Function of endoplasmic reticulum calcium ATPase in innate immunity-mediated programmed cell death

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.

Calmodulin and calmodulin-like protein-mediated plant responses to biotic stresses

Plants have evolved a set of finely regulated mechanisms to respond to various biotic stresses. Transient changes in intracellular calcium (Ca2+ ) concentration have been well documented to act as cellular signals in coupling environmental stimuli to appropriate physiological responses with astonishing accuracy and specificity in plants. Calmodulins (CaMs) and calmodulin-like proteins (CMLs) are extensively characterized as important classes of Ca2+ sensors. The spatial-temporal coordination between Ca2+ transients, CaMs/CMLs and their target proteins is critical for plant responses to environmental stresses. Ca2+ -loaded CaMs/CMLs interact with and regulate a broad spectrum of target proteins, such as ion transporters (including channels, pumps, and antiporters), transcription factors, protein kinases, protein phosphatases, metabolic enzymes and proteins with unknown biological functions. This review focuses on mechanisms underlying how CaMs/CMLs are involved in the regulation of plant responses to diverse biotic stresses including pathogen infections and herbivore attacks. Recent discoveries of crucial functions of CaMs/CMLs and their target proteins in biotic stress resistance revealed through physiological, molecular, biochemical, and genetic analyses have been described, and intriguing insights into the CaM/CML-mediated regulatory network are proposed. Perspectives for future directions in understanding CaM/CML-mediated signalling pathways in plant responses to biotic stresses are discussed. The application of accumulated knowledge of CaM/CML-mediated signalling in biotic stress responses into crop cultivation would improve crop resistance to various biotic stresses and safeguard our food production in the future.

The calcium connection: exploring the intricacies of calcium signaling in plant-microbe interactions

The process of plant immune response is orchestrated by intracellular signaling molecules. Since plants are devoid of a humoral system, they develop extensive mechanism of pathogen recognition, signal perception, and intricate cell signaling for their protection from biotic and abiotic stresses. The pathogenic attack induces calcium ion accumulation in the plant cells, resulting in calcium signatures that regulate the synthesis of proteins of defense system. These calcium signatures induct different calcium dependent proteins such as calmodulins (CaMs), calcineurin B-like proteins (CBLs), calcium-dependent protein kinases (CDPKs) and other signaling molecules to orchestrate the complex defense signaling. Using advanced biotechnological tools, the role of Ca2+ signaling during plant-microbe interactions and the role of CaM/CMLs and CDPKs in plant defense mechanism has been revealed to some extent. The Emerging perspectives on calcium signaling in plant-microbe interactions suggest that this complex interplay could be harnessed to improve plant resistance against pathogenic microbes. We present here an overview of current understanding in calcium signatures during plant-microbe interaction so as to imbibe a future direction of research.

Plant Ca2+-ATPases: From biochemistry to signalling

Calcium (Ca2+)-ATPases are ATP-dependent enzymes that transport Ca2+ ions against their electrochemical gradient playing the fundamental biological function of keeping the free cytosolic Ca2+ concentration in the submicromolar range to prevent cytotoxic effects. In plants, type IIB autoinhibited Ca2+-ATPases (ACAs) are localised both at the plasma membrane and at the endomembranes including endoplasmic reticulum (ER) and tonoplast and their activity is primarily regulated by Ca2+-dependent mechanisms. Instead, type IIA ER-type Ca2+-ATPases (ECAs) are present mainly at the ER and Golgi Apparatus membranes and are active at resting Ca2+. Whereas research in plants has historically focused on the biochemical characterization of these pumps, more recently the attention has been also addressed on the physiological roles played by the different isoforms. This review aims to highlight the main biochemical properties of both type IIB and type IIA Ca2+ pumps and their involvement in the shaping of cellular Ca2+ dynamics induced by different stimuli.

Calcium signaling in plant immunity: a spatiotemporally controlled symphony

Calcium ions (Ca²⁺) are prominent intracellular messengers in all eukaryotic cells. Recent studies have emphasized the crucial roles of Ca²⁺ in plant immunity. Here, we review the latest progress on the spatiotemporal control of Ca²⁺ function in plant immunity. We discuss discoveries of how Ca²⁺ influx is triggered upon the activation of immune receptors, how Ca²⁺-permeable channels are activated, how Ca²⁺ signals are decoded inside plant cells, and how these signals are switched off. Despite recent advances, many open questions remain and we highlight the existing toolkit and the new technologies to address the outstanding questions of Ca²⁺ signaling in plant immunity.

FERONIA-like receptor 1-mediated calcium ion homeostasis is involved in the immune response

Calcium (Ca²⁺) is the most abundant divalent cation in plants, and cellular levels of Ca²⁺, which functions as a nutrient and secondary messenger, play a critical role in plant immunity. In the present study, we found that FERONIA-like receptor 1 (FLR1) positively regulates Magnaporthe oryzae resistance and that expression of FLR1 is strongly induced in response to Ca²⁺ deficiency. In addition, the Ca content in the shoots of flr1 was lower than that in wild-type, and the M. oryzae-sensitive phenotype of the flr1 mutant was not rescued by exogenous application of Ca²⁺. Moreover, RNA sequencing revealed 2,697 differentially expressed genes (DEGs) in the flr1 mutant compared with wild-type, and some of these DEGs are involved in cellular metal ion homeostasis and transition metal ion homeostasis. Changes in expression of overlapping genes between the flr1 mutant and in plants under low-Ca²⁺ treatment were consistent in terms of direction, indicating that FLR1 is involved in Ca²⁺ homeostasis. In summary, we detected FLR1-mediated resistance to M. oryzae, a phenomenon associated with Ca²⁺ homeostasis.

TaBln1, a member of the Blufensin family, negatively regulates wheat resistance to stripe rust by reducing Ca2+ influx

Blufensin1 (Bln1) has been identified as a susceptibility factor of basal defense mechanisms which is unique to the cereal grain crops barley (Hordeum vulgare), wheat (Triticum aestivum), rice (Oryza sativa), and rye (Secale cereale). However, the molecular mechanisms through which Bln1 regulates the wheat immune response are poorly understood. In this study, we found that TaBln1 was significantly induced by Puccinia striiformis f. sp. tritici (Pst) virulent race CYR31 infection. Knockdown of TaBln1 expression by virus-induced gene silencing reduced Pst growth and development, and enhanced the host defense response. In addition, TaBln1 was found to physically interact with a calmodulin, TaCaM3, on the plasma membrane. Silencing TaCaM3 with virus-induced gene silencing increased fungal infection areas and sporulation and reduced wheat resistance to the Pst avirulent race CYR23 (incompatible interaction) and virulent race CYR31 (compatible interaction). Moreover, we found that the accumulation of TaCaM3 transcripts could be induced by treatment with chitin but not flg22. Silencing TaCaM3 decreased the calcium (Ca2+) influx induced by chitin, but silencing TaBln1 increased the Ca2+ influx in vivo using a noninvasive micro-test technique. Taken together, we identified the wheat susceptibility factor TaBln1, which interacts with TaCaM3 to impair Ca2+ influx and inhibit plant defenses.

The signatures of organellar calcium

Recent insights about the transport mechanisms involved in the in and out of calcium ions in plant organelles, and their role in the regulation of cytosolic calcium homeostasis in different signaling pathways.

Calmodulin7: recent insights into emerging roles in plant development and stress

Analyses of the function of Arabidopsis Calmodulin7 (CAM7) in concert with multiple regulatory proteins involved in various signal transduction processes. Calmodulin (CaM) plays various regulatory roles in multiple signaling pathways in eukaryotes. Arabidopsis CALMODULIN 7 (CAM7) is a unique member of the CAM family that works as a transcription factor in light signaling pathways. CAM7 works in concert with CONSTITUTIVE PHOTOMORPHOGENIC 1 and ELONGATED HYPOCOTYL 5, and plays an important role in seedling development. Further, it is involved in the regulation of the activity of various Ca²⁺-gated channels such as cyclic nucleotide gated channel 6 (CNGC6), CNGC14 and auto-inhibited Ca²⁺ ATPase 8. Recent studies further indicate that CAM7 is also an integral part of multiple signaling pathways including hormone, immunity and stress. Here, we review the recent advances in understanding the multifaceted role of CAM7. We highlight the open-ended questions, and also discuss the diverse aspects of CAM7 characterization that need to be addressed for comprehensive understanding of its cellular functions.

