants with improved nutrient acquisition and normal stomatal movement, we generated grafted plants (scion/rootstock: Col-0 (WT)/ost2-2D), and compared their growth, nutrient element content, and transcriptomes with those of control plants (WT/WT) under nutrient-rich or nutrient-poor conditions. WT/ost2-2D shoots had larger weights, rosette diameter, leaf blade area, and content of C, N, K, Ca, S, P, Mg, Na, Mn, B, Co, and Mo under nutrient-poor conditions compared with WT/WT shoots. The root weights and primary root length were greater in WT/ost2-2D plants than in WT/WT plants under both nutrient conditions. Root expression of the high-affinity nitrate transporter NRT2.1, potassium transporter HAK5, and divalent cation transporter IRT1 was higher in WT/ost2-2D plants than in WT/WT plants under nutrient-poor conditions. These results suggest that root-specific activation of PM H⁺-ATPase enhances plant growth by increasing root uptake of nutrient elements under nutrient-poor conditions. Our study presents a novel approach to improving nutrient uptake efficiency in crops for low-input 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.

Plant hormone regulation of abiotic stress responses

Plant hormones are signalling compounds that regulate crucial aspects of growth, development and environmental stress responses. Abiotic stresses, such as drought, salinity, heat, cold and flooding, have profound effects on plant growth and survival. Adaptation and tolerance to such stresses require sophisticated sensing, signalling and stress response mechanisms. In this Review, we discuss recent advances in understanding how diverse plant hormones control abiotic stress responses in plants and highlight points of hormonal crosstalk during abiotic stress signalling. Control mechanisms and stress responses mediated by plant hormones including abscisic acid, auxin, brassinosteroids, cytokinins, ethylene and gibberellins are discussed. We discuss new insights into osmotic stress sensing and signalling mechanisms, hormonal control of gene regulation and plant development during stress, hormone-regulated submergence tolerance and stomatal movements. We further explore how innovative imaging approaches are providing insights into single-cell and tissue hormone dynamics. Understanding stress tolerance mechanisms opens new opportunities for agricultural applications.

Transcriptome analysis of oil palm pistil during pollination and fertilization to unravel the role of phytohormone biosynthesis and signaling genes

Phytohormones play an important role in the pollination and fertilization of crops, but the regulatory mechanisms of oil palm pollination and fertilization are unclear. The purpose of this study is to explore the hormonal changes of oil palm pistils during flowering. We used RNA sequencing to evaluate differentially expressed genes (DEGs) in oil palm pistils at the pollination and non-pollination stages. In this study, we found that the hormone contents of oil palm pistil changed drastically after pollination. The transcriptome of the oil palm pistil without pollination and at 2 h, 4 h, 12 h, 24 h, and 48 h after pollination was comprehensively analyzed, and a large number of differential genes and metabolic pathways were explored. Based on the transcriptome data, it could be recognized that the changes of indoleacetic acid (IAA), zeatin riboside (ZR), and abscisic acid (ABA) during pollination were consistent with the changes in the corresponding gene transcripts. Differentially expressed genes during pollination and fertilization of oil palm were mainly related to energy metabolism and hormone signal transduction. It provides new insights to elucidate the interaction and regulation mechanisms of plant hormones before and after oil palm pollination, providing a theoretical basis and reference for the research on sexual reproduction of oil palm.

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.

Posttranslational regulation of transporters important for symbiotic interactions

Coordinated sharing of nutritional resources is a central feature of symbiotic interactions, and, despite the importance of this topic, many questions remain concerning the identification, activity, and regulation of transporter proteins involved. Recent progress in obtaining genome and transcriptome sequences for symbiotic organisms provides a wealth of information on plant, fungal, and bacterial transporters that can be applied to these questions. In this update, we focus on legume-rhizobia and mycorrhizal symbioses and how transporters at the symbiotic interfaces can be regulated at the protein level. We point out areas where more research is needed and ways that an understanding of transporter mechanism and energetics can focus hypotheses. Protein phosphorylation is a predominant mechanism of posttranslational regulation of transporters in general and at the symbiotic interface specifically. Other mechanisms of transporter regulation, such as protein-protein interaction, including transporter multimerization, polar localization, and regulation by pH and membrane potential are also important at the symbiotic interface. Most of the transporters that function in the symbiotic interface are members of transporter families; we bring in relevant information on posttranslational regulation within transporter families to help generate hypotheses for transporter regulation at the symbiotic interface.

