Alternative splicing-mediated targeting of the Arabidopsis GLUTAMATE RECEPTOR3.5 to mitochondria affects organelle morphology

E3 SUMO ligase SIZ1 splicing variants localize and function according to external conditions

SIZ1 (SAP and MIZ1) is a member of the Siz/PIAS-type RING family of E3 SUMO (small ubiquitin-related modifier) ligases that play key roles in growth, development, and stress responses in plant and animal systems. Nevertheless, splicing variants of SIZ1 have not yet been characterized. Here, we identified four splicing variants of Arabidopsis (Arabidopsis thaliana) SIZ1, which encode three different protein isoforms. The SIZ1 gene encodes an 873-amino acid (aa) protein. Among the four SIZ1 splicing variants (SSVs), SSV1 and SSV4 encode identical 885 aa proteins; SSV2 encodes an 832 aa protein; and SSV3 encodes an 884 aa protein. SSV2 mainly localized to the plasma membrane, whereas SIZ1, SSV1/SSV4, and SSV3 localized to the nucleus. Interestingly, SIZ1 and all SSVs exhibited similar E3 SUMO ligase activities and preferred SUMO1 and SUMO2 for their E3 ligase activity. Transcript levels of SSV2 were substantially increased by heat treatment, while those of SSV1, SSV3, and SSV4 transcripts were unaffected by various abiotic stresses. SSV2 directly interacted with and sumoylated cyclic nucleotide-gated ion channel 6 (CNGC6), a positive thermotolerance regulator, enhancing the stability of CNGC6. Notably, transgenic siz1-2 mutants expressing SSV2 exhibited greater heat stress tolerance than wild-type plants, whereas those expressing SIZ1 were sensitive to heat stress. Furthermore, transgenic cngc6 plants overaccumulating a mutated mCNGC6 protein (K347R, a mutation at the sumoylation site) were sensitive to heat stress, similar to the cngc6 mutants, while transgenic cngc6 plants overaccumulating CNGC6 exhibited restored heat tolerance. Together, we propose that alternative splicing is an important mechanism that regulates the function of SSVs during development or under adverse conditions, including heat stress.

Channels of Evolution: Unveiling Evolutionary Patterns in Diatom Ca2+ Signalling

Diatoms are important primary producers in marine and freshwater environments, but little is known about the signalling mechanisms they use to detect changes in their environment. All eukaryotic organisms use Ca2+ signalling to perceive and respond to environmental stimuli, employing a range of Ca2+-permeable ion channels to facilitate the movement of Ca2+ across cellular membranes. We investigated the distribution of different families of Ca2+ channels in diatom genomes, with comparison to other members of the stramenopile lineage. The four-domain voltage-gated Ca2+ channels (Cav) are present in some centric diatoms but almost completely absent in pennate diatoms, whereas single-domain voltage-gated EukCatA channels were found in all diatoms. Glutamate receptors (GLRs) and pentameric ligand-gated ion channels (pLGICs) also appear to have been lost in several pennate species. Transient receptor potential (TRP) channels are present in all diatoms, but have not undergone the significant expansion seen in brown algae. All diatom species analysed lacked the mitochondrial uniporter (MCU), a highly conserved channel type found in many eukaryotes, including several stramenopile lineages. These results highlight the unique Ca2+-signalling toolkit of diatoms and indicate that evolutionary gains or losses of different Ca2+ channels may contribute to differences in cellular-signalling mechanisms between species.

UMAMIT44 is a key player in glutamate export from Arabidopsis chloroplasts

Selective partitioning of amino acids among organelles, cells, tissues, and organs is essential for cellular metabolism and plant growth. Nitrogen assimilation into glutamine and glutamate and de novo biosynthesis of most protein amino acids occur in chloroplasts; therefore, various transport mechanisms must exist to accommodate their directional efflux from the stroma to the cytosol and feed the amino acids into the extraplastidial metabolic and long-distance transport pathways. Yet, Arabidopsis (Arabidopsis thaliana) transporters functioning in plastidial export of amino acids remained undiscovered. Here, USUALLY MULTIPLE ACIDS MOVE IN AND OUT TRANSPORTER 44 (UMAMIT44) was identified and shown to function in glutamate export from Arabidopsis chloroplasts. UMAMIT44 controls glutamate homeostasis within and outside of chloroplasts and influences nitrogen partitioning from leaves to sinks. Glutamate imbalances in chloroplasts and leaves of umamit44 mutants impact cellular redox state, nitrogen and carbon metabolism, and amino acid (AA) and sucrose supply of growing sinks, leading to negative effects on plant growth. Nonetheless, the mutant lines adjust to some extent by upregulating alternative pathways for glutamate synthesis outside the plastids and by mitigating oxidative stress through the production of other amino acids and antioxidants. Overall, this study establishes that the role of UMAMIT44 in glutamate export from chloroplasts is vital for controlling nitrogen availability within source leaf cells and for sink nutrition, with an impact on growth and seed yield.

In Silico Analysis of Glutamate Receptors in Capsicum chinense: Structure, Evolution, and Molecular Interactions

Plant glutamate receptors (GLRs) are integral membrane proteins that function as non-selective cation channels, involved in the regulation of developmental events crucial in plants. Knowledge of these proteins is restricted to a few species and their true agonists are still unknown in plants. Using tomato SlGLRs, a search was performed in the pepper database to identify GLR sequences in habanero pepper (Capsicum chinense Jacq.). Structural, phylogenetic, and orthology analysis of the CcGLRs, as well as molecular docking and protein interaction networks, were conducted. Seventeen CcGLRs were identified, which contained the characteristic domains of GLR. The variation of conserved residues in the M2 transmembrane domain between members suggests a difference in ion selectivity and/or conduction. Also, new conserved motifs in the ligand-binding regions are reported. Duplication events seem to drive the expansion of the species, and these were located in the evolution by using orthologs. Molecular docking analysis allowed us to identify differences in the agonist binding pocket between CcGLRs, which suggest the existence of different affinities for amino acids. The possible interaction of some CcGLRs with proteins leads to suggesting specific functions for them within the plant. These results offer important functional clues for CcGLR, probably extrapolated to other Solanaceae.

Genome-wide identification of the glutamate receptor-like gene family in Vanilla planifolia and their response to Fusarium oxysporum infection

Glutamate receptor-like genes (GLRs) are essential for plant growth and development and for coping with environmental (biological and non-biological) stresses. In this study, 13 GLR members were identified in the Vanilla planifolia genome and attributed to two subgroups (Clade I and Clade III) based on their physical relationships. Cis-acting element analysis and Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) annotations indicated the GLR gene regulation's complexity and their functional diversity. Expression analysis revealed a relatively higher and more general expression pattern of Clade III members compared to the Clade I subgroup in tissues. Most GLRs showed significant differences in expression during Fusarium oxysporum infection. This suggested that GLRs play a critical role in the response of V. planifolia to pathogenic infection. These results provide helpful information for further functional research and crop improvement of VpGLRs.

