Advances and current challenges in calcium signaling

Proton and calcium pumping P-type ATPases and their regulation of plant responses to the environment

Plant plasma membrane H+-ATPases and Ca2+-ATPases maintain low cytoplasmic concentrations of H+ and Ca2+, respectively, and are essential for plant growth and development. These low concentrations allow plasma membrane H+-ATPases to function as electrogenic voltage stats, and Ca2+-ATPases as "off" mechanisms in Ca2+-based signal transduction. Although these pumps are autoregulated by cytoplasmic concentrations of H+ and Ca2+, respectively, they are also subject to exquisite regulation in response to biotic and abiotic events in the environment. A common paradigm for both types of pumps is the presence of terminal regulatory (R) domains that function as autoinhibitors that can be neutralized by multiple means, including phosphorylation. A picture is emerging in which some of the phosphosites in these R domains appear to be highly, nearly constantly phosphorylated, whereas others seem to be subject to dynamic phosphorylation. Thus, some sites might function as major switches, whereas others might simply reduce activity. Here, we provide an overview of the relevant transport systems and discuss recent advances that address their relation to external stimuli and physiological adaptations.

Flg22-induced Ca2+ increases undergo desensitization and resensitization

The flagellin epitope flg22, a pathogen-associated molecular pattern (PAMP), binds to the receptor-like kinase FLAGELLIN SENSING2 (FLS2), and triggers Ca²⁺ influx across the plasma membrane (PM). The flg22-induced increases in cytosolic Ca²⁺ concentration ([Ca²⁺ ]_i ) (FICA) play a crucial role in plant innate immunity. It's well established that the receptor FLS2 and reactive oxygen species (ROS) burst undergo sensitivity adaptation after flg22 stimulation, referred to as desensitization and resensitization, to prevent over responses to pathogens. However, whether FICA also mount adaptation mechanisms to ensure appropriate and efficient responses against pathogens remains poorly understood. Here, we analysed systematically [Ca²⁺ ]_i increases upon two successive flg22 treatments, recorded and characterized rapid desensitization but slow resensitization of FICA in Arabidopsis thaliana. Pharmacological analyses showed that the rapid desensitization might be synergistically regulated by ligand-induced FLS2 endocytosis as well as the PM depolarization. The resensitization of FICA might require de novo FLS2 protein synthesis. FICA resensitization appeared significantly slower than FLS2 protein recovery, suggesting additional regulatory mechanisms of other components, such as flg22-related Ca²⁺ permeable channels. Taken together, we have carefully defined the FICA sensitivity adaptation, which will facilitate further molecular and genetic dissection of the Ca²⁺ -mediated adaptive mechanisms in PAMP-triggered immunity.

Reducing Drought Stress in Plants by Encapsulating Plant Growth-Promoting Bacteria with Polysaccharides

Drought is a major abiotic stress imposed by climate change that affects crop production and soil microbial functions. Plants respond to water deficits at the morphological, biochemical, and physiological levels, and invoke different adaptation mechanisms to tolerate drought stress. Plant growth-promoting bacteria (PGPB) can help to alleviate drought stress in plants through various strategies, including phytohormone production, the solubilization of mineral nutrients, and the production of 1-aminocyclopropane-1-carboxylate deaminase and osmolytes. However, PGPB populations and functions are influenced by adverse soil factors, such as drought. Therefore, maintaining the viability and stability of PGPB applied to arid soils requires that the PGPB have to be protected by suitable coatings. The encapsulation of PGPB is one of the newest and most efficient techniques for protecting beneficial bacteria against unfavorable soil conditions. Coatings made from polysaccharides, such as sodium alginate, chitosan, starch, cellulose, and their derivatives, can absorb and retain substantial amounts of water in the interstitial sites of their structures, thereby promoting bacterial survival and better plant growth.

TypiCal but DeliCate Ca++re: Dissecting the Essence of Calcium Signaling Network as a Robust Response Coordinator of Versatile Abiotic and Biotic Stimuli in Plants

Plant growth, development, and ultimately crop productivity are largely impacted by the interaction of plants with different abiotic and biotic factors throughout their life cycle. Perception of different abiotic stresses, such as salt, cold, drought, heat, and heavy metals, and interaction with beneficial and harmful biotic agents by plants lead to transient, sustained, or oscillatory changes of [calcium ion, Ca²⁺]_cyt within the cell. Significant progress has been made in the decoding of Ca²⁺ signatures into downstream responses to modulate differential developmental and physiological responses in the whole plant. Ca²⁺ sensor proteins, mainly calmodulins (CaMs), calmodulin-like proteins (CMLs), and others, such as Ca²⁺-dependent protein kinases (CDPKs), calcineurin B-like proteins (CBLs), and calmodulin-binding transcription activators (CAMTAs) have played critical roles in coupling the specific stress stimulus with an appropriate response. This review summarizes the current understanding of the Ca²⁺ influx and efflux system in plant cells and various Ca²⁺ binding protein-mediated signal transduction pathways that are delicately orchestrated to mitigate abiotic and biotic stresses. The probable interactions of different components of Ca²⁺ sensor relays and Ca²⁺ sensor responders in response to various external stimuli have been described diagrammatically focusing on established pathways and latest developments. Present comprehensive insight into key components of the Ca²⁺ signaling toolkit in plants can provide an innovative framework for biotechnological manipulations toward crop improvability in near future.

AtCRY2 Negatively Regulates the Functions of AtANN2 and AtANN3 in Drought Tolerance by Affecting Their Subcellular Localization and Transmembrane Ca2+ Flow

Annexins are a multifunctional class of calcium-binding proteins in plants, and their physiological functions and regulation in response to drought stress remain to be elucidated. Here, we found that AtANN2 and AtANN3 conferred to drought tolerance under short-day and long-day conditions, respectively. Under their functional photoperiod, AtANN2 and AtANN3 gene expression was enhanced in the mannitol-treated roots, and their encoded proteins were rapidly targeted to the plasma membrane, and mediated significant Ca²⁺ flows across the plasma membrane. Cryptochromes as photoreceptors can not only sense the photoperiod and regulate ion channels on the plasma membrane to influence ion flow but also induce downstream physiological responses. AtCRY2 repressed the functions of AtANN2 and AtANN3 by affecting their plasma membrane localization and inhibited AtANN2- and AtANN3-dependent transmembrane Ca²⁺ flow in response to drought stress. Taken together, these results uncover a mechanism linking Annexins-AtCRY2 to transmembrane Ca²⁺ flow and resulting in enhanced drought tolerance in Arabidopsis.

Plasma-Activated Water Triggers Rapid and Sustained Cytosolic Ca2+ Elevations in Arabidopsis thaliana

Increasing evidence indicates that water activated by plasma discharge, termed as plasma-activated water (PAW), can promote plant growth and enhance plant defence responses. Nevertheless, the signalling pathways activated in plants in response to PAW are still largely unknown. In this work, we analysed the potential involvement of calcium as an intracellular messenger in the transduction of PAW by plants. To this aim, Arabidopsis thaliana (Arabidopsis) seedlings stably expressing the bioluminescent Ca²⁺ reporter aequorin in the cytosol were challenged with PAW generated by a plasma torch. Ca²⁺ measurement assays demonstrated the induction by PAW of rapid and sustained cytosolic Ca²⁺ elevations in Arabidopsis seedlings. The dynamics of the recorded Ca²⁺ signals were found to depend upon different parameters, such as the operational conditions of the torch, PAW storage, and dilution. The separate administration of nitrate, nitrite, and hydrogen peroxide at the same doses as those measured in the PAW did not trigger any detectable Ca²⁺ changes, suggesting that the unique mixture of different reactive chemical species contained in the PAW is responsible for the specific Ca²⁺ signatures. Unveiling the signalling mechanisms underlying plant perception of PAW may allow to finely tune its generation for applications in agriculture, with potential advantages in the perspective of a more sustainable agriculture.