Calcium Signaling in Plant Programmed Cell Death

Programmed cell death (PCD) is a process intended for the maintenance of cellular homeostasis by eliminating old, damaged, or unwanted cells. In plants, PCD takes place during developmental processes and in response to biotic and abiotic stresses. In contrast to the field of animal studies, PCD is not well understood in plants. Calcium (Ca²⁺) is a universal cell signaling entity and regulates numerous physiological activities across all the kingdoms of life. The cytosolic increase in Ca²⁺ is a prerequisite for the induction of PCD in plants. Although over the past years, we have witnessed significant progress in understanding the role of Ca²⁺ in the regulation of PCD, it is still unclear how the upstream stress perception leads to the Ca²⁺ elevation and how the signal is further propagated to result in the onset of PCD. In this review article, we discuss recent advancements in the field, and compare the role of Ca²⁺ signaling in PCD in biotic and abiotic stresses. Moreover, we discuss the upstream and downstream components of Ca²⁺ signaling and its crosstalk with other signaling pathways in PCD. The review is expected to provide new insights into the role of Ca²⁺ signaling in PCD and to identify gaps for future research efforts.

Arabidopsis Ca2+-ATPases 1, 2, and 7 in the endoplasmic reticulum contribute to growth and pollen fitness

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.

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

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

Oregano essential oil vapour prevents Plasmopara viticola infection in grapevine (Vitis Vinifera) and primes plant immunity mechanisms

The reduction of synthetic fungicides in agriculture is necessary to guarantee a sustainable production that protects the environment and consumers' health. Downy mildew caused by the oomycete Plasmopara viticola is the major pathogen in viticulture worldwide and responsible for up to 60% of pesticide treatments. Alternatives to reduce fungicides are thus utterly needed to ensure sustainable vineyard-ecosystems, consumer health and public acceptance. Essential oils (EOs) are amongst the most promising natural plant protection alternatives and have shown their antibacterial, antiviral and antifungal properties on several agricultural crops. However, the efficiency of EOs highly depends on timing, application method and the molecular interactions between the host, the pathogen and EO. Despite proven EO efficiency, the underlying processes are still not understood and remain a black box. The objectives of the present study were: a) to evaluate whether a continuous fumigation of a particular EO can control downy mildew in order to circumvent the drawbacks of direct application, b) to decipher molecular mechanisms that could be triggered in the host and the pathogen by EO application and c) to try to differentiate whether essential oils directly repress the oomycete or act as plant resistance primers. To achieve this a custom-made climatic chamber was constructed that enabled a continuous fumigation of potted vines with different EOs during long-term experiments. The grapevine (Vitis vinifera) cv Chasselas was chosen in reason of its high susceptibility to Plasmopara viticola. Grapevine cuttings were infected with P. viticola and subsequently exposed to continuous fumigation of different EOs at different concentrations, during 2 application time spans (24 hours and 10 days). Experiments were stopped when infection symptoms were clearly observed on the leaves of the control plants. Plant physiology (photosynthesis and growth rate parameters) were recorded and leaves were sampled at different time points for subsequent RNA extraction and transcriptomics analysis. Strikingly, the Oregano vulgare EO vapour treatment during 24h post-infection proved to be sufficient to reduce downy mildew development by 95%. Total RNA was extracted from leaves of 24h and 10d treatments and used for whole transcriptome shotgun sequencing (RNA-seq). Sequenced reads were then mapped onto the V. vinifera and P. viticola genomes. Less than 1% of reads could be mapped onto the P. viticola genome from treated samples, whereas up to 30% reads from the controls mapped onto the P. viticola genome, thereby confirming the visual observation of P. viticola absence in the treated plants. On average, 80% of reads could be mapped onto the V. vinifera genome for differential expression analysis, which yielded 4800 modulated genes. Transcriptomic data clearly showed that the treatment triggered the plant's innate immune system with genes involved in salicylic, jasmonic acid and ethylene synthesis and signaling, activating Pathogenesis-Related-proteins as well as phytoalexin synthesis. These results elucidate EO-host-pathogen interactions for the first time and indicate that the antifungal efficiency of EO is mainly due to the triggering of resistance pathways inside the host plants. This is of major importance for the production and research on biopesticides, plant stimulation products and for resistance-breeding strategies.

Using a transcriptome sequencing approach to explore candidate resistance genes against stemphylium blight in the wild lentil species Lens ervoides

BACKGROUND

Stemphylium blight (SB), caused by Stemphylium botryosum, is a devastating disease in lentil production. Although it is known that accessions of Lens ervoides possess superior SB resistance at much higher frequency than the cultivated lentil species, very little is known about the molecular basis regulating SB resistance in L. ervoides. Therefore, a comprehensive molecular study of SB resistance in L. ervoides was needed to exploit this wild resource available at genebanks for use by plant breeders in resistance breeding.

RESULTS

Microscopic and qPCR quantification of fungal growth revealed that 48, 96, and 144 h post-inoculation (hpi) were interesting time points for disease development in L. ervoides recombinant inbred lines (RILs) LR-66-637 (resistant to SB) and LR-66-577 (susceptible to SB). Results of transcriptome sequencing at 0, 48, 96 and 144 hpi showed that 8810 genes were disease-responsive genes after challenge by S. botryosum. Among them, 7526 genes displayed a similar expression trend in both RILs, and some of them were likely involved in non-host resistance. The remaining 1284 genes were differentially expressed genes (DEGs) between RILs. Of those, 712 DEGs upregulated in LR-66-637 were mostly enriched in 'carbohydrate metabolic process', 'cell wall organization or biogenesis', and 'polysaccharide metabolic process'. In contrast, there were another 572 DEGs that were upregulated in LR-66-577, and some of them were enriched in 'oxidation-reduction process', 'asparagine metabolic process' and 'asparagine biosynthetic process'. After comparing DEGs to genes identified in previously described quantitative trait loci (QTLs) for resistance to SB, nine genes were common and three of them showed differential gene expression between a resistant and a susceptible bulk consisting of five RILs each. Results showed that two genes encoding calcium-transporting ATPase and glutamate receptor3.2 were candidate resistance genes, whereas one gene with unknown function was a candidate susceptibility gene.

CONCLUSION

This study provides new insights into the mechanisms of resistance and susceptibility in L. ervoides RILs responding to S. botryosum infection. Furthermore, we identified candidate resistance or susceptibility genes which warrant further gene function analyses, and which could be valuable for resistance breeding, if their role in resistance or susceptibility can be confirmed.

Calcium transport across plant membranes: mechanisms and functions

Contents Summary 49 I. Introduction 49 II. Physiological and structural characteristics of plant Ca²⁺ -permeable ion channels 50 III. Ca²⁺ extrusion systems 61 IV. Concluding remarks 64 Acknowledgements 64 References 64 SUMMARY: Calcium is an essential structural, metabolic and signalling element. The physiological functions of Ca²⁺ are enabled by its orchestrated transport across cell membranes, mediated by Ca²⁺ -permeable ion channels, Ca²⁺ -ATPases and Ca²⁺ /H⁺ exchangers. Bioinformatics analysis has not determined any Ca²⁺ -selective filters in plant ion channels, but electrophysiological tests do reveal Ca²⁺ conductances in plant membranes. The biophysical characteristics of plant Ca²⁺ conductances have been studied in detail and were recently complemented by molecular genetic approaches. Plant Ca²⁺ conductances are mediated by several families of ion channels, including cyclic nucleotide-gated channels (CNGCs), ionotropic glutamate receptors, two-pore channel 1 (TPC1), annexins and several types of mechanosensitive channels. Key Ca²⁺ -mediated reactions (e.g. sensing of temperature, gravity, touch and hormones, and cell elongation and guard cell closure) have now been associated with the activities of specific subunits from these families. Structural studies have demonstrated a unique selectivity filter in TPC1, which is passable for hydrated divalent cations. The hypothesis of a ROS-Ca²⁺ hub is discussed, linking Ca²⁺ transport to ROS generation. CNGC inactivation by cytosolic Ca²⁺ , leading to the termination of Ca²⁺ signals, is now mechanistically explained. The structure-function relationships of Ca²⁺ -ATPases and Ca²⁺ /H⁺ exchangers, and their regulation and physiological roles are analysed.