An Arabidopsis mutant line lacking the mitochondrial calcium transport regulator MICU shows an altered metabolite profile

Plant metabolism is constantly changing and requires input signals for efficient regulation. The mitochondrial calcium uniporter (MCU) couples organellar and cytoplasmic calcium oscillations leading to oxidative metabolism regulation in a vast array of species. In Arabidopsis thaliana, genetic deletion of AtMICU leads to altered mitochondrial calcium handling and ultrastructure. Here we aimed to further assess the consequences upon genetic deletion of AtMICU. Our results confirm that AtMICU safeguards intracellular calcium transport associated with carbohydrate, amino acid, and phytol metabolism modifications. The implications of such alterations are discussed.

Genome-wide identification of glutamate receptor-like gene family in soybean

Glutamate receptor-like genes (GLRs) are essential in the growth and development of plants and many physiological and biochemical processes; however, related information in soybean is lacking. In this study, 105 GLRs, including 67 Glycine soja and 38 Glycine max GLRs, were identified and divided into two clades (Clades II and III) according to their phylogenetic relationships. GLR members in the same branch had a relatively conservative motif composition and genetic structure. Furthermore, the soybean GLR family mainly experienced purification selection during evolution. Cis-acting element analysis, gene ontology, and Kyoto Encyclopedia of Genes and Genomic annotations indicated the complexity of the gene regulation and functional diversity of the soybean GLR. Moreover, transcriptome data analysis showed that these GLRs had different expression profiles in different tissues, and Clade III members had higher and more common expression patterns. Additionally, the expression profiles under jasmonic acid treatment and salt stress indicate that the GLR participated in the jasmonic acid signaling pathway and plays a role in salt treatment. This study provides information for a comprehensive understanding of the soybean GLR family and a reference for further functional research and genetic improvement.

Comparative analysis of wild-type and chloroplast MCU-deficient plants reveals multiple consequences of chloroplast calcium handling under drought stress

Introduction

Chloroplast calcium homeostasis plays an important role in modulating the response of plants to abiotic and biotic stresses. One of the greatest challenges is to understand how chloroplast calcium-permeable pathways and sensors are regulated in a concerted manner to translate specific information into a calcium signature and to elucidate the downstream effects of specific chloroplast calcium dynamics. One of the six homologs of the mitochondrial calcium uniporter (MCU) was found to be located in chloroplasts in the leaves and to crucially contribute to drought- and oxidative stress-triggered uptake of calcium into this organelle.

Methods

In the present study we integrated comparative proteomic analysis with biochemical, genetic, cellular, ionomic and hormone analysis in order to gain an insight into how chloroplast calcium channels are integrated into signaling circuits under watered condition and under drought stress.

Results

Altogether, our results indicate for the first time a link between chloroplast calcium channels and hormone levels, showing an enhanced ABA level in the cmcu mutant already in well-watered condition. Furthermore, we show that the lack of cMCU results in an upregulation of the calcium sensor CAS and of enzymes of chlorophyll synthesis, which are also involved in retrograde signaling upon drought stress, in two independent KO lines generated in Col-0 and Col-4 ecotypes.

Conclusions

These observations point to chloroplasts as important signaling hubs linked to their calcium dynamics. Our results obtained in the model plant Arabidopsis thaliana are discussed also in light of our limited knowledge regarding organellar calcium signaling in crops and raise the possibility of an involvement of such signaling in response to drought stress also in crops.

Glutamate receptor like channels: Emerging players in calcium mediated signaling in plants

Glutamate receptors like channels (GLRs) are ligand gated non-selective cation channels and are multigenic in nature. They are homologs of mammalian ionic glutamate receptors (iGLRs) that play an important role in neurotransmission. It has been more than 25 years of discovery of plant GLRs, since then, significant progress has been made to unravel their structure and function in plants. Recently, the first crystal structure of plant GLR has been resolved that suggests that, though, plant GLRs contain the conserved signature domains of iGLRs, their unique features enable agonist/antagonist-dependent change in their activity. GLRs exhibit diverse subcellular localization and undergo dynamic expression variation in response to developmental and environmental stress conditions in plants. The combined use of genetic, electrophysiology and calcium imaging using different genetically encoded calcium indicators has revealed that GLRs are involved in generating calcium (Ca²⁺) influx across the plasma membrane and are involved in shaping the Ca²⁺ signature in response to different developmental and environmental stimuli. These findings indicate that GLRs influence cytosolic Ca²⁺ dynamics, thus, highlighting "GLR-Ca²⁺-crosstalk (GCC)" in developmental and stress-responsive signaling pathways. With this background, the present review summarises the recent developments pertaining to GLR function, in the broader context of regulation of stress tolerance in plants.

Bioengineering Techniques to Improve Nitrogen Transformation and Utilization: Implications for Nitrogen Use Efficiency and Future Sustainable Crop Production

Nitrogen (N) is crucial for plant growth and development, especially in physiological and biochemical processes such as component of different proteins, enzymes, nucleic acids, and plant growth regulators. Six categories, such as transporters, nitrate absorption, signal molecules, amino acid biosynthesis, transcription factors, and miscellaneous genes, broadly encompass the genes regulating NUE in various cereal crops. Herein, we outline detailed research on bioengineering modifications of N metabolism to improve the different crop yields and biomass. We emphasize effective and precise molecular approaches and technologies, including N transporters, transgenics, omics, etc., which are opening up fascinating opportunities for a complete analysis of the molecular elements that contribute to NUE. Moreover, the detection of various types of N compounds and associated signaling pathways within plant organs have been discussed. Finally, we highlight the broader impacts of increasing NUE in crops, crucial for better agricultural yield and in the greater context of global climate change.

Roles of Glutamate Receptor-Like Channels (GLRs) in Plant Growth and Response to Environmental Stimuli

Plant glutamate receptor-like channels (GLRs) are the homologues of ionotropic glutamate receptors (iGluRs) that mediate neurotransmission in mammals, and they play important roles in various plant-specific physiological processes, such as pollen tube growth, sexual reproduction, root meristem proliferation, internode cell elongation, stomata aperture regulation, and innate immune and wound responses. Notably, these biological functions of GLRs have been mostly linked to the Ca²⁺-permeable channel activity as GLRs can directly channel the transmembrane flux of Ca²⁺, which acts as a key second messenger in plant cell responses to both endogenous and exogenous stimuli. Thus, it was hypothesized that GLRs are mainly involved in Ca²⁺ signaling processes in plant cells. Recently, great progress has been made in GLRs for their roles in long-distance signal transduction pathways mediated by electrical activity and Ca²⁺ signaling. Here, we review the recent progress on plant GLRs, and special attention is paid to recent insights into the roles of GLRs in response to environmental stimuli via Ca²⁺ signaling, electrical activity, ROS, as well as hormone signaling networks. Understanding the roles of GLRs in integrating internal and external signaling for plant developmental adaptations to a changing environment will definitely help to enhance abiotic stress tolerance.

