Cell-specific vacuolar calcium storage mediated by CAX1 regulates apoplastic calcium concentration, gas exchange, and plant productivity in Arabidopsis

Function of key ion channels in abiotic stresses and stomatal dynamics

Climate changes disrupt environmental and soil conditions that affect ionic balance in plants, presenting significant challenges to their survival and productivity. Membrane transporters are crucial for maintaining ionic homeostasis and regulating the movement of substances across plasma and organellar membranes, particularly under abiotic stresses. Among these abiotic stress-responsive mechanisms, stomata are critical for regulating water loss and carbon dioxide uptake, reflecting a plant's ability to respond and adapt to abiotic stresses effectively. This review highlights the role of ion transporters, including both anion and cation transporters in plant abiotic stress responses. It explores the interplay between different ion channels and regulatory components that enable plants to withstand key abiotic stresses such as drought, salinity, and heat. Moreover, we emphasized the contributions of three essential types of ion channels - potassium, anion, and calcium to abiotic stress-related stomatal regulation. These ion channels orchestrate complex signaling networks that allow plants to modulate stomatal behavior and maintain physiological balance under adverse conditions. This article provides valuable molecular and physiological insights into the mechanisms of ion transport and regulation for plants to adapt to environmental challenges. Thus, this review offers a useful foundation for developing innovative strategies to enhance crop resilience and performance in an era of increasingly unpredictable and harsh climates.

Cellular calcium homeostasis and regulation of its dynamic perturbation

Calcium ions (Ca2+) play pivotal roles in a host of cellular signalling processes. The requirement to maintain resting cytosolic Ca2+ levels in the 100-200 nM range provides a baseline for dynamic excursions from resting levels that determine the nature of many physiological responses to external stimuli and developmental processes. This review provides an overview of the key components of the Ca2+ homeostatic machinery, including known channel-mediated Ca2+ entry pathways along with transporters that act to shape the cytosolic Ca2+ signature. The relative roles of the vacuole and endoplasmic reticulum as sources or sinks for cytosolic Ca2+ are considered, highlighting significant gaps in our understanding. The components contributing to mitochondrial, chloroplast and nuclear Ca2+ homeostasis and organellar Ca2+ signals are also considered. Taken together, a complex picture of the cellular Ca2+ homeostatic machinery emerges with some clear differences from mechanisms operating in many animal cells.

Transcriptome Analysis Revealed Hub Genes Related to Tipburn Resistance in Chinese Cabbage (Brassica rapa L. ssp. pekinensis)

Tipburn is a physiological disease in Chinese cabbage. In recent years, this disease has become increasingly serious, affecting the quality and economic benefits of Chinese cabbage. However, little is known about the molecular mechanism by which calcium deficiency induces tipburn. Therefore, we performed transcriptome analysis on Y578-2 (tipburn-resistant accession) and Y920-2 (tipburn-susceptible accession) to identify the genes involved in the tipburn defense mechanism in Chinese cabbage. In this study, phenotypic observation showed that Y920-2 began to display symptoms on the 10th day of calcium deficiency treatment. Through weighted gene co-expression network analysis (WGCNA), three gene modules that were highly related to tipburn resistance were identified. Analysis of gene expression regulation in the three modules revealed 13 hub genes related to tipburn resistance, which were involved in the cell wall, photosynthesis, transcription factors, hormones, and the stress response, indicating that these factors play an important role in the tipburn response of Chinese cabbage. These transcriptome data and analysis results provide a basis for the study of the molecular mechanism of calcium deficiency-induced tipburn in Chinese cabbage.

Comparative Analysis of Ca2+/Cation Antiporter Gene Family in Rosa roxburghii and Enhanced Calcium Stress Tolerance via Heterologous Expression of RrCAX1a in Tobacco

Rosa roxburghii, a calciphilic species native to the mountainous regions of Southwest China, is renowned for its high vitamin C and bioactive components, making it valuable for culinary and medicinal uses. This species exhibits remarkable tolerance to the high-calcium conditions typical of karst terrains. However, the underlying mechanisms of this calcium resilience remain unclear. The Ca2+/cation antiporter (CaCA) superfamily plays a vital role in the transport of Ca2+ and other cations and is crucial for plant tolerance to metal stress. However, the roles and evolutionary significance of the CaCA superfamily members in R. roxburghii remain poorly understood. This study identified 22 CaCA superfamily genes in R. roxburghii, categorized into four subfamilies. The gene structures of these RrCaCAs show considerable conservation across related species. Selection pressure analysis revealed that all RrCaCAs are subject to purifying selection. The promoter regions of these genes contain numerous hormone-responsive and stress-related elements. qRT-PCR analyses demonstrated that H+/cation exchanger (CAX) RrCAX1a and RrCAX3a were highly responsive to Ca2+ stress, cation/Ca2+ exchanger (CCX) RrCCX4 to Mg2+ stress, and RrCCX11a to Na+ stress. Subcellular localization indicated that RrCAX1a is localized to the plant cell membrane, and its stable transformation in tobacco confirmed its ability to confer enhanced resistance to heavy Ca2+ stresses, highlighting its crucial role in the high-calcium tolerance mechanisms of R. roxburghii. This research establishes a foundation for further molecular-level functional analyses of the adaptation mechanisms of R. roxburghii to high-calcium environments.

Growing on calcareous soils and facing climate change

Soil calcium carbonate (CaCO3) impacts plant mineral nutrition far beyond Fe metabolism, imposing constraints for crop growth and quality in calcareous agrosystems. Our knowledge on plant strategies to tolerate CaCO3 effects mainly refers to Fe acquisition. This review provides an update on plant cellular and molecular mechanisms recently described to counteract the negative effects of CaCO3 in soils, as well as recent efforts to identify genetic bases involved in CaCO3 tolerance from natural populations, that could be exploited to breed CaCO3-tolerant crops. Finally, we review the impact of environmental factors (soil water content, air CO2, and temperature) affecting soil CaCO3 equilibrium and plant tolerance to calcareous soils, and we propose strategies for improvement in the context of climate change.

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.

STOP1 regulates CCX1-mediated Ca2+ homeostasis for plant adaptation to Ca2+ deprivation

Calcium (Ca) is essential for plant growth and stress adaptation, yet its availability is often limited in acidic soils, posing a major threat to crop production. Understanding the intricate mechanisms orchestrating plant adaptation to Ca deficiency remains elusive. Here, we show that the Ca deficiency-enhanced nuclear accumulation of the transcription factor SENSITIVE TO PROTON RHIZOTOXICITY 1 (STOP1) in Arabidopsis thaliana confers tolerance to Ca deprivation, with the global transcriptional responses triggered by Ca deprivation largely impaired in the stop1 mutant. Notably, STOP1 activates the Ca deprivation-induced expression of CATION/Ca2+ EXCHANGER 1 (CCX1) by directly binding to its promoter region, which facilitates Ca2+ efflux from endoplasmic reticulum to cytosol to maintain Ca homeostasis. Consequently, the constitutive expression of CCX1 in the stop1 mutant partially rescues the Ca deficiency phenotype by increasing Ca content in the shoots. These findings uncover the pivotal role of the STOP1-CCX1 axis in plant adaptation to low Ca, offering alternative manipulating strategies to improve plant Ca nutrition in acidic soils and extending our understanding of the multifaceted role of STOP1.

CAX control: multiple roles of vacuolar cation/H+ exchangers in metal tolerance, mineral nutrition and environmental signalling

Plant vacuolar transporters, particularly CAX (Cation/H+ Exchangers) responsible for Ca2+/H+ exchange on the vacuole tonoplast, play a central role in governing cellular pH, ion balance, nutrient storage, metal accumulation, and stress responses. Furthermore, CAX variants have been employed to enhance the calcium content of crops, contributing to biofortification efforts. Recent research has uncovered the broader significance of these transporters in plant signal transduction and element partitioning. The use of genetically encoded Ca2+ sensors has begun to highlight the crucial role of CAX isoforms in generating cytosolic Ca2+ signals, underscoring their function as pivotal hubs in diverse environmental and developmental signalling networks. Interestingly, it has been observed that the loss of CAX function can be advantageous in specific stress conditions, both for biotic and abiotic stressors. Determining the optimal timing and approach for modulating the expression of CAX is a critical concern. In the future, strategically manipulating the temporal loss of CAX function in agriculturally important crops holds promise to bolster plant immunity, enhance cold tolerance, and fortify resilience against one of agriculture's most significant challenges, namely flooding.