MIZ1 regulates ECA1 to generate a slow, long-distance phloem-transmitted Ca2+ signal essential for root water tracking in Arabidopsis

Ever since Darwin postulated that the tip of the root is sensitive to moisture differences and that it "transmits an influence to the upper adjoining part, which bends towards the source of moisture" [Darwin C, Darwin F (1880) The Power of Movement in Plants, pp 572-574], the signal underlying this tropic response has remained elusive. Using the FRET-based Cameleon Ca²⁺ sensor in planta, we show that a water potential gradient applied across the root tip generates a slow, long-distance asymmetric cytosolic Ca²⁺ signal in the phloem, which peaks at the elongation zone, where it is dispersed laterally and asymmetrically to peripheral cells, where cell elongation occurs. In addition, the MIZ1 protein, whose biochemical function is unknown but is required for root curvature toward water, is indispensable for generating the slow, long-distance Ca²⁺ signal. Furthermore, biochemical and genetic manipulations that elevate cytosolic Ca²⁺ levels, including mutants of the endoplasmic reticulum (ER) Ca²⁺-ATPase isoform ECA1, enhance root curvature toward water. Finally, coimmunoprecipitation of plant proteins and functional complementation assays in yeast cells revealed that MIZ1 directly binds to ECA1 and inhibits its activity. We suggest that the inhibition of ECA1 by MIZ1 changes the balance between cytosolic Ca²⁺ influx and efflux and generates the cytosolic Ca²⁺ signal required for water tracking.

The contribution of organelles to plant intracellular Calcium signalling

Calcium (Ca2+) is among the most important intracellular messengers in living organisms. Understanding of the players and dynamics of Ca2+ signalling pathways in plants may help to unravel the molecular basis of their exceptional flexibility to respond and to adapt to different stimuli. In the present review we focus on new tools that have recently revolutionized our view of organellar Ca2+ signalling as well as on the current knowledge regarding the pathways mediating Ca2+ fluxes across intracellular membranes. The contribution of organelles and cellular subcompartments to the orchestrated response via Ca2+ signalling within a cell is also discussed, underlining the fact that one of the greatest challenges in the field is the elucidation of how influx and efflux Ca2+ transporters/channels are regulated in a concerted manner to translate specific information into a Ca2+ signature.

The pepper Bs4C proteins are localized to the endoplasmic reticulum (ER) membrane and confer disease resistance to bacterial blight in transgenic rice

Transcription activator-like effector (TALE)-dependent dominant disease resistance (R) genes in plants, also referred to as executor R genes, are induced on infection by phytopathogenic bacteria of the genus Xanthomonas harbouring the corresponding TALE genes. Unlike the traditional R proteins, the executor R proteins do not determine the resistance specificity and may function broadly in different plant species. The executor R gene Bs4C-R in the resistant genotype PI 235047 of the pepper species Capsicum pubescens (CpBs4C-R) confers disease resistance to Xanthomonas campestris pv. vesicatoria (Xcv) harbouring the TALE genes avrBsP/avrBs4. In this study, the synthetic genes of CpBs4C-R and two other Bs4C-like genes, the susceptible allele in the genotype PI585270 of C. pubescens (CpBs4C-S) and the CaBs4C-R homologue gene in the cultivar 'CM334' of Capsicum annum (CaBs4C), were characterized in tobacco (Nicotiana benthamiana) and rice (Oryza sativa). The Bs4C genes induced cell death in N. benthamiana. The functional Bs4C-eCFP fusion proteins were localized to the endoplasmic reticulum (ER) membrane in the leaf epidermal cells of N. benthamiana. The Xa10 promoter-Bs4C fusion genes in transgenic rice conferred strain-specific disease resistance to Xanthomonas oryzae pv. oryzae (Xoo), the causal agent of bacterial blight in rice, and were specifically induced by the Xa10-incompatible Xoo strain PXO99A (pHM1avrXa10). The results indicate that the Bs4C proteins from pepper species function broadly in rice and the Bs4C protein-mediated cell death from the ER is conserved between dicotyledonous and monocotyledonous plants, which can be utilized to engineer novel and enhanced disease resistance in heterologous plants.

Inhibition of breast cancer cell growth by methyl pyropheophenylchlorin photodynamic therapy is mediated though endoplasmic reticulum stress-induced autophagy in vitro and vivo

Autophagy and ER stress participated in the inhibition of MPPa‐PDT on tumor growth, but the molecular links between them remain undefined. We just explore the molecular mechanism between them in vitro and vivo. CCK‐8 assay and flow cytometer were used to detect the cytotoxicity and mode of cell death after MPPa‐PDT. Furthermore, the role of autophagy was verified in MPPa‐PDT. Confocal microscopy was used to show the intracellular distribution of MPPa. ER stress markers and PERK signaling pathway were detected by western blot. While in vivo, tumor histology and immunohistochemistry were performed to show the effect of MPPa‐PDT in mice. After MPPa‐PDT, cells viability decreased in dose‐dependent manner. Besides, the cell apoptosis increased along with the increasing of Beclin‐1and LC3B II but declining of P62. When pretreated with 3‐MA, LC3B II formation and the cytotoxicity declined. MPPa‐PDT caused increasing of ER stress markers (GRP78, CHOP) as MPPa accumulated in ER. However, pretreatment with ER stress inhibitor 4PBA, the expression of GRP78 and LC3B II was blocked but the PERK signaling pathway activated and the expression of P62 increased. In vivo, the tumor growth was significantly inhibited by MPPa‐PDT. Besides, the appearance of ER stress and autophagy was further demonstrated by immunohistochemistry. Our findings demonstrate that autophagy mediated by MPPa‐PDT was regulated by ER stress, via PERK signaling pathway, to kill MDA‐MB‐231 cells in vitro and vivo.

Plant ion channels and transporters in herbivory-induced signalling

In contrast to many biotic stresses that plants face, feeding by herbivores produces unique mechanical and chemical signatures. Plants have evolved effective systems to recognise these mechanical stimuli and chemical elicitors at the plasma membrane (PM), where this recognition generates ion fluxes, including an influx of Ca2+ that elicits cellular Ca2+ signalling, production of reactive oxygen species (ROS), and variation in transmembrane potential. These signalling events also function in propagation of long-distance signals (Ca2+ waves, ROS waves, and electrical signals), which contribute to rapid, systemic induction of defence responses. Recent studies have identified several candidate channels or transporters that likely produce these ion fluxes at the PM. Here, we describe the important roles of these channels/transporters in transduction or transmission of herbivory-induced early signalling events, long-distance signals, and jasmonic acid and green leaf volatile signalling in plants.

Grapevine immune signaling network in response to drought stress as revealed by transcriptomic analysis

Drought is a ubiquitous abiotic factor that severely impedes growth and development of horticulture crops. The challenge postured by global climate change is the evolution of drought-tolerant cultivars that could cope with concurrent stress. Hence, in this study, biochemical, physiological and transcriptome analysis were investigated in drought-treated grapevine leaves. The results revealed that photosynthetic activity and reducing sugars were significantly diminished which were positively correlated with low stomatal conductance and CO₂ exchange in drought-stressed leaves. Further, the activities of superoxide dismutase, peroxidase, and catalase were significantly actuated in the drought-responsive grapevine leaves. Similarly, the levels of abscisic acid and jasmonic acid were also significantly increased in the drought-stressed leaves. In transcriptome analysis, 12,451 differentially-expressed genes (DEGs) were annotated, out of which 8021 DEGs were up-regulated and 4430 DEGs were down-regulated in response to drought stress. In addition, the genes encoding pathogen-associated molecular pattern (PAMP) triggered immunity (PTI), including calcium signals, protein phosphatase 2C, calcineurin B-like proteins, MAPKs, and phosphorylation (FLS2 and MEKK1) cascades were up-regulated in response to drought stress. Several genes related to plant-pathogen interaction pathway (RPM1, PBS1, RPS5, RIN4, MIN7, PR1, and WRKYs) were also found up-regulated in response to drought stress. Overall the results of present study showed the dynamic interaction of DEG in grapevine physiology which provides the premise for selection of defense-related genes against drought stress for subsequent grapevine breeding programs.