MCU proteins dominate in vivo mitochondrial Ca2+ uptake in Arabidopsis roots

Ca2+ signaling is central to plant development and acclimation. While Ca2+-responsive proteins have been investigated intensely in plants, only a few Ca2+-permeable channels have been identified, and our understanding of how intracellular Ca2+ fluxes is facilitated remains limited. Arabidopsis thaliana homologs of the mammalian channel-forming mitochondrial calcium uniporter (MCU) protein showed Ca2+ transport activity in vitro. Yet, the evolutionary complexity of MCU proteins, as well as reports about alternative systems and unperturbed mitochondrial Ca2+ uptake in knockout lines of MCU genes, leave critical questions about the in vivo functions of the MCU protein family in plants unanswered. Here, we demonstrate that MCU proteins mediate mitochondrial Ca2+ transport in planta and that this mechanism is the major route for fast Ca2+ uptake. Guided by the subcellular localization, expression, and conservation of MCU proteins, we generated an mcu triple knockout line. Using Ca2+ imaging in living root tips and the stimulation of Ca2+ transients of different amplitudes, we demonstrated that mitochondrial Ca2+ uptake became limiting in the triple mutant. The drastic cell physiological phenotype of impaired subcellular Ca2+ transport coincided with deregulated jasmonic acid-related signaling and thigmomorphogenesis. Our findings establish MCUs as a major mitochondrial Ca2+ entry route in planta and link mitochondrial Ca2+ transport with phytohormone signaling.

Morphology and permeability transitions in plant mitochondria: Different aspects of the same event?

Plant mitochondria are sensitive organelles affected by changing environmental stressors. Upon heat shock or the presence of reactive oxygen species, plant mitochondria undergo in vivo morphological derangements associated with the extensively characterized opening of the mitochondrial permeability transition pore. Nevertheless, the classic mitochondrial permeability transition is known to be triggered by calcium overload causing mitochondrial swelling and dysfunction. Here we review evidence concerning calcium handling, permeability transition and mitochondrial impairments in plants, supporting the notion that the mitochondrial morphology transition is an in vivo indicator of the permeability transition.

An Insight into Animal Glutamate Receptors Homolog of Arabidopsis thaliana and Their Potential Applications-A Review

Most excitatory impulses received by neurons are mediated by ionotropic glutamate receptors (iGluRs). These receptors are located at the apex and play an important role in memory, neuronal development, and synaptic plasticity. These receptors are ligand-dependent ion channels that allow a wide range of cations to pass through. Glutamate, a neurotransmitter, activates three central ionotropic receptors: N-methyl-D-aspartic acid (NMDA), -amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA), and kainic acid (KA). According to the available research, excessive glutamate release causes neuronal cell death and promotes neurodegenerative disorders. Arabidopsis thaliana contains 20 glutamate receptor genes (AtGluR) comparable to the human ionotropic glutamate (iGluRs) receptor. Many studies have proved that AtGL-rec genes are involved in a number of plant growth and physiological activities, such as in the germination of seeds, roots, abiotic and biotic stress, and cell signaling, which clarify the place of these genes in plant biology. In spite of these, the iGluRs, Arabidopsis glutamate receptors (AtGluR), is associated with the ligand binding activity, which confirms the evolutionary relationship between animal and plant glutamate receptors. Along with the above activities, the impact of mammalian agonists and antagonists on Arabidopsis suggests a correlation between plant and animal glutamate receptors. In addition, these glutamate receptors (plant/animal) are being utilized for the early detection of neurogenerative diseases using the fluorescence resonance energy transfer (FRET) approach. However, a number of scientific laboratories and institutes are consistently working on glutamate receptors with different aspects. Currently, we are also focusing on Arabidopsis glutamate receptors. The current review is focused on updating knowledge on AtGluR genes, their evolution, functions, and expression, and as well as in comparison with iGluRs. Furthermore, a high throughput approach based on FRET nanosensors developed for understanding neurotransmitter signaling in animals and plants via glutamate receptors has been discussed. The updated information will aid in the future comprehension of the complex molecular dynamics of glutamate receptors and the exploration of new facts in plant/animal biology.

Both metaxin and Tom20 together with two mitochondria-specific motifs support mitochondrial targeting of dual-targeting AtSufE1

Plant cells have two endosymbiotic organelles, chloroplasts, and mitochondria. These organelles perform specific functions that depend on organelle-specific proteins. The majority of chloroplast and mitochondrial proteins are specifically imported by the transit peptide and presequence, respectively. However, a significant number of proteins are also dually targeted to these two organelles. Currently, it is not fully understood how proteins are dually targeted to both chloroplasts and mitochondria. In this study, the mechanism underlying mitochondrial targeting of dual targeting AtSufE1 in Arabidopsis was elucidated. The N-terminal fragment containing 80 residues of AtSufE1 (AtSufE1N80) was sufficient to confer dual targeting of reporter protein, AtSufE1N80:GFP, in protoplasts. Two sequence motifs, two arginine residues at 15^th and 21^st positions, and amino acid (aa) sequence motif AKTLLLRPLK from the 31^st to 40^th aa position, were responsible for targeting to mitochondria a portion of reporter proteins amid the chloroplast targeting. The sequence motif PSEVPFRRT from the 41^st to 50^th aa position constitutes a common motif for targeting to both chloroplasts and mitochondria. For mitochondrial import of AtSufE1:N80, Metaxin played a critical role. In addition, BiFC and protein pull-down experiments showed that AtSufE1N80 specifically interacts with import receptors, Metaxin and Tom20. The interaction of AtSufE1N80 with Metaxin was required for the interaction with Tom20. Based on these results, we propose that mitochondrial targeting of dual-targeting AtSufE1 is mediated by both mitochondria-specific and common sequence motifs in the signal sequence through the interaction with import receptors, Metaxin and Tom20, in a successive manner.

Signaling by plant glutamate receptor-like channels: What else!

Plant glutamate receptor-like channels (GLRs) are transmembrane proteins that allow the movement of several ions across membranes. In the model plant Arabidopsis, there are 20 GLR isoforms grouped in three clades and, since their discovery, it was hypothesized that GLRs were mainly involved in signaling processes. Indeed, in the last years, several pieces of evidence demonstrate different signaling roles played by GLRs, related to pollen development, sexual reproduction, chemotaxis, root development, regulation of stomatal aperture, and response to pathogens. Recently, GLRs have gained attention for their role in long-distance electric and calcium signaling. In this review, we resume the evidence about the role of GLRs in signaling processes. This role is mostly linked to the GLRs involvement in the regulation of ion fluxes across membranes and, in particular, of calcium, which represents a key second messenger in plant cell responses to both endogenous and exogenous stimuli.

