Plasmodesmata-localized proteins and ROS orchestrate light-induced rapid systemic signaling in Arabidopsis

The roles of movement and coat proteins in the transport of tobamoviruses between plant cells

Tobamovirus is a large group of positive-sense, single-stranded RNA viruses that cause diseases in a broad range of plant species, including many agronomically important crops. The number of known Tobamovirus species has been on the rise in recent years, and currently, this genus includes 47 viruses. Tobamoviruses are transmitted mainly by mechanical contact, such as physical touching by hands or agricultural tools; and some are also transmitted on seeds, or through pollinator insects. The tobamoviral genome encodes proteins that have evolved to fulfill the main conceptual task of the viral infection cycle - the spread of the invading virus throughout the host plant cells, tissues, and organs. Here, we discuss this aspect of the infection cycle of tobamoviruses, focusing on the advances in our understanding of the local, i.e., cell-to-cell, and systemic, i.e., organ-to-organ, virus movement, and the viral and host plant determinants of these processes. Specifically, we spotlight two viral proteins-the movement protein (MP) and the coat protein (CP), which are directly involved in the local and systemic spread of tobamoviruses-with respect to their phylogeny, activities during viral movement, and interactions with the host determinants of the movement process.

More questions than answers: insights into potential cysteine-rich receptor-like kinases redox signalling in Arabidopsis

Over the past few decades, significant advancements have been made in understanding how plasma-membrane localised receptor kinases (RKs) detect signals and activate responses to various stimuli. Numerous examples of ligand-induced receptor activation mechanisms and their downstream consequences have been characterised in detail. The crucial role of post-translational modifications (PTMs), such as the phosphorylation of receptor kinases, has been demonstrated concerning different cellular responses. Given the diverse structures and architectures of the extracellular domains (ECDs) of RKs, it is probable that various forms of PTMs also play an essential role in receptor activation, including cysteine oxidative modifications triggered by reactive oxygen species (ROS). The function of cysteine oxidative modifications as functional redox switches that modulate protein structure and function has been extensively studied across various multicellular organisms. Based on biochemical and structural characteristics, the family of cysteine-rich receptor-like kinases (CRK) emerges as excellent candidates for proteins regulated in a redox-dependent manner. This review provides a concise overview of cysteine's biochemical and structural properties in its role as a molecular redox switch. Drawing on the currently available literature, we describe how cysteine-redox signalling is maintained, particularly in plant cells. We further focus on extracellular ROS perception and the role of CRKs as promising candidates for ROS sensors in Arabidopsis thaliana. We discuss the structural and biochemical properties of CRKs, their involvement in plant growth and defence processes, and our perspective on why CRKs could be key components of the ROS sensing machinery or ROS sensors, especially regarding the dimerization abilities of CRKs. Finally, we highlight the current challenges in identifying and quantifying cysteine oxidative modifications and propose methods for detecting ROS-modified cysteines that may be promising for investigating the role of CRKs in extracellular ROS perception and signalling.

A mechanistic integration of hypoxia signaling with energy, redox, and hormonal cues

Oxygen deficiency (hypoxia) occurs naturally in many developing plant tissues but can become a major threat during acute flooding stress. Consequently, plants as aerobic organisms must rapidly acclimate to hypoxia and the associated energy crisis to ensure cellular and ultimately organismal survival. In plants, oxygen sensing is tightly linked with oxygen-controlled protein stability of group VII ETHYLENE-RESPONSE FACTORs (ERFVII), which, when stabilized under hypoxia, act as key transcriptional regulators of hypoxia-responsive genes (HRGs). Multiple signaling pathways feed into hypoxia signaling to fine-tune cellular decision-making under stress. First, ATP shortage upon hypoxia directly affects the energy status and adjusts anaerobic metabolism. Secondly, altered redox homeostasis leads to reactive oxygen and nitrogen species (ROS and RNS) accumulation, evoking signaling and oxidative stress acclimation. Finally, the phytohormone ethylene promotes hypoxia signaling to improve acute stress acclimation, while hypoxia signaling in turn can alter ethylene, auxin, abscisic acid, salicylic acid, and jasmonate signaling to guide development and stress responses. In this Update, we summarize the current knowledge on how energy, redox, and hormone signaling pathways are induced under hypoxia and subsequently integrated at the molecular level to ensure stress-tailored cellular responses. We show that some HRGs are responsive to changes in redox, energy, and ethylene independently of the oxygen status, and we propose an updated HRG list that is more representative for hypoxia marker gene expression. We discuss the synergistic effects of hypoxia, energy, redox, and hormone signaling and their phenotypic consequences in the context of both environmental and developmental hypoxia.

GhCNGC31 is critical for conferring resistance to Verticillium wilt in cotton

In the past decades, cyclic nucleotide-gated ion channels (CNGCs) have been extensively studied in diploid species Arabidopsis thaliana. However, the functional diversification of CNGCs in crop plants, mostly polyploid, remains poorly understood. In allotetraploid Upland cotton (Gossypium hirsutum), GhCNGC31 is one of the multiple orthologs of AtCNGC2, being present in the plasma membrane, capable of interacting with itself and binding to calmodulins and cyclic nucleotides. GhCNGC31 knockdown plants exhibited slight growth inhibition, and became more susceptible to Verticillium dahliae infection, which was associated with the reduced lignin and flavonoid accumulation, impaired ROS (reactive oxygen species) burst, and down-regulation of defense-related genes PR1, JAZ2, LOX2, and RBOH10. RNA-Seq analysis identified 1817 differentially expressed genes from GhCNGC31 knockdown, of which 1184 (65%) were responsive to V. dahliae infection and accounted for 57% among a total of 2065 V. dahliae-responsive genes identified in this study. These GhCNGC31-regulated genes mainly function with cell wall organization and biogenesis, cellular carbohydrate metabolic or biosynthetic process, cellular component macromolecule biosynthetic process, and rhythmic process. They are significantly enriched in the pathways of plant MAPK signaling, plant-pathogen interaction, phenylpropanoid biosynthesis, and plant hormone signal transduction. A set of transcription factors (TFs) and resistance (R) genes are among the GhCNGC31-regulated genes, which are significantly over-represented with the TCP and WRKY TFs families, as well as with the R genes of T (TIR) and TNL (TIR-NB-LRR) classes. Together, our results unraveled a critical role of GhCNGC31 for conferring resistance to Verticillium wilt in cotton.

Analysis of the Mechanisms Underlying the Specificity of the Variation Potential Induced by Different Stimuli

Plants are able to perceive diverse environmental factors and form an appropriate systemic functional response. Systemic responses are induced by stimulus-specific long-distance signals that carry information about the stimulus. Variation potential is proposed as a candidate for the role of such a signal. Here, we focus on the mechanisms that determine the specificity of the variation potential under the action of different local stimuli. Local stimuli such as heating, burning and wounding cause variation potential, the parameters of which differ depending on the type of stimulus. It was found that the stimulus-specific features of the hydraulic signal monitored by changes in leaf thickness and variation potential, such as a greater amplitude upon heating and burning and a significant amplitude decrement upon burning and wounding, were similar. The main features of these signals are the greater amplitude upon heating and burning, and a significant amplitude decrement upon burning and wounding. Together with the temporal correspondence of signal propagation, this evidence indicates a role for the hydraulic signal in the induction of stimulus-specific variation potential. Experiments using mechanosensitive channel inhibitors have demonstrated that the hydraulic signal contributes more to the induction of the variation potential in the case of rapidly growing stimuli, such as burning and wounding, than in the case of gradual heating. For thermal stimuli (gradual heating and burning), a greater contribution, compared to wounding, of the chemical signal related to reactive oxygen species to the induction of the variation potential was demonstrated. Thus, the specificity of the parameters of the variation potential is determined by the different contributions of hydraulic and chemical signals.