Inducible tolerance to low Ca:Mg in serpentine ecotype of Erythranthe guttata

In serpentine soils, the low level of calcium relative to magnesium (Ca:Mg) is detrimental to the growth of most plant species. Ecotypic variation in Erythranthe guttata allows for some populations to maintain high photosynthetic rates and biomass despite low Ca:Mg. In this study, the mechanism of tolerance was investigated by treating hydroponically grown plants with either high (1.0) or low (0.02) Ca:Mg growth solutions and assaying excised leaf discs for rates of photosynthesis and disc expansion, and for starch, Ca2+ and Mg2+ ion concentrations. Low Ca:Mg in the assay solutions reduced both photosynthesis and leaf disc expansion after one week of treatment. However, serpentine tissues show stable photosynthetic rates after one week and a recovery in leaf tissue expansion after two weeks exposure to low Ca:Mg conditions. Values for non-serpentine tissues continued to decline. Increased growth of low Ca:Mg treated discs supplied with exogenous sucrose suggests that growth in serpentine-exposed tissues is limited by availability of carbon products from photosynthesis. Serpentine leaves had higher vacuole Mg concentrations than non-serpentine leaves after three weeks of treatment with low Ca:Mg. The combination of elevated starch concentrations, reduced growth and lower vacuolar Mg concentrations in leaves of non-serpentine plants grown in low Ca:Mg indicate an inefficient use of carbon resources and starch degradation as an observed response to Mg toxicity. Together, these results suggest that serpentine E. guttata exhibits an inducible tolerance to low Ca:Mg through gradual compartmentalization of magnesium to maintain the production and metabolism of photosynthates necessary for growth.

Ammonium nutrition modifies cellular calcium distribution influencing ammonium-induced growth inhibition

Proper plant growth requires balanced nutrient levels. In this study, we analyzed the relationship between ammonium (NH4+) nutrition and calcium (Ca2+) homeostasis in the leaf tissues of wild-type and mutant Arabidopsis specimens provided with different nitrogen sources (NH4+ and nitrate, NO3-). Providing plants with NH4+ as the sole nitrogen source disrupts Ca2+ homeostasis, which is essential for activating signaling pathways and maintaining the cell wall structure. The results revealed that the lower Ca2+ content in Arabidopsis leaves under NH4+ stress might result from reduced transpiration pull, which could impair root-to-shoot Ca2+ transport. Moreover, NH4+ nutrition increased the expression of genes encoding proteins responsible for exporting Ca2+ from the cytosol of leaf cells. Furthermore, overexpression of the Ca2+/H+ antiporter 1 (CAX1) gene alleviates the effects of NH4+ syndrome, including stunted growth. The oeCAX1 plants, characterized by a lower apoplastic Ca2+ level, grew better under NH4+ stress than wild-type plants. Evaluation of the mechanical properties of the leaf blades, including stiffness, strength, toughness, and extensibility, showed that the wild-type and oeCAX1 plants responded differently to the nitrogen source, highlighting the role of cell wall metabolism in inhibiting the growth of NH4+-stressed plants.

Activating plant immunity: the hidden dance of intracellular Ca2+ stores

Calcium ion (Ca2+) serves as a versatile and conserved second messenger in orchestrating immune responses. In plants, plasma membrane-localized Ca2+-permeable channels can be activated to induce Ca2+ influx from extracellular space to cytosol upon pathogen infection. Notably, different immune elicitors can induce dynamic Ca2+ signatures in the cytosol. During pattern-triggered immunity, there is a rapid and transient increase in cytosolic Ca2+, whereas in effector-triggered immunity, the elevation of cytosolic Ca2+ is strong and sustained. Numerous Ca2+ sensors are localized in the cytosol or different intracellular organelles, which are responsible for detecting and converting Ca2+ signals. In fact, Ca2+ signaling coordinated by cytosol and subcellular compartments plays a crucial role in activating plant immune responses. However, the complete Ca2+ signaling network in plant cells is still largely ambiguous. This review offers a comprehensive insight into the collaborative role of intracellular Ca2+ stores in shaping the Ca2+ signaling network during plant immunity, and several intriguing questions for future research are highlighted.

Ectopic overexpression of Eleusine coracana CAX3 confers tolerance to metal and ion stress in yeast and Arabidopsis

Ionic and metal toxicity in plants is still a global problem for the environment, agricultural productivity and ultimately poses human health threats when these metal ions accumulate in edible organs of plants. Metal and ion transport from cytosol to the vacuole is considered an important component of metal and ion tolerance and a plant's potential utility in phytoremediation. Finger millet (Eleusine coracana) is an orphan crop but has prominent nutritional value in comparison to other cereals. Previous transcriptomic studies suggested that one of the calcium/proton exchanger (EcCAX3) is strongly upregulated during different developmental stages of spikes development in plant. This finding led us to speculate that high calcium accumulation in the grain might be because of CAX3 function. Moreover, phylogenetic analysis shows that EcCAX3 is more closely related to foxtail millet, sorghum and rice CAX3 protein. To decipher the functional role of EcCAX3, we have adopted complementation of yeast triple mutant K677 (Δpmc1Δvcx1Δcnb1), which has defective calcium transport machinery. Furthermore, metal tolerance assay shows that EcCAX3 expression conferred tolerance to different metal stresses in yeast. The gain-of-function study suggests that EcCAX3 overexpressing Arabidopsis plants shows better tolerance to higher concentration of different metal ions as compared to wild type Col-0 plants. EcCAX3-overexpression transgenic lines exhibits abundance of metal transporters and cation exchanger transporter transcripts under metal stress conditions. Furthermore, EcCAX3-overexpression lines have higher accumulation of macro- and micro-elements under different metal stress. Overall, this finding highlights the functional role of EcCAX3 in the regulation of metal and ion homeostasis and this could be potentially utilized to engineer metal fortification and generation of stress tolerant crops in near future.

Ca2+ regulates developmental timing in Arabidopsis

This article is a Commentary on Wang et al. (2024), 242: 1043–1054.

Mechanisms of calcium homeostasis orchestrate plant growth and immunity

Calcium (Ca2+) is an essential nutrient for plants and a cellular signal, but excessive levels can be toxic and inhibit growth1,2. To thrive in dynamic environments, plants must monitor and maintain cytosolic Ca2+ homeostasis by regulating numerous Ca2+ transporters3. Here we report two signalling pathways in Arabidopsis thaliana that converge on the activation of vacuolar Ca2+/H+ exchangers (CAXs) to scavenge excess cytosolic Ca2+ in plants. One mechanism, activated in response to an elevated external Ca2+ level, entails calcineurin B-like (CBL) Ca2+ sensors and CBL-interacting protein kinases (CIPKs), which activate CAXs by phosphorylating a serine (S) cluster in the auto-inhibitory domain. The second pathway, triggered by molecular patterns associated with microorganisms, engages the immune receptor complex FLS2-BAK1 and the associated cytoplasmic kinases BIK1 and PBL1, which phosphorylate the same S-cluster in CAXs to modulate Ca2+ signals in immunity. These Ca2+-dependent (CBL-CIPK) and Ca2+-independent (FLS2-BAK1-BIK1/PBL1) mechanisms combine to balance plant growth and immunity by regulating cytosolic Ca2+ homeostasis.

Types of Membrane Transporters and the Mechanisms of Interaction between Them and Reactive Oxygen Species in Plants

Membrane transporters are proteins that mediate the entry and exit of substances through the plasma membrane and organellar membranes and are capable of recognizing and binding to specific substances, thereby facilitating substance transport. Membrane transporters are divided into different types, e.g., ion transporters, sugar transporters, amino acid transporters, and aquaporins, based on the substances they transport. These membrane transporters inhibit reactive oxygen species (ROS) generation through ion regulation, sugar and amino acid transport, hormone induction, and other mechanisms. They can also promote enzymatic and nonenzymatic reactions in plants, activate antioxidant enzyme activity, and promote ROS scavenging. Moreover, membrane transporters can transport plant growth regulators, solute proteins, redox potential regulators, and other substances involved in ROS metabolism through corresponding metabolic pathways, ultimately achieving ROS homeostasis in plants. In turn, ROS, as signaling molecules, can affect the activity of membrane transporters under abiotic stress through collaboration with ions and involvement in hormone metabolic pathways. The research described in this review provides a theoretical basis for improving plant stress resistance, promoting plant growth and development, and breeding high-quality plant varieties.

Calcium homeostasis and signaling in plant immunity

Calcium (Ca2+) signaling consists of three steps: (1) initiation of a change in cellular Ca2+ concentration in response to a stimulus, (2) recognition of the change through direct binding of Ca2+ by its sensors, (3) transduction of the signal to elicit downstream responses. Recent studies have uncovered a central role for Ca2+ signaling in both layers of immune responses initiated by plasma membrane (PM) and intracellular receptors, respectively. These advances in our understanding are attributed to several lines of research, including invention of genetically-encoded Ca2+ reporters for the recording of intracellular Ca2+ signals, identification of Ca2+ channels and their gating mechanisms, and functional analysis of Ca2+ binding proteins (Ca2+ sensors). This review analyzes the recent literature that illustrates the importance of Ca2+ homeostasis and signaling in plant innate immunity, featuring intricate Ca2+dependent positive and negative regulations.