Calcium Pumps and Interacting BON1 Protein Modulate Calcium Signature, Stomatal Closure, and Plant Immunity

Calcium signaling is essential for environmental responses including immune responses. Here, we provide evidence that the evolutionarily conserved protein BONZAI1 (BON1) functions together with autoinhibited calcium ATPase10 (ACA10) and ACA8 to regulate calcium signals in Arabidopsis. BON1 is a plasma membrane localized protein that negatively regulates the expression of immune receptor genes and positively regulates stomatal closure. We found that BON1 interacts with the autoinhibitory domains of ACA10 and ACA8, and the aca10 loss-of-function (LOF) mutants have an autoimmune phenotype similar to that of the bon1 LOF mutants. Genetic evidences indicate that BON1 positively regulates the activities of ACA10 and ACA8. Consistent with this idea, the steady level of calcium concentration is increased in both aca10 and bon1 mutants. Most strikingly, cytosolic calcium oscillation imposed by external calcium treatment was altered in aca10, aca8, and bon1 mutants in guard cells. In addition, calcium- and pathogen-induced stomatal closure was compromised in the aca10 and bon1 mutants. Taken together, this study indicates that ACA10/8 and BON1 physically interact on plasma membrane and function in the generation of cytosol calcium signatures that are critical for stomatal movement and impact plant immunity.

From Chaos to Harmony: Responses and Signaling upon Microbial Pattern Recognition

Pathogen- or microbe-associated molecular patterns (PAMPs/MAMPs) are detected as nonself by host pattern recognition receptors (PRRs) and activate pattern-triggered immunity (PTI). Microbial invasions often trigger the production of host-derived endogenous signals referred to as danger- or damage-associated molecular patterns (DAMPs), which are also perceived by PRRs to modulate PTI responses. Collectively, PTI contributes to host defense against infections by a broad range of pathogens. Remarkable progress has been made toward demonstrating the cellular and physiological responses upon pattern recognition, elucidating the molecular, biochemical, and genetic mechanisms of PRR activation, and dissecting the complex signaling networks that orchestrate PTI responses. In this review, we present an update on the current understanding of how plants recognize and respond to nonself patterns, a process from which the seemingly chaotic responses form into a harmonic defense.

Using GCaMP3 to Study Ca2+ Signaling in Nicotiana Species

Ca2+ signaling is a central component of plant biology; however, direct analysis of in vivo Ca2+ levels is experimentally challenging. In recent years, the use of genetically encoded Ca2+ indicators has revolutionized the study of plant Ca2+ signaling, although such studies have been largely restricted to the model plant Arabidopsis. We have developed stable transgenic Nicotiana benthamiana and Nicotiana tabacum lines expressing the single-wavelength fluorescent Ca2+ indicator, GCaMP3. Ca2+ levels in these plants can be imaged in situ using fluorescence microscopy, and these plants can be used qualitatively and semi-quantitatively to evaluate Ca2+ signals in response to a broad array of abiotic or biotic stimuli, such as cold shock or pathogen-associated molecular patterns (PAMPs). Furthermore, these tools can be used in conjunction with well-established N. benthamiana techniques such as virus-induced gene silencing (VIGS) or transient heterologous expression to assay the effects of loss or gain of function on Ca2+ signaling, an approach which we validated via silencing or transient expression of the PAMP receptors FLS2 (Flagellin Sensing 2) or EFR (EF-Tu receptor), respectively. Using these techniques, along with chemical inhibitor treatments, we demonstrate how these plants can be used to elucidate the molecular components governing Ca2+ signaling in response to specific stimuli.

Induction of Xa10-like Genes in Rice Cultivar Nipponbare Confers Disease Resistance to Rice Bacterial Blight

Bacterial blight of rice, caused by Xanthomonas oryzae pv. oryzae, is one of the most destructive bacterial diseases throughout the major rice-growing regions in the world. The rice disease resistance (R) gene Xa10 confers race-specific disease resistance to X. oryzae pv. oryzae strains that deliver the corresponding transcription activator-like (TAL) effector AvrXa10. Upon bacterial infection, AvrXa10 binds specifically to the effector binding element in the promoter of the R gene and activates its expression. Xa10 encodes an executor R protein that triggers hypersensitive response and activates disease resistance. 'Nipponbare' rice carries two Xa10-like genes in its genome, of which one is the susceptible allele of the Xa23 gene, a Xa10-like TAL effector-dependent executor R gene isolated recently from 'CBB23' rice. However, the function of the two Xa10-like genes in disease resistance to X. oryzae pv. oryzae strains has not been investigated. Here, we designated the two Xa10-like genes as Xa10-Ni and Xa23-Ni and characterized their function for disease resistance to rice bacterial blight. Both Xa10-Ni and Xa23-Ni provided disease resistance to X. oryzae pv. oryzae strains that deliver the matching artificially designed TAL effectors (dTALE). Transgenic rice plants containing Xa10-Ni and Xa23-Ni under the Xa10 promoter provided specific disease resistance to X. oryzae pv. oryzae strains that deliver AvrXa10. Xa10-Ni and Xa23-Ni knock-out mutants abolished dTALE-dependent disease resistance to X. oryzae pv. oryzae. Heterologous expression of Xa10-Ni and Xa23-Ni in Nicotiana benthamiana triggered cell death. The 19-amino-acid residues at the N-terminal regions of XA10 or XA10-Ni are dispensable for their function in inducing cell death in N. benthamiana and the C-terminal regions of XA10, XA10-Ni, and XA23-Ni are interchangeable among each other without affecting their function. Like XA10, both XA10-Ni and XA23-Ni locate to the endoplasmic reticulum (ER) membrane, show self-interaction, and induce ER Ca2+ depletion in leaf cells of N. benthamiana. The results indicate that Xa10-Ni and Xa23-Ni in Nipponbare encode functional executor R proteins, which induce cell death in both monocotyledonous and dicotyledonous plants and have the potential of being engineered to provide broad-spectrum disease resistance to plant-pathogenic Xanthomonas spp.

Ca2+-dependent phosphoregulation of the plasma membrane Ca2+-ATPase ACA8 modulates stimulus-induced calcium signatures

Ca2+ signals are transient, hence, upon a stimulus-induced increase in cytosolic Ca2+ concentration, cells have to re-establish resting Ca2+ levels. Ca2+ extrusion is operated by a wealth of transporters, such as Ca2+ pumps and Ca2+/H+ antiporters, which often require a rise in Ca2+ concentration to be activated. Here, we report a regulatory fine-tuning mechanism of the Arabidopsis thaliana plasma membrane-localized Ca2+-ATPase isoform ACA8 that is mediated by calcineurin B-like protein (CBL) and CBL-interacting protein kinase (CIPK) complexes. We show that two CIPKs (CIPK9 and CIPK14) are able to interact with ACA8 in vivo and phosphorylate it in vitro. Transient co-overexpression of ACA8 with CIPK9 and the plasma membrane Ca2+ sensor CBL1 in tobacco leaf cells influences nuclear Ca2+ dynamics, specifically reducing the height of the second peak of the wound-induced Ca2+ transient. Stimulus-induced Ca2+ transients in mature leaves and seedlings of an aca8 T-DNA insertion line exhibit altered dynamics when compared with the wild type. Altogether our results identify ACA8 as a prominent in vivo regulator of cellular Ca2+ dynamics and reveal the existence of a Ca2+-dependent CBL-CIPK-mediated regulatory feedback mechanism, which crucially functions in the termination of Ca2+ signals.

