Cation channels are involved in brassinosteroid signalling in higher plants

Exogenous brassinosteroids alleviate calcium deficiency-induced tip-burn by maintaining cell wall structural stability and higher photosynthesis in mini Chinese Cabbage

Tip-burn has seriously affected the yield, quality and commodity value of mini Chinese cabbage. Calcium (Ca²⁺) deficiency is the main cause of tip-burn. In order to investigate whether exogenous brassinosteroids (BRs) can alleviate tip-burn induced by calcium (Ca²⁺) deficiency and its mechanism, in this study, Ca²⁺ deficiency in nutrient solution was used to induced tip-burn, and then distilled water and BRs were sprayed on leaves to observe the tip-burn incidence of mini Chinese cabbage. The tip-burn incidence and disease index, leaf area, fluorescence parameters (Fv/Fm, NPQ, qP andφPSII) and gas exchange parameters (Tr, Pn, Gs and Ci), pigment contents, cell wall components, mesophyll cell ultrastructure and the expression of genes related to chlorophyll degradation were measured. The results showed that exogenous BRs reduced the tip-burn incidence rate and disease index of mini Chinese cabbage, and the tip-burn incidence rate reached the highest on the ninth day after treatment. Exogenous BRs increased the contents of cellulose, hemifiber, water-soluble pectin in Ca²⁺ deficiency treated leaves, maintaining the stability of cell wall structure. In addition, BRs increased photosynthetic rate by increasing the activities of Ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) and fructose 1,6-bisphosphatase (FBPase) related to Calvin cycle, maintaining relatively complete chloroplast structure and higher chlorophyll content via down-regulating the expression of BrPPH1 and BrPAO1 genes related to chlorophyll degradation. In conclusion, exogenous BRs alleviated calcium deficiency-induced tip-burn by maintaining cell wall structural stability and higher photosynthesis.

Modes of Brassinosteroid Activity in Cold Stress Tolerance

Cold stress is a significant environmental factor that negatively affects plant growth and development in particular when it occurs during the growth phase. Plants have evolved means to protect themselves from damage caused by chilling or freezing temperatures and some plant species, in particular those from temperate geographical zones, can increase their basal level of freezing tolerance in a process termed cold acclimation. Cold acclimation improves plant survival, but also represses growth, since it inhibits activity of the growth-promoting hormones gibberellins (GAs). In addition to GAs, the steroid hormones brassinosteroids (BRs) also take part in growth promotion and cold stress signaling; however, in contrast to Gas, BRs can improve cold stress tolerance with fewer trade-offs in terms of growth and yields. Here we summarize our current understanding of the roles of BRs in cold stress responses with a focus on freezing tolerance and cold acclimation pathways.

Molecular Mechanisms of Brassinosteroid-Mediated Responses to Changing Environments in Arabidopsis

Plant adaptations to changing environments rely on integrating external stimuli into internal responses. Brassinosteroids (BRs), a group of growth-promoting phytohormones, have been reported to act as signal molecules mediating these processes. BRs are perceived by cell surface receptor complex including receptor BRI1 and coreceptor BAK1, which subsequently triggers a signaling cascade that leads to inhibition of BIN2 and activation of BES1/BZR1 transcription factors. BES1/BZR1 can directly regulate the expression of thousands of downstream responsive genes. Recent studies in the model plant Arabidopsis demonstrated that BR biosynthesis and signal transduction, especially the regulatory components BIN2 and BES1/BZR1, are finely tuned by various environmental cues. Here, we summarize these research updates and give a comprehensive review of how BR biosynthesis and signaling are modulated by changing environments and how these changes regulate plant adaptive growth or stress tolerance.