News about amino acid metabolism in plant-microbe interactions

Plants constantly come into contact with a diverse mix of pathogenic and beneficial microbes. The ability to distinguish between them and to respond appropriately is essential for plant health. Here we review recent progress in understanding the role of amino acid sensing, signaling, transport, and metabolism during plant-microbe interactions. Biochemical pathways converting individual amino acids into active compounds have recently been elucidated, and comprehensive large-scale approaches have brought amino acid sensors and transporters into focus. These findings show that plant central amino acid metabolism is closely interwoven with stress signaling and defense responses at various levels. The individual biochemical mechanisms and the interconnections between the different processes are just beginning to emerge and might serve as a foundation for new plant protection strategies.

Upregulation of GLRs expression by light in Arabidopsis leaves

BACKGROUND

Glutamate receptor-like (GLR) channels are plant homologs of iGluRs, animal ionotropic glutamate receptors which participate in neurotransmission. GLRs mediate plant adaptive processes and photomorphogenesis. Despite their contribution to light-dependent processes, signaling mechanisms that modulate GLR response to light remain unknown. Here we show that leaf expression of 7 out of 20 Arabidopsis GLRs is significantly up-regulated by monochromatic irradiation.

RESULTS

Our data indicates that both red and blue light stimulate the expression of selected AtGLRs. Using a photosynthesis inhibitor and different irradiation regimes, we demonstrated that retrograde signaling from photosystem II is unlikely to be involved in light-dependent GLR up-regulation. Analysis of transcriptional patterns in mutants of key photoreceptors allowed us to observe that both phytochromes and cryptochromes are likely to be involved in the control of light-dependent up-regulation of AtGLR expression, with phytochromes playing a clearly dominating role in this process.

CONCLUSIONS

In mature Arabidopsis leaves, phytochromes, assisted by cryptochromes, mediate light-driven transcriptional up-regulation of several genes encoding GLR proteins. Since GLRs are known to be involved in a wide range of plant developmental processes our results provide mechanistic insight into how light may influence plant growth and development.

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.

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

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

Adaptive Responses of Citrus grandis Leaves to Copper Toxicity Revealed by RNA-Seq and Physiology

Copper (Cu)-toxic effects on Citrus grandis growth and Cu uptake, as well as gene expression and physiological parameters in leaves were investigated. Using RNA-Seq, 715 upregulated and 573 downregulated genes were identified in leaves of C. grandis seedlings exposed to Cu-toxicity (LCGSEC). Cu-toxicity altered the expression of 52 genes related to cell wall metabolism, thus impairing cell wall metabolism and lowering leaf growth. Cu-toxicity downregulated the expression of photosynthetic electron transport-related genes, thus reducing CO₂ assimilation. Some genes involved in thermal energy dissipation, photorespiration, reactive oxygen species scavenging and cell redox homeostasis and some antioxidants (reduced glutathione, phytochelatins, metallothioneins, l-tryptophan and total phenolics) were upregulated in LCGSEC, but they could not protect LCGSEC from oxidative damage. Several adaptive responses might occur in LCGSEC. LCGSEC displayed both enhanced capacities to maintain homeostasis of Cu via reducing Cu uptake by leaves and preventing release of vacuolar Cu into the cytoplasm, and to improve internal detoxification of Cu by accumulating Cu chelators (lignin, reduced glutathione, phytochelatins, metallothioneins, l-tryptophan and total phenolics). The capacities to maintain both energy homeostasis and Ca homeostasis might be upregulated in LCGSEC. Cu-toxicity increased abscisates (auxins) level, thus stimulating stomatal closure and lowering water loss (enhancing water use efficiency and photosynthesis).

Deciphering the Role of Ion Channels in Early Defense Signaling against Herbivorous Insects

Plants and insect herbivores are in a relentless battle to outwit each other. Plants have evolved various strategies to detect herbivores and mount an effective defense system against them. These defenses include physical and structural barriers such as spines, trichomes, cuticle, or chemical compounds, including secondary metabolites such as phenolics and terpenes. Plants perceive herbivory by both mechanical and chemical means. Mechanical sensing can occur through the perception of insect biting, piercing, or chewing, while chemical signaling occurs through the perception of various herbivore-derived compounds such as oral secretions (OS) or regurgitant, insect excreta (frass), or oviposition fluids. Interestingly, ion channels or transporters are the first responders for the perception of these mechanical and chemical cues. These transmembrane pore proteins can play an important role in plant defense through the induction of early signaling components such as plasma transmembrane potential (V_m) fluctuation, intracellular calcium (Ca²⁺), and reactive oxygen species (ROS) generation, followed by defense gene expression, and, ultimately, plant defense responses. In recent years, studies on early plant defense signaling in response to herbivory have been gaining momentum with the application of genetically encoded GFP-based sensors for real-time monitoring of early signaling events and genetic tools to manipulate ion channels involved in plant-herbivore interactions. In this review, we provide an update on recent developments and advances on early signaling events in plant-herbivore interactions, with an emphasis on the role of ion channels in early plant defense signaling.

Ferroportin 3 is a dual-targeted mitochondrial/chloroplast iron exporter necessary for iron homeostasis in Arabidopsis

Mitochondria and chloroplasts are organelles with high iron demand that are particularly susceptible to iron-induced oxidative stress. Despite the necessity of strict iron regulation in these organelles, much remains unknown about mitochondrial and chloroplast iron transport in plants. Here, we propose that Arabidopsis ferroportin 3 (FPN3) is an iron exporter that is dual-targeted to mitochondria and chloroplasts. FPN3 is expressed in shoots, regardless of iron conditions, but its transcripts accumulate under iron deficiency in roots. fpn3 mutants cannot grow as well as the wild type under iron-deficient conditions and their shoot iron levels are lower compared with the wild type. Analyses of iron homeostasis gene expression in fpn3 mutants and inductively coupled plasma mass spectrometry (ICP-MS) measurements show that iron levels in the mitochondria and chloroplasts are increased relative to the wild type, consistent with the proposed role of FPN3 as a mitochondrial/plastid iron exporter. In iron-deficient fpn3 mutants, abnormal mitochondrial ultrastructure was observed, whereas chloroplast ultrastructure was not affected, implying that FPN3 plays a critical role in the mitochondria. Overall, our study suggests that FPN3 is essential for optimal iron homeostasis.

Structure of the Arabidopsis thaliana glutamate receptor-like channel GLR3.4

Glutamate receptor-like channels (GLRs) play vital roles in various physiological processes in plants, such as wound response, stomatal aperture control, seed germination, root development, innate immune response, pollen tube growth, and morphogenesis. Despite the importance of GLRs, knowledge about their molecular organization is limited. Here we use X-ray crystallography and single-particle cryo-EM to solve structures of the Arabidopsis thaliana GLR3.4. Our structures reveal the tetrameric assembly of GLR3.4 subunits into a three-layer domain architecture, reminiscent of animal ionotropic glutamate receptors (iGluRs). However, the non-swapped arrangement between layers of GLR3.4 domains, binding of glutathione through S-glutathionylation of cysteine C205 inside the amino-terminal domain clamshell, unique symmetry, inter-domain interfaces, and ligand specificity distinguish GLR3.4 from representatives of the iGluR family and suggest distinct features of the GLR gating mechanism. Our work elaborates on the principles of GLR architecture and symmetry and provides a molecular template for deciphering GLR-dependent signaling mechanisms in plants.