Interplay of Plasma Membrane and Vacuolar Ion Channels, Together with BAK1, Elicits Rapid Cytosolic Calcium Elevations in Arabidopsis during Aphid Feeding

A transient rise in cytosolic calcium ion concentration is one of the main signals used by plants in perception of their environment. The role of calcium in the detection of abiotic stress is well documented; however, its role during biotic interactions remains unclear. Here, we use a fluorescent calcium biosensor (GCaMP3) in combination with the green peach aphid (Myzus persicae) as a tool to study Arabidopsis thaliana calcium dynamics in vivo and in real time during a live biotic interaction. We demonstrate rapid and highly localized plant calcium elevations around the feeding sites of M. persicae, and by monitoring aphid feeding behavior electrophysiologically, we demonstrate that these elevations correlate with aphid probing of epidermal and mesophyll cells. Furthermore, we dissect the molecular mechanisms involved, showing that interplay between the plant defense coreceptor BRASSINOSTEROID INSENSITIVE-ASSOCIATED KINASE1 (BAK1), the plasma membrane ion channels GLUTAMATE RECEPTOR-LIKE 3.3 and 3.6 (GLR3.3 and GLR3.6), and the vacuolar ion channel TWO-PORE CHANNEL1 (TPC1) mediate these calcium elevations. Consequently, we identify a link between plant perception of biotic threats by BAK1, cellular calcium entry mediated by GLRs, and intracellular calcium release by TPC1 during a biologically relevant interaction.

Cyclopiazonic Acid Is a Pathogenicity Factor for Aspergillus flavus and a Promising Target for Screening Germplasm for Ear Rot Resistance

Aspergillus flavus, an opportunistic pathogen, contaminates maize and other key crops with carcinogenic aflatoxins (AFs). Besides AFs, A. flavus makes many more secondary metabolites (SMs) whose toxicity in insects or vertebrates has been studied. However, the role of SMs in the invasion of plant hosts by A. flavus remains to be investigated. Cyclopiazonic acid (CPA), a neurotoxic SM made by A. flavus, is a nanomolar inhibitor of endoplasmic reticulum calcium ATPases (ECAs) and a potent inducer of cell death in plants. We hypothesized that CPA, by virtue of its cytotoxicity, may serve as a key pathogenicity factor that kills plant cells and supports the saprophytic life style of the fungus while compromising the host defense response. This proposal was tested by two complementary approaches. A comparison of CPA levels among A. flavus isolates indicated that CPA may be a determinant of niche adaptation, i.e., isolates that colonize maize make more CPA than those restricted only to the soil. Further, mutants in the CPA biosynthetic pathway are less virulent in causing ear rot than their wild-type parent in field inoculation assays. Additionally, genes encoding ECAs are expressed in developing maize seeds and are induced by A. flavus infection. Building on these results, we developed a seedling assay in which maize roots were exposed to CPA, and cell death was measured as Evans Blue uptake. Among >40 maize inbreds screened for CPA tolerance, inbreds with proven susceptibility to ear rot were also highly CPA sensitive. The publicly available data on resistance to silk colonization or AF contamination for many of the lines was also broadly correlated with their CPA sensitivity. In summary, our studies show that i) CPA serves as a key pathogenicity factor that enables the saprophytic life style of A. flavus and ii) maize inbreds are diverse in their tolerance to CPA. Taking advantage of this natural variation, we are currently pursuing both genome-wide and candidate gene approaches to identify novel components of maize resistance to Aspergillus ear rot.

Characterization and Functional Analysis of Calmodulin and Calmodulin-Like Genes in Fragaria vesca

Calcium is a universal messenger that is involved in the modulation of diverse developmental and adaptive processes in response to various stimuli. Calmodulin (CaM) and calmodulin-like (CML) proteins are major calcium sensors in all eukaryotes, and they have been extensively investigated for many years in plants and animals. However, little is known about CaMs and CMLs in woodland strawberry (Fragaria vesca). In this study, we performed a genome-wide analysis of the strawberry genome and identified 4 CaM and 36 CML genes. Bioinformatics analyses, including gene structure, phylogenetic tree, synteny and three-dimensional model assessments, revealed the conservation and divergence of FvCaMs and FvCMLs, thus providing insight regarding their functions. In addition, the transcript abundance of four FvCaM genes and the four most related FvCML genes were examined in different tissues and in response to multiple stress and hormone treatments. Moreover, we investigated the subcellular localization of several FvCaMs and FvCMLs, revealing their potential interactions based on the localizations and potential functions. Furthermore, overexpression of five FvCaM and FvCML genes could not induce a hypersensitive response, but four of the five genes could increase resistance to Agrobacterium tumefaciens in Nicotiana benthamiana leaves. This study provides evidence for the biological roles of FvCaM and CML genes, and the results lay the foundation for future functional studies of these genes.

The Multivesicular Bodies (MVBs)-Localized AAA ATPase LRD6-6 Inhibits Immunity and Cell Death Likely through Regulating MVBs-Mediated Vesicular Trafficking in Rice

Previous studies have shown that multivesicular bodies (MVBs)/endosomes-mediated vesicular trafficking may play key roles in plant immunity and cell death. However, the molecular regulation is poorly understood in rice. Here we report the identification and characterization of a MVBs-localized AAA ATPase LRD6-6 in rice. Disruption of LRD6-6 leads to enhanced immunity and cell death in rice. The ATPase activity and homo-dimerization of LRD6-6 is essential for its regulation on plant immunity and cell death. An ATPase inactive mutation (LRD6-6^E315Q) leads to dominant-negative inhibition in plants. The LRD6-6 protein co-localizes with the MVBs marker protein RabF1/ARA6 and interacts with ESCRT-III components OsSNF7 and OsVPS2. Further analysis reveals that LRD6-6 is required for MVBs-mediated vesicular trafficking and inhibits the biosynthesis of antimicrobial compounds. Collectively, our study shows that the AAA ATPase LRD6-6 inhibits plant immunity and cell death most likely through modulating MVBs-mediated vesicular trafficking in rice.

Plants have evolved sophistical immunity system in fighting against pathogenic micro-organisms including bacteria, fungi and oomycetes. Upon perception of pathogens, the immune system activates rapid cell death, characterized as a form of hypersensitive response typically in and around the infection sites to restrict pathogen invasion and prevent disease development. Recent studies have suggested that MVBs-mediated vesicular trafficking might play key roles in plant immunity and cell death. However, the molecular regulation is poorly known. By using the lesion resembling disease (lrd) mutant, lrd6-6, which exhibits autoimmunity and spontaneous cell death, we characterized LRD6-6 as a MVBs-localized AAA ATPase. We found that the ATPase LRD6-6 was required for MVBs-mediated vesicular trafficking and inhibited the biosynthesis of antimicrobial compounds for immune response in rice. Both the ATPase activity and homo-dimerization of LRD6-6 were essential for its inhibition on immunity and cell death. The catalytically inactive ATPase, LRD6-6^E315Q, played dominant-negative effect on inhibition of immunity in plants. In addition, the LRD6-6 protein co-localized with the MVBs-spread marker protein RabF1/ARA6 and also interacted with ESCRT-III components OsSNF7 and OsVPS2. In summary, our study has shown that the AAA ATPase LRD6-6 inhibits plant immunity and cell death most likely through modulating MVBs-mediated vesicular trafficking in rice.

Endoplasmic Reticulum Stress Signaling in Plant Immunity--At the Crossroad of Life and Death

Rapid and complex immune responses are induced in plants upon pathogen recognition. One form of plant defense response is a programmed burst in transcription and translation of pathogenesis-related proteins, of which many rely on ER processing. Interestingly, several ER stress marker genes are up-regulated during early stages of immune responses, suggesting that enhanced ER capacity is needed for immunity. Eukaryotic cells respond to ER stress through conserved signaling networks initiated by specific ER stress sensors tethered to the ER membrane. Depending on the nature of ER stress the cell prioritizes either survival or initiates programmed cell death (PCD). At present two plant ER stress sensors, bZIP28 and IRE1, have been described. Both sensor proteins are involved in ER stress-induced signaling, but only IRE1 has been additionally linked to immunity. A second branch of immune responses relies on PCD. In mammals, ER stress sensors are involved in activation of PCD, but it is unclear if plant ER stress sensors play a role in PCD. Nevertheless, some ER resident proteins have been linked to pathogen-induced cell death in plants. In this review, we will discuss the current understanding of plant ER stress signaling and its cross-talk with immune signaling.