Permeation mechanisms of hydrogen peroxide and water through Plasma Membrane Intrinsic Protein aquaporins

Hydrogen peroxide (H2O2) transport by aquaporins (AQP) is a critical feature for cellular redox signaling. However, the H2O2 permeation mechanism through these channels remains poorly understood. Through functional assays, two Plasma membrane Intrinsic Protein (PIP) AQP from Medicago truncatula, MtPIP2;2 and MtPIP2;3 have been identified as pH-gated channels capable of facilitating the permeation of both water (H2O) and H2O2. Employing a combination of unbiased and enhanced sampling molecular dynamics simulations, we investigated the key barriers and translocation mechanisms governing H2O2 permeation through these AQP in both open and closed conformational states. Our findings reveal that both H2O and H2O2 encounter their primary permeation barrier within the selectivity filter (SF) region of MtPIP2;3. In addition to the SF barrier, a second energetic barrier at the NPA (asparagine-proline-alanine) region that is more restrictive for the passage of H2O2 than for H2O, was found. This behavior can be attributed to a dissimilar geometric arrangement and hydrogen bonding profile between both molecules in this area. Collectively, these findings suggest mechanistic heterogeneity in H2O and H2O2 permeation through PIPs.

C-Type LECTIN receptor-like kinase 1 and ACTIN DEPOLYMERIZING FACTOR 3 are key components of plasmodesmata callose modulation

Plasmodesmata (PDs) are intercellular organelles carrying multiple membranous nanochannels that allow the trafficking of cellular signalling molecules. The channel regulation of PDs occurs dynamically and is required in various developmental and physiological processes. It is well known that callose is a critical component in regulating PD permeability or symplasmic connectivity, but the understanding of the signalling pathways and mechanisms of its regulation is limited. Here, we used the reverse genetic approach to investigate the role of C-type lectin receptor-like kinase 1 (CLRLK1) in the aspect of PD callose-modulated symplasmic continuity. Here, we found that loss-of-function mutations in CLRLK1 resulted in excessive PD callose deposits and reduced symplasmic continuity, resulting in an accelerated gravitropic response. The protein interactome study also found that CLRLK1 interacted with actin depolymerizing factor 3 (ADF3) in vitro and in plants. Moreover, mutations in ADF3 result in elevated PD callose deposits and faster gravitropic response. Our results indicate that CLRLK1 and ADF3 negatively regulate PD callose accumulation, contributing to fine-tuning symplasmic opening apertures. Overall, our studies identified two key components involved in the deposits of PD callose and provided new insights into how symplasmic connectivity is maintained by the control of PD callose homoeostasis.

New insights into plasmodesmata: complex 'protoplasmic connecting threads'

Intercellular communication in plants, as in other multicellular organisms, allows cells in tissues to coordinate their responses for development and in response to environmental stimuli. Much of this communication is facilitated by plasmodesmata (PD), consisting of membranes and cytoplasm, that connect adjacent cells to each other. PD have long been viewed as passive conduits for the movement of a variety of metabolites and molecular cargoes, but this perception has been changing over the last two decades or so. Research from the last few years has revealed the importance of PD as signaling hubs and as crucial players in hormone signaling. The adoption of advanced biochemical approaches, molecular tools, and high-resolution imaging modalities has led to several recent breakthroughs in our understanding of the roles of PD, revealing the structural and regulatory complexity of these 'protoplasmic connecting threads'. We highlight several of these findings that we think well illustrate the current understanding of PD as functioning at the nexus of plant physiology, development, and acclimation to the environment.

The Respiratory Burst Oxidase Homologue OsRBOHE is crucial for root hair formation, drought resistance and tillering in rice

Respiratory Burst Oxidase Homologues (RBOHs) are involved in plant growth, development, and stress adaptation. How OsRBOHs affect root hair formation and consequently nutrient acquisition and drought resistance in rice is not well understood. We knocked out six OsRBOH genes in rice that were expressed in roots and identified OsRBOHE as the only one affecting root hair formation. OsRBOHE was strongly expressed in the root epidermis, root hairs and tiller buds. OsRBOHE is localised at the plasma membrane. Knockout of OsRBOHE decreased reactive oxygen species generation in the root hairs and tiller buds, downregulated genes involved in cell wall biogenesis, and decreased root hair length and tillering by 90% and 30%, respectively. Knockout of OsRBOHE decreased phosphorus acquisition only in low available P soil under aerobic conditions, but not in high P soil or under flooded conditions when P was likely not limited by diffusion. Knockout of OsRBOHE markedly decreased drought resistance of rice plants through the effect on root hair formation and the associated rhizosheath. Taken together, OsRBOHE is crucial for root hair formation and tillering and consequently on drought resistance in rice. The contribution of root hairs to P acquisition in rice is limited to aerobic soil.

PLASMODESMATA-LOCATED PROTEIN 6 regulates plasmodesmal function in Arabidopsis vasculature

Plasmodesmata connect adjoining plant cells, allowing molecules to move between the connected cells for communication and sharing resources. It has been well established that the plant polysaccharide callose is deposited at plasmodesmata, regulating their aperture and function. Among proteins involved in maintaining callose homeostasis, PLASMODESMATA-LOCATED PROTEINSs (PDLPs) promote callose deposition at plasmodesmata. This study explored the function of PDLP5 and PDLP6 in different cell types. We discovered that PDLP5 and PDLP6 are expressed in nonoverlapping cell types in Arabidopsis (Arabidopsis thaliana). The overexpression of PDLP5 and PDLP6 results in the overaccumulation of plasmodesmal callose at different cell interfaces, indicating that PDLP5 and PDLP6 are active in different cell types. We also observed 2 distinct patterns of starch accumulation in mature leaves of PDLP5 and PDLP6 overexpressors. An enzyme-catalyzed proximity labeling approach was used to identify putative functional partners of the PDLPs. We identified SUCROSE SYNTHASE 6 (SUS6) as a functional partner of PDLP6 in the vasculature. We further demonstrated that PDLP6 physically and genetically interacts with SUS6. In addition, CALLOSE SYNTHASE 7 (CALS7) physically interacts with SUS6 and PDLP6. Genetic interaction studies showed that CALS7 is required for PDLP6 function. We propose that PDLP6 functions with SUS6 and CALS7 in the vasculature to regulate plasmodesmal function.

The redox code of plants

Central metabolism is organised through high-flux, Nicotinamide Adenine Dinucleotide (NAD+/NADH) and NADP+/NADPH systems operating at near equilibrium. As oxygen is indispensable for aerobic organisms, these systems are also linked to the levels of reactive oxygen species, such as H2O2, and through H2O2 to the regulation of macromolecular structures and activities, via kinetically controlled sulphur switches in the redox proteome. Dynamic changes in H2O2 production, scavenging and transport, associated with development, growth and responses to the environment are, therefore, linked to the redox state of the cell and regulate cellular function. These basic principles form the 'redox code' of cells and were first defined by D. P. Jones and H. Sies in 2015. Here, we apply these principles to plants in which recent studies have shown that they can also explain cell-to-cell and even plant-to-plant signalling processes. The redox code is, therefore, an integral part of biological systems and can be used to explain multiple processes in plants at the subcellular, cellular, tissue, whole organism and perhaps even community and ecosystem levels. As the environmental conditions on our planet are worsening due to global warming, climate change and increased pollution levels, new studies are needed applying the redox code of plants to these changes.