Sequential removal of cation/H+ exchangers reveals their additive role in elemental distribution, calcium depletion and anoxia tolerance

Multiple Arabidopsis H+ /Cation exchangers (CAXs) participate in high-capacity transport into the vacuole. Previous studies have analysed single and double mutants that marginally reduced transport; however, assessing phenotypes caused by transport loss has proven enigmatic. Here, we generated quadruple mutants (cax1-4: qKO) that exhibited growth inhibition, an 85% reduction in tonoplast-localised H+ /Ca transport, and enhanced tolerance to anoxic conditions compared to CAX1 mutants. Leveraging inductively coupled plasma mass spectrometry (ICP-MS) and synchrotron X-ray fluorescence (SXRF), we demonstrate CAX transporters work together to regulate leaf elemental content: ICP-MS analysis showed that the elemental concentrations in leaves strongly correlated with the number of CAX mutations; SXRF imaging showed changes in element partitioning not present in single CAX mutants and qKO had a 40% reduction in calcium (Ca) abundance. Reduced endogenous Ca may promote anoxia tolerance; wild-type plants grown in Ca-limited conditions were anoxia tolerant. Sequential reduction of CAXs increased mRNA expression and protein abundance changes associated with reactive oxygen species and stress signalling pathways. Multiple CAXs participate in postanoxia recovery as their concerted removal heightened changes in postanoxia Ca signalling. This work showcases the integrated and diverse function of H+ /Cation transporters and demonstrates the ability to improve anoxia tolerance through diminishing endogenous Ca levels.

Golgi apparatus-localized CATION CALCIUM EXCHANGER4 promotes osmotolerance of Arabidopsis

Calcium (Ca2+) is a major ion in living organisms, where it acts as a second messenger for various biological phenomena. The Golgi apparatus retains a higher Ca2+ concentration than the cytosol and returns cytosolic Ca2+ to basal levels after transient elevation in response to environmental stimuli such as osmotic stress. However, the Ca2+ transporters localized in the Golgi apparatus of plants have not been clarified. We previously found that a wild-type (WT) salt-tolerant Arabidopsis (Arabidopsis thaliana) accession, Bu-5, showed osmotic tolerance after salt acclimatization, whereas the Col-0 WT did not. Here, we isolated a Bu-5 background mutant gene, acquired osmotolerance-defective 6 (aod6), which reduces tolerance to osmotic, salt, and oxidative stresses, with a smaller plant size than the WT. The causal gene of the aod6 mutant encodes CATION CALCIUM EXCHANGER4 (CCX4). The aod6 mutant was more sensitive than the WT to both deficient and excessive Ca2+. In addition, aod6 accumulated higher Ca2+ than the WT in the shoots, suggesting that Ca2+ homeostasis is disturbed in aod6. CCX4 expression suppressed the Ca2+ hypersensitivity of the csg2 (calcium sensitive growth 2) yeast (Saccharomyces cerevisiae) mutant under excess CaCl2 conditions. We also found that aod6 enhanced MAP kinase 3/6 (MPK3/6)-mediated immune responses under osmotic stress. Subcellular localization analysis of mGFP-CCX4 showed GFP signals adjacent to the trans-Golgi apparatus network and co-localization with Golgi apparatus-localized markers, suggesting that CCX4 localizes in the Golgi apparatus. These results suggest that CCX4 is a Golgi apparatus-localized transporter involved in the Ca2+ response and plays important roles in osmotic tolerance, shoot Ca2+ content, and normal growth of Arabidopsis.

Auxin as an architect of the pectin matrix

Auxin is a versatile plant growth regulator that triggers multiple signalling pathways at different spatial and temporal resolutions. A plant cell is surrounded by the cell wall, a complex and dynamic network of polysaccharides. The cell wall needs to be rigid to provide mechanical support and protection and highly flexible to allow cell growth and shape acquisition. The modification of the pectin components, among other processes, is a mechanism by which auxin activity alters the mechanical properties of the cell wall. Auxin signalling precisely controls the transcriptional output of several genes encoding pectin remodelling enzymes, their local activity, pectin deposition, and modulation in different developmental contexts. This review examines the mechanism of auxin activity in regulating pectin chemistry at organ, cellular, and subcellular levels across diverse plant species. Moreover, we ask questions that remain to be addressed to fully understand the interplay between auxin and pectin in plant growth and development.

Genome-Wide Identification of BrCAX Genes and Functional Analysis of BrCAX1 Involved in Ca2+ Transport and Ca2+ Deficiency-Induced Tip-Burn in Chinese Cabbage (Brassica rapa L. ssp. pekinensis)

Calcium (Ca2+) plays essential roles in plant growth and development. Ca2+ deficiency causes a physiological disorder of tip-burn in Brassiceae crops and is involved in the regulation of cellular Ca2+ homeostasis. Although the functions of Ca2+/H+ exchanger antiporters (CAXs) in mediating transmembrane transport of Ca2+ have been extensively characterized in multiple plant species, the potential roles of BrCAX genes remain unclear in Chinese cabbage. In this study, eight genes of the BrCAX family were genome-widely identified in Chinese cabbage. These BrCAX proteins contained conserved Na_Ca_ex domain and belonged to five members of the CAX family. Molecular evolutionary analysis and sequence alignment revealed the evolutionary conservation of BrCAX family genes. Expression profiling demonstrated that eight BrCAX genes exhibited differential expression in different tissues and under heat stress. Furthermore, Ca2+ deficiency treatment induced the typical symptoms of tip-burn in Chinese cabbage seedlings and a significant decrease in total Ca2+ content in both roots and leaves. The expression changes in BrCAX genes were related to the response to Ca2+ deficiency-induced tip-burn of Chinese cabbage. Specially, BrCAX1-1 and BrCAX1-2 genes were highly expressed gene members of the BrCAX family in the leaves and were significantly differentially expressed under Ca2+ deficiency stress. Moreover, overexpression of BrCAX1-1 and BrCAX1-2 genes in yeast and Chinese cabbage cotyledons exhibited a higher Ca2+ tolerance, indicating the Ca2+ transport capacity of BrCAX1-1 and BrCAX1-2. In addition, suppression expression of BrCAX1-1 and BrCAX1-2 genes reduced cytosolic Ca2+ levels in the root tips of Chinese cabbage. These results provide references for functional studies of BrCAX genes and to investigate the regulatory mechanisms underlying Ca2+ deficiency disorder in Brassiceae vegetables.

Effects of exogenous calcium on cadmium accumulation in amaranth

Calcium oxalate (CaOx) crystals in plants act as a sink for excess Ca and play an essential role in detoxifying heavy metals (HMs). However, the mechanism and related influencing factors remain unclear. Amaranth (Amaranthus tricolor L.) is a common edible vegetable rich in CaOx and a potential Cd hyperaccumulation species. In this study, the hydroponic experiment was carried out to investigate the effect of exogenous Ca concentrations on Cd uptake by amaranth. The results showed that either insufficient or excess Ca supply inhibited amaranth growth, while the Cd bioconcentration factor (BCF) increased with Ca concentration. Meanwhile, the sequence extraction results demonstrated that Cd mainly accumulated as pectate and protein-bound species (NaCl extracted) in the root and stem, compared to pectate, protein, and phosphate-bound (acetic acid extractable) species in the leaf. Correlation analysis showed that the concentration of exogenous Ca was positively correlated with amaranth-produced CaOx crystals but negatively correlated with insoluble oxalate-bound Cd in the leaf. However, since the accumulated insoluble oxalate-bound Cd was relatively low, Cd detoxification via the CaOx pathway in amaranth is limited.

Evaluation of the alkalinity stress tolerance of three Brassica rapa CAX1 TILLING mutants

Alkalinity is an important environmental factor that affects crop production and will be exacerbated in the current climate change scenario. Thus, the presence of carbonates and high pH in soils negatively impacts nutrient assimilation and photosynthesis and causes oxidative stress. A potential strategy to improve tolerance to alkalinity could be the modification of cation exchanger (CAX) activity, given that these transporters are involved in calcium (Ca²⁺) signaling under stresses. In this study, we used three Brassica rapa mutants (BraA.cax1a-4, BraA.cax1a-7, and BraA.cax1a-12) from the parental line 'R-o-18' that were generated by Targeting Induced Local Lesions in Genomes (TILLING) and grown under control and alkaline conditions. The objective was to assess the tolerance of these mutants to alkalinity stress. Biomass, nutrient accumulation, oxidative stress, and photosynthesis parameters were analyzed. ^{The results showed} that BraA.cax1a-7 mutation was negative for alkalinity tolerance because it reduced plant biomass, increased oxidative stress, partially inhibited antioxidant response, and lowered photosynthesis performance. Conversely, the BraA.cax1a-12 mutation increased plant biomass and Ca²⁺ accumulation, reduced oxidative stress, and improved antioxidant response and photosynthesis performance. Hence, this study identifies BraA.cax1a-12 as a useful CAX1 mutation to enhance the tolerance of plants grown under alkaline conditions.