Temporal transcriptome changes induced by methyl jasmonate in Salvia sclarea

Salvia sclarea is a traditional medicinal and aromatic plant that grows in Europe and produces various economically important compounds, including phenylpropanoid derivatives and terpenoids. Methyl jasmonate (MeJA) is commonly used to elicit plant stress responses. However, how MeJA enhances production of secondary metabolites in S. sclarea is not well understood. We performed a genome-wide analysis of temporal gene expression in S. sclarea leaves and roots. The transcriptome profiles 0, 10 and 26 h after MeJA treatment were analyzed by Illumina RNA-Seq. A total of 16,142 isogenes (average length 866bp; N50 1035bp) were obtained by de novo assembly of 35,757,567 raw sequencing reads. When these sequencing reads were mapped onto the assembled Unigenes, 3236, 2792 and 798 Unigenes were found to be expressed differentially between 0 and 10h, 0 and 26 h, and 10 and 26h, respectively. These included many secondary metabolite biosynthesis, stress and defense-related genes. A qRT-PCR analysis confirmed the expression profiles of selected differentially expressed genes (DEGs) revealed by RNA-Seq data, and also extended our analysis of differential gene expression to 73 h. Our investigations revealed temporal differences in the responses of S. sclarea to MeJA treatment. MeJA treatment induced the expression of a large number of genes involved in phenylpropanoid biosynthesis, especially between 0 and 10h, and 0 and 26 h. Additionally, many genes encoding transcription factors, cytochrome P450s, glycosyltransferases, methyltransferases and transporters were shown to respond to MeJA elicitation. DEGs related to structural molecule activity and cell death showed a significant temporal variation. A chromatographic analysis of metabolites at 26h, 73h and six days after MeJA treatment indicated that these transcriptomic changes precede MeJA-induced changes in secondary metabolite content. This study sheds light on the molecular mechanisms of MeJA elicitation and is helpful in understanding how exogenous MeJA treatment mediates extensive plant transcriptome reprogramming/remodeling. Our results can be utilized to characterize genes related to secondary metabolism and their regulation, and in breeding S. sclarea for desirable chemotypes.

Ca2+ signalling in plant immune response: from pattern recognition receptors to Ca2+ decoding mechanisms

Ca2+ is a ubiquitous second messenger for cellular signalling in various stresses and developmental processes. Here, we summarize current developments in the roles of Ca2+ during plant immunity responses. We discuss the early perception events preceding and necessary for triggering cellular Ca2+ fluxes, the potential Ca2+-permeable channels, the decoding of Ca2+ signals predominantly via Ca2+-dependent phosphorylation events and transcriptional reprogramming. To highlight the complexity of the cellular signal network, we briefly touch on the interplay between Ca2+-dependent signalling and selected major signalling mechanisms--with special emphasis on reactive oxygen species at local and systemic levels.

Measuring spatial and temporal Ca2+ signals in Arabidopsis plants

Developmental and environmental cues induce Ca(2+) fluctuations in plant cells. Stimulus-specific spatial-temporal Ca(2+) patterns are sensed by cellular Ca(2+) binding proteins that initiate Ca(2+) signaling cascades. However, we still know little about how stimulus specific Ca(2+) signals are generated. The specificity of a Ca(2+) signal may be attributed to the sophisticated regulation of the activities of Ca(2+) channels and/or transporters in response to a given stimulus. To identify these cellular components and understand their functions, it is crucial to use systems that allow a sensitive and robust recording of Ca(2+) signals at both the tissue and cellular levels. Genetically encoded Ca(2+) indicators that are targeted to different cellular compartments have provided a platform for live cell confocal imaging of cellular Ca(2+) signals. Here we describe instructions for the use of two Ca(2+) detection systems: aequorin based FAS (film adhesive seedlings) luminescence Ca(2+) imaging and case12 based live cell confocal fluorescence Ca(2+) imaging. Luminescence imaging using the FAS system provides a simple, robust and sensitive detection of spatial and temporal Ca(2+) signals at the tissue level, while live cell confocal imaging using Case12 provides simultaneous detection of cytosolic and nuclear Ca(2+) signals at a high resolution.

When supply does not meet demand-ER stress and plant programmed cell death

The endoplasmic reticulum (ER) is the central organelle in the eukaryotic secretory pathway. The ER functions in protein synthesis and maturation and is crucial for proper maintenance of cellular homeostasis and adaptation to adverse environments. Acting as a cellular sentinel, the ER is exquisitely sensitive to changing environments principally via the ER quality control machinery. When perturbed, ER-stress triggers a tightly regulated and highly conserved, signal transduction pathway known as the unfolded protein response (UPR) that prevents the dangerous accumulation of unfolded/misfolded proteins. In situations where excessive UPR activity surpasses threshold levels, cells deteriorate and eventually trigger programmed cell death (PCD) as a way for the organism to cope with dysfunctional or toxic signals. The programmed cell death that results from excessive ER stress in mammalian systems contributes to several important diseases including hypoxia, neurodegeneration, and diabetes. Importantly, hallmark features and markers of cell death that are associated with ER stress in mammals are also found in plants. In particular, there is a common, conserved set of chaperones that modulate ER cell death signaling. Here we review the elements of plant cell death responses to ER stress and note that an increasing number of plant-pathogen interactions are being identified in which the host ER is targeted by plant pathogens to establish compatibility.

When supply does not meet demand-ER stress and plant programmed cell death

The endoplasmic reticulum (ER) is the central organelle in the eukaryotic secretory pathway. The ER functions in protein synthesis and maturation and is crucial for proper maintenance of cellular homeostasis and adaptation to adverse environments. Acting as a cellular sentinel, the ER is exquisitely sensitive to changing environments principally via the ER quality control machinery. When perturbed, ER-stress triggers a tightly regulated and highly conserved, signal transduction pathway known as the unfolded protein response (UPR) that prevents the dangerous accumulation of unfolded/misfolded proteins. In situations where excessive UPR activity surpasses threshold levels, cells deteriorate and eventually trigger programmed cell death (PCD) as a way for the organism to cope with dysfunctional or toxic signals. The programmed cell death that results from excessive ER stress in mammalian systems contributes to several important diseases including hypoxia, neurodegeneration, and diabetes. Importantly, hallmark features and markers of cell death that are associated with ER stress in mammals are also found in plants. In particular, there is a common, conserved set of chaperones that modulate ER cell death signaling. Here we review the elements of plant cell death responses to ER stress and note that an increasing number of plant-pathogen interactions are being identified in which the host ER is targeted by plant pathogens to establish compatibility.

OsACA6, a P-type IIB Ca²⁺ ATPase promotes salinity and drought stress tolerance in tobacco by ROS scavenging and enhancing the expression of stress-responsive genes

Calcium (Ca²⁺) regulates several signalling pathways involved in growth, development and stress tolerance. Cellular Ca²⁺ homeostasis is achieved by the combined action of channels, pumps and antiporters, but direct evidence for a role of Ca²⁺ATPase pumps in stress tolerance is lacking. Here we report the characterization of a Ca²⁺ ATPase gene (OsACA6) from Oryza sativa, and elucidate its functions in stress tolerance. OsACA6 transcript levels are enhanced in response to salt, drought, abscisic acid and heat. In vivo localization identified plasma membranes as an integration site for the OsACA6-GFP fusion protein. Using transgenic tobacco lines, we demonstrate that over-expression of OsACA6 is triggered during salinity and drought stresses. The enhanced tolerance to these stresses was confirmed by changes in several physiological indices, including water loss rate, photosynthetic efficiency, cell membrane stability, germination, survival rate, malondialdehyde content, electrolyte leakage and increased proline accumulation. Furthermore, over-expressing lines also showed higher leaf chlorophyll and reduced accumulation of H₂O₂ and Na⁺ ions compared to the wild-type. Reduced accumulation of reactive oxygen species (ROS) was observed in transgenic lines. The increased proline accumulation and ROS scavenging enzyme activities in transgenic plants over-expressing OsACA6 efficiently modulate the ROS machinery and proline biosynthesis through an integrative mechanism. Transcriptional profiling of these plants revealed altered expression of genes encoding many transcription factors, stress- and disease-related proteins, as well as signalling components. These results suggest that Ca²⁺ ATPases have diverse roles as regulators of many stress signalling pathways, leading to plant growth, development and stress tolerance.