Roles of ROS and redox in regulating cell-to-cell communication: Spotlight on viral modulation of redox for local spread

Reactive oxygen species (ROS) are important signalling molecules that influence many aspects of plant biology. One way in which ROS influence plant growth and development is by modifying intercellular trafficking through plasmodesmata (PD). Viruses have evolved to use PD for their local cell-to-cell spread between plant cells, so it is therefore not surprising that they have found ways to modulate ROS and redox signalling to optimise PD function for their benefit. This review examines how intracellular signalling via ROS and redox pathways regulate intercellular trafficking via PD during development and stress. The relationship between viruses and ROS-redox systems, and the strategies viruses employ to control PD function by interfering with ROS-redox in plants is also discussed.

Functionality of Reactive Oxygen Species (ROS) in Plants: Toxicity and Control in Poaceae Crops Exposed to Abiotic Stress

Agriculture and changing environmental conditions are closely related, as weather changes could adversely affect living organisms or regions of crop cultivation. Changing environmental conditions trigger different abiotic stresses, which ultimately cause the accumulation of reactive oxygen species (ROS) in plants. Common ROS production sites are the chloroplast, endoplasmic reticulum, plasma membrane, mitochondria, peroxisomes, etc. The imbalance in ROS production and ROS detoxification in plant cells leads to oxidative damage to biomolecules such as lipids, nucleic acids, and proteins. At low concentrations, ROS initiates signaling events related to development and adaptations to abiotic stress in plants by inducing signal transduction pathways. In plants, a stress signal is perceived by various receptors that induce a signal transduction pathway that activates numerous signaling networks, which disrupt gene expression, impair the diversity of kinase/phosphatase signaling cascades that manage the stress response in the plant, and result in changes in physiological responses under various stresses. ROS production also regulates ABA-dependent and ABA-independent pathways to mitigate drought stress. This review focuses on the common subcellular location of manufacturing, complex signaling mechanisms, and networks of ROS, with an emphasis on cellular effects and enzymatic and non-enzymatic antioxidant scavenging mechanisms of ROS in Poaceae crops against drought stress and how the manipulation of ROS regulates stress tolerance in plants. Understanding ROS systems in plants could help to create innovative strategies to evolve paths of cell protection against the negative effects of excessive ROS in attempts to improve crop productivity in adverse environments.

ROS are universal cell-to-cell stress signals

The interplay between reactive oxygen species (ROS) and the redox state of cells is deeply rooted in the biology of almost all organisms, regulating development, growth, and responses to the environment. Recent studies revealed that the ROS levels and redox state of one cell can be transmitted, as an information 'state' or 'currency', to other cells and spread by cell-to-cell communication within an entire community of cells or an organism. Here, we discuss the different pathways that mediate cell-to-cell signaling in plants, their hierarchy, and the different mechanisms that transmit ROS/redox signaling between different cells. We further hypothesize that ROS/redox signaling between different organisms could play a key role within the 'one world' principle, impacting human health and our future.

Natural variation in CYCLIC NUCLEOTIDE-GATED ION CHANNEL 4 reveals a novel role of calcium signaling in vegetative phase change in Arabidopsis

The timing of vegetative phase change (VPC) in plants is regulated by a temporal decline in the expression of miR156. Both exogenous cues and endogenous factors, such as temperature, light, sugar, nutrients, and epigenetic regulators, have been shown to affect VPC by altering miR156 expression. However, the genetic basis of natural variation in VPC remains largely unexplored. Here, we conducted a genome-wide association study on the variation of the timing of VPC in Arabidopsis. We identified CYCLIC NUCLEOTIDE-GATED ION CHANNEL 4 (CNGC4) as a significant locus associated with the diversity of VPC. Mutations in CNGC4 delayed VPC, accompanied by an increased expression level of miR156 and a corresponding decrease in SQUAMOSA PROMOTER BINDING-LIKE (SPL) gene expression. Furthermore, mutations in CNGC2 and CATION EXCHANGER 1/3 (CAX1/3) also led to a delay in VPC. Polymorphisms in the CNGC4 promoter contribute to the natural variation in CNGC4 expression and the diversity of VPC. Specifically, the early CNGC4 variant promotes VPC and enhances plant adaptation to local environments. In summary, our findings offer genetic insights into the natural variation in VPC in Arabidopsis, and reveal a previously unidentified role of calcium signaling in the regulation of VPC.

Propagation of a rapid cell-to-cell H2O2 signal over long distances in a monolayer of cardiomyocyte cells

Cell-to-cell communication plays a cardinal role in the biology of multicellular organisms. H2O2 is an important cell-to-cell signaling molecule involved in the response of mammalian cells to wounding and other stimuli. We previously identified a signaling pathway that transmits wound-induced cell-to-cell H2O2 signals within minutes over long distances, measured in centimeters, in a monolayer of cardiomyocytes. Here we report that this long-distance H2O2 signaling pathway is accompanied by enhanced accumulation of cytosolic H2O2 and altered redox state in cells along its path. We further show that it requires the production of superoxide, as well as the function of gap junctions, and that it is accompanied by changes in the abundance of hundreds of proteins in cells along its path. Our findings highlight the existence of a unique and rapid long-distance H2O2 signaling pathway that could play an important role in different inflammatory responses, wound responses/healing, cardiovascular disease, and/or other conditions.

Systemic stomatal responses in plants: Coordinating development, stress, and pathogen defense under a changing climate

To successfully survive, develop, grow and reproduce, multicellular organisms must coordinate their molecular, physiological, developmental and metabolic responses among their different cells and tissues. This process is mediated by cell-to-cell, vascular and/or volatile communication, and involves electric, chemical and/or hydraulic signals. Within this context, stomata serve a dual role by coordinating their responses to the environment with their neighbouring cells at the epidermis, but also with other stomata present on other parts of the plant. As stomata represent one of the most important conduits between the plant and its above-ground environment, as well as directly affect photosynthesis, respiration and the hydraulic status of the plant by controlling its gas and vapour exchange with the atmosphere, coordinating the overall response of stomata within and between different leaves and tissues plays a cardinal role in plant growth, development and reproduction. Here, we discuss different examples of local and systemic stomatal coordination, the different signalling pathways that mediate them, and the importance of systemic stomatal coordination to our food supply, ecosystems and weather patterns, under our changing climate. We further discuss the potential biotechnological implications of regulating systemic stomatal responses for enhancing agricultural productivity in a warmer and CO2 -rich environment.

METACASPASE8 (MC8) Is a Crucial Protein in the LSD1-Dependent Cell Death Pathway in Response to Ultraviolet Stress

LESION-SIMULATING DISEASE1 (LSD1) is one of the well-known cell death regulatory proteins in Arabidopsis thaliana. The lsd1 mutant exhibits runaway cell death (RCD) in response to various biotic and abiotic stresses. The phenotype of the lsd1 mutant strongly depends on two other proteins, ENHANCED DISEASE SUSCEPTIBILITY 1 (EDS1) and PHYTOALEXIN-DEFICIENT 4 (PAD4) as well as on the synthesis/metabolism/signaling of salicylic acid (SA) and reactive oxygen species (ROS). However, the most interesting aspect of the lsd1 mutant is its conditional-dependent RCD phenotype, and thus, the defined role and function of LSD1 in the suppression of EDS1 and PAD4 in controlled laboratory conditions is different in comparison to a multivariable field environment. Analysis of the lsd1 mutant transcriptome in ambient laboratory and field conditions indicated that there were some candidate genes and proteins that might be involved in the regulation of the lsd1 conditional-dependent RCD phenotype. One of them is METACASPASE 8 (AT1G16420). This type II metacaspase was described as a cell death-positive regulator induced by UV-C irradiation and ROS accumulation. In the double mc8/lsd1 mutant, we discovered reversion of the lsd1 RCD phenotype in response to UV radiation applied in controlled laboratory conditions. This cell death deregulation observed in the lsd1 mutant was reverted like in double mutants of lsd1/eds1 and lsd1/pad4. To summarize, in this work, we demonstrated that MC8 is positively involved in EDS1 and PAD4 conditional-dependent regulation of cell death when LSD1 function is suppressed in Arabidopsis thaliana. Thus, we identified a new protein compound of the conditional LSD1-EDS1-PAD4 regulatory hub. We proposed a working model of MC8 involvement in the regulation of cell death and we postulated that MC8 is a crucial protein in this regulatory pathway.