Ca2+ deficiency triggers panicle degeneration in rice mediated by Ca2+ /H+ exchanger OsCAX1a

Increasing rice yield has always been one of the primary objectives of rice breeding. However, panicle degeneration often occurs in rice-growing regions and severely curbs rice yield. In this study, we obtained a new apical panicle degeneration mutant, which induces a marked degeneration rate and diminishes the final grain yield. Cellular and physiological analyses revealed that the apical panicle undergoes programmed cell death, accompanied by excessive accumulations of peroxides. Following, the panicle degeneration gene OsCAX1a was identified in the mutant, which was involved in Ca²⁺ transport. Hydroponics assays and Ca²⁺ quantification confirmed that Ca²⁺ transport and distribution to apical tissues were restricted and over-accumulated in the mutant sheath. Ca²⁺ transport between cytoplasm and vacuole was affected, and the reduced Ca²⁺ content in the vacuole and cell wall of the apical panicle and the decreased Ca²⁺ absorption appeared in the mutant. RNA-Seq data indicated that the abnormal CBL (calcineurin b-like proteins) pathway mediated by deficient Ca²⁺ might occur in the mutant, resulting in the burst of ROS and programmed cell death in panicles. Our results explained the key role of OsCAX1a in Ca²⁺ transport and distribution and laid a foundation to further explore the genetic and molecular mechanisms of panicle degeneration in rice.

Membrane Proteins in Plant Salinity Stress Perception, Sensing, and Response

Plants have several mechanisms to endure salinity stress. The degree of salt tolerance varies significantly among different terrestrial crops. Proteins at the plant's cell wall and membrane mediate different physiological roles owing to their critical positioning between two distinct environments. A specific membrane protein is responsible for a single type of activity, such as a specific group of ion transport or a similar group of small molecule binding to exert multiple cellular effects. During salinity stress in plants, membrane protein functions: ion homeostasis, signal transduction, redox homeostasis, and solute transport are essential for stress perception, signaling, and recovery. Therefore, comprehensive knowledge about plant membrane proteins is essential to modulate crop salinity tolerance. This review gives a detailed overview of the membrane proteins involved in plant salinity stress highlighting the recent findings. Also, it discusses the role of solute transporters, accessory polypeptides, and proteins in salinity tolerance. Finally, some aspects of membrane proteins are discussed with potential applications to developing salt tolerance in crops.

Exogenous brassinosteroid alleviates calcium deficiency induced tip-burn by regulating calcium transport in Brassica rapa L. ssp. pekinensis

Mini Chinese cabbage (Brassica rapa L. ssp. Pekinensis) plays an important role in the supply of summer vegetables on the plateau in western China. In recent years, tip-burn has seriously affected the yield, quality and commodity value of mini Chinese cabbage. Calcium (Ca²⁺) deficiency is a key inducer of tip-burn. As a new type plant hormone, brassinolide (BR) is involved in regulating a variety of biotic and abiotic stresses. To explore the alleviation role of BR in tip-burn caused by Ca²⁺ deficiency, a hydroponic experiment was conducted to study the relationship between BR and Ca²⁺ absorption and transport. The results showed that foliar spraying with 0.5 µM BR significantly reduced tip-burn incidence rate and disease index of mini Chinese cabbage caused by Ca²⁺ deficiency. Moreover, the dynamic monitoring results of tip-burn incidence rate showed that the value reached the highest on the ninth day after treatment. BR promoted the Ca²⁺ transport from roots to shoots and from outer leaves to inner leaves by increasing the activities of Ca²⁺-ATPase and H⁺-ATPase as well as the total ATP content, which provided power for Ca²⁺ transport. In addition, exogenous BR upregulated the relative expression levels of BrACA4, BrACA11, BrECA1, BrECA3, BrECA4, BrCAX1, BrCAS and BrCRT2, whereas Ca²⁺ deficiency induced down-regulation. In conclusion, exogenous BR can alleviate the Ca²⁺-deficiency induced tip-burn of mini Chinese cabbage by promoting the transport and distribution of Ca²⁺.

The vacuolar H+/Ca transporter CAX1 participates in submergence and anoxia stress responses

A plant's oxygen supply can vary from normal (normoxia) to total depletion (anoxia). Tolerance to anoxia is relevant to wetland species, rice (Oryza sativa) cultivation, and submergence tolerance of crops. Decoding and transmitting calcium (Ca) signals may be an important component to anoxia tolerance; however, the contribution of intracellular Ca transporters to this process is poorly understood. Four functional cation/proton exchangers (CAX1-4) in Arabidopsis (Arabidopsis thaliana) help regulate Ca homeostasis around the vacuole. Our results demonstrate that cax1 mutants are more tolerant to both anoxic conditions and submergence. Using phenotypic measurements, RNA-sequencing, and proteomic approaches, we identified cax1-mediated anoxia changes that phenocopy changes present in anoxia-tolerant crops: altered metabolic processes, diminished reactive oxygen species production post anoxia, and altered hormone signaling. Comparing wild-type and cax1 expressing genetically encoded Ca indicators demonstrated altered cytosolic Ca signals in cax1 during reoxygenation. Anoxia-induced Ca signals around the plant vacuole are involved in the control of numerous signaling events related to adaptation to low oxygen stress. This work suggests that cax1 anoxia response pathway could be engineered to circumvent the adverse effects of flooding that impair production agriculture.

Calcium channels and transporters: Roles in response to biotic and abiotic stresses

Calcium (Ca2+) serves as a ubiquitous second messenger by mediating various signaling pathways and responding to numerous environmental conditions in eukaryotes. Therefore, plant cells have developed complex mechanisms of Ca2+ communication across the membrane, receiving the message from their surroundings and transducing the information into cells and organelles. A wide range of biotic and abiotic stresses cause the increase in [Ca2+]cyt as a result of the Ca2+ influx permitted by membrane-localized Ca2+ permeable cation channels such as CYCLIC NUCLEOTIDE-GATE CHANNELs (CNGCs), and voltage-dependent HYPERPOLARIZATION-ACTIVATED CALCIUM2+ PERMEABLE CHANNELs (HACCs), as well as GLUTAMATE RECEPTOR-LIKE RECEPTORs (GLRs) and TWO-PORE CHANNELs (TPCs). Recently, resistosomes formed by some NUCLEOTIDE-BINDING LEUCINE-RICH REPEAT RECEPTORs (NLRs) are also proposed as a new type of Ca2+ permeable cation channels. On the contrary, some Ca2+ transporting membrane proteins, mainly Ca2+-ATPase and Ca2+/H+ exchangers, are involved in Ca2+ efflux for removal of the excessive [Ca2+]cyt in order to maintain the Ca2+ homeostasis in cells. The Ca2+ efflux mechanisms mediate the wide ranges of cellular activities responding to external and internal stimuli. In this review, we will summarize and discuss the recent discoveries of various membrane proteins involved in Ca2+ influx and efflux which play an essential role in fine-tuning the processing of information for plant responses to abiotic and biotic stresses.

Beneficial Effects of Sodium Nitroprusside on the Aroma, Flavors, and Anthocyanin Accumulation in Blood Orange Fruits

The quality of Tarocco blood orange (Citrus sinensis (L.) Osbeck), which has been cultivated for many years, has degraded substantially. Decreased sugar content, decreased blood color, and increased sour flavor have developed as a result. To improve fruit quality, we studied the effects of bagging and sodium nitroprusside, as a nitric oxide (NO) donor, on the fruit quality of Tarocco blood orange two months before picking. The results showed that NO treatment effectively improved the content of total soluble solids and limonene in the fruit, as well as the color and hardness of the fruit, but reduced the tannin content. It also increased the contents of soluble sugar, fructose, sucrose, vitamin C, amino acids, and mineral elements. NO treatment inhibited the activities of polygalacturonase and pectin esterase, delayed the degradation of protopectin, and promoted the accumulation of anthocyanins, total flavonoids, and flavonoids synthesis. Thus, NO treatment improved the aroma, flavors, and physical properties of blood orange fruit.