Molecular Evolution of Calcium Signaling and Transport in Plant Adaptation to Abiotic Stress

Adaptation to unfavorable abiotic stresses is one of the key processes in the evolution of plants. Calcium (Ca2+) signaling is characterized by the spatiotemporal pattern of Ca2+ distribution and the activities of multi-domain proteins in integrating environmental stimuli and cellular responses, which are crucial early events in abiotic stress responses in plants. However, a comprehensive summary and explanation for evolutionary and functional synergies in Ca2+ signaling remains elusive in green plants. We review mechanisms of Ca2+ membrane transporters and intracellular Ca2+ sensors with evolutionary imprinting and structural clues. These may provide molecular and bioinformatics insights for the functional analysis of some non-model species in the evolutionarily important green plant lineages. We summarize the chronological order, spatial location, and characteristics of Ca2+ functional proteins. Furthermore, we highlight the integral functions of calcium-signaling components in various nodes of the Ca2+ signaling pathway through conserved or variant evolutionary processes. These ultimately bridge the Ca2+ cascade reactions into regulatory networks, particularly in the hormonal signaling pathways. In summary, this review provides new perspectives towards a better understanding of the evolution, interaction and integration of Ca2+ signaling components in green plants, which is likely to benefit future research in agriculture, evolutionary biology, ecology and the environment.

The Cell Fate Controlling CLE40 Peptide Requires CNGCs to Trigger Highly Localized Ca2+ Transients in Arabidopsis thaliana Root Meristems

Communication between plant cells and their biotic environment largely depends on the function of plasma membrane localized receptor-like kinases (RLKs). Major players in this communication within root meristems are secreted peptides, including CLAVATA3/EMBRYO SURROUNDING REGION40 (CLE40). In the distal root meristem, CLE40 acts through the RLK ARABIDOPSIS CRINKLY4 (ACR4) and the leucine-rich repeat (LRR) RLK CLAVATA1 (CLV1) to promote cell differentiation. In the proximal meristem, CLE40 signaling requires the LRR receptor-like protein CLAVATA2 (CLV2) and the membrane localized pseudokinase CORYNE (CRN) and serves to inhibit cell differentiation. The molecular components that act immediately downstream of the CLE40-activated receptors are not yet known. Here, we show that active CLE40 signaling triggers the release of intracellular Ca2+ leading to increased cytosolic Ca2+ concentration ([Ca2+]cyt) in a small subset of proximal root meristem cells. This rise in [Ca2+]cyt depends on the CYCLIC NUCLEOTIDE GATED CHANNELS (CNGCs) 6 and 9 and on CLV1. The precise function of changes in [Ca2+]cyt is not yet known but might form a central part of a fine-tuned response to CLE40 peptide that serves to integrate root meristem growth with stem cell fate decisions and initiation of lateral root primordia.

Herbivore-induced volatiles influence moth preference by increasing the β-Ocimene emission of neighbouring tea plants

Herbivore-induced plant volatiles prime neighbouring plants to respond more strongly to subsequent attacks. However, the key volatiles that trigger this state and their priming mechanisms remain largely unknown. The tea geometrid Ectropis obliqua is one of the most devastating leaf-feeding pests of tea plants. Here, plant-plant communication experiments demonstrated that volatiles emitted from tea plants infested by E. obliqua larvae triggered neighbouring plants to release volatiles that repel E. obliqua adult, especially mated females. Volatile analyses revealed that the quantity of eight volatiles increased dramatically when plants were exposed to volatiles emitted by infested tea plants, including (Z)-3-hexenol, linalool, α-farnesene, β-Ocimene and (E)-4,8-dimethyl-1,3,7-nonatriene (DMNT). The results of behavioural bioassays demonstrated that β-Ocimene strongly repelled mated E. obliqua females. Individual volatile compound exposure experiments revealed that (Z)-3-hexenol, linalool, α-farnesene and DMNT triggered the emission of β-Ocimene from tea plants. Chemical inhibition experiments demonstrated that the emission of β-Ocimene induced by (Z)-3-hexenol, linalool, α-farnesene and DMNT were dependent on Ca²⁺ and JA signalling. These findings help us to understand how E. obliqua moths respond to volatiles emitted from tea plants and provide new insight into volatile-mediated plant-plant interactions. They have potential significance for the development of novel insect and pest control strategies in crops.

Characteristics of SlCML39, a Tomato Calmodulin-like Gene, and Its Negative Role in High Temperature Tolerance of Arabidopsis thaliana during Germination and Seedling Growth

Calmodulin-like (CML) proteins are primary calcium sensors and function in plant growth and response to stress stimuli. However, so far, the function of plant CML proteins, including tomato, is still unclear. Previously, it was found that a tomato (Solanum lycopersicum) CML, here named SlCML39, was significantly induced by high temperature (HT) at transcription level, but its biological function is scarce. In this study, the characteristics of SlCML39 and its role in HT tolerance were studied. SlCML39 encodes a protein of 201 amino acids containing four EF hand motifs. Many cis-acting elements related to plant stress and hormone response appear in the promoter regions of SlCML39. SlCML39 is mainly expressed in the root, stem, and leaf and can be regulated by HT, cold, drought, and salt stresses as well as ABA and H₂O₂. Furthermore, heterologous overexpression of SlCML39 reduces HT tolerance in Arabidopsis thaliana at the germination and seedling growth stages. To better understand the molecular mechanism of SlCML39, the downstream gene network regulated by SlCML39 under HT was analyzed by RNA-Seq. Interestingly, we found that many genes involved in stress responses as well as ABA signal pathway are down-regulated in the transgenic seedlings under HT stress, such as KIN1, RD29B, RD26, and MAP3K18. Collectively, these data indicate that SlCML39 acts as an important negative regulator in response to HT stress, which might be mediated by the ABA signal pathway.

Soil Salinity, a Serious Environmental Issue and Plant Responses: A Metabolomics Perspective

The effects of global warming have increasingly led to devastating environmental stresses, such as heat, salinity, and drought. Soil salinization is a serious environmental issue and results in detrimental abiotic stress, affecting 7% of land area and 33% of irrigated lands worldwide. The proportion of arable land facing salinity is expected to rise due to increasing climate change fuelled by anthropogenic activities, exacerbating the threat to global food security for the exponentially growing populace. As sessile organisms, plants have evolutionarily developed mechanisms that allow ad hoc responses to salinity stress. The orchestrated mechanisms include signalling cascades involving phytohormones, kinases, reactive oxygen species (ROS), and calcium regulatory networks. As a pillar in a systems biology approach, metabolomics allows for comprehensive interrogation of the biochemistry and a deconvolution of molecular mechanisms involved in plant responses to salinity. Thus, this review highlights soil salinization as a serious environmental issue and points to the negative impacts of salinity on plants. Furthermore, the review summarises mechanisms regulating salinity tolerance on molecular, cellular, and biochemical levels with a focus on metabolomics perspectives. This critical synthesis of current literature is an opportunity to revisit the current models regarding plant responses to salinity, with an invitation to further fundamental research for novel and actionable insights.