Organellar calcium signaling in plants: An update

Calcium ion (Ca²⁺) is a versatile signaling transducer in all eukaryotic organisms. In plants, intracellular changes in free Ca²⁺ levels act as regulators in many growth and developmental processes. Ca²⁺ also mediates the cellular responses to environmental stimuli and thus plays an important role in providing stress tolerance to plants. Ca²⁺ signals are decoded by a tool kit of various families of Ca²⁺-binding proteins and their downstream targets, which mediate the transformation of the Ca²⁺ signal into appropriate cellular response. Early interest and research on Ca²⁺ signaling focused on its function in the cytosol, however it has become evident that this important regulatory pathway also exists in organelles such as nucleus, chloroplast, mitochondria, peroxisomes and the endomembrane system. In this review, we give an overview on the knowledge about organellar Ca²⁺ signaling with a focus on recent advances and developments.

Mitochondrial calcium transport and permeability transition as rational targets for plant protection

The mitochondrial permeability transition (MPT) is a death-inducing mechanism that collapses electrochemical gradients across inner mitochondrial membranes. Several studies in model plants have detailed potential MPT-dependent cell death upon abiotic stress in response to heat shock, ultraviolet radiation, heavy metal toxicity and waterlogging. However, the molecular specifics of the MPT and its possible role on plant cell death remain controversial. This review addresses previous and recent developments on the role(s) of the MPT in plants. Considering these advances, MPT targeting can constitute a plausible strategy to ameliorate cell death in plants upon abiotic stress.

Effects of artificial ultraviolet B radiation on the macrophyte Lemna minor: a conceptual study for toxicity pathway characterization

UVB radiation caused irradiance-dependent and target-specific responses in non-UVB acclimated Lemna minor. Conceptual toxicity pathways were developed to propose causal relationships between UVB-mediated effects at multiple levels of biological organisation. Macrophytes inhabit waterways around the world and are used in hydroponics or aquaponics for different purposes such as feed and wastewater treatment and are thus exposed to elevated levels of UVB from natural and artificial sources. Although high UVB levels are harmful to macrophytes, mechanistic understanding of irradiance-dependent effects and associated modes of action in non-UVB acclimated plants still remains low. The present study was conducted to characterise the irradiance-dependent mechanisms of UVB leading to growth inhibition in Lemna minor as an aquatic macrophyte model. The L. minor were continuously exposed to UVB (0.008-4.2 W m^-2) and constant UVA (4 W m^-2) and photosynthetically active radiation, PAR (80 µmol m^-2 s^-1) for 7 days. A suite of bioassays was deployed to assess effects on oxidative stress, photosynthesis, DNA damage, and transcription of antioxidant biosynthesis, DNA repair, programmed cell death, pigment metabolism and respiration. The results showed that UVB triggered both irradiance-dependent and target-specific effects at multiple levels of biological organization, whereas exposure to UVA alone did not cause any effects. Inhibition of photosystem II and induction of carotenoids were observed at 0.23 W m^-2, whereas growth inhibition, excessive reactive oxygen species, lipid peroxidation, cyclobutane pyrimidine dimer formation, mitochondrial membrane potential reduction and chlorophyll depletion were observed at 0.5-1 W m^-2. Relationships between responses at different levels of biological organization were used to establish a putative network of toxicity pathways to improve our understanding of UVB effects in aquatic macrophytes under continuous UVB exposures. Additional studies under natural illuminations were proposed to assess whether these putative toxicity pathways may also be relevant for more ecologically relevant exposure scenarios.

Architecture and function of NMDA receptors: an evolutionary perspective

Ionotropic glutamate receptors (iGluRs) are a major class of ligand-gated ion channels that are widespread in the living kingdom. Their critical role in excitatory neurotransmission and brain function of arthropods and vertebrates has made them a compelling subject of interest for neurophysiologists and pharmacologists. This is particularly true for NMDA receptor (NMDARs), a subclass of iGluRs that act as central drivers of synaptic plasticity in the CNS. How and when the unique properties of NMDARs arose during evolution, and how they relate to the evolution of the nervous system, remain open questions. Recent years have witnessed a boom in both genomic and structural data, such that it is now possible to analyse the evolution of iGluR genes on an unprecedented scale and within a solid molecular framework. In this review, combining insights from phylogeny, atomic structure and physiological and mechanistic data, we discuss how evolution of NMDAR motifs and sequences shaped their architecture and functionalities. We trace differences and commonalities between NMDARs and other iGluRs, emphasizing a few distinctive properties of the former regarding ligand binding and gating, permeation, allosteric modulation and intracellular signalling. Finally, we speculate on how specific molecular properties of iGuRs arose to supply new functions to the evolving structure of the nervous system, from early metazoan to present mammals.

Channels and transporters for inorganic ions in plant mitochondria: Prediction and facts

Mitochondria are crucial bioenergetic organelles for providing different metabolites, including ATP, to sustain cell growth both in animals and in plants. These organelles, delimited by two membranes (outer and inner mitochondrial membrane), maintain their function by an intensive communication with other organelles as well as with the cytosol. Transport of metabolites across the two membranes, but also that of inorganic ions, takes place through specific ion channels and transporters and plays a crucial role in ensuring an adequate ionic milieu within the mitochondria. In the present review we briefly summarize the current knowledge about plant mitochondrial ion channels and transporters in comparison to those of animal mitochondria and examine the possible molecular identity of the so far unidentified transport systems taking into account subcellular targeting predictions and data from literature.

Chloroplast Calcium Signaling in the Spotlight

Calcium has long been known to regulate the metabolism of chloroplasts, concerning both light and carbon reactions of photosynthesis, as well as additional non photosynthesis-related processes. In addition to undergo Ca²⁺ regulation, chloroplasts can also influence the overall Ca²⁺ signaling pathways of the plant cell. Compelling evidence indicate that chloroplasts can generate specific stromal Ca²⁺ signals and contribute to the fine tuning of cytoplasmic Ca²⁺ signaling in response to different environmental stimuli. The recent set up of a toolkit of genetically encoded Ca²⁺ indicators, targeted to different chloroplast subcompartments (envelope, stroma, thylakoids) has helped to unravel the participation of chloroplasts in intracellular Ca²⁺ handling in resting conditions and during signal transduction. Intra-chloroplast Ca²⁺ signals have been demonstrated to occur in response to specific environmental stimuli, suggesting a role for these plant-unique organelles in transducing Ca²⁺-mediated stress signals. In this mini-review we present current knowledge of stimulus-specific intra-chloroplast Ca²⁺ transients, as well as recent advances in the identification and characterization of Ca²⁺-permeable channels/transporters localized at chloroplast membranes. In particular, the potential role played by cMCU, a chloroplast-localized member of the mitochondrial calcium uniporter (MCU) family, as component of plant environmental sensing is discussed in detail, taking into account some specific structural features of cMCU. In summary, the recent molecular identification of some players of chloroplast Ca²⁺ signaling has opened new avenues in this rapidly developing field and will hopefully allow a deeper understanding of the role of chloroplasts in shaping physiological responses in plants.