Infection by phloem-limited phytoplasma affects mineral nutrient homeostasis in tomato leaf tissues

Phytoplasmas are sieve-elements restricted wall-less, pleomorphic pathogenic microorganisms causing devastating damage to over 700 plant species worldwide. The invasion of sieve elements by phytoplasmas has several consequences on nutrient transport and metabolism, anyway studies about changes of the mineral-nutrient profile following phytoplasma infections are scarce and offer contrasting results. Here, we examined changes in macro- and micronutrient concentration in tomato plant upon 'Candidatus Phytoplasma solani' infection. To investigate possible effects of 'Ca. P. solani' infection on mineral element allocation, the mineral elements were separately analysed in leaf midrib, leaf lamina and root. Moreover, we focused our analysis on the transcriptional regulation of genes encoding trans-membrane transporters of mineral nutrients. To this aim, a manually curated inventory of differentially expressed genes encoding transporters in tomato leaf midribs was mined from the transcriptional profile of healthy and infected tomato leaf midribs. Results highlighted changes in ion homeostasis in the host plant, and significant modulations at transcriptional level of genes encoding ion transporters and channels.

Targeting calcium-mediated inter-organellar crosstalk in cardiac diseases

INTRODUCTION

Abnormal calcium signaling between organelles such as the sarcoplasmic reticulum (SR), mitochondria and lysosomes is a key feature of heart diseases. Calcium serves as a secondary messenger mediating inter-organellar crosstalk, essential for maintaining the cardiomyocyte function.

AREAS COVERED

This article examines the available literature related to calcium channels and transporters involved in inter-organellar calcium signaling. The SR calcium-release channels ryanodine receptor type-2 (RyR2) and inositol 1,4,5-trisphosphate receptor (IP3R), and calcium-transporter SR/ER-ATPase 2a (SERCA2a) are illuminated. The roles of mitochondrial voltage-dependent anion channels (VDAC), the mitochondria Ca2+ uniporter complex (MCUC), and the lysosomal H+/Ca2+ exchanger, two pore channels (TPC), and transient receptor potential mucolipin (TRPML) are discussed. Furthermore, recent studies showing calcium-mediated crosstalk between the SR, mitochondria, and lysosomes as well as how this crosstalk is dysregulated in cardiac diseases are placed under the spotlight.

EXPERT OPINION

Enhanced SR calcium release via RyR2 and reduced SR reuptake via SERCA2a, increased VDAC and MCUC-mediated calcium uptake into mitochondria, and enhanced lysosomal calcium-release via lysosomal TPC and TRPML may all contribute to aberrant calcium homeostasis causing heart disease. While mechanisms of this crosstalk need to be studied further, interventions targeting these calcium channels or combinations thereof might represent a promising therapeutic strategy.

ZEITLUPE Promotes ABA-Induced Stomatal Closure in Arabidopsis and Populus

Plants balance water availability with gas exchange and photosynthesis by controlling stomatal aperture. This control is regulated in part by the circadian clock, but it remains unclear how signalling pathways of daily rhythms are integrated into stress responses. The serine/threonine protein kinase OPEN STOMATA 1 (OST1) contributes to the regulation of stomatal closure via activation of S-type anion channels. OST1 also mediates gene regulation in response to ABA/drought stress. We show that ZEITLUPE (ZTL), a blue light photoreceptor and clock component, also regulates ABA-induced stomatal closure in Arabidopsis thaliana, establishing a link between clock and ABA-signalling pathways. ZTL sustains expression of OST1 and ABA-signalling genes. Stomatal closure in response to ABA is reduced in ztl mutants, which maintain wider stomatal apertures and show higher rates of gas exchange and water loss than wild-type plants. Detached rosette leaf assays revealed a stronger water loss phenotype in ztl-3, ost1-3 double mutants, indicating that ZTL and OST1 contributed synergistically to the control of stomatal aperture. Experimental studies of Populus sp., revealed that ZTL regulated the circadian clock and stomata, indicating ZTL function was similar in these trees and Arabidopsis. PSEUDO-RESPONSE REGULATOR 5 (PRR5), a known target of ZTL, affects ABA-induced responses, including stomatal regulation. Like ZTL, PRR5 interacted physically with OST1 and contributed to the integration of ABA responses with circadian clock signalling. This suggests a novel mechanism whereby the PRR proteins-which are expressed from dawn to dusk-interact with OST1 to mediate ABA-dependent plant responses to reduce water loss in time of stress.

Calcium Regulates Growth and Nutrient Absorption in Poplar Seedlings

As a crucial element for plants, calcium (Ca) is involved in photosynthesis and nutrient absorption, and affects the growth of plants. Poplar is an important economic forest and shelter forest species in China. However, the optimum calcium concentration for its growth is still unclear. Herein, we investigated the growth, biomass, photosynthetic pigments, photosynthetic parameters and products, chlorophyll fluorescence parameters, water use efficiency (iWUE), and antioxidant enzyme activity of "Liao Hu NO.1" poplar (P. simonii × P. euphratica) seedlings at 0, 2.5, 5, 10, and 20 mmol·L^-1 concentrations of Ca²⁺, and further studied the absorption, distribution, and utilization of nutrient elements (C, N, P, K, and Ca) in plants. We found that with increasing calcium gradient, plant height and diameter; root, stem, leaf, and total biomasses; net photosynthetic rate (Pn); stomatal conductance (Gs); intercellular carbon dioxide (Ci) level; transpiration rate (Tr); Fv/Fm ratio; Fv/F0 ratio; chlorophyll-a; chlorophyll-b; soluble sugar and starch content; superoxide dismutase (SOD), catalase (CAT), and peroxidase (POD) levels; and long-term water use efficiency (iWUE) of poplar seedlings first increased and then decreased. These parameters attained maximum values when the calcium concentration was 5 mmol·L^-1, which was significantly different from the other treatments (P < 0.05). Moreover, a suitable Ca²⁺ level promoted the absorption of C, N, P, K, and Ca by various organs of poplar seedlings. The absorption of C, N, P, and K increased first and then decreased with the increased calcium concentration, but the optimum calcium concentrations for the absorption of different elements by different organs were different, and the calcium concentration in leaves, stems, and roots increased gradually. Furthermore, the increase in exogenous calcium content led to a decreasing trend in the C/N ratio in different organs of poplar seedlings. C/P and N/P ratios showed different results in different parts, and only the N/P ratio in leaves showed a significant positive correlation with Ca²⁺ concentration. In conclusion, the results of this study indicate that 5 mmol·L^-1 concentration of Ca²⁺ is the optimal level, as it increased growth by enhancing photosynthesis, stress resistance, and nutrient absorption.

New insights into the evolution of SPX gene family from algae to legumes; a focus on soybean

BACKGROUND

SPX-containing proteins have been known as key players in phosphate signaling and homeostasis. In Arabidopsis and rice, functions of some SPXs have been characterized, but little is known about their function in other plants, especially in the legumes.

RESULTS

We analyzed SPX gene family evolution in legumes and in a number of key species from algae to angiosperms. We found that SPX harboring proteins showed fluctuations in domain fusions from algae to the angiosperms with, finally, four classes appearing and being retained in the land plants. Despite these fluctuations, Lysine Surface Cluster (KSC), and the third residue of Phosphate Binding Sites (PBS) showed complete conservation in almost all of SPXs except few proteins in Selaginella moellendorffii and Papaver sumniferum, suggesting they might have different ligand preferences. In addition, we found that the WGD/segmentally or dispersed duplication types were the most frequent contributors to the SPX expansion, and that there is a positive correlation between the amount of WGD contribution to the SPX expansion in individual species and its number of EXS genes. We could also reveal that except SPX class genes, other classes lost the collinearity relationships among Arabidopsis and legume genomes. The sub- or neo-functionalization of the duplicated genes in the legumes makes it difficult to find the functional orthologous genes. Therefore, we used two different methods to identify functional orthologs in soybean and Medicago. High variance in the dynamic and spatial expression pattern of GmSPXs proved the new or sub-functionalization in the paralogs.

CONCLUSION

This comprehensive analysis revealed how SPX gene family evolved from algae to legumes and also discovered several new domains fused to SPX domain in algae. In addition, we hypothesized that there different phosphate sensing mechanisms might occur in S. moellendorffii and P. sumniferum. Finally, we predicted putative functional orthologs of AtSPXs in the legumes, especially, orthologs of AtPHO1, involved in long-distance Pi transportation. These findings help to understand evolution of phosphate signaling and might underpin development of new legume varieties with improved phosphate use efficiency.