OsANN4 modulates ROS production and mediates Ca2+ influx in response to ABA

BACKGROUND

Plant annexins are calcium- and lipid-binding proteins that have multiple functions, and a significant amount of research on plant annexins has been reported in recent years. However, the functions of annexins in diverse biological processes in rice are largely unclear.

RESULTS

Herein, we report that OsANN4, a calcium-binding rice annexin protein, was induced by abscisic acid (ABA). Under ABA treatment, the plants in which OsANN4 was knocked down by RNA interference showed some visible phenotypic changes compared to the wild type, such as a lower rooting rate and shorter shoot and root lengths. Moreover, the superoxide dismutase (SOD) and catalase (CAT) activities of the RNAi lines were significantly lower and further resulted in higher accumulation of O₂^.- and H₂O₂ than those of the wild-type. A Non-invasive Micro-test Technology (NMT) assay showed that ABA-induced net Ca²⁺ influx was inhibited in OsANN4 knockdown plants. Interestingly, the phenotypic differences caused by ABA were eliminated in the presence of LaCl₃ (Ca²⁺ channel inhibitor). Apart from this, we demonstrated that OsCDPK24 interacted with and phosphorylated OsANN4. When the phosphorylated serine residue of OsANN4 was substituted by alanine, the interaction between OsANN4 and OsCDPK24 was still observed, however, both the conformation of OsANN4 and its binding activity with Ca²⁺ might be changed.

CONCLUSIONS

OsANN4 plays a crucial role in the ABA response, partially by modulating ROS production, mediating Ca²⁺ influx or interacting with OsCDPK24.

Comprehensive genome-wide analysis of calmodulin-binding transcription activator (CAMTA) in Durio zibethinus and identification of fruit ripening-associated DzCAMTAs

Background

Fruit ripening is an intricate developmental process driven by a highly coordinated action of complex hormonal networks. Ethylene is considered as the main phytohormone that regulates the ripening of climacteric fruits. Concomitantly, several ethylene-responsive transcription factors (TFs) are pivotal components of the regulatory network underlying fruit ripening. Calmodulin-binding transcription activator (CAMTA) is one such ethylene-induced TF implicated in various stress and plant developmental processes.

Results

Our comprehensive analysis of the CAMTA gene family in Durio zibethinus (durian, Dz) identified 10 CAMTAs with conserved domains. Phylogenetic analysis of DzCAMTAs, positioned DzCAMTA3 with its tomato ortholog that has already been validated for its role in the fruit ripening process through ethylene-mediated signaling. Furthermore, the transcriptome-wide analysis revealed DzCAMTA3 and DzCAMTA8 as the highest expressing durian CAMTA genes. These two DzCAMTAs possessed a distinct ripening-associated expression pattern during post-harvest ripening in Monthong, a durian cultivar native to Thailand. The expression profiling of DzCAMTA3 and DzCAMTA8 under natural ripening conditions and ethylene-induced/delayed ripening conditions substantiated their roles as ethylene-induced transcriptional activators of ripening. Similarly, auxin-suppressed expression of DzCAMTA3 and DzCAMTA8 confirmed their responsiveness to exogenous auxin treatment in a time-dependent manner. Accordingly, we propose that DzCAMTA3 and DzCAMTA8 synergistically crosstalk with ethylene during durian fruit ripening. In contrast, DzCAMTA3 and DzCAMTA8 antagonistically with auxin could affect the post-harvest ripening process in durian. Furthermore, DzCAMTA3 and DzCAMTA8 interacting genes contain significant CAMTA recognition motifs and regulated several pivotal fruit-ripening-associated pathways.

Conclusion

Taken together, the present study contributes to an in-depth understanding of the structure and probable function of CAMTA genes in the post-harvest ripening of durian.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-021-08022-1.

BAK1-induced RPK1 phosphorylation is essential for RPK1-mediated cell death in Arabidopsis

Being sessile, plants must deploy highly exquisite systems to respond to various internal and external signals for modulating growth and development throughout their lifespan. Many studies on Arabidopsis have shown that leucine-rich repeat-containing receptor-like kinases, including BRI1-associated receptor kinase 1 (BAK1) and receptor-like protein kinase 1 (RPK1), are suitable for such pleiotropic demands of plants. Previously, BAK1 and RPK1 were independently proven to be involved in the regulation of premature cell death. BAK1 inhibits spontaneous cell death and promotes defense-induced cell death. Meanwhile, RPK1 mediates reactive oxygen species (ROS) production through complexation with CaM4 and RbohF in an age-dependent manner. In the present study, RPK1-induced cell death and growth retardation were abolished both with respect to the phenotype and ROS production in bak1 mutants. Moreover, BAK1 interacts with RPK1 and mediates its unidirectional phosphorylation in plants. Further, BAK1-mediated RPK1 phosphorylation is indispensable for RPK1-CaM4 interaction, which is vital for ROS production, resulting in cell death. The presence of BAK1 enhanced the expression of cell death- and senescence-related genes, such as ORE1, PR1, SAG12, and SIRK in RPK1-mediated signaling cascades. Overall, in Arabidopsis, in addition to independent cell death regulation by BAK1 and RPK1, multiple-layers control cell death and premature senescence via the coordinated action of BAK1 and RPK1.

Advances and prospects of rhodopsin-based optogenetics in plant research

Microbial rhodopsins have advanced optogenetics since the discovery of channelrhodopsins almost two decades ago. During this time an abundance of microbial rhodopsins has been discovered, engineered, and improved for studies in neuroscience and other animal research fields. Optogenetic applications in plant research, however, lagged largely behind. Starting with light-regulated gene expression, optogenetics has slowly expanded into plant research. The recently established all-trans retinal production in plants now enables the use of many microbial opsins, bringing extra opportunities to plant research. In this review, we summarize the recent advances of rhodopsin-based plant optogenetics and provide a perspective for future use, combined with fluorescent sensors to monitor physiological parameters.

Illuminating the hidden world of calcium ions in plants with a universe of indicators

The tools available to carry out in vivo analysis of Ca²⁺ dynamics in plants are powerful and mature technologies that still require the proper controls.

Multiparameter in vivo imaging in plants using genetically encoded fluorescent indicator multiplexing

Biological processes are highly dynamic, and during plant growth, development, and environmental interactions, they occur and influence each other on diverse spatiotemporal scales. Understanding plant physiology on an organismic scale requires analyzing biological processes from various perspectives, down to the cellular and molecular levels. Ideally, such analyses should be conducted on intact and living plant tissues. Fluorescent protein (FP)-based in vivo biosensing using genetically encoded fluorescent indicators (GEFIs) is a state-of-the-art methodology for directly monitoring cellular ion, redox, sugar, hormone, ATP and phosphatidic acid dynamics, and protein kinase activities in plants. The steadily growing number of diverse but technically compatible genetically encoded biosensors, the development of dual-reporting indicators, and recent achievements in plate-reader-based analyses now allow for GEFI multiplexing: the simultaneous recording of multiple GEFIs in a single experiment. This in turn enables in vivo multiparameter analyses: the simultaneous recording of various biological processes in living organisms. Here, we provide an update on currently established direct FP-based biosensors in plants, discuss their functional principles, and highlight important biological findings accomplished by employing various approaches of GEFI-based multiplexing. We also discuss challenges and provide advice for FP-based biosensor analyses in plants.