Rather rule than exception? How to evaluate the relevance of dual protein targeting to mitochondria and chloroplasts

Dual targeting of a nuclearly encoded protein into two different cell organelles is an exceptional event in eukaryotic cells. Yet, the frequency of such dual targeting is remarkably high in case of mitochondria and chloroplasts, the two endosymbiotic organelles of plant cells. In most instances, it is mediated by "ambiguous" transit peptides, which recognize both organelles as the target. A number of different approaches including in silico, in organello as well as both transient and stable in vivo assays are established to determine the targeting specificity of such transit peptides. In this review, we will describe and compare these approaches and discuss the potential role of this unusual targeting process. Furthermore, we will present a hypothetical scenario how dual targeting might have arisen during evolution.

Identification of cell populations necessary for leaf-to-leaf electrical signaling in a wounded plant

Numerous modes of long-distance electrical signaling exist in nature. The best known of these, axonal conduction, requires one primary cell population, i.e., neurons. In contrast, the cell types that mediate leaf-to-leaf electrical signaling in wounded plants have not been defined rigorously. Using genetic approaches, we find that two distinct populations of cells in the vasculature matrix are needed to perform this function. Surprisingly, these cells do not contact each other directly. As we further defined the plant wound response, we found that wound-induced membrane depolarizations preceded large intravasculature calcium fluxes. We reveal a two-cell-type mode of electrical signaling in leaves and discuss parallels and differences in electrical signaling outside the plant kingdom.

The identity of the cell files necessary for the leaf-to-leaf transmission of wound signals plants has been debated for decades. In Arabidopsis, wounding initiates the glutamate receptor-like (GLR)–dependent propagation of membrane depolarizations that lead to defense gene activation. Using a vein extraction procedure we found pools of GLR-fusion proteins in endomembranes in phloem sieve elements and/or in xylem contact cells. Strikingly, only double mutants that eliminated GLRs from both of these spatially separated cell types strongly attenuated leaf-to-leaf electrical signaling. glr3.3 mutants were also compromised in their defense against herbivores. Since wounding is known to cause increases in cytosolic calcium, we monitored electrical signals and Ca²⁺ transients simultaneously. This revealed that wound-induced membrane depolarizations in the wild-type preceded cytosolic Ca²⁺ maxima. The axial and radial distributions of calcium fluxes were differentially affected in each glr mutant. Resolving a debate over which cell types are necessary for electrical signaling between leaves, we show that phloem sieve elements and xylem contact cells function together in this process.

Genome-Wide Association Studies of Free Amino Acid Levels by Six Multi-Locus Models in Bread Wheat

Genome-wide association studies (GWAS) have been widely used to dissect the complex biosynthetic processes of plant metabolome. Most studies have used single-locus GWAS approaches, such as mixed linear model (MLM), and little is known about more efficient algorithms to implement multi-locus GWAS. Here, we report a comprehensive GWAS of 20 free amino acid (FAA) levels in kernels of bread wheat (Triticumaestivum L.) based on 14,646 SNPs by six multi-locus models (FASTmrEMMA, FASTmrMLM, ISISEM-BLASSO, mrMLM, pKWmEB, and pLARmEB). Our results showed that 328 significant quantitative trait nucleotides (QTNs) were identified in total (38, 8, 92, 45, 117, and 28, respectively, for the above six models). Among them, 66 were repeatedly detected by more than two models, and 155 QTNs appeared only in one model, indicating the reliability and complementarity of these models. We also found that the number of significant QTNs for different FAAs varied from 8 to 41, which revealed the complexity of the genetic regulation of metabolism, and further demonstrated the necessity of the multi-locus GWAS. Around these significant QTNs, 15 candidate genes were found to be involved in FAA biosynthesis, and one candidate gene (TraesCS1D01G052500, annotated as tryptophan decarboxylase) was functionally identified to influence the content of tryptamine in vitro. Our study demonstrated the power and efficiency of multi-locus GWAS models in crop metabolome research and provided new insights into understanding FAA biosynthesis in wheat.

Pharmacological Strategies for Manipulating Plant Ca2+ Signalling

Calcium is one of the most pleiotropic second messengers in all living organisms. However, signalling specificity is encoded via spatio-temporally regulated signatures that act with surgical precision to elicit highly specific cellular responses. How this is brought about remains a big challenge in the plant field, in part due to a lack of specific tools to manipulate/interrogate the plant Ca2+ toolkit. In many cases, researchers resort to tools that were optimized in animal cells. However, the obviously large evolutionary distance between plants and animals implies that there is a good chance observed effects may not be specific to the intended plant target. Here, we provide an overview of pharmacological strategies that are commonly used to activate or inhibit plant Ca2+ signalling. We focus on highlighting modes of action where possible, and warn for potential pitfalls. Together, this review aims at guiding plant researchers through the Ca2+ pharmacology swamp.

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.

Comparing Plant and Animal Glutamate Receptors: Common Traits but Different Fates?

Animal ionotropic glutamate receptors (iGluRs) are ligand-gated channels whose evolution is intimately linked to the one of the nervous system, where the agonist glutamate and co-agonists glycine/D-serine act as neuro-transmitters or -modulators. While iGluRs are specialized in neuronal communication, plant glutamate receptor-like (GLR) homologues have evolved many plant-specific physiological functions, such as sperm signaling in moss, pollen tube growth, root meristem proliferation, innate immune and wound responses. GLRs have been associated with Ca2+ signaling by directly channeling its extracellular influx into the cytosol. Nevertheless, very limited information on functional properties of GLRs is available, and we mostly rely on structure/function data obtained for animal iGluRs to interpret experimental results obtained for plant GLRs. Yet, a deeper characterization and better understanding of plant GLRs is progressively unveiling original and different mode of functions when compared to their mammalian counterparts. Here, we review the function of plant GLRs comparing their predicted structure and physiological roles to the well-documented ones of iGluRs. We conclude that interpreting GLR function based on comparison to their animal counterparts calls for caution, especially when presuming physiological roles and mode of action for plant GLRs from comparison to iGluRs in peripheral, non-neuronal tissues.