Transport, functions, and interaction of calcium and manganese in plant organellar compartments

Calcium (Ca2+) and manganese (Mn2+) are essential elements for plants and have similar ionic radii and binding coordination. They are assigned specific functions within organelles, but share many transport mechanisms to cross organellar membranes. Despite their points of interaction, those elements are usually investigated and reviewed separately. This review takes them out of this isolation. It highlights our current mechanistic understanding and points to open questions of their functions, their transport, and their interplay in the endoplasmic reticulum (ER), vesicular compartments (Golgi apparatus, trans-Golgi network, pre-vacuolar compartment), vacuoles, chloroplasts, mitochondria, and peroxisomes. Complex processes demanding these cations, such as Mn2+-dependent glycosylation or systemic Ca2+ signaling, are covered in some detail if they have not been reviewed recently or if recent findings add to current models. The function of Ca2+ as signaling agent released from organelles into the cytosol and within the organelles themselves is a recurrent theme of this review, again keeping the interference by Mn2+ in mind. The involvement of organellar channels [e.g. glutamate receptor-likes (GLR), cyclic nucleotide-gated channels (CNGC), mitochondrial conductivity units (MCU), and two-pore channel1 (TPC1)], transporters (e.g. natural resistance-associated macrophage proteins (NRAMP), Ca2+ exchangers (CAX), metal tolerance proteins (MTP), and bivalent cation transporters (BICAT)], and pumps [autoinhibited Ca2+-ATPases (ACA) and ER Ca2+-ATPases (ECA)] in the import and export of organellar Ca2+ and Mn2+ is scrutinized, whereby current controversial issues are pointed out. Mechanisms in animals and yeast are taken into account where they may provide a blueprint for processes in plants, in particular, with respect to tunable molecular mechanisms of Ca2+ versus Mn2+ selectivity.

Identification of genes related to tipburn resistance in Chinese cabbage and preliminary exploration of its molecular mechanism

BACKGROUND

Tipburn, also known as leaf tip necrosis, is a severe issue in Chinese cabbage production. One known cause is that plants are unable to provide adequate Ca²⁺ to rapidly expanding leaves. Bacterial infection is also a contributing factor. Different cultivars have varying degrees of tolerance to tipburn. Two inbred lines of Chinese cabbage were employed as resources in this research.

RESULTS

We determined that the inbred line 'J39290' was the tipburn resistant material and the inbred line 'J95822' was the tipburn sensitive material based on the severity of tipburn, and the integrity of cell membrane structure. Ca²⁺ concentration measurements revealed no significant difference in Ca²⁺ concentration between the two materials inner leaves. Transcriptome sequencing technology was also used to find the differentially expressed genes (DEGs) of 'J95822' and 'J39290', and there was no significant difference in the previously reported Ca²⁺ uptake and transport related genes in the two materials. However, it is evident through DEG screening and classification that 23 genes are highly linked to plant-pathogen interactions, and they encode three different types of proteins: CaM/CML, Rboh, and CDPK. These 23 genes mainly function through Ca²⁺-CaM/CML-CDPK signal pathway based on KEGG pathway analysis, protein interaction prediction, and quantitative real-time PCR (qRT-PCR) of key genes.

CONCLUSIONS

By analyzing the Ca²⁺ concentration in the above two materials, the transcription of previously reported genes related to Ca²⁺ uptake and transport, the functional annotation and KEGG pathway of DEGs, it was found that Ca²⁺ deficiency was not the main cause of tipburn in 'J95822', but was probably caused by bacterial infection. This study lays a theoretical foundation for exploring the molecular mechanism of resistance to tipburn in Chinese cabbage, and has important guiding significance for genetics and breeding.

Recent advances in physiological and molecular mechanisms of heavy metal accumulation in plants

Among abiotic stress, the toxicity of metals impacts negatively on plants' growth and productivity. This toxicity promotes various perturbations in plants at different levels. To withstand stress, plants involve efficient mechanisms through the implication of various signaling pathways. These pathways enhance the expression of many target genes among them gene coding for metal transporters. Various metal transporters which are localized at the plasma membrane and/or at the tonoplast are crucial in metal stress response. Furthermore, metal detoxification is provided by metal-binding proteins like phytochelatins and metallothioneins. The understanding of the molecular basis of metal toxicities signaling pathways and tolerance mechanisms is crucial for genetic engineering to produce transgenic plants that enhance phytoremediation. This review presents an overview of the recent advances in our understanding of metal stress response. Firstly, we described the effect of metal stress on plants. Then, we highlight the mechanisms involved in metal detoxification and the importance of the regulation in the response to heavy metal stress. Finally, we mentioned the importance of genetic engineering for enhancing the phytoremediation technique. In the end, the response to heavy metal stress is complex and implicates various components. Thus, further studies are needed to better understand the mechanisms involved in response to this abiotic stress.

Guard cell endomembrane Ca2+-ATPases underpin a 'carbon memory' of photosynthetic assimilation that impacts on water-use efficiency

Stomata of most plants close to preserve water when the demand for CO₂ by photosynthesis is reduced. Stomatal responses are slow compared with photosynthesis, and this kinetic difference erodes assimilation and water-use efficiency under fluctuating light. Despite a deep knowledge of guard cells that regulate the stoma, efforts to enhance stomatal kinetics are limited by our understanding of its control by foliar CO₂. Guided by mechanistic modelling that incorporates foliar CO₂ diffusion and mesophyll photosynthesis, here we uncover a central role for endomembrane Ca²⁺ stores in guard cell responsiveness to fluctuating light and CO₂. Modelling predicted and experiments demonstrated a delay in Ca²⁺ cycling that was enhanced by endomembrane Ca²⁺-ATPase mutants, altering stomatal conductance and reducing assimilation and water-use efficiency. Our findings illustrate the power of modelling to bridge the gap from the guard cell to whole-plant photosynthesis, and they demonstrate an unforeseen latency, or 'carbon memory', of guard cells that affects stomatal dynamics, photosynthesis and water-use efficiency.

High-Resolution Microstructure Analysis of Cork Spot Disordered Pear Fruit "Akizuki" (Pyrus pyrifolia Nakai) Using X-Ray CT

Cork spot is one of the most damaging physiological disorders in pear fruit, causing considerable economic loss every year. However, the mechanism of cork spot occurrence requires further examination. In this study, X-ray CT scanning was applied to analyze the microstructure of pear fruit "Akizuki" (Pyrus pyrifolia), a cultivar susceptible to cork spot disorder, to elucidate the fruit texture alteration between healthy and cork spotted fruit. Results showed that cork spotted fruit had much higher porosity (9.37%) than healthy fruit (3.52%). Reconstructed three-dimensional (3D) network skeleton models showed highly branched pore channels in cork spotted fruit and a low degree of pore connectivity in healthy fruit. Even in areas of disordered fruit without cork spot, the pore throat diameter, pore length, and coordinated core number (i.e., 77, 160, and 16, respectively) were much higher than that of healthy fruit. The structure analysis of fruit core showed that core deformation only occurred in cork spotted fruit. A much more highly branched network was observed in cork spotted fruit cores compared with healthy fruit cores. High-resolution observation of flesh tissue directly demonstrated that pore size in cork spotted fruit (87 μm) was four times larger than that of healthy fruit (22 μm). Altered expression of genes related to Ca²⁺ transport and the uneven distribution of intracellular Ca²⁺ were also shown to associate with the development of cork spot disorder. Our results suggest that flesh tissue damage likely occurred prior to the initiation of cork spot. The dysfunction of long-distance and transmembrane Ca²⁺ transport channels could be responsible for the imbalanced distribution of Ca²⁺ inside the fruit, thus resulting in the development of cork spot.

The CaCA superfamily genes in Saccharum: comparative analysis and their functional implications in response to biotic and abiotic stress

BACKGROUND

In plants, Calcium (Ca2+) acts as a universal messenger in various signal transduction pathways, including responses to biotic and abiotic stresses and regulation of cellular and developmental processes. The Ca2+/cation antiporter (CaCA) superfamily proteins play vital roles in the transport of Ca2+ and/or other cations. However, the characteristics of these superfamily members in Saccharum and their evolutionary and functional implications have remained unclear.

RESULTS

A total of 34 CaCA genes in Saccharum spontaneum, 5 CaCA genes in Saccharum spp. R570, and 14 CaCA genes in Sorghum bicolor were identified and characterized. These genes consisted of the H+/cation exchanger (CAX), cation/Ca2+ exchanger (CCX), EF-hand / CAX (EFCAX), and Mg2+/H+ exchanger (MHX) families, among which the CCX and EFCAX could be classified into three groups while the CAX could be divided into two groups. The exon/intron structures and motif compositions suggested that the members in the same group were highly conserved. Synteny analysis of CaCAs established their orthologous and paralogous relationships among the superfamily in S. spontaneum, R570, and S. bicolor. The results of protein-protein interactions indicated that these CaCA proteins had direct or indirect interactions. Quantitative reverse transcription polymerase chain reaction (qRT-PCR) analysis demonstrated that most members of Saccharum CaCA genes exhibited a similar expression pattern in response to hormonal (abscisic acid, ABA) treatment but played various roles in response to biotic (Sporisorium scitamineum) and abiotic (cold) stresses. Furthermore, ScCAX4, a gene encoding a cytoplasm, plasma membrane and nucleus positioning protein, was isolated from sugarcane. This gene was constitutively expressed in different sugarcane tissues and its expression was only induced at 3 and 6 h time points after ABA treatment, however was inhibited and indued in the whole process under cold and S. scitamineum stresses, respectively.