Broadening the definition of a nervous system to better understand the evolution of plants and animals

Most textbook definitions recognize only animals as having nervous systems. However, for the past couple decades, botanists have been meticulously studying long-distance signaling systems in plants, and some researchers have stated that plants have a simple nervous system. Thus, an academic conflict has emerged between those who defend and those who deny the existence of a nervous system in plants. This article analyses that debate, and we propose an alternative to answering yes or no: broadening the definition of a nervous system to include plants. We claim that a definition broader than the current one, which is based only on a phylogenetic viewpoint, would be helpful in obtaining a deeper understanding of how evolution has driven the features of signal generation, transmission and processing in multicellular beings. Also, we propose two possible definitions and exemplify how broader a definition allows for new viewpoints on the evolution of plants, animals and the nervous system.

Calcium signaling networks mediate nitrate sensing and responses in Arabidopsis

Nitrate signaling integrates and coordinates the expression of a wide range of genes, metabolic pathways and ultimately, plant growth and development. Calcium signaling is proved to be involved in the primary nitrate response pathway. However, it is much less understood how calcium signaling mediates nitrate sensing and responses from the extracellular space to cytoplasm, then to the nucleus. In this review, we describe how transceptor-channel complex (cyclic nucleotide-gated channel protein 15 interacting with nitrate transceptor, CNGC15-NRT1.1), calcineurin B-like proteins (CBLs, CBL1, CBL9), CBL-interacting protein kinases (CIPKs), phospholipase C (PLC) and calcium-dependent protein kinases (CDPKs, also CPKs), acting as key players, complete a potential backbone of the nitrate-signaling pathway, from the plasma membrane to the nucleus. NRT1.1 together with CBL1/9-CIPK23 and CBL-CIPK8 links the NO₃^- signaling to cytoplasmic and nuclear regulators and triggers downstream NO₃^- responses. PLCs and inositol 1, 4, 5-triphosphate (IP3) connect NO₃^- signaling and cytoplasmic Ca²⁺ signature. CPK10/30/32 fill the gap between NRT1.1 and NIN-like protein (NLP) transcription factors. The arabidopsis nitrate regulated1 (ANR1) is induced from the endosome by the Ca²⁺-CPKs-NLPs signaling pathway activated by the unphosphorylated form of NRT1.1 (NRT1.1 ^T101A) at high nitrate condition. Understanding how calcium signaling interconnects the upstream nitrate sensor complex with downstream multiple sensors of the nitrate-signaling pathway is key to completing the nutrient-growth regulatory networks.

Fe acquisition at the crossroad of calcium and reactive oxygen species signaling

Due to its redox properties, iron is both essential and toxic. Therefore, soil iron availability variations pose a significant problem for plants. Recent evidence suggests that calcium and reactive oxygen species coordinate signaling events related to soil iron acquisition. Calcium was found to affect directly IRT1-mediated iron import through the lipid-binding protein EHB1 and to trigger a CBL-CIPK-mediated signaling influencing the activity of the key iron-acquisition transcription factor FIT. In parallel, under prolonged iron deficiency, reactive oxygen species both inhibit FIT function and depend on FIT through the function of the catalase CAT2. We discuss the role of calcium and reactive oxygen species signaling in iron acquisition, with post-translational mechanisms influencing the localization and activity of iron-acquisition regulators and effectors.

The CaCIPK3 gene positively regulates drought tolerance in pepper

Drought stress is a major agricultural problem restricting the growth, development, and productivity of plants. Calcineurin B-like proteins (CBLs) and CBL-interacting protein kinases (CIPKs) significantly influence the plant response to different stresses. However, the molecular mechanisms of CBL-CIPK in the drought stress response of pepper are still unknown. Here, the function of CaCIPK3 in the regulation of drought stress in pepper (Capsicum annuum L.) was explored. Transcriptomic data and quantitative real-time PCR (qRT-PCR) analysis revealed that CaCIPK3 participates in the response to multiple stresses. Knockdown of CaCIPK3 in pepper increased the sensitivity to mannitol and methyl jasmonate (MeJA). Transient overexpression of CaCIPK3 improved drought tolerance by enhancing the activities of the antioxidant system and positively regulating jasmonate (JA)-related genes. Ectopic expression of CaCIPK3 in tomato also improved drought and MeJA resistance. As the CaCIPK3-interacting partner, CaCBL2 positively influenced drought resistance. Additionally, CaWRKY1 and CaWRKY41 directly bound the CaCIPK3 promoter to influence its expression. This study shows that CaCIPK3 acts as a positive regulator in drought stress resistance via the CBL-CIPK network to regulate MeJA signaling and the antioxidant defense system.

Calcium-dependent protein kinase CPK32 mediates calcium signaling in regulating Arabidopsis flowering time

Appropriate flowering time is critical for the reproductive success of plant species. Emerging evidence indicates that calcium may play an important role in the regulation of flowering time. However, the underlying molecular mechanisms remain unclear. In this study, we demonstrate that calcium-dependent protein kinase 32 (CPK32) regulates flowering time by affecting the alternative polyadenylation of FLOWERING CONTROL LOCUS A (FCA) and altering the transcription of FLOWERING LOCUS C (FLC), a central repressor of flowering time. The knockdown of CPK32 results in an obvious late flowering phenotype and dramatically enhanced FLC transcription. CPK32 interacts with FCA, and phosphorylates the serine⁵⁹² of FCA in a Ca²⁺-dependent manner. Moreover, the ratio of abundance of the FCA transcripts (FCA-D and FCA-P) changes significantly in the cpk32 mutant, which subsequently affects FLC expression and consequently regulates floral transition. The present evidence demonstrates that CPK32 modulates flowering time by regulating FCA alternative polyadenylation and consequent FLC expression.

Regulation of Root Nutrient Transporters by CIPK23: 'One Kinase to Rule Them All'

Protein kinases constitute essential regulatory components in the majority of cellular processes in eukaryotic cells. The CBL-INTERACTING PROTEIN KINASE (CIPK) family of plant protein kinases functions in calcium (Ca2+)-related signaling pathways and is therefore involved in the response to a wide variety of signals in plants. By covalently linking phosphate groups to their target proteins, CIPKs regulate the activity of downstream targets, their localization, their stability and their ability to interact with other proteins. In Arabidopsis, the CIPK23 kinase has emerged as a major hub driving root responses to diverse environmental stresses, including drought, salinity and nutrient imbalances, such as potassium, nitrate and iron deficiencies, as well as ammonium, magnesium and non-iron metal toxicities. This review will chiefly report on the prominent roles of CIPK23 in the regulation of plant nutrient transporters and on the underlying molecular mechanisms. We will also discuss the different scenarios explaining how a single promiscuous kinase, such as CIPK23, may convey specific responses to a myriad of signals.

Chloroplast calcium signalling regulates thermomemory

After an episode of heat stress plants retain a cellular "memory" of this event, a phenomenon known as thermomemory. This mechanism allows plants to better cope against a subsequent heat event. Thermomemory occurs through the persistence of heat shock proteins (HSPs) synthesized after the first "priming" event. Maintenance of this thermomemory comes at the cost to growth though, therefore it is vital that the memory is reset when no longer required. Recently, it has been reported that autophagy is important for resetting the thermomemory. It has also been shown recently that in response to heat, Arabidopsis displays an increase in chloroplast free calcium concentration which is partially dependent on calcium sensing receptor (CAS) protein. It is not known what the purpose of this heat-activated calcium signal is. Therefore, we compared downstream responses to heat in wild type (WT) and cas mutants, as the latter produce a reduced chloroplast calcium signal to heat. We found that after thermopriming the cas mutants displayed a greater biomass and a reduced level of the small heat shock protein HSP 17.6 degradation compared to WT. cas mutants did not show an increase in free amino acid levels after thermopriming, suggesting reduced autophagy. These results suggest that heat-induced chloroplast calcium elevation is a positive signal for resetting of the thermomemory.

