Genes for calcium-permeable channels in the plasma membrane of plant root cells

Genome-wide association study reveals the genetic architecture for calcium accumulation in grains of hexaploid wheat (Triticum aestivum L.)

BACKGROUND

Hexaploid wheat (Triticum aestivum L.) is a leading cereal crop worldwide. Understanding the mechanism of calcium (Ca) accumulation in wheat is important to reduce the risk of human micronutrient deficiencies. However, the mechanisms of Ca accumulation in wheat grain are only partly understood.

RESULTS

Here, a genome-wide association study (GWAS) was performed to dissect the genetic basis of Ca accumulation in wheat grain using an association population consisting of 207 varieties, with phenotypic data from three locations. In total, 11 non-redundant genetic loci associated with Ca concentration were identified and they explained, on average, 9.61-26.93% of the phenotypic variation. Cultivars containing more superior alleles had increased grain Ca concentrations. Notably, four non-redundant loci were mutually verified by different statistical models in at least two environments, indicating their stability across different environments. Four putative candidate genes linked to Ca accumulation were revealed from the stable genetic loci. Among them, two genes, associated with the stable genetic loci on chromosomes 4A (AX-108912427) and 3B (AX-110922471), encode the subunits of V-type Proton ATPase (TraesCS4A02G428900 and TraesCS3B02G241000), which annotated as the typical generators of a proton gradient that might be involved in Ca homeostasis in wheat grain.

CONCLUSION

To identify genetic loci associated with Ca accumulation, we conducted GWAS on Ca concentrations and detected 11 genetic loci; whereas four genetic loci were stable across different environments. A genetic loci hot spot exists at the end of chromosome 4A and associated with the putative candidate gene TraesCS4A02G428900. The candidate gene TraesCS4A02G428900 encodes V-type proton ATPase subunit e and highly expressed in wheat grains, and it possibly involved in Ca accumulation. This study increases our understanding of the genetic architecture of Ca accumulation in wheat grains, which is potentially helpful for wheat Ca biofortification pyramid breeding.

Effect of strontium on nutrient uptake, physiological parameters, and strontium localization in lettuce

Human activities increase the risk of stable and radioactive strontium (Sr) isotopes entering the environment and food chain. In this study, the effects of Sr on the nutrient uptake and physiological responses of lettuce under different "Sr treatment" concentrations (0, control, 1, 2, 3, 4, and 5 mM) and "times" (7, 14, and 21 day) were studied in a hydroponic system. In addition, the distribution of Sr on the surfaces and cross-sections of lettuce leaves was revealed by scanning electron microscopy with energy-dispersive X-ray (SEM-EDX) analysis. A two-way analysis of variance (ANOVA) method was used to analyze the significance of "Sr treatment," "time," and their "interaction." The results showed that an increase in Sr uptake in lettuce could significantly reduce the uptake of calcium (Ca). The contents of sulfur (S), potassium (K), and iron (Fe) in lettuce leaves showed significant differences with the sampling day. Similarly, the fresh weight of lettuce leaves and roots as well as the photosynthetic pigment contents of lettuce leaves was also significantly different with the sampling day. The activities of antioxidant enzymes (superoxide dismutase (SOD), catalase (CAT), and peroxidase (POD)) showed significant differences with the sampling day. The activities of SOD and CAT decreased significantly with the sampling day, while POD increased significantly. The MDA content increased significantly with increasing hydroponic Sr concentration on the 21st day. SEM-EDX analysis showed that the weight percentage of Sr in the vascular bundle sheath in the cross-section of lettuce leaves was relatively higher than that in the mesophyll. This study aids our understanding of the distribution of Sr in lettuce leaf tissues and the effect of Sr on lettuce physiology.

High-value pleiotropic genes for developing multiple stress-tolerant biofortified crops for 21st-century challenges

The agriculture-based livelihood systems that are already vulnerable due to multiple challenges face immediate risk of increased crop failures due to weather vagaries. As breeders and biotechnologists, our strategy is to advance and innovate breeding for weather-proofing crops. Plant stress tolerance is a genetically complex trait. Additionally, crops rarely face a single type of stress in isolation, and it is difficult for plants to deal with multiple stresses simultaneously. One of the most helpful approaches to creating stress-resilient crops is genome editing and trans- or cis-genesis. Out of hundreds of stress-responsive genes, many have been used to impart tolerance against a particular stress factor, while a few used in combination for gene pyramiding against multiple stresses. However, a better approach would be to use multi-role pleiotropic genes that enable plants to adapt to numerous environmental stresses simultaneously. Herein we attempt to integrate and present the scattered information published in the past three decades about these pleiotropic genes for crop improvement and remodeling future cropping systems. Research articles validating functional roles of genes in transgenic plants were used to create groups of multi-role pleiotropic genes that could be candidate genes for developing weather-proof crop varieties. These biotech crop varieties will help create 'high-value farms' to meet the goal of a sustainable increase in global food productivity and stabilize food prices by ensuring a fluctuation-free assured food supply. It could also help create a gene repository through artificial gene synthesis for 'resilient high-value food production' for the 21st century.