Sodium Influx and Potassium Efflux Currents in Sunflower Root Cells Under High Salinity

Helianthus annuus L. is an important oilseed crop, which exhibits moderate salt tolerance and can be cultivated in areas affected by salinity. Using patch-clamp electrophysiology, we have characterized Na⁺ influx and K⁺ efflux conductances in protoplasts of salt-tolerant H. annuus L. hybrid KBSH-53 under high salinity. This work demonstrates that the plasma membrane of sunflower root cells has a classic set of ionic conductances dominated by K⁺ outwardly rectifying channels (KORs) and non-selective cation channels (NSCCs). KORs in sunflower show extreme Na⁺ sensitivity at high extracellular [Ca²⁺] that can potentially have a positive adaptive effect under salt stress (decreasing K⁺ loss). Na⁺ influx currents in sunflower roots demonstrate voltage-independent activation, lack time-dependent component, and are sensitive to Gd³⁺. Sunflower Na⁺-permeable NSCCs mediate a much weaker Na⁺ influx currents on the background of physiological levels of Ca²⁺ as compared to other species. This suggests that sunflower NSCCs have greater Ca²⁺ sensitivity. The responses of Na⁺ influx to Ca²⁺ correlates well with protection of sunflower growth by external Ca²⁺ in seedlings treated with NaCl. It can be, thus, hypothesized that NaCl tolerance in sunflower seedling roots is programmed at the ion channel level via their sensitivity to Ca²⁺ and Na⁺.

Calcium transport across plant membranes: mechanisms and functions

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

Novel roles of ascorbate in plants: induction of cytosolic Ca2+ signals and efflux from cells via anion channels

Ascorbate is not often considered as a signalling molecule in plants. This study demonstrates that, in Arabidopsis roots, exogenous l-ascorbic acid triggers a transient increase of the cytosolic free calcium activity ([Ca2+]cyt.) that is central to plant signalling. Exogenous copper and iron stimulate the ascorbate-induced [Ca2+]cyt. elevation, while cation channel blockers, free radical scavengers, low extracellular [Ca2+], transition metal chelators, and removal of the cell wall inhibit this reaction. These data show that apoplastic redox-active transition metals are involved in the ascorbate-induced [Ca2+]cyt. elevation. Exogenous ascorbate also induces a moderate increase in programmed cell death symptoms in intact roots, but it does not activate Ca2+ influx currents in patch-clamped root protoplasts. Intriguingly, the replacement of gluconate with ascorbate in the patch-clamp pipette reveals a large ascorbate efflux current, which shows sensitivity to the anion channel blocker, anthracene-9-carboxylic acid (A9C), indicative of the ascorbate release via anion channels. EPR spectroscopy measurements demonstrate that salinity (NaCl) triggers the accumulation of root apoplastic ascorbyl radicals in an A9C-dependent manner, confirming that l-ascorbate leaks through anion channels under depolarization. This mechanism may underlie ascorbate release, signalling phenomena, apoplastic redox reactions, iron acquisition, and control the ionic and electrical equilibrium (together with K+ efflux via GORK channels).

Mechanisms of cytosolic calcium elevation in plants: the role of ion channels, calcium extrusion systems and NADPH oxidase-mediated 'ROS-Ca2+ Hub'

Elevation in the cytosolic free calcium is crucial for plant growth, development and adaptation. Calcium influx into plant cells is mediated by Ca2+ depolarisation-activated, hyperpolarisation-activated and voltage-independent Ca2+-permeable channels (DACCs, HACCs and VICCs respectively). These channels are encoded by the following gene families: (1) cyclic nucleotide-gated channels (CNGCs), (2) ionotropic glutamate receptors (GLRs), (3) annexins, (4) 'mechanosensitive channels of small (MscS) conductance'-like channels (MSLs), (5) 'mid1-complementing activity' channels (MCAs), Piezo channels, and hyperosmolality-induced [Ca2+]cyt. channel 1 (OSCA1). Also, a 'tandem-pore channel1' (TPC1) catalyses Ca2+ efflux from the vacuole in response to the plasma membrane-mediated Ca2+ elevation. Recent experimental data demonstrated that Arabidopsis thaliana (L.) Heynh. CNGCs 2, 5-10, 14, 16 and 18, GLRs 1.2, 3.3, 3.4, 3.6 and 3.7, TPC1, ANNEXIN1, MSL9 and MSL10,MCA1 and MCA2, OSCA1, and some their homologues counterparts in other species, are responsible for Ca2+ currents and/or cytosolic Ca2+ elevation. Extrusion of Ca2+ from the cytosol is mediated by Ca2+-ATPases and Ca2+/H+ exchangers which were recently examined at the level of high resolution crystal structure. Calcium-activated NADPH oxidases and reactive oxygen species (ROS)-activated Ca2+ conductances form a self-amplifying 'ROS-Ca2+hub', enhancing and transducing Ca2+ and redox signals. The ROS-Ca2+ hub contributes to physiological reactions controlled by ROS and Ca2+, demonstrating synergism and unity of Ca2+ and ROS signalling mechanisms.