What can mechanistic models tell us about guard cells, photosynthesis, and water use efficiency?

Stomatal pores facilitate gaseous exchange between the inner air spaces of the leaf and the atmosphere. The pores open to enable CO₂ entry for photosynthesis and close to reduce transpirational water loss. How stomata respond to the environment has long attracted interest in modeling as a tool to understand the consequences for the plant and for the ecosystem. Models that focus on stomatal conductance for gas exchange make intuitive sense, but such models need also to connect with the mechanics of the guard cells that regulate pore aperture if we are to understand the 'decisions made' by stomata, their impacts on the plant and on the global environment.

The SAUR gene family in coffee: genome-wide identification and gene expression analysis during somatic embryogenesis

BACKGROUND

Small auxin-up RNA (SAUR) genes form a wide family supposedly involved in different physiological and developmental processes in plants such as leaf senescence, auxin signaling and transport, hypocotyl development and tolerance to abiotic stresses. The transcription of SAUR genes is quickly induced by auxins, a group of phytohormones of major importance on embryo development. To better understand the distribution and expression profile of such still not explored family in Coffea sp., especially during the development of somatic embryogenesis (SE), SAUR members were characterized in silico using the available Coffea canephora genome data and analyzed for gene expression by RT-qPCR in C. arabica embryogenic samples.

METHODS AND RESULTS

Over C. canephora genome 31 CcSAURs were distributed by 11 chromosomes. Out of these 31 gene members, 5 SAURs were selected for gene expression analysis in C. arabica embryogenic materials. CaSAUR12 and CaSAUR18 were the members highly expressed through almost all plant materials. The other genes had more expression in at least one of the developing embryo stages or plantlets. The CaSAUR12 was the only member to exhibit an increased expression in both non-embryogenic calli and the developing embryo stages.

CONCLUSION

The identification of SAUR family on C. canephora genome followed by the analysis of gene expression profile across coffee somatic embryogenesis process on C. arabica represents a further additional step towards a better comprehension of molecular components acting on SE. Along with new research about this gene family such knowledge may support studies about clonal propagation methods via somatic embryogenesis to help the scientific community towards improvements into coffee crop.

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.

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.

Overexpression of MusaSNAC1 improves shoot proliferation in transgenic banana lines

Augmenting shoot multiplication through genetic engineering is an emerging biotechnological application desirable in optimizing regeneration of genetically modified plants on selection medium and rapid clonal propagation of elite cultivars. Here, we report the improved shoot multiplication in transgenic banana lines with overexpression of MusaSNAC1, a drought-associated NAC transcription factor in banana. Overexpression of MusaSNAC1 induces hypersensitivity of transgenic banana lines toward 6-benzylaminopurine ensuing higher shoot number on different concentrations of 6-benzylaminopurine. Altered transcript levels of multiple genes involved in auxin signaling (Aux/IAA and ARFs) and cytokinin signaling pathways (ARRs) in banana plants overexpressing MusaSNAC1 corroborate the hypersensitivity of transgenic banana plants toward 6-benzylaminopurine. Modulation in expression of ARRs reported to be involved in ABA-hypersensitivity and closure of stomatal aperture correlates with the function of MusaSNAC1 as a drought-responsive NAC transcription factor. Present study suggests a prospective cross talk between shoot multiplication and drought responses coordinated by MusaSNAC1 in banana plants.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s13205-021-02744-5.