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Summary Flooding represents a major threat to global agricultural productivity and food security, but plants are capable of deploying a suite of adaptive responses that can lead to short‐ or longer‐term survival to this stress. One cellular pathway thought to help coordinate these responses is via flooding‐triggered Ca²⁺signaling.We have mined publicly available transcriptomic data from Arabidopsis subjected to flooding or low oxygen stress to identify rapidly upregulated, Ca²⁺‐related transcripts. We then focused on transporters likely to modulate Ca²⁺signals. Candidates emerging from this analysis includedAUTOINHIBITED Ca²⁺ATPASE 1andCATION EXCHANGER 2. We therefore assayed mutants in these genes for flooding sensitivity at levels from growth to patterns of gene expression and the kinetics of flooding‐related Ca²⁺changes.Knockout mutants inCAX2especially showed enhanced survival to soil waterlogging coupled with suppressed induction of many marker genes for hypoxic response and constitutive activation of others.CAX2mutants also generated larger and more sustained Ca²⁺signals in response to both flooding and hypoxic challenges.CAX2 is a Ca²⁺transporter located on the tonoplast, and so these results are consistent with an important role for vacuolar Ca²⁺transport in the signaling systems that trigger flooding response. 

Iron- and reactive oxygen species (ROS)-dependent ferroptosis occurs in plant cells. Ca²⁺acts as a conserved key mediator to control plant immune responses. Here, we report a novel role of cytoplasmic Ca²⁺influx regulating ferroptotic cell death in rice immunity using pharmacological approaches. High Ca²⁺influx triggered iron-dependent ROS accumulation, lipid peroxidation, and subsequent hypersensitive response (HR) cell death in rice (Oryza sativa). DuringMagnaporthe oryzaeinfection, 14 different Ca²⁺influx regulators altered Ca²⁺, ROS and Fe²⁺accumulation,glutathione reductase(GR) expression, glutathione (GSH) depletion and lipid peroxidation, leading to ferroptotic cell death in rice. High Ca²⁺levels inhibited the reduction of glutathione isulphide (GSSG) to GSHin vitro. Ca²⁺chelation by ethylene glycol-bis (2-aminoethylether)-N, N, N’, N’-tetra-acetic acid (EGTA) suppressed apoplastic Ca²⁺influx in rice leaf sheaths during infection. Blocking apoplastic Ca²⁺influx into the cytoplasm by Ca²⁺chelation effectively suppressed Ca²⁺-mediated iron-dependent ROS accumulation and ferroptotic cell death. By contrast, acibenzolar-S-methyl (ASM), a plant defense activator, significantly enhanced Ca²⁺influx, as well as ROS and iron accumulation to trigger ferroptotic cell death in rice. The cytoplasmic Ca²⁺influx through calcium-permeable cation channels, including the putative resistosomes, could mediate iron- and ROS-dependent ferroptotic cell death under reducedGRexpression levels in rice immune responses. 

Abstract A potential strategy to mitigate oxidative damage in plants is to increase the abundance of antioxidants, such as ascorbate (i.e. vitamin C). In Arabidopsis (A. thaliana), a rate-limiting step in ascorbate biosynthesis is a phosphorylase encoded by Vitamin C Defective 2 (VTC2). To specifically overexpress VTC2 (VTC2 OE) in pollen, the coding region was expressed using a promoter from a gene with ∼150-fold higher expression in pollen, leading to pollen grains with an eight-fold increased VTC2 mRNA. VTC2 OE resulted in a near-sterile phenotype with a 50-fold decrease in pollen transmission efficiency and a five-fold reduction in the number of seeds per silique. In vitro assays revealed pollen grains were more prone to bursting (greater than two-fold) or produced shorter, morphologically abnormal pollen tubes. The inclusion of a genetically encoded Ca2+ reporter, mCherry-GCaMP6fast (CGf), revealed pollen tubes with altered tip-focused Ca2+ dynamics and increased bursting frequency during periods of oscillatory and arrested growth. Despite these phenotypes, VTC2 OE pollen failed to show expected increases in ascorbate or reductions in reactive oxygen species, as measured using a redox-sensitive dye or a roGFP2. However, mRNA expression analyses revealed greater than two-fold reductions in mRNA encoding two enzymes critical to biosynthetic pathways related to cell walls or glyco-modifications of lipids and proteins: GDP-d-mannose pyrophosphorylase (GMP) and GDP-d-mannose 3′,5′ epimerase (GME). These results support a model in which the near-sterile defects resulting from VTC2 OE in pollen are associated with feedback mechanisms that can alter one or more signaling or metabolic pathways critical to pollen tube growth and fertility. 