The GABA shunt contributes to ROS homeostasis in guard cells of Arabidopsis

γ-Aminobutyric acid (GABA) accumulates rapidly under stress via the GABA shunt pathway, which has been implicated in reducing the accumulation of stress-induced reactive oxygen species (ROS) in plants. γ-Aminobutyric acid has been demonstrated to act as a guard-cell signal in Arabidopsis thaliana, modulating stomatal opening. Knockout of the major GABA synthesis enzyme Glutamate Decarboxylase 2 (GAD2) increases the aperture of gad2 mutants, which results in greater stomatal conductance and reduces water-use efficiency compared with wild-type plants. Here, we found that the additional loss of GAD1, GAD4, and GAD5 in gad2 leaves increased GABA deficiency but abolished the more open stomatal pore phenotype of gad2, which we link to increased cytosolic calcium (Ca2+ ) and ROS accumulation in gad1/2/4/5 guard cells. Compared with wild-type and gad2 plants, glutamate was ineffective in closing gad1/2/4/5 stomatal pores, whereas lowering apoplastic calcium, applying ROS inhibitors or complementation with GAD2 reduced gad1/2/4/5 guard-cell ROS, restored the gad2-like greater stomatal apertures of gad1/2/4/5 beyond that of wild-type. We conclude that GADs are important contributors to ROS homeostasis in guard cells likely via a Ca2+ -mediated pathway. As such, this study reveals greater complexity in GABA's role as a guard-cell signal and the interactions it has with other established signals.

Propagation of a rapid cell-to-cell H 2 O 2 signal over long distances in a monolayer of cardiomyocyte cells

Cell-to-cell communication plays a cardinal role in the biology of multicellular organisms. H 2 O 2 is an important cell-to-cell signaling molecule involved in the response of mammalian cells to wounding and other stimuli. We previously identified a signaling pathway that transmits wound-induced cell-to-cell H 2 O 2 signals within minutes over long distances, measured in centimeters, in a monolayer of cardiomyocytes. Here we report that this long-distance H 2 O 2 signaling pathway is accompanied by enhanced accumulation of cytosolic H 2 O 2 and altered redox state in cells along its path. We further show that it requires the production of superoxide, as well as the function of gap junctions, and that it is accompanied by changes in the abundance of hundreds of proteins in cells along its path. Our findings highlight the existence of a unique and rapid long-distance H 2 O 2 signaling pathway that could play an important role in different inflammatory responses, wound responses/healing, cardiovascular disease, and/or other conditions.

Highlights

Wounding induces an H 2 O 2 cell-to-cell signal in a monolayer of cardiomyocytes. The cell-to-cell signal requires H 2 O 2 and O 2 · - accumulation along its path. The signal propagates over several centimeters changing the redox state of cells.Changes in the abundance of hundreds of proteins accompanies the signal.The cell-to-cell signal requires paracrine and juxtacrine signaling.

GhCNGC13 and 32 Act as Critical Links between Growth and Immunity in Cotton

Cyclic nucleotide-gated ion channels (CNGCs) remain poorly studied in crop plants, most of which are polyploid. In allotetraploid Upland cotton (Gossypium hirsutum), silencing GhCNGC13 and 32 impaired plant growth and shoot apical meristem (SAM) development, while triggering plant autoimmunity. Both growth hormones (indole-3-acetic acid and gibberellin) and stress hormones (abscisic acid, salicylic acid, and jasmonate) increased, while leaf photosynthesis decreased. The silenced plants exhibited an enhanced resistance to Botrytis cinerea; however, Verticillium wilt resistance was weakened, which was associated with LIPOXYGENASE2 (LOX2) downregulation. Transcriptomic analysis of silenced plants revealed 4835 differentially expressed genes (DEGs) with functional enrichment in immunity and photosynthesis. These DEGs included a set of transcription factors with significant over-representation in the HSF, NAC, and WRKY families. Moreover, numerous members of the GhCNGC family were identified among the DEGs, which may indicate a coordinated action. Collectively, our results suggested that GhCNGC13 and 32 functionally link to photosynthesis, plant growth, and plant immunity. We proposed that GhCNGC13 and 32 play a critical role in the "growth-defense tradeoff" widely observed in crops.

Rapid systemic responses of Arabidopsis to waterlogging stress

Waterlogging stress (WLS) negatively impacts the growth and yield of crops resulting in heavy losses to agricultural production. Previous studies have revealed that WLS induces a systemic response in shoots that is partially dependent on the plant hormones ethylene and abscisic acid. However, the role of rapid cell-to-cell signaling pathways, such as the reactive oxygen species (ROS) and calcium waves, in systemic responses of plants to WLS is unknown at present. Here, we reveal that an abrupt WLS treatment of Arabidopsis (Arabidopsis thaliana) plants growing in peat moss triggers systemic ROS and calcium wave responses and that the WLS-triggered ROS wave response of Arabidopsis is dependent on the ROS-generating RESPIRATORY BURST OXIDASE HOMOLOG D (RBOHD), calcium-permeable channels GLUTAMATE-LIKE RECEPTOR 3.3 and 3.6 (GLR3.3 and GLR3.6), and aquaporin PLASMA MEMBRANE INTRINSIC PROTEIN 2;1 (PIP2;1) proteins. We further show that WLS is accompanied by a rapid systemic transcriptomic response that is evident as early as 10 min following waterlogging initiation, includes many hypoxia-response transcripts, and is partially dependent on RBOHD. Interestingly, the abrupt WLS of Arabidopsis resulted in the triggering of a rapid hydraulic wave response and the transient opening of stomata on leaves. In addition, it induced in plants a heightened state of tolerance to a subsequent submergence stress. Taken together, our findings reveal that the initiation of WLS in plants is accompanied by rapid systemic physiological and transcriptomic responses that involve the ROS, calcium, and hydraulic waves, as well as the induction of hypoxia acclimation mechanisms in systemic tissues.

Reactive Oxygen Species: A Crosslink between Plant and Human Eukaryotic Cell Systems

Reactive oxygen species (ROS) are important regulating factors that play a dual role in plant and human cells. As the first messenger response in organisms, ROS coordinate signals in growth, development, and metabolic activity pathways. They also can act as an alarm mechanism, triggering cellular responses to harmful stimuli. However, excess ROS cause oxidative stress-related damage and oxidize organic substances, leading to cellular malfunctions. This review summarizes the current research status and mechanisms of ROS in plant and human eukaryotic cells, highlighting the differences and similarities between the two and elucidating their interactions with other reactive substances and ROS. Based on the similar regulatory and metabolic ROS pathways in the two kingdoms, this review proposes future developments that can provide opportunities to develop novel strategies for treating human diseases or creating greater agricultural value.