CONCLUSIONS

This study systematically conducted comparative analyses of CaCA superfamily genes among S. spontaneum, R570, and S. bicolor, delineating their sequence and structure characteristics, classification, evolutionary history, and putative functions. These results not only provided rich gene resources for exploring the molecular mechanism of the CaCA superfamily genes but also offered guidance and reference for research on other gene families in Saccharum.

Relationships Between Leaf Carbon and Macronutrients Across Woody Species and Forest Ecosystems Highlight How Carbon Is Allocated to Leaf Structural Function

Stoichiometry of leaf macronutrients can provide insight into the tradeoffs between leaf structural and metabolic investments. Structural carbon (C) in cell walls is contained in lignin and polysaccharides (cellulose, hemicellulose, and pectins). Much of leaf calcium (Ca) and a fraction of magnesium (Mg) were further bounded with cell wall pectins. The macronutrients phosphorus (P), potassium (K), and nitrogen (N) are primarily involved in cell metabolic functions. There is limited information on the functional interrelations among leaf C and macronutrients, and the functional dimensions characterizing the leaf structural and metabolic tradeoffs are not widely appreciated. We investigated the relationships between leaf C and macronutrient (N, P, K, Ca, Mg) concentrations in two widespread broad-leaved deciduous woody species Quercus wutaishanica (90 individuals) and Betula platyphylla (47 individuals), and further tested the generality of the observed relationships in 222 woody eudicots from 15 forest ecosystems. In a subsample of 20 broad-leaved species, we also analyzed the relationships among C, Ca, lignin, and pectin concentrations in leaf cell walls. We found a significant leaf C-Ca tradeoff operating within and across species and across ecosystems. This basic relationship was explained by variations in the share of cell wall lignin and pectin investments at the cell scale. The C-Ca tradeoffs were mainly driven by soil pH and mean annual temperature and precipitation, suggesting that leaves were more economically built with less C and more Ca as soil pH increased and at lower temperature and lower precipitation. However, we did not detect consistent patterns among C-N, and C-Mg at different levels of biological organization, suggesting substantial plasticity in N and Mg distribution among cell organelles and cell protoplast and cell wall. We observed two major axes of macronutrient differentiation: the cell-wall structural axis consisting of protein-free C and Ca and the protoplasm metabolic axis consisting of P and K, underscoring the decoupling of structural and metabolic elements inherently linked with cell wall from protoplasm investment strategies. We conclude that the tradeoffs between leaf C and Ca highlight how carbon is allocated to leaf structural function and suggest that this might indicate biogeochemical niche differentiation of species.

Phylogenetic and ion-response analyses reveal a relationship between gene expansion and functional divergence in the Ca2+/cation antiporter family in Angiosperms

Plant CaCA superfamily genes with higher tendency to retain after WGD are more gene expression and function differentiated in ion-response. Plants and animals face different environmental stresses but share conserved Ca²⁺ signaling pathways, such as Ca²⁺/Cation transport. The Ca²⁺/cation antiporters superfamily (CaCAs) is an ancient and widespread family of ion-coupled cation transporters found in all kingdoms of life. We analyzed the molecular evolution progress of the family through comparative genomics and phylogenetics of CaCAs genes from plants and animals, grouping these genes into several families and clades, and identified multiple gene duplication retention events, particularly in the CAX (H⁺/cation exchanger), CCX (cation/Ca²⁺ exchanger), and NCL (Na⁺/Ca²⁺ exchanger-like) families. The tendency of duplication retention differs between families and gene clades. The gene duplication events were probably the result of whole-genome duplication (WGD) in plants and might have led to functional divergence. Tissue and ion-response expression analyses revealed that CaCAs genes with more highly differentiated expression patterns are more likely to be retained as duplicates than those with more conserved expression profiles. Phenotype of Arabidopsis thaliana mutants showed that loss of genes with a greater tendency to be retained after duplication resulted in more severe growth deficiency. CaCAs genes in salt-tolerant species tended to inherit the expression characteristics of their most recent common ancestral genes, with conservative ion-response expression. This study indicates a possible evolutionary scheme for cation transport and illustrates distinct fates and a mechanism for the evolution of gene duplicates. The increased copy numbers of genes and divergences in expression might have contributed to the divergent functions of CaCAs protein, allowing plants to cope with environmental stresses and adapt to a larger number of ecological niches.

Accumulation of phosphorus and calcium in different cells protects the phosphorus-hyperaccumulator Ptilotus exaltatus from phosphorus toxicity in high-phosphorus soils

Ptilotus exaltatus accumulates phosphorus (P) to > 40 mg g^-1 without toxicity symptoms, while Kennedia prostrata is intolerant of increased P supply. What physiological mechanisms underlie this difference and protect P. exaltatus from P toxicity? Ptilotus exaltatus and K. prostrata were grown in a sandy soil with low-P, high-P and P-pulse treatments. Both species hyperaccumulated P (>20 mg g^-1) under high-P and P-pulse treatments; shoot dry weight was unchanged for P. exaltatus, but decreased by >50% for K. prostrata. Under high-P, in young fully-expanded leaves, both species accumulated P predominantly as inorganic P. However, P. exaltatus preferentially allocated P to mesophyll cells and stored calcium (Ca) as occasional crystals in specific lower mesophyll cells, separate from P, while K. prostrata preferentially allocated P to epidermal and spongy mesophyll cells, but co-located P and Ca in palisade mesophyll cells where granules with high [P] and [Ca] were evident. Mesophyll cellular [P] correlated positively with [potassium] for both species, and negatively with [sulfur] for P. exaltatus. Thus, P. exaltatus tolerated a very high leaf [inorganic P] (17 mg g^-1), associated with P and Ca allocation to different cell types and formation of Ca crystals, thereby avoiding deleterious precipitation of Ca₃(PO₄)₂. It also showed enhanced [potassium] and decreased [sulfur] to balance high cellular [P]. Phosphorus toxicity in K. prostrata arose from co-location of Ca and P in palisade mesophyll cells. This study advances understanding of leaf physiological mechanisms for high P tolerance in a P-hyperaccumulator and indicates P. exaltatus as a promising candidate for P-phytoextraction.

Description of AtCAX4 in Response to Abiotic Stress in Arabidopsis

High-capacity tonoplast cation/H⁺ antiport in plants is partially mediated by a family of CAX transporters. Previous studies have reported that CAX activity is affected by an N-terminal autoinhibitory region. CAXs may be present as heterodimers in plant cells, and this phenomenon necessitates further study. In this study, we demonstrate that there is an interaction between CAX4 and CAX1 as determined by the use of a yeast two-hybrid system and a bimolecular fluorescence complementation assay. More specifically, the N-terminal of CAX4 interacts with CAX1. We further observed the over-expression and either a single or double mutant of CAX1 and CAX4 in response to abiotic stress in Arabidopsis. These results suggest that CAX1 and CAX4 can interact to form a heterodimer, and the N-terminal regions of CAX4 play important roles in vivo; this may provide a foundation for a deep study of CAX4 function in the future.

Calcium Binding by Arabinogalactan Polysaccharides Is Important for Normal Plant Development

Arabinogalactan proteins (AGPs) are a family of plant extracellular proteoglycans involved in many physiological events. AGPs are often anchored to the extracellular side of the plasma membrane and are highly glycosylated with arabinogalactan (AG) polysaccharides, but the molecular function of this glycosylation remains largely unknown. The β-linked glucuronic acid (GlcA) residues in AG polysaccharides have been shown in vitro to bind to calcium in a pH-dependent manner. Here, we used Arabidopsis (Arabidopsis thaliana) mutants in four AG β-glucuronyltransferases (GlcAT14A, -B, -D, and -E) to understand the role of glucuronidation of AG. AG isolated from glcat14 triple mutants had a strong reduction in glucuronidation. AG from a glcat14a/b/d triple mutant had lower calcium binding capacity in vitro than AG from wild-type plants. Some mutants had multiple developmental defects such as reduced trichome branching. glcat14a/b/e triple mutant plants had severely limited seedling growth and were sterile, and the propagation of calcium waves was perturbed in roots. Several of the developmental phenotypes were suppressed by increasing the calcium concentration in the growth medium. Our results show that AG glucuronidation is crucial for multiple developmental processes in plants and suggest that a function of AGPs might be to bind and release cell-surface apoplastic calcium.