Calmodulin-binding transcription activator AtSR1/CAMTA3 fine-tunes plant immune response by transcriptional regulation of the salicylate receptor NPR1

Calcium (Ca²⁺ ) signalling regulates salicylic acid (SA)-mediated immune response through calmodulin-meditated transcriptional activators, AtSRs/CAMTAs, but its mechanism is not fully understood. Here, we report an AtSR1/CAMTA3-mediated regulatory mechanism involving the expression of the SA receptor, NPR1. Results indicate that the transcriptional expression of NPR1 was regulated by AtSR1 binding to a CGCG box in the NPR1 promotor. The atsr1 mutant exhibited resistance to the virulent strain of Pseudomonas syringae pv. tomato (Pst), however, was susceptible to an avirulent Pst strain carrying avrRpt2, due to the failure of the induction of hypersensitive responses. These resistant/susceptible phenotypes in the atsr1 mutant were reversed in the npr1 mutant background, suggesting that AtSR1 regulates NPR1 as a downstream target during plant immune response. The virulent Pst strain triggered a transient elevation in intracellular Ca²⁺ concentration, whereas the avirulent Pst strain triggered a prolonged change. The distinct Ca²⁺ signatures were decoded into the regulation of NPR1 expression through AtSR1's IQ motif binding with Ca²⁺ -free-CaM2, while AtSR1's calmodulin-binding domain with Ca²⁺ -bound-CaM2. These observations reveal a role for AtSR1 as a Ca²⁺ -mediated transcription regulator in controlling the NPR1-mediated plant immune response.

Electrical Signals, Plant Tolerance to Actions of Stressors, and Programmed Cell Death: Is Interaction Possible?

In environmental conditions, plants are affected by abiotic and biotic stressors which can be heterogenous. This means that the systemic plant adaptive responses on their actions require long-distance stress signals including electrical signals (ESs). ESs are based on transient changes in the activities of ion channels and H⁺-ATP-ase in the plasma membrane. They influence numerous physiological processes, including gene expression, phytohormone synthesis, photosynthesis, respiration, phloem mass flow, ATP content, and many others. It is considered that these changes increase plant tolerance to the action of stressors; the effect can be related to stimulation of damages of specific molecular structures. In this review, we hypothesize that programmed cell death (PCD) in plant cells can be interconnected with ESs. There are the following points supporting this hypothesis. (i) Propagation of ESs can be related to ROS waves; these waves are a probable mechanism of PCD initiation. (ii) ESs induce the inactivation of photosynthetic dark reactions and activation of respiration. Both responses can also produce ROS and, probably, induce PCD. (iii) ESs stimulate the synthesis of stress phytohormones (e.g., jasmonic acid, salicylic acid, and ethylene) which are known to contribute to the induction of PCD. (iv) Generation of ESs accompanies K⁺ efflux from the cytoplasm that is also a mechanism of induction of PCD. Our review argues for the possibility of PCD induction by electrical signals and shows some directions of future investigations in the field.

MPK3/6-induced degradation of ARR1/10/12 promotes salt tolerance in Arabidopsis

Cytokinins are phytohormones that regulate plant development, growth, and responses to stress. In particular, cytokinin has been reported to negatively regulate plant adaptation to high salinity; however, the molecular mechanisms that counteract cytokinin signaling and enable salt tolerance are not fully understood. Here, we provide evidence that salt stress induces the degradation of the cytokinin signaling components Arabidopsis (Arabidopisis thaliana) response regulator 1 (ARR1), ARR10 and ARR12. Furthermore, the stress-activated mitogen-activated protein kinase 3 (MPK3) and MPK6 interact with and phosphorylate ARR1/10/12 to promote their degradation in response to salt stress. As expected, salt tolerance is decreased in the mpk3/6 double mutant, but enhanced upon ectopic MPK3/MPK6 activation in an MKK5^DD line. Importantly, salt hypersensitivity phenotypes of the mpk3/6 line were significantly alleviated by mutation of ARR1/12. The above results indicate that MPK3/6 enhance salt tolerance in part via their negative regulation of ARR1/10/12 protein stability. Thus, our work reveals a new molecular mechanism underlying salt-induced stress adaptation and the inhibition of plant growth, via enhanced degradation of cytokinin signaling components.

VDACs: An Outlook on Biochemical Regulation and Function in Animal and Plant Systems

The voltage-dependent anion channels (VDACs) are the most abundant proteins present on the outer mitochondrial membrane. They serve a myriad of functions ranging from energy and metabolite exchange to highly debatable roles in apoptosis. Their role in molecular transport puts them on the center stage as communicators between cytoplasmic and mitochondrial signaling events. Beyond their general role as interchangeable pores, members of this family may exhibit specific functions. Even after nearly five decades of their discovery, their role in plant systems is still a new and rapidly emerging field. The information on biochemical regulation of VDACs is limited. Various interacting proteins and post-translational modifications (PTMs) modulate VDAC functions, amongst these, phosphorylation is quite noticeable. In this review, we have tried to give a glimpse of the recent advancements in the biochemical/interactional regulation of plant VDACs. We also cover a critical analysis on the importance of PTMs in the functional regulation of VDACs. Besides, the review also encompasses numerous studies which can identify VDACs as a connecting link between Ca2+ and reactive oxygen species signaling in special reference to the plant systems.

Ca2+talyzing Initial Responses to Environmental Stresses

Plants have evolved stress-sensing machineries that initiate rapid adaptive environmental stress responses. Cytosolic calcium ion (Ca²⁺) is the most prominent second messenger that couples extracellular signals with specific intracellular responses. Essential early events that generate a cytosolic Ca²⁺ spike in response to environmental stress are starting to emerge. We review sensory machineries, including ion channels and transporters, which perceive various stress stimuli and allow cytosolic Ca²⁺ influx. We highlight integrative roles of Ca²⁺ channels in plant responses to various environmental stresses, as well as possible interplay of Ca²⁺ with other early signaling components, which facilitates signal propagation for systemic spread and spatiotemporal variations in respect to external cues. The early Ca²⁺ signaling schemes inspire the identification of additional stress sensors.

Managing activity and Ca2+ dependence through mutation in the Junction of the AtCPK1 coordinates the salt tolerance in transgenic tobacco plants

Calcium-dependent protein kinases (CDPKs) are Ca2+ decoders in plants. AtCPK1 is a positive regulator in the plant response to biotic and abiotic stress. Inactivation of the autoinhibitory domain of AtCPK1 in the mutated form KJM23 provides constitutive activity of the kinase. In the present study, we investigated the effect of overexpressed native and mutant KJM23 forms on salinity tolerance in Nicotiana tabacum. Overexpression of native AtCPK1 provided tobacco resistance to 120 mM NaCl during germination and 180 mM NaCl during long-term growth, while the resistance of plants increased to 240 mM NaCl during both phases of plant development when transformed with KJM23. Mutation in the junction KJM4, which disrupted Ca2+ induced activation, completely nullified the acquired salt tolerance up to levels of normal plants. Analysis by confocal microscopy showed that under high salinity conditions, overexpression of AtCPK1 and KJM23 inhibited reactive oxygen species (ROS) accumulation to levels observed in untreated plants. Quantitative real-time PCR analysis showed that overexpression of AtCPK1 and KJM23 was associated with changes in expression of genes encoding heat shock factors. In all cases, the KJM23 mutation enhanced the effect of AtCPK1, while the KJM4 mutation reduced it to the control level. We suggest that the autoinhibitory domains in CDPKs could be promising targets for manipulation in engineering salt-tolerant plants.