L-Met Activates Arabidopsis GLR Ca2+ Channels Upstream of ROS Production and Regulates Stomatal Movement

Plant glutamate receptor homologs (GLRs) have long been proposed to function as ligand-gated Ca²⁺ channels, but no in planta evidence has been provided. Here, we present genetic evidence that Arabidopsis GLR3.1 and GLR3.5 form Ca²⁺ channels activated by L-methionine (L-Met) at physiological concentrations and regulate stomatal apertures and plant growth. The glr3.1/3.5 mutations resulted in a lower cytosolic Ca²⁺ level, defective Ca²⁺-induced stomatal closure, and Ca²⁺-deficient growth disorder, all of which involved L-Met. Patch-clamp analyses of guard cells showed that GLR3.1/3.5 Ca²⁺ channels are activated specifically by L-Met, with the activation abolished in glr3.1/3.5. Moreover, GLR3.1/3.5 Ca²⁺ channels are distinct from previously characterized ROS-activated Ca²⁺ channels and act upstream of ROS, providing Ca²⁺ transients necessary for the activation of NADPH oxidases. Our data indicate that GLR3.1/3.5 constitute L-Met-activated Ca²⁺ channels responsible for maintaining basal [Ca²⁺]_cyt, play a pivotal role in plant growth, and act upstream of ROS, thereby regulating stomatal aperture.

Impact of the ion transportome of chloroplasts on the optimization of photosynthesis

Ions play fundamental roles in all living cells, and their gradients are often essential to fuel transport, regulate enzyme activities, and transduce energy within cells. Regulation of their homeostasis is essential for cell metabolism. Recent results indicate that modulation of ion fluxes might also represent a useful strategy to regulate one of the most important physiological processes taking place in chloroplasts, photosynthesis. Photosynthesis is highly regulated, due to its unique role as a cellular engine for growth in the light. Controlling the balance between ATP and NADPH synthesis is a critical task, and availability of these molecules can limit the overall photosynthetic yield. Photosynthetic organisms optimize photosynthesis in low light, where excitation energy limits CO2 fixation, and minimize photo-oxidative damage in high light by dissipating excess photons. Despite extensive studies of these phenomena, the mechanism governing light utilization in plants is still poorly understood. In this review, we provide an update of the recently identified chloroplast-located ion channels and transporters whose function impacts photosynthetic efficiency in plants.

Physiological Characterization of a Plant Mitochondrial Calcium Uniporter in Vitro and in Vivo

Over the recent years, several proteins that make up the mitochondrial calcium uniporter complex (MCUC) mediating Ca²⁺uptake into the mitochondrial matrix have been identified in mammals, including the channel-forming protein MCU. Although six MCU gene homologs are conserved in the model plant Arabidopsis (Arabidopsis thaliana) in which mitochondria can accumulate Ca²⁺, a functional characterization of plant MCU homologs has been lacking. Using electrophysiology, we show that one isoform, AtMCU1, gives rise to a Ca²⁺-permeable channel activity that can be observed even in the absence of accessory proteins implicated in the formation of the active mammalian channel. Furthermore, we provide direct evidence that AtMCU1 activity is sensitive to the mitochondrial calcium uniporter inhibitors Ruthenium Red and Gd³⁺, as well as to the Arabidopsis protein MICU, a regulatory MCUC component. AtMCU1 is prevalently expressed in roots, localizes to mitochondria, and its absence causes mild changes in Ca²⁺ dynamics as assessed by in vivo measurements in Arabidopsis root tips. Plants either lacking or overexpressing AtMCU1 display root mitochondria with altered ultrastructure and show shorter primary roots under restrictive growth conditions. In summary, our work adds evolutionary depth to the investigation of mitochondrial Ca²⁺ transport, indicates that AtMCU1, together with MICU as a regulator, represents a functional configuration of the plant mitochondrial Ca²⁺ uptake complex with differences to the mammalian MCUC, and identifies a new player of the intracellular Ca²⁺ regulation network in plants.

Signaling with Ions: The Keystone for Apical Cell Growth and Morphogenesis in Pollen Tubes

Ion homeostasis and signaling are crucial to regulate pollen tube growth and morphogenesis and affect upstream membrane transporters and downstream targets.

Plants grow with a little help from their organelle friends

Chloroplasts and mitochondria are indispensable for plant development. They not only provide energy and carbon sources to cells, but also have evolved to become major players in a variety of processes such as amino acid metabolism, hormone biosynthesis and cellular signalling. As semi-autonomous organelles, they contain a small genome that relies largely on nuclear factors for its maintenance and expression. An intensive crosstalk between the nucleus and the organelles is therefore essential to ensure proper functioning, and the nuclear genes encoding organellar proteins involved in photosynthesis and oxidative phosphorylation are obviously crucial for plant growth. Organ growth is determined by two main cellular processes: cell proliferation and cell expansion. Here, we review how plant growth is affected in mutants of organellar proteins that are differentially expressed during leaf and root development. Our findings indicate a clear role for organellar proteins in plant organ growth, primarily during cell proliferation. However, to date, the role of the nuclear-encoded organellar proteins in the cellular processes driving organ growth has not been investigated in much detail. We therefore encourage researchers to extend their phenotypic characterization beyond macroscopic features in order to get a better view on how chloroplasts and mitochondria regulate the basic processes of cell proliferation and cell expansion, essential to driving growth.

Heterologous Expression and Functional Analysis of Rice GLUTAMATE RECEPTOR-LIKE Family Indicates its Role in Glutamate Triggered Calcium Flux in Rice Roots

BACKGROUND

Tremendous progress has been made in understanding the functions of the GLUTAMATE RECEPTOR-LIKE (GLR) family in Arabidopsis. Still, the functions of OsGLRs in rice, especially the ion channel activities, are largely unknown.

RESULTS

Using the aequorin-based luminescence imaging system, we screened the specificity of amino acids involved in the induction of Ca(2+) flux in rice roots. Of all the amino acids tested, glutamate (Glu) was the only one to trigger Ca(2+) flux significantly in rice roots. Detailed analysis showed a dose response of Ca(2+) increase to different concentrations of Glu. In addition, the Ca(2+) spike response to Glu was rapid, within 20 s after the application. A desensitization assay and pharmacological tests showed that the Glu-triggered Ca(2+) flux is mediated by OsGLRs. Whole genome analysis identified 13 OsGLR genes in rice, and these genes have various expression patterns in different tissues. Subcellular localization studies showed that all the OsGLRs examined are likely localized to the plasma membrane. Bacteria growth assays showed that at least OsGLR2.1 and OsGLR3.2 have the potential to mediate ion uptake in bacteria. Further analysis using Fura-2-based Ca(2+) imaging revealed a Glu-triggered Ca(2+) increase in OsGLR2.1-expressing human embryonic kidney (HEK) cells.

CONCLUSIONS

Our work provides a molecular basis for investigating mechanisms of Glu-triggered Ca(2+) flux in rice.

Calcium-Mediated Abiotic Stress Signaling in Roots

Roots are subjected to a range of abiotic stresses as they forage for water and nutrients. Cytosolic free calcium is a common second messenger in the signaling of abiotic stress. In addition, roots take up calcium both as a nutrient and to stimulate exocytosis in growth. For calcium to fulfill its multiple roles must require strict spatio-temporal regulation of its uptake and efflux across the plasma membrane, its buffering in the cytosol and its sequestration or release from internal stores. This prompts the question of how specificity of signaling output can be achieved against the background of calcium's other uses. Threats to agriculture such as salinity, water availability and hypoxia are signaled through calcium. Nutrient deficiency is also emerging as a stress that is signaled through cytosolic free calcium, with progress in potassium, nitrate and boron deficiency signaling now being made. Heavy metals have the capacity to trigger or modulate root calcium signaling depending on their dose and their capacity to catalyze production of hydroxyl radicals. Mechanical stress and cold stress can both trigger an increase in root cytosolic free calcium, with the possibility of membrane deformation playing a part in initiating the calcium signal. This review addresses progress in identifying the calcium transporting proteins (particularly channels such as annexins and cyclic nucleotide-gated channels) that effect stress-induced calcium increases in roots and explores links to reactive oxygen species, lipid signaling, and the unfolded protein response.