Land plants evolved to quickly sense and adapt to temperature changes, such as hot days and cold nights. Given that calcium (Ca 2+ ) signaling networks are implicated in most abiotic stress responses, heat-triggered changes in cytosolic Ca 2+ were investigated in Arabidopsis leaves and pollen. Plants were engineered with a reporter called CGf, a ratiometric, genetically encoded Ca 2+ reporter with an m C herry reference domain fused to an intensiometric Ca 2+ reporter G CaMP6 f . Relative changes in [Ca 2+ ] cyt were estimated based on CGf’s apparent K D around 220 nM. The ratiometric output provided an opportunity to compare Ca 2+ dynamics between different tissues, cell types, or subcellular locations. In leaves, CGf detected heat-triggered cytosolic Ca 2+ signals, comprised of three different signatures showing similarly rapid rates of Ca 2+ influx followed by differing rates of efflux (50% durations ranging from 5 to 19 min). These heat-triggered Ca 2+ signals were approximately 1.5-fold greater in magnitude than blue light-triggered signals in the same leaves. In contrast, growing pollen tubes showed two different heat-triggered responses. Exposure to heat caused tip-focused steady growth [Ca 2+ ] cyt oscillations to shift to a pattern characteristic of a growth arrest (22%), or an almost undetectable [Ca 2+ ] cyt (78%). Together, these contrasting examples of heat-triggered Ca 2+ responses in leaves and pollen highlight the diversity of Ca 2+ signals in plants, inviting speculations about their differing kinetic features and biological functions. 

In natural ecosystems, plants are constantly exposed to changes in their surroundings as they grow, caused by a lifestyle that requires them to live where their seeds fall. Thus, plants strive to adapt and respond to changes in their exposed environment that change every moment. Heat stress that naturally occurs when plants grow in the summer or a tropical area adversely affects plants’ growth and poses a risk to plant development. When plants are subjected to heat stress, they recognize heat stress and respond using highly complex intracellular signaling systems such as reactive oxygen species (ROS). ROS was previously considered a byproduct that impairs plant growth. However, in recent studies, ROS gained attention for its function as a signaling molecule when plants respond to environmental stresses such as heat stress. In particular, ROS, produced in response to heat stress in various plant cell compartments such as mitochondria and chloroplasts, plays a crucial role as a signaling molecule that promotes plant growth and triggers subsequent downstream reactions. Therefore, this review aims to address the latest research trends and understandings, focusing on the function and role of ROS in responding and adapting plants to heat stress. 

Abstract Generating cellular Ca2+ signals requires coordinated transport activities from both Ca2+ influx and efflux pathways. In Arabidopsis (Arabidopsis thaliana), multiple efflux pathways exist, some of which involve Ca2+-pumps belonging to the Autoinhibited Ca2+-ATPase (ACA) family. Here, we show that ACA1, 2, and 7 localize to the endoplasmic reticulum (ER) and are important for plant growth and pollen fertility. While phenotypes for plants harboring single-gene knockouts (KOs) were weak or undetected, a triple KO of aca1/2/7 displayed a 2.6-fold decrease in pollen transmission efficiency, whereas inheritance through female gametes was normal. The triple KO also resulted in smaller rosettes showing a high frequency of lesions. Both vegetative and reproductive phenotypes were rescued by transgenes encoding either ACA1, 2, or 7, suggesting that all three isoforms are biochemically redundant. Lesions were suppressed by expression of a transgene encoding NahG, an enzyme that degrades salicylic acid (SA). Triple KO mutants showed elevated mRNA expression for two SA-inducible marker genes, Pathogenesis-related1 (PR1) and PR2. The aca1/2/7 lesion phenotype was similar but less severe than SA-dependent lesions associated with a double KO of vacuolar pumps aca4 and 11. Imaging of Ca2+ dynamics triggered by blue light or the pathogen elicitor flg22 revealed that aca1/2/7 mutants display Ca2+ transients with increased magnitudes and durations. Together, these results indicate that ER-localized ACAs play important roles in regulating Ca2+ signals, and that the loss of these pumps results in male fertility and vegetative growth deficiencies.