Exploring aquaporin functions during changes in leaf water potential

Maintenance of optimal leaf tissue humidity is important for plant productivity and food security. Leaf humidity is influenced by soil and atmospheric water availability, by transpiration and by the coordination of water flux across cell membranes throughout the plant. Flux of water and solutes across plant cell membranes is influenced by the function of aquaporin proteins. Plants have numerous aquaporin proteins required for a multitude of physiological roles in various plant tissues and the membrane flux contribution of each aquaporin can be regulated by changes in protein abundance, gating, localisation, post-translational modifications, protein:protein interactions and aquaporin stoichiometry. Resolving which aquaporins are candidates for influencing leaf humidity and determining how their regulation impacts changes in leaf cell solute flux and leaf cavity humidity is challenging. This challenge involves resolving the dynamics of the cell membrane aquaporin abundance, aquaporin sub-cellular localisation and location-specific post-translational regulation of aquaporins in membranes of leaf cells during plant responses to changes in water availability and determining the influence of cell signalling on aquaporin permeability to a range of relevant solutes, as well as determining aquaporin influence on cell signalling. Here we review recent developments, current challenges and suggest open opportunities for assessing the role of aquaporins in leaf substomatal cavity humidity regulation.

Metabolic regulation of quiescence in plants

Quiescence is a crucial survival attribute in which cell division is repressed in a reversible manner. Although quiescence has long been viewed as an inactive state, recent studies have shown that it is an actively monitored process that is influenced by environmental stimuli. Here, we provide a perspective of the quiescent state and discuss how this process is tuned by energy, nutrient and oxygen status, and the pathways that sense and transmit these signals. We not only highlight the governance of canonical regulators and signalling mechanisms that respond to changes in nutrient and energy status, but also consider the central significance of mitochondrial functions and cues as key regulators of nuclear gene expression. Furthermore, we discuss how reactive oxygen species and the associated redox processes, which are intrinsically linked to energy carbohydrate metabolism, also play a key role in the orchestration of quiescence.

Merging Signaling with Structure: Functions and Mechanisms of Plant Glutamate Receptor Ion Channels

Plant glutamate receptor-like (GLR) genes encode ion channels with demonstrated roles in electrical and calcium (Ca2+) signaling. The expansion of the GLR family along the lineage of land plants, culminating in the appearance of a multiclade system among flowering plants, has been a topic of interest since their discovery nearly 25 years ago. GLRs are involved in many physiological processes, from wound signaling to transcriptional regulation to sexual reproduction. Emerging evidence supports the notion that their fundamental functions are conserved among different groups of plants as well. In this review, we update the physiological and genetic evidence for GLRs, establishing their role in signaling and cell-cell communication. Special emphasis is given to the recent discussion of GLRs' atomic structures. Along with functional assays, a structural view of GLRs' molecular organization presents a window for novel hypotheses regarding the molecular mechanisms underpinning signaling associated with the ionic fluxes that GLRs regulate. Newly uncovered transcriptional regulations associated with GLRs-which propose the involvement of genes from all clades ofArabidopsis thaliana in ways not previously observed-are discussed in the context of the broader impacts of GLR activity. We posit that the functions of GLRs in plant biology are probably much broader than anticipated, but describing their widespread involvement will only be possible with (a) a comprehensive understanding of the channel's properties at the molecular and structural levels, including protein-protein interactions, and (b) the design of new genetic approaches to explore stress and pathogen responses where precise transcriptional control may result in more precise testable hypotheses to overcome their apparent functional redundancies.

Phytochrome B regulates reactive oxygen signaling during abiotic and biotic stress in plants

Reactive oxygen species (ROS) and the photoreceptor protein phytochrome B (phyB) play a key role in plant acclimation to stress. However, how phyB that primarily functions in the nuclei impacts ROS signaling mediated by respiratory burst oxidase homolog (RBOH) proteins that reside on the plasma membrane, during stress, is unknown. Arabidopsis thaliana and Oryza sativa mutants, RNA-Seq, bioinformatics, biochemistry, molecular biology, and whole-plant ROS imaging were used to address this question. Here, we reveal that phyB and RBOHs function as part of a key regulatory module that controls apoplastic ROS production, stress-response transcript expression, and plant acclimation in response to excess light stress. We further show that phyB can regulate ROS production during stress even if it is restricted to the cytosol and that phyB, respiratory burst oxidase protein D (RBOHD), and respiratory burst oxidase protein F (RBOHF) coregulate thousands of transcripts in response to light stress. Surprisingly, we found that phyB is also required for ROS accumulation in response to heat, wounding, cold, and bacterial infection. Our findings reveal that phyB plays a canonical role in plant responses to biotic and abiotic stresses, regulating apoplastic ROS production, possibly while at the cytosol, and that phyB and RBOHD/RBOHF function in the same regulatory pathway.

Phytochrome B regulates reactive oxygen signaling during abiotic and biotic stress in plants

Reactive oxygen species (ROS) and the photoreceptor protein phytochrome B (phyB) play a key role in plant acclimation to stress. However, how phyB that primarily functions in the nuclei impacts ROS signaling mediated by respiratory burst oxidase homolog (RBOH) proteins that reside on the plasma membrane, during stress, is unknown. Arabidopsis thaliana and Oryza sativa mutants, RNA-Seq, bioinformatics, biochemistry, molecular biology, and whole-plant ROS imaging were used to address this question. Here, we reveal that phyB and RBOHs function as part of a key regulatory module that controls apoplastic ROS production, stress-response transcript expression, and plant acclimation in response to excess light stress. We further show that phyB can regulate ROS production during stress even if it is restricted to the cytosol and that phyB, respiratory burst oxidase protein D (RBOHD), and respiratory burst oxidase protein F (RBOHF) coregulate thousands of transcripts in response to light stress. Surprisingly, we found that phyB is also required for ROS accumulation in response to heat, wounding, cold, and bacterial infection. Our findings reveal that phyB plays a canonical role in plant responses to biotic and abiotic stresses, regulating apoplastic ROS production, possibly while at the cytosol, and that phyB and RBOHD/RBOHF function in the same regulatory pathway.

Imaging systemic calcium response and its molecular dissection using virus-induced gene silencing

Many biotic and abiotic stimuli arrive locally on the plant. For example, attack by an insect or invasion by a fungal pathogen generally starts with a single leaf. However, the responses that are then elicited are often systemic, triggering effects throughout the entire plant body. One of the rapid signaling systems that helps coordinate these plant-wide response networks is changes in cytoplasmic Ca²⁺ that rapidly propagate throughout the plant. These Ca²⁺ signals are readily visualized using plants expressing green fluorescent protein-based Ca²⁺-sensitive bioreporters, such as those of the GCaMP and GECO families. Dissecting the underlying molecular machinery behind this systemic spread of information is often approached by imaging the Ca²⁺ response in mutants in candidate genes. Introducing the GFP sensor into the relevant genetic backgrounds and then selecting lines usable for imaging can be very time consuming. An alternative, more rapid approach to screening these candidates is through virus-induced gene silencing (VIGS), where target genes are suppressed in the wild-type bioreporter expressing plants. This chapter describes how to generate VIGS constructs targeted to candidate genes and then how to visualize wound-induced, systemic Ca²⁺ signaling in the VIGS suppressed plants.