Calcium Signals from the Vacuole

The vacuole is by far the largest intracellular Ca(2+) store in most plant cells. Here, the current knowledge about the molecular mechanisms of vacuolar Ca(2+) release and Ca(2+) uptake is summarized, and how different vacuolar Ca(2+) channels and Ca(2+) pumps may contribute to Ca(2+) signaling in plant cells is discussed. To provide a phylogenetic perspective, the distribution of potential vacuolar Ca(2+) transporters is compared for different clades of photosynthetic eukaryotes. There are several candidates for vacuolar Ca(2+) channels that could elicit cytosolic [Ca(2+)] transients. Typical second messengers, such as InsP₃ and cADPR, seem to trigger vacuolar Ca(2+) release, but the molecular mechanism of this Ca(2+) release still awaits elucidation. Some vacuolar Ca(2+) channels have been identified on a molecular level, the voltage-dependent SV/TPC1 channel, and recently two cyclic-nucleotide-gated cation channels. However, their function in Ca(2+) signaling still has to be demonstrated. Ca(2+) pumps in addition to establishing long-term Ca(2+) homeostasis can shape cytosolic [Ca(2+)] transients by limiting their amplitude and duration, and may thus affect Ca(2+) signaling.

Aphanomyces euteiches cell wall fractions containing novel glucan-chitosaccharides induce defense genes and nuclear calcium oscillations in the plant host Medicago truncatula

N-acetylglucosamine-based saccharides (chitosaccharides) are components of microbial cell walls and act as molecular signals during host-microbe interactions. In the legume plant Medicago truncatula, the perception of lipochitooligosaccharide signals produced by symbiotic rhizobia and arbuscular mycorrhizal fungi involves the Nod Factor Perception (NFP) lysin motif receptor-like protein and leads to the activation of the so-called common symbiotic pathway. In rice and Arabidopsis, lysin motif receptors are involved in the perception of chitooligosaccharides released by pathogenic fungi, resulting in the activation of plant immunity. Here we report the structural characterization of atypical chitosaccharides from the oomycete pathogen Aphanomyces euteiches, and their biological activity on the host Medicago truncatula. Using a combination of biochemical and biophysical approaches, we show that these chitosaccharides are linked to β-1,6-glucans, and contain a β-(1,3;1,4)-glucan backbone whose β-1,3-linked glucose units are substituted on their C-6 carbon by either glucose or N-acetylglucosamine residues. This is the first description of this type of structural motif in eukaryotic cell walls. Glucan-chitosaccharide fractions of A. euteiches induced the expression of defense marker genes in Medicago truncatula seedlings independently from the presence of a functional Nod Factor Perception protein. Furthermore, one of the glucan-chitosaccharide fractions elicited calcium oscillations in the nucleus of root cells. In contrast to the asymmetric oscillatory calcium spiking induced by symbiotic lipochitooligosaccharides, this response depends neither on the Nod Factor Perception protein nor on the common symbiotic pathway. These findings open new perspectives in oomycete cell wall biology and elicitor recognition and signaling in legumes.

Global calcium transducer P-type Ca²⁺-ATPases open new avenues for agriculture by regulating stress signalling

Food security is in danger under the continuous growing threat of various stresses including climate change and global warming, which ultimately leads to a reduction in crop yields. Calcium plays a very important role in many signal transduction pathways including stress signalling. Different extracellular stimuli trigger increases in cytosolic calcium, which is detrimental to plants. To cope with such stresses, plants need to develop efficient efflux mechanisms to maintain ionic homeostasis. The Ca(2+)-ATPases are members of the P-type ATPase superfamily, which perform many fundamental processes in organisms by actively transporting ions across cellular membranes. In recent years, many studies have revealed that, as well as efflux mechanisms, Ca(2+)-ATPases also play critical roles in sensing calcium fluctuations and relaying downstream signals by activating definitive targets, thus modulating corresponding metabolic pathways. As calcium-activated calmodulin (CaM) is reported to play vital roles in stress tolerance, the presence of a unique CaM-binding site in type IIB Ca(2+)-ATPases indicates their potential role in biotic as well as abiotic stress tolerance. The key roles of Ca(2+)-ATPases in transport systems and stress signalling in cellular homeostasis are addressed in this review. A complete understanding of plant defence mechanisms under stress will allow bioengineering of improved crop plants, which will be crucial for food security currently observed worldwide in the context of global climate changes. Overall, this article covers classification, evolution, structural aspects of Ca(2+)-ATPases, and their emerging roles in plant stress signalling.

Genome-wide analysis of plant-type II Ca(2+)ATPases gene family from rice and Arabidopsis: potential role in abiotic stresses

The Plant Ca(2+)ATPases are members of the P-type ATPase superfamily and play essential roles in pollen tube growth, vegetative development, inflorescence architecture, stomatal opening or closing as well as transport of Ca(2+), Mn(2+) and Zn(2+). Their role in abiotic stress adaptation by activation of different signaling pathways is emerging. In Arabidopsis, the P-type Ca(2+)ATPases can be classified in two distinct groups: type IIA (ECA) and type IIB (ACA). The availability of rice genome sequence allowed performing a genome-wide search for P-type Ca(2+)ATPases proteins, and the comparison of the identified proteins with their homologs in Arabidopsis model plant. In the present study, we identified the P-type II Ca(2+)ATPases from rice by analyzing their phylogenetic relationship, multiple alignment, cis-regulatory elements, protein domains, motifs and homology percentage. The phylogenetic analysis revealed that rice type IIA Ca(2+)ATPases clustered with Arabidopsis type IIA Ca(2+)ATPases and showed high sequence similarity within the group, whereas rice type IIB Ca(2+)ATPases presented variable sequence similarities with Arabidopsis type IIB members. The protein homology modeling, identification of putative transmembrane domains and conserved motifs of rice P-type II Ca(2+)ATPases provided information on their functions and structural architecture. The analysis of P-type II Ca(2+)ATPases promoter regions in rice showed multiple stress-induced cis-acting elements. The expression profile analysis indicated vital roles of P-type II Ca(2+)ATPases in stress signaling, plant development and abiotic stress responses. The comprehensive analysis and expression profiling provided a critical platform for functional characterization of P-type II Ca(2+)ATPase genes that could be applied in engineering crop plants with modified calcium signaling and homeostatic pathways.

Aequorin-based luminescence imaging reveals stimulus- and tissue-specific Ca2+ dynamics in Arabidopsis plants

Calcium ion is a versatile second messenger for diverse cell signaling in response to developmental and environmental cues. The specificity of Ca(2+)-mediated signaling is defined by stimulus-elicited Ca(2+) signature and downstream decoding processes. Here, an Aequorin-based luminescence recording system was developed for monitoring Ca(2+) in response to various stimuli in Arabidopsis. With the simple, highly sensitive, and robust Ca(2+) recording, this system revealed stimulus- and tissue-specific Ca(2+) signatures in seedlings. Cellular Ca(2+) dynamics and relationship to Aequorin-based Ca(2+) recording were explored using a GFP-based Ca(2+) indicator, which suggested that a synchronous cellular Ca(2+) signal is responsible for cold-induced Ca(2+) response in seedlings, whereas asynchronous Ca(2+) oscillation contributes to osmotic stress-induced Ca(2+) increase in seedlings. The optimized recording system would be a powerful tool for the identification and characterization of novel components in Ca(2+)-mediated stress-signaling pathways.