Genome-Wide Identification and Characterization of the Potato IQD Family During Development and Stress

Calmodulin-binding proteins belong to the IQ67 domain (IQD) gene family and play essential roles in plant development and stress responses. However, the role of IQD gene family in potato (Solanum tuberosum L.) is yet to be known. In the present study, 23 StIQDs were identified in the potato genome and named StIQD1 to StIQD23. They were unevenly distributed on 10 of the 12 chromosomes. Phylogenetic analysis divided the IQDs into four subfamilies (IQD I–IV). StIQDs found in three of the four subfamilies. Synteny analysis confirmed that potato and tomato shared a close evolutionary relationship. Besides, RNA-Seq data analysis revealed that the expression of 19 of the 23 StIQDs was detected in at least one of the 12 tissues, and some of which showed a tissue-specific pattern. Quantitative reverse transcriptase–polymerase chain reaction results further confirmed that 14 StIQDs responded differently to various abiotic stresses, including drought, extreme temperature, and CaCl₂ treatment, suggesting their significance in stress response. This study presents a comprehensive overview of the potato IQD gene family and lays a foundation for further analysis of the StIQDs functions in plant development and stress response.

Revealing the Role of the Calcineurin B-Like Protein-Interacting Protein Kinase 9 (CIPK9) in Rice Adaptive Responses to Salinity, Osmotic Stress, and K+ Deficiency

In plants, calcineurin B-like (CBL) proteins and their interacting protein kinases (CIPK) form functional complexes that transduce downstream signals to membrane effectors assisting in their adaptation to adverse environmental conditions. This study addresses the issue of the physiological role of CIPK9 in adaptive responses to salinity, osmotic stress, and K⁺ deficiency in rice plants. Whole-plant physiological studies revealed that Oscipk9 rice mutant lacks a functional CIPK9 gene and displayed a mildly stronger phenotype, both under saline and osmotic stress conditions. The reported difference was attributed to the ability of Oscipk9 to maintain significantly higher stomatal conductance (thus, a greater carbon gain). Oscipk9 plants contained much less K⁺ in their tissues, implying the role of CIPK9 in K⁺ acquisition and homeostasis in rice. Oscipk9 roots also showed hypersensitivity to ROS under conditions of low K⁺ availability suggesting an important role of H₂O₂ signalling as a component of plant adaptive responses to a low-K environment. The likely mechanistic basis of above physiological responses is discussed.

Plant Responses to Abiotic Stresses and Rhizobacterial Biostimulants: Metabolomics and Epigenetics Perspectives

In response to abiotic stresses, plants mount comprehensive stress-specific responses which mediate signal transduction cascades, transcription of relevant responsive genes and the accumulation of numerous different stress-specific transcripts and metabolites, as well as coordinated stress-specific biochemical and physiological readjustments. These natural mechanisms employed by plants are however not always sufficient to ensure plant survival under abiotic stress conditions. Biostimulants such as plant growth-promoting rhizobacteria (PGPR) formulation are emerging as novel strategies for improving crop quality, yield and resilience against adverse environmental conditions. However, to successfully formulate these microbial-based biostimulants and design efficient application programs, the understanding of molecular and physiological mechanisms that govern biostimulant-plant interactions is imperatively required. Systems biology approaches, such as metabolomics, can unravel insights on the complex network of plant-PGPR interactions allowing for the identification of molecular targets responsible for improved growth and crop quality. Thus, this review highlights the current models on plant defence responses to abiotic stresses, from perception to the activation of cellular and molecular events. It further highlights the current knowledge on the application of microbial biostimulants and the use of epigenetics and metabolomics approaches to elucidate mechanisms of action of microbial biostimulants.

FERONIA Confers Resistance to Photooxidative Stress in Arabidopsis

Plants absorb light energy required for photosynthesis, but excess light can damage plant cells. To protect themselves, plants have developed diverse signaling pathways which are activated under high-intensity light. Plant photoprotection mechanisms have been mainly investigated under conditions of extremely high amount of light; thus, it is largely unknown how plants manage photooxidative damage under moderate light intensities. In the present study, we found that FERONIA (FER) is a key protein that confers resistance to photooxidative stress in plants under moderate light intensity. FER-deficient mutants were highly susceptible to increasing light intensity and exhibited photobleaching even under moderately elevated light intensity (ML). Light-induced expression of stress genes was largely diminished by the fer-4 mutation. In addition, excitation pressure on Photosystem II was significantly increased in fer-4 mutants under ML. Consistently, reactive oxygen species, particularly singlet oxygen, accumulated in fer-4 mutants grown under ML. FER protein abundance was found to be elevated after exposure to ML, which is indirectly affected by the ubiquitin-proteasome pathway. Altogether, our findings showed that plants require FER-mediated photoprotection to maintain their photosystems even under moderate light intensity.

Tissue-level transcriptomic responses to local and distal chilling reveal potential chilling survival mechanisms in maize

Chilling is a major stress to plants of subtropical and tropical origins including maize (Zea mays L.). To reveal molecular mechanisms underlying chilling tolerance and survival, we investigated transcriptomic responses to chilling stress in differentiated leaves and roots as well as in crowns with meristem activity in maize. Chilling stress on shoots and roots is found to each contributes to seedling lethality in maize. Comparison of maize lines with different chilling tolerance capacities reveals that chilling survival is highly associated with upregulation of abscisic acid biosynthesis and response as well as transcriptional regulators in leaves and crowns. It is also associated with the downregulation of translation in leaves and heat response in crowns. Chilling treatment on whole or part of the plants reveals that response to distal-chilling is very distinct from, and sometimes opposite to, response to local- or whole-plant chilling in both leaves and roots, suggesting a communication between shoots and roots in environmental response. This study thus provides transcriptomic responses in leaves, roots and crowns under differential chilling stresses in maize and reveals potential chilling tolerance and survival mechanisms which lays ground for improving chilling tolerance in crop plants.

Integrated global analysis reveals a vitamin E-vitamin K1 sub-network, downstream of COLD1, underlying rice chilling tolerance divergence

Rice, a staple food with tropical/subtropical origination, is susceptible to cold stress, one of the major constraints on its yield and distribution. Asian cultivated rice consists of two subspecies with diverged chilling tolerance to adapt to different environments. The mechanism underlying this divergence remains obscure with a few known factors, including membrane protein CHILLING-TOLERANCE DIVERGENCE 1 (COLD1). Here, we reveal a vitamin E-vitamin K1 sub-network responsible for chilling tolerance divergence through global analyses. Rice genome regions responsible for tolerance divergence are identified with chromosome segment substitution lines (CSSLs). Comparative transcriptomic and metabolomic analysis of chilling-tolerant CSSL4-1 and parent lines uncovered a vitamin E-vitamin K1 sub-network in chloroplast with tocopherol (vitamin E) mediating chloroplast-to-nucleus signaling. COLD1, located in the substitution segment in CSSL4-1, is confirmed as its upstream regulator by transgenic material analysis. Our work uncovers a pathway downstream of COLD1, through which rice modulates chilling tolerance for thermal adaptation, with potential utility in crop improvement.