ROS-mediated plasmodesmal regulation requires a network of an Arabidopsis receptor-like kinase, calmodulin-like proteins, and callose synthases

Plasmodesmata (PD) play a critical role in symplasmic communication, coordinating plant activities related to growth & development, and environmental stress responses. Most developmental and environmental stress signals induce reactive oxygen species (ROS)-mediated signaling in the apoplast that causes PD closure by callose deposition. Although the apoplastic ROS signals are primarily perceived at the plasma membrane (PM) by receptor-like kinases (RLKs), such components involved in PD regulation are not yet known. Here, we show that an Arabidopsis NOVEL CYS-RICH RECEPTOR KINASE (NCRK), a PD-localized protein, is required for plasmodesmal callose deposition in response to ROS stress. We identified the involvement of NCRK in callose accumulation at PD channels in either basal level or ROS-dependent manner. Loss-of-function mutant (ncrk) of NCRK induces impaired callose accumulation at the PD under the ROS stress resembling a phenotype of the PD-regulating GLUCAN SYNTHASE-LIKE 4 (gsl4) knock-out plant. The overexpression of transgenic NCRK can complement the callose and the PD permeability phenotypes of ncrk mutants but not kinase-inactive NCRK variants or Cys-mutant NCRK, in which Cys residues were mutated in Cys-rich repeat ectodomain. Interestingly, NCRK mediates plasmodesmal permeability in mechanical injury-mediated signaling pathways regulated by GSL4. Furthermore, we show that NCRK interacts with calmodulin-like protein 41 (CML41) and GSL4 in response to ROS stress. Altogether, our data indicate that NCRK functions as an upstream regulator of PD callose accumulation in response to ROS-mediated stress signaling pathways.

A high-throughput yeast approach to characterize aquaporin permeabilities: Profiling the Arabidopsis PIP aquaporin sub-family

INTRODUCTION

Engineering membrane transporters to achieve desired functionality is reliant on availability of experimental data informing structure-function relationships and intelligent design. Plant aquaporin (AQP) isoforms are capable of transporting diverse substrates such as signaling molecules, nutrients, metalloids, and gases, as well as water. AQPs can act as multifunctional channels and their transport function is reliant on many factors, with few studies having assessed transport function of specific isoforms for multiple substrates.

METHODS

High-throughput yeast assays were developed to screen for transport function of plant AQPs, providing a platform for fast data generation and cataloguing of substrate transport profiles. We applied our high-throughput growth-based yeast assays to screen all 13 Arabidopsis PIPs (AtPIPs) for transport of water and several neutral solutes: hydrogen peroxide (H2O2), boric acid (BA), and urea. Sodium (Na+) transport was assessed using elemental analysis techniques.

RESULTS

All AtPIPs facilitated water and H2O2 transport, although their growth phenotypes varied, and none were candidates for urea transport. For BA and Na+ transport, AtPIP2;2 and AtPIP2;7 were the top candidates, with yeast expressing these isoforms having the most pronounced toxicity response to BA exposure and accumulating the highest amounts of Na+. Linking putative AtPIP isoform substrate transport profiles with phylogenetics and gene expression data, enabled us to align possible substrate preferences with known and hypothesized biological roles of AtPIPs.

DISCUSSION

This testing framework enables efficient cataloguing of putative transport functionality of diverse AQPs at a scale that can help accelerate our understanding of AQP biology through big data approaches (e.g. association studies). The principles of the individual assays could be further adapted to test additional substrates. Data generated from this framework could inform future testing of AQP physiological roles, and address knowledge gaps in structure-function relationships to improve engineering efforts.

Phosphate-deprivation and damage signalling by extracellular ATP

Phosphate deprivation compromises plant productivity and modulates immunity. DAMP signalling by extracellular ATP (eATP) could be compromised under phosphate deprivation by the lowered production of cytosolic ATP and the need to salvage eATP as a nutritional phosphate source. Phosphate-starved roots of Arabidopsis can still sense eATP, indicating robustness in receptor function. However, the resultant cytosolic free Ca²⁺ signature is impaired, indicating modulation of downstream components. This perspective on DAMP signalling by extracellular ATP (eATP) addresses the salvage of eATP under phosphate deprivation and its promotion of immunity, how Ca²⁺ signals are generated and how the Ca²⁺ signalling pathway could be overcome to allow beneficial fungal root colonization to fulfill phosphate demands. Safe passage for an endophytic fungus allowing root colonization could be achieved by its down-regulation of the Ca²⁺ channels that act downstream of the eATP receptors and by also preventing ROS accumulation, thus further impairing DAMP signalling.

To Be or Not to Be? Are Reactive Oxygen Species, Antioxidants, and Stress Signalling Universal Determinants of Life or Death?

In the environmental and organism context, oxidative stress is complex and unavoidable. Organisms simultaneously cope with a various combination of stress factors in natural conditions. For example, excess light stress is accompanied by UV stress, heat shock stress, and/or water stress. Reactive oxygen species (ROS) and antioxidant molecules, coordinated by electrical signalling (ES), are an integral part of the stress signalling network in cells and organisms. They together regulate gene expression to redirect energy to growth, acclimation, or defence, and thereby, determine cellular stress memory and stress crosstalk. In plants, both abiotic and biotic stress increase energy quenching, photorespiration, stomatal closure, and leaf temperature, while toning down photosynthesis and transpiration. Locally applied stress induces ES, ROS, retrograde signalling, cell death, and cellular light memory, then acclimation and defence responses in the local organs, whole plant, or even plant community (systemic acquired acclimation, systemic acquired resistance, network acquired acclimation). A simplified analogy can be found in animals where diseases vs. fitness and prolonged lifespan vs. faster aging, are dependent on mitochondrial ROS production and ES, and body temperature is regulated by sweating, temperature-dependent respiration, and gene regulation. In this review, we discuss the universal features of stress factors, ES, the cellular production of ROS molecules, ROS scavengers, hormones, and other regulators that coordinate life and death.

ROS and redox regulation of cell-to-cell and systemic signaling in plants during stress

Stress results in the enhanced accumulation of reactive oxygen species (ROS) in plants, altering the redox state of cells and triggering the activation of multiple defense and acclimation mechanisms. In addition to activating ROS and redox responses in tissues that are directly subjected to stress (termed 'local' tissues), the sensing of stress in plants triggers different systemic signals that travel to other parts of the plant (termed 'systemic' tissues) and activate acclimation and defense mechanisms in them; even before they are subjected to stress. Among the different systemic signals triggered by stress in plants are electric, calcium, ROS, and redox waves that are mobilized in a cell-to-cell fashion from local to systemic tissues over long distances, sometimes at speeds of up to several millimeters per second. Here, we discuss new studies that identified various molecular mechanisms and proteins involved in mediating systemic signals in plants. In addition, we highlight recent studies that are beginning to unravel the mode of integration and hierarchy of the different systemic signals and underline open questions that require further attention. Unraveling the role of ROS and redox in plant stress responses is highly important for the development of climate resilient crops.

HPCA1 is required for systemic reactive oxygen species and calcium cell-to-cell signaling and plant acclimation to stress

Reactive oxygen species (ROS), produced by respiratory burst oxidase homologs (RBOHs) at the apoplast, play a key role in local and systemic cell-to-cell signaling, required for plant acclimation to stress. Here we reveal that the Arabidopsis thaliana leucine-rich-repeat receptor-like kinase H2O2-INDUCED CA2+ INCREASES 1 (HPCA1) acts as a central ROS receptor required for the propagation of cell-to-cell ROS signals, systemic signaling in response to different biotic and abiotic stresses, stress responses at the local and systemic tissues, and plant acclimation to stress, following a local treatment of high light (HL) stress. We further report that HPCA1 is required for systemic calcium signals, but not systemic membrane depolarization responses, and identify the calcium-permeable channel MECHANOSENSITIVE ION CHANNEL LIKE 3, CALCINEURIN B-LIKE CALCIUM SENSOR 4 (CBL4), CBL4-INTERACTING PROTEIN KINASE 26 and Sucrose-non-fermenting-1-related Protein Kinase 2.6/OPEN STOMATA 1 (OST1) as required for the propagation of cell-to-cell ROS signals. In addition, we identify serine residues S343 and S347 of RBOHD (the putative targets of OST1) as playing a key role in cell-to-cell ROS signaling in response to a local application of HL stress. Our findings reveal that HPCA1 plays a key role in mediating and coordinating systemic cell-to-cell ROS and calcium signals required for plant acclimation to stress.