Evolutionary insight into the ionotropic glutamate receptor superfamily of photosynthetic organisms

Photosynthetic eukaryotes have a complex evolutionary history shaped by multiple endosymbiosis events that required a tight coordination between the organelles and the rest of the cell. Plant ionotropic glutamate receptors (iGLRs) form a large superfamily of proteins with a predicted or proven non-selective cation channel activity regulated by a broad range of amino acids. They are involved in different physiological processes such as C/N sensing, resistance against fungal infection, root and pollen tube growth and response to wounding and pathogens. Most of the present knowledge is limited to iGLRs located in plasma membranes. However, recent studies localized different iGLR isoforms to mitochondria and/or chloroplasts, suggesting the possibility that they play a specific role in bioenergetic processes. In this work, we performed a comparative analysis of GLR sequences from bacteria and various photosynthetic eukaryotes. In particular, novel types of selectivity filters of bacteria are reported adding new examples of the great diversity of the GLR superfamily. The highest variability in GLR sequences was found among the algal sequences (cryptophytes, diatoms, brown and green algae). GLRs of land plants are not closely related to the GLRs of green algae analyzed in this work. The GLR family underwent a great expansion in vascular plants. Among plant GLRs, Clade III includes sequences from Physcomitrella patens, Marchantia polymorpha and gymnosperms and can be considered the most ancient, while other clades likely emerged later. In silico analysis allowed the identification of sequences with a putative target to organelles. Sequences with a predicted localization to mitochondria and chloroplasts are randomly distributed among different type of GLRs, suggesting that no compartment-related specific function has been maintained across the species.

The Plant Mitochondrial Transportome: Balancing Metabolic Demands with Energetic Constraints

In plants, mitochondrial function is associated with hundreds of metabolic reactions. To facilitate these reactions, charged substrates and cofactors move across the charge-impermeable inner mitochondrial membrane via specialized transporters and must work cooperatively with the electrochemical gradient which is essential for mitochondrial function. The regulatory framework for mitochondrial metabolite transport is expected to be more complex in plants than in mammals owing to the close metabolic association between mitochondrial, plastids, and peroxisome metabolism, as well as to the major diurnal fluctuations in plant metabolic function. We propose here how recent advances can be integrated towards defining the mitochondrial transportome in plants. We also discuss what this reveals about sustaining cooperativity between bioenergetics, metabolism, and transport in typical and challenging environments.

Regulation of mitochondrial calcium in plants versus animals

Ca(2+) acts as an important cellular second messenger in eukaryotes. In both plants and animals, a wide variety of environmental and developmental stimuli trigger Ca(2+) transients of a specific signature that can modulate gene expression and metabolism. In animals, mitochondrial energy metabolism has long been considered a hotspot of Ca(2+) regulation, with a range of pathophysiology linked to altered Ca(2+) control. Recently, several molecular players involved in mitochondrial Ca(2+) signalling have been identified, including those of the mitochondrial Ca(2+) uniporter. Despite strong evidence for sophisticated Ca(2+) regulation in plant mitochondria, the picture has remained much less clear. This is currently changing aided by live imaging and genetic approaches which allow dissection of subcellular Ca(2+) dynamics and identification of the proteins involved. We provide an update on our current understanding in the regulation of mitochondrial Ca(2+) and signalling by comparing work in plants and animals. The significance of mitochondrial Ca(2+) control is discussed in the light of the specific metabolic and energetic needs of plant and animal cells.

Dissecting stimulus-specific Ca2+ signals in amyloplasts and chloroplasts of Arabidopsis thaliana cell suspension cultures

Calcium is used by plants as an intracellular messenger in the detection of and response to a plethora of environmental stimuli and contributes to a fine-tuned internal regulation. Interest in the role of different subcellular compartments in Ca(2+) homeostasis and signalling has been growing in recent years. This work has evaluated the potential participation of non-green plastids and chloroplasts in the plant Ca(2+) signalling network using heterotrophic and autotrophic cell suspension cultures from Arabidopsis thaliana plant lines stably expressing the bioluminescent Ca(2+) reporter aequorin targeted to the plastid stroma. Our results indicate that both amyloplasts and chloroplasts are involved in transient Ca(2+) increases in the plastid stroma induced by several environmental stimuli, suggesting that these two functional types of plastids are endowed with similar mechanisms for handling Ca(2+) A comparison of the Ca(2+) trace kinetics recorded in parallel in the plastid stroma, the surface of the outer membrane of the plastid envelope, and the cytosol indicated that plastids play an essential role in switching off different cytosolic Ca(2+) signals. Interestingly, a transient stromal Ca(2+) signal in response to the light-to-dark transition was observed in chloroplasts, but not amyloplasts. Moreover, significant differences in the amplitude of specific plastidial Ca(2+) changes emerged when the photosynthetic metabolism of chloroplasts was reactivated by light. In summary, our work highlights differences between non-green plastids and chloroplasts in terms of Ca(2+) dynamics in response to environmental stimuli.

Calcium Flux across Plant Mitochondrial Membranes: Possible Molecular Players

Plants, being sessile organisms, have evolved the ability to integrate external stimuli into metabolic and developmental signals. A wide variety of signals, including abiotic, biotic, and developmental stimuli, were observed to evoke specific spatio-temporal Ca(2+) transients which are further transduced by Ca(2+) sensor proteins into a transcriptional and metabolic response. Most of the research on Ca(2+) signaling in plants has been focused on the transport mechanisms for Ca(2+) across the plasma- and the vacuolar membranes as well as on the components involved in decoding of cytoplasmic Ca(2+) signals, but how intracellular organelles such as mitochondria are involved in the process of Ca(2+) signaling is just emerging. The combination of the molecular players and the elicitors of Ca(2+) signaling in mitochondria together with newly generated detection systems for measuring organellar Ca(2+) concentrations in plants has started to provide fruitful grounds for further discoveries. In the present review we give an updated overview of the currently identified/hypothesized pathways, such as voltage-dependent anion channels, homologs of the mammalian mitochondrial uniporter (MCU), LETM1, a plant glutamate receptor family member, adenine nucleotide/phosphate carriers and the permeability transition pore (PTP), that may contribute to the transport of Ca(2+) across the outer and inner mitochondrial membranes in plants. We briefly discuss the relevance of the mitochondrial Ca(2+) homeostasis for ensuring optimal bioenergetic performance of this organelle.