Shared and tailored common bean transcriptomic responses to combined fusarium wilt and water deficit

Common bean (Phaseolus vulgaris L.), one of the most consumed food legumes worldwide, is threatened by two main constraints that are found frequently together in nature, water deficit (WD) and fusarium wilt (Fop). To understand the shared and unique responses of common bean to Fop and WD, we analyzed the transcriptomic changes and phenotypic responses in two accessions, one resistant and one susceptible to both stresses, exposed to single and combined stresses. Physiological responses (photosynthetic performance and pigments quantification) and disease progression were also assessed. The combined FopWD imposition negatively affected the photosynthetic performance and increased the susceptible accession disease symptoms. The susceptible accession revealed a higher level of transcriptional changes than the resistant one, and WD single stress triggered the highest transcriptional changes. While 89 differentially expressed genes were identified exclusively in combined stresses for the susceptible accession, 35 were identified in the resistant one. These genes belong mainly to "stress", "signaling", "cell wall", "hormone metabolism", and "secondary metabolism" functional categories. Among the up-regulated genes with higher expression in the resistant accession, the cysteine-rich secretory, antigen 5 and Pr-1 (CAP) superfamily protein, a ribulose bisphosphate carboxylase family protein, and a chitinase A seem promising targets for multiple stress breeding.

Long Non-Coding RNA and Its Regulatory Network Response to Cold Stress in Eucalyptus urophylla S.T.Blake

Long non-coding RNA (lncRNA) plays an important regulatory role in plant growth and development, but its systematic identification and analysis in Eucalyptus has not yet been reported. Cold stress has a huge impact on the survival and yield of Eucalyptus seedlings, but the regulatory mechanism of lncRNA in Eucalyptus in response to cold stress is still unclear. In this study, the transcriptomes of young leaves of Eucalyptus urophylla S.T.Blake under low-temperature treatment and restoration were analyzed by RNA-seq. A total of 11,394 lncRNAs and 46,276 mRNAs were identified, of which 300 were differentially expressed lncRNAs (DE_lncRNAs) and 5606 were differentially expressed target genes of lncRNAs under cold stress, with the total number of target genes of DE_lncRNAs being 1681. A total of 677 differentially expressed transcription factors (TFs) were also identified, mainly including ERF, MYB and the NAC transcription factor family. Gene ontology (GO) analysis of the differentially expressed genes (DEGs) and target genes of DE_lncRNAs was mostly related to the response to cold stress and external stimuli. Furthermore, lncRNA–miRNA–mRNA regulatory networks were constructed, and 22 DE_lncRNAs were predicted to be targets or targeting mimics of 20 miRNAs. A qRT-PCR was used to verify the relative expression of genes in the regulatory EuGBF3-EUC_00002677-MSTRG.7690 network, and it matched the transcriptome data, indicating that it may play an important role in the response to cold stress in E. urophylla. This study provides a new insight into lncRNA and its regulatory network under abiotic stress, especially cold stress in E. urophylla.

Overexpression of OsCASP1 Improves Calcium Tolerance in Rice

The Casparian strip domain protein 1 (OsCASP1) is necessary for the formation of the Casparian strip (CS) in the rice endodermis. It also controls Ca²⁺ transport to the stele. Here, we demonstrated that OsCASP1 overexpression enhanced Ca tolerance in rice. Under normal conditions, OsCASP1-overexpressed lines showed similar concentrations of essential metals in the roots and shoots compared to the wild type, while under high Ca conditions, Ca in the roots, shoots, and xylem sap of the OsCASP1-overexpressed lines was significantly decreased. This did not apply to other essential metals. Ca-inhibited growth was significantly alleviated in the OsCASP1-overexpressed lines. Furthermore, OsCASP1 overexpression resulted in earlier formation of both the CS and functional apoplastic barrier in the endodermis but did not induce ectopic CS formation in non-endodermal cell layers and affect suberin accumulation in the endodermis. These results indicate that the overexpression of OsCASP1 promotes CS formation in endodermal cells and inhibits Ca²⁺ transport by the apoplastic pathway, restricting Ca accumulation in the roots and shoots under high Ca conditions. Taken together, the results suggest that OsCASP1 overexpression is an effective way to improve rice adaptation to high Ca environments.

Carbon/nitrogen metabolism and stress response networks - calcium-dependent protein kinases as the missing link?

Calcium-dependent protein kinases (CDPKs) play essential roles in plant development and stress responses. CDPKs have a conserved kinase domain, followed by an auto-inhibitory junction connected to the calmodulin-like domain that binds Ca2+. These structural features allow CDPKs to decode the dynamic changes in cytoplasmic Ca2+ concentrations triggered by hormones and by biotic and abiotic stresses. In response to these signals, CDPKs phosphorylate downstream protein targets to regulate growth and stress responses according to the environmental and developmental circumstances. The latest advances in our understanding of the metabolic, transcriptional, and protein-protein interaction networks involving CDPKs suggest that they have a direct influence on plant carbon/nitrogen (C/N) balance. In this review, we discuss how CDPKs could be key signaling nodes connecting stress responses with metabolic homeostasis, and acting together with the sugar and nutrient signaling hubs SnRK1, HXK1, and TOR to improve plant fitness.

Kinase SnRK1.1 regulates nitrate channel SLAH3 engaged in nitrate-dependent alleviation of ammonium toxicity

Nitrate (NO3-) and ammonium (NH4+) are major inorganic nitrogen (N) supplies for plants, but NH4+ as the sole or dominant N source causes growth inhibition in many plants, known as ammonium toxicity. Small amounts of NO3- can significantly mitigate ammonium toxicity, and the anion channel SLAC1 homolog 3 (SLAH3) is involved in this process, but the mechanistic detail of how SLAH3 regulates nitrate-dependent alleviation of ammonium toxicity is still largely unknown. In this study, we identified SnRK1.1, a central regulator involved in energy homeostasis, and various stress responses, as a SLAH3 interactor in Arabidopsis (Arabidopsis thaliana). Our results suggest that SNF1-related protein kinase 1 (SnRK1.1) functions as a negative regulator of SLAH3. Kinase assays indicate SnRK1.1 strongly phosphorylates the C-terminal of SLAH3 at the site S601. Under high-NH4+/low-pH condition, phospho-mimetic and phospho-dead mutations in SLAH3 S601 result in barely rescued phenotypes and fully complemented phenotypes in slah3. Furthermore, SnRK1.1 migrates from cytoplasm to nucleus under high-NH4+/low-pH conditions. The translocation of SnRK1.1 from cytosol to nucleus under high-ammonium stress releases the inhibition on SLAH3, which allows SLAH3-mediated NO3- efflux leading to alleviation of high-NH4+/low-pH stress. Our study reveals that the C-terminal phosphorylation also plays important role in SLAH3 regulation and provides additional insights into nitrate-dependent alleviation of ammonium toxicity in plants.

Plant Transcription Factors Involved in Drought and Associated Stresses

Transcription factors (TFs) play a significant role in signal transduction networks spanning the perception of a stress signal and the expression of corresponding stress-responsive genes. TFs are multi-functional proteins that may simultaneously control numerous pathways during stresses in plants-this makes them powerful tools for the manipulation of regulatory and stress-responsive pathways. In recent years, the structure-function relationships of numerous plant TFs involved in drought and associated stresses have been defined, which prompted devising practical strategies for engineering plants with enhanced stress tolerance. Vast data have emerged on purposely basic leucine zipper (bZIP), WRKY, homeodomain-leucine zipper (HD-Zip), myeloblastoma (MYB), drought-response elements binding proteins/C-repeat binding factor (DREB/CBF), shine (SHN), and wax production-like (WXPL) TFs that reflect the understanding of their 3D structure and how the structure relates to function. Consequently, this information is useful in the tailored design of variant TFs that enhances our understanding of their functional states, such as oligomerization, post-translational modification patterns, protein-protein interactions, and their abilities to recognize downstream target DNA sequences. Here, we report on the progress of TFs based on their interaction pathway participation in stress-responsive networks, and pinpoint strategies and applications for crops and the impact of these strategies for improving plant stress tolerance.