Integration of long-range signals in plants: A model for wound-induced Ca2+, electrical, ROS, and glutamate waves

Plants show long-range cytosolic Ca²⁺ signal transduction in response to wounding. Recent advances in in vivo imaging techniques have helped visualize spatiotemporal dynamics of the systemic Ca²⁺ signals and provided new insights into underlying molecular mechanisms, in which ion channels of the GLUTAMATE RECEPTOR-LIKE (GLR) family are critical for the sensory system. These, along with MECHANOSENSITIVE CHANNEL OF SMALL CONDUCTANCE-LIKE 10 (MSL10) and Arabidopsis H⁺-ATPase (AHA1) regulate the propagation system. In addition, membrane potential, reactive oxygen species (ROS), and glutamate waves operate in parallel to long-range signal transduction. We summarize these findings and introduce a model that integrates long-range Ca²⁺, electrical, ROS, and glutamate signals in systemic wound responses.

ROS production and signalling in chloroplasts: cornerstones and evolving concepts

Reactive oxygen species (ROS) such as singlet oxygen, superoxide (O₂^●- ) and hydrogen peroxide (H₂ O₂ ) are the markers of living cells. Oxygenic photosynthesis produces ROS in abundance, which act as a readout of a functional electron transport system and metabolism. The concept that photosynthetic ROS production is a major driving force in chloroplast to nucleus retrograde signalling is embedded in the literature, as is the role of chloroplasts as environmental sensors. The different complexes and components of the photosynthetic electron transport chain (PETC) regulate O₂^●- production in relation to light energy availability and the redox state of the stromal Cys-based redox systems. All of the ROS generated in chloroplasts have the potential to act as signals and there are many sulphhydryl-containing proteins and peptides in chloroplasts that have the potential to act as H₂ O₂ sensors and function in signal transduction. While ROS may directly move out of the chloroplasts to other cellular compartments, ROS signalling pathways can only be triggered if appropriate ROS-sensing proteins are present at or near the site of ROS production. Chloroplast antioxidant systems serve either to propagate these signals or to remove excess ROS that cannot effectively be harnessed in signalling. The key challenge is to understand how regulated ROS delivery from the PETC to the Cys-based redox machinery is organised to transmit redox signals from the environment to the nucleus. Redox changes associated with stromal carbohydrate metabolism also play a key role in chloroplast signalling pathways.

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.

The receptor kinase SRF3 coordinates iron-level and flagellin dependent defense and growth responses in plants

Iron is critical for host–pathogen interactions. While pathogens seek to scavenge iron to spread, the host aims at decreasing iron availability to reduce pathogen virulence. Thus, iron sensing and homeostasis are of particular importance to prevent host infection and part of nutritional immunity. While the link between iron homeostasis and immunity pathways is well established in plants, how iron levels are sensed and integrated with immune response pathways remains unknown. Here we report a receptor kinase SRF3, with a role in coordinating root growth, iron homeostasis and immunity pathways via regulation of callose synthases. These processes are modulated by iron levels and rely on SRF3 extracellular and kinase domains which tune its accumulation and partitioning at the cell surface. Mimicking bacterial elicitation with the flagellin peptide flg22 phenocopies SRF3 regulation upon low iron levels and subsequent SRF3-dependent responses. We propose that SRF3 is part of nutritional immunity responses involved in sensing external iron levels.

Iron homeostasis is known to influence plant immune signaling. Here the authors characterize SRF3, a receptor kinase that acts as a negative regulator of callose synthesis, that is required for root responses to iron deficiency and pathogen signals.

Aboveground plant-to-plant electrical signaling mediates network acquired acclimation

Systemic acquired acclimation and wound signaling require the transmission of electrical, calcium, and reactive oxygen species (ROS) signals between local and systemic tissues of the same plant. However, whether such signals can be transmitted between two different plants is largely unknown. Here, we reveal a new type of plant-to-plant aboveground direct communication involving electrical signaling detected at the surface of leaves, ROS, and photosystem networks. A foliar electrical signal induced by wounding or high light stress applied to a single dandelion leaf can be transmitted to a neighboring plant that is in direct contact with the stimulated plant, resulting in systemic photosynthetic, oxidative, molecular, and physiological changes in both plants. Furthermore, similar aboveground changes can be induced in a network of plants serially connected via touch. Such signals can also induce responses even if the neighboring plant is from a different plant species. Our study demonstrates that electrical signals can function as a communication link between transmitter and receiver plants that are organized as a network (community) of plants. This process can be described as network-acquired acclimation.

Reactive oxygen species signalling in plant stress responses

Reactive oxygen species (ROS) are key signalling molecules that enable cells to rapidly respond to different stimuli. In plants, ROS play a crucial role in abiotic and biotic stress sensing, integration of different environmental signals and activation of stress-response networks, thus contributing to the establishment of defence mechanisms and plant resilience. Recent advances in the study of ROS signalling in plants include the identification of ROS receptors and key regulatory hubs that connect ROS signalling with other important stress-response signal transduction pathways and hormones, as well as new roles for ROS in organelle-to-organelle and cell-to-cell signalling. Our understanding of how ROS are regulated in cells by balancing production, scavenging and transport has also increased. In this Review, we discuss these promising developments and how they might be used to increase plant resilience to environmental stress.

Empagliflozin attenuates cardiac microvascular ischemia/reperfusion through activating the AMPKα1/ULK1/FUNDC1/mitophagy pathway

Mitophagy preserves microvascular structure and function during myocardial ischemia/reperfusion (I/R) injury. Empagliflozin, an anti-diabetes drug, may also protect mitochondria. We explored whether empagliflozin could reduce cardiac microvascular I/R injury by enhancing mitophagy. In mice, I/R injury induced luminal stenosis, microvessel wall damage, erythrocyte accumulation and perfusion defects in the myocardial microcirculation. Additionally, I/R triggered endothelial hyperpermeability and myocardial neutrophil infiltration, which upregulated adhesive factors and endothelin-1 but downregulated vascular endothelial cadherin and endothelial nitric oxide synthase in heart tissue. In vitro, I/R impaired the endothelial barrier function and integrity of cardiac microvascular endothelial cells (CMECs), while empagliflozin preserved CMEC homeostasis and thus maintained cardiac microvascular structure and function. I/R activated mitochondrial fission, oxidative stress and apoptotic signaling in CMECs, whereas empagliflozin normalized mitochondrial fission and fusion, neutralized supraphysiologic reactive oxygen species concentrations and suppressed mitochondrial apoptosis. Empagliflozin exerted these protective effects by activating FUNDC1-dependent mitophagy through the AMPKα1/ULK1 pathway. Both in vitro and in vivo, genetic ablation of AMPKα1 or FUNDC1 abolished the beneficial effects of empagliflozin on the myocardial microvasculature and CMECs. Taken together, the preservation of mitochondrial function through an activation of the AMPKα1/ULK1/FUNDC1/mitophagy pathway is the working mechanism of empagliflozin in attenuating cardiac microvascular I/R injury.

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Empagliflozin reduces I/R-induced microvascular damage.

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Empagliflozin suppresses I/R-induced endothelial cell damage.

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Empagliflozin activates FUNDC1-dependent mitophagy through the AMPKα1/ULK1 pathway.