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.

The CLAVATA signaling pathway mediating stem cell fate in shoot meristems requires Ca(2+) as a secondary cytosolic messenger

CLAVATA1 (CLV1) is a receptor protein expressed in the shoot apical meristem (SAM) that translates perception of a non-cell-autonomous CLAVATA3 (CLV3) peptide signal into altered stem cell fate. CLV3 reduces expression of WUSCHEL (WUS) and FANTASTIC FOUR 2 (FAF2) in the SAM. Expression of WUS and FAF2 leads to maintenance of undifferentiated stem cells in the SAM. CLV3 binding to CLV1 inhibits expression of these genes and controls stem cell fate in the SAM through an unidentified signaling pathway. Cytosolic Ca(2+) elevations, cyclic nucleotide (cGMP)-activated Ca(2+) channels, and cGMP have been linked to signaling downstream of receptors similar to CLV1. Hence, we hypothesized that cytosolic Ca(2+) elevation mediates the CLV3 ligand/CLV1 receptor signaling that controls meristem stem cell fate. CLV3 application to Arabidopsis seedlings results in elevation of cytosolic Ca(2+) and cGMP. CLV3 control of WUS was prevented in a genotype lacking a functional cGMP-activated Ca(2+) channel. In wild-type plants, CLV3 inhibition of WUS and FAF2 expression was impaired by treatment with either a Ca(2+) channel blocker or a guanylyl cyclase inhibitor. When CLV3-dependent repression of WUS is blocked, altered control of stem cell fate leads to an increase in SAM size; we observed a larger SAM size in seedlings treated with the Ca(2+) channel blocker. These results suggest that the CLV3 ligand/CLV1 receptor system initiates a signaling cascade that elevates cytosolic Ca(2+), and that this cytosolic secondary messenger is involved in the signal transduction cascade linking CLV3/CLV1 to control of gene expression and stem cell fate in the SAM.

NPK macronutrients and microRNA homeostasis

Macronutrients are essential elements for plant growth and development. In natural, non-cultivated systems, the availability of macronutrients is not a limiting factor of growth, due to fast recycling mechanisms. However, their availability might be an issue in modern agricultural practices, since soil has been frequently over exploited. From a crop management perspective, the nitrogen (N), phosphorus (P), and potassium (K) are three important limiting factors and therefore frequently added as fertilizers. NPK are among the nutrients that have been reported to alter post-embryonic root developmental processes and consequently, impairs crop yield. To cope with nutrients scarcity, plants have evolved several mechanisms involved in metabolic, physiological, and developmental adaptations. In this scenario, microRNAs (miRNAs) have emerged as additional key regulators of nutrients uptake and assimilation. Some studies have demonstrated the intrinsic relation between miRNAs and their targets, and how they can modulate plants to deal with the NPK availability. In this review, we focus on miRNAs and their regulation of targets involved in NPK metabolism. In general, NPK starvation is related with miRNAs that are involved in root-architectural changes and uptake activity modulation. We further show that several miRNAs were discovered to be involved in plant-microbe symbiosis during N and P uptake, and in this way we present a global view of some studies that were conducted in the last years. The integration of current knowledge about miRNA-NPK signaling may help future studies to focus in good candidates genes for the development of important tools for plant nutritional breeding.