A voltage-dependent Ca2+ homeostat operates in the plant vacuolar membrane

Cytosolic calcium signals are evoked by a large variety of biotic and abiotic stimuli and play an important role in cellular and long distance signalling in plants. While the function of the plasma membrane in cytosolic Ca²⁺ signalling has been intensively studied, the role of the vacuolar membrane remains elusive. A newly developed vacuolar voltage clamp technique was used in combination with live-cell imaging, to study the role of the vacuolar membrane in Ca²⁺ and pH homeostasis of bulging root hair cells of Arabidopsis. Depolarisation of the vacuolar membrane caused a rapid increase in the Ca²⁺ concentration and alkalised the cytosol, while hyperpolarisation led to the opposite responses. The relationship between the vacuolar membrane potential, the cytosolic pH and Ca²⁺ concentration suggests that a vacuolar H⁺ /Ca²⁺ exchange mechanism plays a central role in cytosolic Ca²⁺ homeostasis. Mathematical modelling further suggests that the voltage-dependent vacuolar Ca²⁺ homeostat could contribute to calcium signalling when coupled to a recently discovered K⁺ channel-dependent module for electrical excitability of the vacuolar membrane.

Characterization of two group III potato CDPKs, StCDPK22 and StCDPK24, that contain three EF-Hand motifs in their CLDs

Four members of the potato (Solanum tuberosum L.) calcium-dependent protein kinase (CDPK) family StCDPK22/23/24 and StCDPK27, present three functional EF-hands motifs in their calmodulin-like domain (CLD). StCDPK22/23/24 are clustered in clade III-b1 with tomato and Arabidopsis CDPKs that lack the first EF-hand motif, while StCDPK27 is clustered in clade III-b3 with CDPKs that lack EF-hand 2. Members of each clade share similar intron-exon structures and acylation profiles. 3D model predictions suggested that StCDPK22 and StCDPK24 are active kinases that undergo a conformational switch in the presence of Ca²⁺ even when lacking one functional EF-hand motif; however, assays performed with recombinant proteins indicated that StCDPK24:6xHis was active in all the conditions tested, and its activity was enhanced in the presence of Ca²⁺, but StCDPK22:6xHis had scarce or null activity. Both kinases share with AtCPK8 the same autophosphorylation pattern in the autoinhibitory (AD) and C-terminal variable (CTV) domains, suggesting that it could be a characteristic of clade III-b1. RT-qPCR analysis revealed that StCDPK22 is mainly expressed in early stages of tuberization, but not limited to, while StCDPK24 expression is more ubiquitous. In silico analysis predicted several abiotic stress-responsive elements in its promoters. Accordingly, StCDPK24 expression peaked at 10 h in in vitro plants exposed to salt shock and then declined. Moreover, a significant increase was observed at 2 h in stems of salt-treated greenhouse plants, suggesting that this CDPK could participate in the early events of the signaling cascade triggered in response to salt.

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.

cROStalk for Life: Uncovering ROS Signaling in Plants and Animal Systems, from Gametogenesis to Early Embryonic Development

This review explores the role of reactive oxygen species (ROS)/Ca2+ in communication within reproductive structures in plants and animals. Many concepts have been described during the last years regarding how biosynthesis, generation products, antioxidant systems, and signal transduction involve ROS signaling, as well as its possible link with developmental processes and response to biotic and abiotic stresses. In this review, we first addressed classic key concepts in ROS and Ca2+ signaling in plants, both at the subcellular, cellular, and organ level. In the plant science field, during the last decades, new techniques have facilitated the in vivo monitoring of ROS signaling cascades. We will describe these powerful techniques in plants and compare them to those existing in animals. Development of new analytical techniques will facilitate the understanding of ROS signaling and their signal transduction pathways in plants and mammals. Many among those signaling pathways already have been studied in animals; therefore, a specific effort should be made to integrate this knowledge into plant biology. We here discuss examples of how changes in the ROS and Ca2+ signaling pathways can affect differentiation processes in plants, focusing specifically on reproductive processes where the ROS and Ca2+ signaling pathways influence the gametophyte functioning, sexual reproduction, and embryo formation in plants and animals. The study field regarding the role of ROS and Ca2+ in signal transduction is evolving continuously, which is why we reviewed the recent literature and propose here the potential targets affecting ROS in reproductive processes. We discuss the opportunities to integrate comparative developmental studies and experimental approaches into studies on the role of ROS/ Ca2+ in both plant and animal developmental biology studies, to further elucidate these crucial signaling pathways.

Indole primes defence signalling and increases herbivore resistance in tea plants

Upon herbivore attack, plants emit herbivore-induced plant volatiles (HIPVs). HIPVs can prime defences and resistance of intact plants. However, how HIPVs are decoded and translated into functional defence responses is not well understood, especially in long-lived woody plants. Here, we investigated the impact of the aromatic HIPV indole on defence-related early signalling, phytohormone accumulation, secondary metabolite biosynthesis and herbivore resistance in tea plants. We find that tea plants infested with tea geometrid caterpillars release indole at concentrations >450 ng*hr^-1 . Exposure to corresponding doses of synthetic indole primes the expression of early defence genes involved in calcium (Ca²⁺ ) signalling, MPK signalling and jasmonate biosynthesis. Indole exposure also primes the production of jasmonates and defence-related secondary metabolites. These changes are associated with higher herbivore resistance of indole-exposed tea plants. Chemical inhibition of Ca²⁺ and jasmonate signalling provides evidence that both are required for indole-mediated defence priming and herbivore resistance. Our systematic assessment of the impact of indole on defence signalling and deployment shows that indole acts by boosting Ca²⁺ signalling, resulting in enhanced jasmonate-dependent defence and resistance in a woody plant. Our work extends the molecular basis of HIPV-induced defence priming from annual plants to an economically important tree species.

Jasmonic Acid Signaling and Molecular Crosstalk with Other Phytohormones

Plants continually monitor their innate developmental status and external environment and make adjustments to balance growth, differentiation and stress responses using a complex and highly interconnected regulatory network composed of various signaling molecules and regulatory proteins. Phytohormones are an essential group of signaling molecules that work through a variety of different pathways conferring plasticity to adapt to the everchanging developmental and environmental cues. Of these, jasmonic acid (JA), a lipid-derived molecule, plays an essential function in controlling many different plant developmental and stress responses. In the past decades, significant progress has been made in our understanding of the molecular mechanisms that underlie JA metabolism, perception, signal transduction and its crosstalk with other phytohormone signaling pathways. In this review, we discuss the JA signaling pathways starting from its biosynthesis to JA-responsive gene expression, highlighting recent advances made in defining the key transcription factors and transcriptional regulatory proteins involved. We also discuss the nature and degree of crosstalk between JA and other phytohormone signaling pathways, highlighting recent breakthroughs that broaden our knowledge of the molecular bases underlying JA-regulated processes during plant development and biotic stress responses.