Linking ligand perception by PEPR pattern recognition receptors to cytosolic Ca2+ elevation and downstream immune signaling in plants

Little is known about molecular steps linking perception of pathogen invasion by cell surface sentry proteins acting as pattern recognition receptors (PRRs) to downstream cytosolic Ca(2+) elevation, a critical step in plant immune signaling cascades. Some PRRs recognize molecules (such as flagellin) associated with microbial pathogens (pathogen-associated molecular patterns, PAMPs), whereas others bind endogenous plant compounds (damage-associated molecular patterns, DAMPs) such as peptides released from cells upon attack. This work focuses on the Arabidopsis DAMPs plant elicitor peptides (Peps) and their receptors, PEPR1 and PEPR2. Pep application causes in vivo cGMP generation and downstream signaling that is lost when the predicted PEPR receptor guanylyl cyclase (GC) active site is mutated. Pep-induced Ca(2+) elevation is attributable to cGMP activation of a Ca(2+) channel. Some differences were identified between Pep/PEPR signaling and the Ca(2+)-dependent immune signaling initiated by the flagellin peptide flg22 and its cognate receptor Flagellin-sensing 2 (FLS2). FLS2 signaling may have a greater requirement for intracellular Ca(2+) stores and inositol phosphate signaling, whereas Pep/PEPR signaling requires extracellular Ca(2+). Maximal FLS2 signaling requires a functional Pep/PEPR system. This dependence was evidenced as a requirement for functional PEPR receptors for maximal flg22-dependent Ca(2+) elevation, H(2)O(2) generation, defense gene [WRKY33 and Plant Defensin 1.2 (PDF1.2)] expression, and flg22/FLS2-dependent impairment of pathogen growth. In a corresponding fashion, FLS2 loss of function impaired Pep signaling. In addition, a role for PAMP and DAMP perception in bolstering effector-triggered immunity (ETI) is reported; loss of function of either FLS2 or PEPR receptors impaired the hypersensitive response (HR) to an avirulent pathogen.

Molecular communications between plant heat shock responses and disease resistance

As sessile, plants are continuously exposed to potential dangers including various abiotic stresses and pathogen attack. Although most studies focus on plant responses under an ideal condition to a specific stimulus, plants in nature must cope with a variety of stimuli at the same time. This indicates that it is critical for plants to fine-control distinct signaling pathways temporally and spatially for simultaneous and effective responses to various stresses. Global warming is currently a big issue threatening the future of humans. Reponses to high temperature affect many physiological processes in plants including growth and disease resistance, resulting in decrease of crop yield. Although plant heat stress and defense responses share important mediators such as calcium ions and heat shock proteins, it is thought that high temperature generally suppresses plant immunity. We therefore specifically discuss on interactions between plant heat and defense responses in this review hopefully for an integrated understanding of these responses in plants.

MAP Kinase Cascades in Arabidopsis Innate Immunity

Plant mitogen-activated protein kinase (MAPK) cascades generally transduce extracellular stimuli into cellular responses. These stimuli include the perception of pathogen-associated molecular patterns (PAMPs) by host transmembrane pattern recognition receptors which trigger MAPK-dependent innate immune responses. In the model Arabidopsis, molecular genetic evidence implicates a number of MAPK cascade components in PAMP signaling, and in responses to immunity-related phytohormones such as ethylene, jasmonate, and salicylate. In a few cases, cascade components have been directly linked to the transcription of target genes or to the regulation of phytohormone synthesis. Thus MAPKs are obvious targets for bacterial effector proteins and are likely guardees of resistance proteins, which mediate defense signaling in response to the action of effectors, or effector-triggered immunity. This mini-review discusses recent progress in this field with a focus on the Arabidopsis MAPKs MPK3, MPK4, MPK6, and MPK11 in their apparent pathways.

Calcium signatures and signaling in cytosol and organelles of tobacco cells induced by plant defense elicitors

Calcium signatures induced by two elicitors of plant defense reactions, namely cryptogein and oligogalacturonides, were monitored at the subcellular level, using apoaequorin-transformed Nicotiana tabacum var Xanthi cells, in which the apoaequorin calcium sensor was targeted either to cytosol, mitochondria or chloroplasts. Our study showed that both elicitors induced specific Ca(2+) signatures in each compartment, with the most striking difference relying on duration. Common properties also emerged from the analysis of Ca(2+) signatures: both elicitors induced a biphasic cytosolic [Ca(2+)] elevation together with a single mitochondrial [Ca(2+)] elevation concomitant with the first cytosolic [Ca(2+)] peak. In addition, both elicitors induced a chloroplastic [Ca(2+)] elevation peaking later in comparison to cytosolic [Ca(2+)] elevation. In cryptogein-treated cells, pharmacological studies indicated that IP(3) should play an important role in Ca(2+) signaling contrarily to cADPR or nitric oxide, which have limited or no effect on [Ca(2+)] variations. Our data also showed that, depending on [Ca(2+)] fluxes at the plasma membrane, cryptogein triggered a mitochondrial respiration increase and affected excess energy dissipation mechanisms in chloroplasts. Altogether the results indicate that cryptogein profoundly impacted cell functions at many levels, including organelles.

Plasma membrane calcium ATPases are important components of receptor-mediated signaling in plant immune responses and development

Plasma membrane-resident receptor kinases (RKs) initiate signaling pathways important for plant immunity and development. In Arabidopsis (Arabidopsis thaliana), the receptor for the elicitor-active peptide epitope of bacterial flagellin, flg22, is encoded by FLAGELLIN SENSING2 (FLS2), which promotes plant immunity. Despite its relevance, the molecular components regulating FLS2-mediated signaling remain largely unknown. We show that plasma membrane ARABIDOPSIS-AUTOINHIBITED Ca(2+)-ATPase (ACA8) forms a complex with FLS2 in planta. ACA8 and its closest homolog ACA10 are required for limiting the growth of virulent bacteria. One of the earliest flg22 responses is the transient increase of cytosolic Ca(2+) ions, which is crucial for many of the well-described downstream responses (e.g. generation of reactive oxygen species and the transcriptional activation of defense-associated genes). Mutant aca8 aca10 plants show decreased flg22-induced Ca(2+) and reactive oxygen species bursts and exhibit altered transcriptional reprogramming. In particular, mitogen-activated protein kinase-dependent flg22-induced gene expression is elevated, whereas calcium-dependent protein kinase-dependent flg22-induced gene expression is reduced. These results demonstrate that the fine regulation of Ca(2+) fluxes across the plasma membrane is critical for the coordination of the downstream microbe-associated molecular pattern responses and suggest a mechanistic link between the FLS2 receptor complex and signaling kinases via the secondary messenger Ca(2+). ACA8 also interacts with other RKs such as BRI1 and CLV1 known to regulate plant development, and both aca8 and aca10 mutants show morphological phenotypes, suggesting additional roles for ACA8 and ACA10 in developmental processes. Thus, Ca(2+) ATPases appear to represent general regulatory components of RK-mediated signaling pathways.

Virus-induced gene silencing and its application in plant functional genomics

Virus-induced gene silencing is regarded as a powerful and efficient tool for the analysis of gene function in plants because it is simple, rapid and transformation-free. It has been used to perform both forward and reverse genetics to identify plant functional genes. Many viruses have been developed into virus-induced gene silencing vectors and gene functions involved in development, biotic and abiotic stresses, metabolism, and cellular signaling have been reported. In this review, we discuss the development and application of virus-induced gene silencing in plant functional genomics.

Evolution of plant p-type ATPases

Five organisms having completely sequenced genomes and belonging to all major branches of green plants (Viridiplantae) were analyzed with respect to their content of P-type ATPases encoding genes. These were the chlorophytes Ostreococcus tauri and Chlamydomonas reinhardtii, and the streptophytes Physcomitrella patens (a non-vascular moss), Selaginella moellendorffii (a primitive vascular plant), and Arabidopsis thaliana (a model flowering plant). Each organism contained sequences for all five subfamilies of P-type ATPases. Whereas Na(+) and H(+) pumps seem to mutually exclude each other in flowering plants and animals, they co-exist in chlorophytes, which show representatives for two kinds of Na(+) pumps (P2C and P2D ATPases) as well as a primitive H(+)-ATPase. Both Na(+) and H(+) pumps also co-exist in the moss P. patens, which has a P2D Na(+)-ATPase. In contrast to the primitive H(+)-ATPases in chlorophytes and P. patens, the H(+)-ATPases from vascular plants all have a large C-terminal regulatory domain as well as a conserved Arg in transmembrane segment 5 that is predicted to function as part of a backflow protection mechanism. Together these features are predicted to enable H(+) pumps in vascular plants to create large electrochemical gradients that can be modulated in response to diverse physiological cues. The complete inventory of P-type ATPases in the major branches of Viridiplantae is an important starting point for elucidating the evolution in plants of these important pumps.