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Ablation of FUNDC1 or AMPKα1 abolishes the protective effects of empagliflozin against I/R-induced microvascular damage.

Systemic Signaling: A Role in Propelling Crop Yield

Food security has become a topic of great concern in many countries. Global food security depends heavily on agriculture that has access to proper resources and best practices to generate higher crop yields. Crops, as with other plants, have a variety of strategies to adapt their growth to external environments and internal needs. In plants, the distal organs are interconnected through the vascular system and intricate hierarchical signaling networks, to communicate and enhance survival within fluctuating environments. Photosynthesis and carbon allocation are fundamental to crop production and agricultural outputs. Despite tremendous progress achieved by analyzing local responses to environmental cues, and bioengineering of critical enzymatic processes, little is known about the regulatory mechanisms underlying carbon assimilation, allocation, and utilization. This review provides insights into vascular-based systemic regulation of photosynthesis and resource allocation, thereby opening the way for the engineering of source and sink activities to optimize the yield performance of major crops.

A tale of many families: calcium channels in plant immunity

Plants launch a concerted immune response to dampen potential infections upon sensing microbial pathogen and insect invasions. The transient and rapid elevation of the cytosolic calcium concentration [Ca2+]cyt is among the essential early cellular responses in plant immunity. The free Ca2+ concentration in the apoplast is far higher than that in the resting cytoplasm. Thus, the precise regulation of calcium channel activities upon infection is the key for an immediate and dynamic Ca2+ influx to trigger downstream signaling. Specific Ca2+ signatures in different branches of the plant immune system vary in timing, amplitude, duration, kinetics, and sources of Ca2+. Recent breakthroughs in the studies of diverse groups of classical calcium channels highlight the instrumental role of Ca2+ homeostasis in plant immunity and cell survival. Additionally, the identification of some immune receptors as noncanonical Ca2+-permeable channels opens a new view of how immune receptors initiate cell death and signaling. This review aims to provide an overview of different Ca2+-conducting channels in plant immunity and highlight their molecular and genetic mode-of-actions in facilitating immune signaling. We also discuss the regulatory mechanisms that control the stability and activity of these channels.

Auxin analog-induced Ca2+ signaling is independent of inhibition of endosomal aggregation in Arabidopsis roots

Much of what we know about the role of auxin in plant development derives from exogenous manipulations of auxin distribution and signaling, using inhibitors, auxins, and auxin analogs. In this context, synthetic auxin analogs, such as 1-naphthalene acetic acid (1-NAA), are often favored over the endogenous auxin, indole-3-acetic acid (IAA), in part due to their higher stability. While such auxin analogs have proven instrumental in revealing the various faces of auxin, they display in some cases bioactivities distinct from IAA. Here, we focused on the effect of auxin analogs on the accumulation of PIN proteins in brefeldin A-sensitive endosomal aggregations (BFA bodies), and correlation with the ability to elicit Ca2+ responses. For a set of commonly used auxin analogs, we evaluated if auxin analog-induced Ca2+ signaling inhibits PIN accumulation. Not all auxin analogs elicited a Ca2+ response, and their differential ability to elicit Ca2+ responses correlated partially with their ability to inhibit BFA-body formation. However, in tir1/afb and cngc14, 1-NAA-induced Ca2+ signaling was strongly impaired, yet 1-NAA still could inhibit PIN accumulation in BFA bodies. This demonstrates that TIR1/AFB-CNGC14-dependent Ca2+ signaling does not inhibit BFA body formation in Arabidopsis roots.

Plant cell mechanobiology: Greater than the sum of its parts

The ability to sense and respond to physical forces is critical for the proper function of cells, tissues, and organisms across the evolutionary tree. Plants sense gravity, osmotic conditions, pathogen invasion, wind, and the presence of barriers in the soil, and dynamically integrate internal and external stimuli during every stage of growth and development. While the field of plant mechanobiology is growing, much is still poorly understood-including the interplay between mechanical and biochemical information at the single-cell level. In this review, we provide an overview of the mechanical properties of three main components of the plant cell and the mechanoperceptive pathways that link them, with an emphasis on areas of complexity and interaction. We discuss the concept of mechanical homeostasis, or "mechanostasis," and examine the ways in which cellular structures and pathways serve to maintain it. We argue that viewing mechanics and mechanotransduction as emergent properties of the plant cell can be a useful conceptual framework for synthesizing current knowledge and driving future research.

Plasmodesmata Structural Components and Their Role in Signaling and Plant Development

Plasmodesmata are plant intercellular channels that mediate the transport of small and large molecules including RNAs and transcription factors (TFs) that regulate plant development. In this review, we present current research on plasmodesmata form and function and discuss the main regulatory pathways. We show the progress made in the development of approaches and tools to dissect the plasmodesmata proteome in diverse plant species and discuss future perspectives and challenges in this field of research.

Improving oxidative stress resilience in plants

Originally conceived as harmful metabolic byproducts, reactive oxygen species (ROS) are now recognized as an integral part of numerous cellular programs. Thanks to their diverse physicochemical properties, compartmentalized production, and tight control exerted by the antioxidant machinery they activate signaling pathways that govern plant growth, development, and defense. Excessive ROS levels are often driven by adverse changes in environmental conditions, ultimately causing oxidative stress. The associated negative impact on cellular constituents have been a major focus of decade-long research efforts to improve the oxidative stress resilience by boosting the antioxidant machinery in model and crop species. We highlight the role of enzymatic and non-enzymatic antioxidants as integral factors of multiple signaling cascades beyond their mere function to prevent oxidative damage under adverse abiotic stress conditions.

The emerging role of GABA as a transport regulator and physiological signal

While the proposal that γ-aminobutyric acid (GABA) acts a signal in plants is decades old, a signaling mode of action for plant GABA has been unveiled only relatively recently. Here, we review the recent research that demonstrates how GABA regulates anion transport through aluminum-activated malate transporters (ALMTs) and speculation that GABA also targets other proteins. The ALMT family of anion channels modulates multiple physiological processes in plants, with many members still to be characterized, opening up the possibility that GABA has broad regulatory roles in plants. We focus on the role of GABA in regulating pollen tube growth and stomatal pore aperture, and we speculate on its role in long-distance signaling and how it might be involved in cross talk with hormonal signals. We show that in barley (Hordeum vulgare), guard cell opening is regulated by GABA, as it is in Arabidopsis (Arabidopsis thaliana), to regulate water use efficiency, which impacts drought tolerance. We also discuss the links between glutamate and GABA in generating signals in plants, particularly related to pollen tube growth, wounding, and long-distance electrical signaling, and explore potential interactions of GABA signals with hormones, such as abscisic acid, jasmonic acid, and ethylene. We conclude by postulating that GABA encodes a signal that links plant primary metabolism to physiological status to fine tune plant responses to the environment.

Circadian coordination of cellular processes and abiotic stress responses

Diel changes in the environment are perceived by the circadian clock which transmits temporal information throughout the plant cell to synchronize daily and seasonal environmental signals with internal biological processes. Dynamic modulations of diverse levels of clock gene regulation within the plant cell are impacted by stress. Recent insights into circadian control of cellular processes such as alternative splicing, polyadenylation, and noncoding RNAs are discussed. We highlight studies on the circadian regulation of reactive oxygen species, calcium signaling, and gating of temperature stress responses. Finally, we briefly summarize recent work on the translation-specific rhythmicity of cell cycle genes and the control of subcellular localization and relocalization of oscillator components. Together, this mini-review highlights these cellular events in the context of clock gene regulation and stress responses in Arabidopsis.