Extracellular Ca2+ induces desensitized cytosolic Ca2+ rise sensitive to phospholipase C inhibitor which suppresses root growth with Ca2+ dependence

Calcium (Ca) is an essential element for all organisms. In animal cells, the plasma membrane-localized Ca receptor CaSR coupled to a phospholipase C (PLC)-dependent signaling cascade monitors extracellular Ca²⁺ concentrations ([Ca²⁺]_ext) and responds with increases in cytosolic calcium concentrations ([Ca²⁺]_cyt). Plant roots encounter variable soil conditions, but how they sense changes in [Ca²⁺]_ext is largely unknown. In this study, we demonstrate that increasing [Ca²⁺]_ext evokes a transient increase in [Ca²⁺] in the cytosol, mitochondria, and nuclei of Arabidopsis thaliana root cells. These increases were strongly desensitized to repeat applications of [Ca²⁺]_ext, a typical feature of receptor-mediated cellular signaling in animal and plant cells. Treatment with gadolinium (Gd³⁺), a CaSR activator in animal cells, induced concentration-dependent increases in [Ca²⁺]_cyt in roots, which showed self-desensitization and cross-desensitization to [Ca²⁺]_ext-induced increases in [Ca²⁺]_cyt (EICC). EICC was sensitive to extracellular H⁺, K⁺, Na⁺, and Mg²⁺ levels. Treatment with the PLC inhibitor neomycin suppressed EICC and Ca accumulation in roots. The inhibitory effect of neomycin on root elongation was fully rescued by increasing [Ca²⁺]_ext but not [Mg²⁺] or [K⁺] in the growth medium. These results suggest that [Ca²⁺]_ext and the movement of Ca²⁺ into the cytosol of plant roots are regulated by a receptor-mediated signaling pathway involving PLC.

An SMU Splicing Factor Complex Within Nuclear Speckles Contributes to Magnesium Homeostasis in Arabidopsis

Mg2+ is among the most abundant divalent cations in living cells. In plants, investigations on magnesium (Mg) homeostasis are restricted to the functional characterization of Mg2+ transporters. Here, we demonstrate that the splicing factors SUPPRESSORS OF MEC-8 AND UNC-52 1 (SMU1) and SMU2 mediate Mg homeostasis in Arabidopsis (Arabidopsis thaliana). A low-Mg sensitive Arabidopsis mutant was isolated, and the causal gene was identified as SMU1 Disruption of SMU2, a protein that can form a complex with SMU1, resulted in a similar low-Mg sensitive phenotype. In both mutants, an Mg2+ transporter gene, Mitochondrial RNA Splicing 2 (MRS2-7), showed altered splicing patterns. Genetic evidence indicated that MRS2-7 functions in the same pathway as SMU1 and SMU2 for low-Mg adaptation. In contrast with previous results showing that the SMU1-SMU2 complex is the active form in RNA splicing, MRS2-7 splicing was promoted in the smu2 mutant overexpressing SMU1, indicating that complex formation is not a prerequisite for the splicing. We found here that formation of the SMU1-SMU2 complex is an essential step for their compartmentation in the nuclear speckles, a type of nuclear body enriched with proteins that participate in various aspects of RNA metabolism. Taken together, our study reveals the involvement of the SMU splicing factors in plant Mg homeostasis and provides evidence that complex formation is required for their intranuclear compartmentation.

The grapevine CAX-interacting protein VvCXIP4 is exported from the nucleus to activate the tonoplast Ca2+/H+ exchanger VvCAX3

The nuclear-localized CAX-interacting protein VvCXIP4 is exported to the cytosol after a Ca²⁺ pulse, to activate the tonoplast-localized Ca²⁺/H⁺ exchanger VvCAX3. Vacuolar cation/H⁺ exchangers (CAXs) have long been recognized as 'housekeeping' components in cellular Ca²⁺ and trace metal homeostasis, being involved in a range of key cellular and physiological processes. However, the mechanisms that drive functional activation of the transporters are largely unknown. In the present study, we investigated the function of a putative grapevine CAX-interacting protein, VvCXIP4, by testing its ability to activate VvCAX3, previously characterized as a tonoplast-localized Ca²⁺/H⁺ exchanger. VvCAX3 contains an autoinhibitory domain that drives inactivation of the transporter and thus, is incapable of suppressing the Ca²⁺-hypersensitive phenotype of the S. cerevisiae mutant K667. In this study, the co-expression of VvCXIP4 and VvCAX3 in this strain efficiently rescued its growth defect at high Ca²⁺ levels. Flow cytometry experiments showed that yeast harboring both proteins effectively accumulated higher Ca²⁺ levels than cells expressing each of the proteins separately. Bimolecular fluorescence complementation (BiFC) assays allowed visualization of the direct interaction between the proteins in tobacco plants and in yeast, and also showed the self-interaction of VvCAX3 but not of VvCXIP4. Subcellular localization studies showed that, despite being primarily localized to the nucleus, VvCXIP4 is able to move to other cell compartments upon a Ca²⁺ stimulus, becoming prone to interaction with the tonoplast-localized VvCAX3. qPCR analysis showed that both genes are more expressed in grapevine stems and leaves, followed by the roots, and that the steady-state transcript levels were higher in the pulp than in the skin of grape berries. Also, both VvCXIP4 and VvCAX3 were upregulated by Ca²⁺ and Na⁺, indicating they share common regulatory mechanisms. However, VvCXIP4 was also upregulated by Li⁺, Cu²⁺ and Mn²⁺, and its expression increased steadily throughout grape berry development, contrary to VvCAX3, suggesting additional physiological roles for VvCXIP4, including the regulation of VvCAXs not yet functionally characterized. The main novelty of the present study was the demonstration of physical interaction between CXIP and CAX proteins from a woody plant model by BiFC assays, demonstrating the intracellular mobilization of CXIPs in response to Ca²⁺.

Plant tissue succulence engineering improves water-use efficiency, water-deficit stress attenuation and salinity tolerance in Arabidopsis

Tissue succulence (ratio of tissue water/leaf area or dry mass) or the ability to store water within living tissues is among the most successful adaptations to drought in the plant kingdom. This taxonomically widespread adaptation helps plants avoid the damaging effects of drought, and is often associated with the occupancy of epiphytic, epilithic, semi-arid and arid environments. Tissue succulence was engineered in Arabidopsis thaliana by overexpression of a codon-optimized helix-loop-helix transcription factor (VvCEB1_opt ) from wine grape involved in the cell expansion phase of berry development. VvCEB1_opt -overexpressing lines displayed significant increases in cell size, succulence and decreased intercellular air space. VvCEB1_opt -overexpressing lines showed increased instantaneous and integrated water-use efficiency (WUE) due to reduced stomatal conductance caused by reduced stomatal aperture and density resulting in increased attenuation of water-deficit stress. VvCEB1_opt -overexpressing lines also showed increased salinity tolerance due to reduced salinity uptake and dilution of internal Na⁺ and Cl^- as well as other ions. Alterations in transporter activities were further suggested by media and apoplastic acidification, hygromycin B tolerance and changes in relative transcript abundance patterns of various transporters with known functions in salinity tolerance. Engineered tissue succulence might provide an effective strategy for improving WUE, drought avoidance or attenuation, salinity tolerance, and for crassulacean acid metabolism biodesign.

Loss of proton/calcium exchange 1 results in the activation of plant defense and accelerated senescence in Arabidopsis

Cytosolic Ca²⁺ increases in response to many stimuli. CAX1 (H⁺/Ca²⁺ exchanger 1) maintains calcium homeostasis by transporting calcium from the cytosol to vacuoles. Here, we determined that the cax1 mutant exhibits enhanced resistance against both an avirulent biotrophic pathogen Pst-avrRpm1 (Pseudomonas syringae pv tomato DC3000 avrRpm1), and a necrotrophic pathogen, B. cinerea (Botrytis cinerea). The defense hormone SA (salicylic acid) and phytoalexin scopoletin, which fight against biotrophs and necrotrophs respectively, accumulated more in cax1 than wild-type. Moreover, the cax1 mutant exhibited early senescence after exogenous Ca²⁺ application. The accelerated senescence in the cax1 mutant was dependent on SID2 (salicylic acid induction deficient 2) but not on NPR1 (nonexpressor of pathogenesis-related genes1). Additionally, the introduction of CAX1 into the cax1 mutant resulted in phenotypes similar to that of wild-type in terms of Ca²⁺-conditioned senescence and Pst-avrRpm1 and B. cinerea infections. However, disruption of CAX3, the homolog of CAX1, did not produce an obvious phenotype. Moreover, exogenous Ca²⁺ application on plants resulted in increased resistance to both Pst-avrRpm1 and B. cinerea. Therefore, we conclude that the disruption of CAX1, but not CAX3, causes the activation of pathogen defense mechanisms, probably through the manipulation of calcium homeostasis or other signals.