Calmodulin and Its Interactive Proteins Participate in Regulating the Explosive Growth of Alexandrium pacificum (Dinoflagellate)

Alexandrium pacificum is a typical dinoflagellate that can cause harmful algal blooms, resulting in negative impacts on ecology and human health. The calcium (Ca2+) signal transduction pathway plays an important role in cell proliferation. Calmodulin (CaM) and CaM-related proteins are the main cellular Ca2+ sensors, and can act as an intermediate in the Ca2+ signal transduction pathway. In this study, the proteins that interacted with CaM of A. pacificum were screened by two-dimensional electrophoresis analysis and far western blots under different growth conditions including lag phase and high phosphorus and manganese induced log phase (HPM). The interactive proteins were then identified using matrix-assisted laser desorption ionization time-of-flight mass spectrometry. Four proteins were identified, including Ca2+/CaM-dependent protein kinase, serine/threonine kinase, annexin, and inositol-3-phosphate synthase, which all showed high expression levels under HPM. The gene expression levels encoding these four proteins were also up-regulated under HPM, as revealed by quantitative polymerase chain reaction, suggesting that the identified proteins participate in the Ca2+ transport channel and cell cycle regulation to promote cell division. A network of proteins interacting with CaM and their target proteins involved in the regulation of cell proliferation was raised, which provided new insights into the mechanisms behind the explosive growth of A. pacificum.

Phytoremediation of radionuclides in soil, sediments and water

Soil and water contaminated with radionuclides threaten the environment and public health during leaks from nuclear power plants. Remediation of radionuclides at the contaminated sites uses mainly physical and chemical methods such as vitrification, chemical immobilization, electro-kinetic remediation and soil excavation, capping and washing being among the preferred methods. These traditional technologies are however costly and less suitable for dealing with large-area pollution. In contrast to this, cost-effective and environment-friendly alternatives such as phytoremediation using plants to remove radionuclides from polluted sites in situ represent promising alternatives for environmental cleanup. Understanding the physiology and molecular mechanisms of radionuclides accumulation in plants is essential to optimize and improve this new remediation technology. Here, we give an overview of radionuclide contamination in the environment and biochemical characteristics for uptake, transport, and compartmentation of radionuclides in plants that characterize phytoextraction and its efficiency. Phytoextraction is an eco-friendly and efficient method for environmental removal of radionuclides at contaminated sites such as mine tailings. Selecting the most proper plant for the specific purpose, however, is important to obtain the best result together with, for example, applying soil amendments such as citric acid. In addition, using genetic engineering and optimizing agronomic management practices including regulation of atmospheric CO₂ concentration, reasonable measures of fertilization and rational water management are important as well. For future application, the technique needs commercialization in order to fully exploit the technique at mining activities and nuclear industries.

Interrelations of nutrient and water transporters in plants under abiotic stress

Environmental changes cause abiotic stress in plants, primarily through alterations in the uptake of the nutrients and water they require for their metabolism and growth and to maintain their cellular homeostasis. The plasma membranes of cells contain transporter proteins, encoded by their specific genes, responsible for the uptake of nutrients and water (aquaporins). However, their interregulation has rarely been taken into account. Therefore, in this review we identify how the plant genome responds to abiotic stresses such as nutrient deficiency, drought, salinity and low temperature, in relation to both nutrient transporters and aquaporins. Some general responses or regulation mechanisms can be observed under each abiotic stress such as the induction of plasma membrane transporter expression during macronutrient deficiency, the induction of tonoplast transporters and reduction of aquaporins during micronutrients deficiency. However, drought, salinity and low temperatures generally cause an increase in expression of nutrient transporters and aquaporins in tolerant plants. We propose that both types of transporters (nutrients and water) should be considered jointly in order to better understand plant tolerance of stresses.

Drought stress-induced physiological mechanisms, signaling pathways and molecular response of chloroplasts in common vegetable crops

Drought stress is one of the most adverse abiotic stresses that hinder plants' growth and productivity, threatening sustainable crop production. It impairs normal growth, disturbs water relations and reduces water-use efficiency in plants. However, plants have evolved many physiological and biochemical responses at the cellular and organism levels, in order to cope with drought stress. Photosynthesis, which is considered one of the most crucial biological processes for survival of plants, is greatly affected by drought stress. A gradual decrease in CO₂ assimilation rates, reduced leaf size, stem extension and root proliferation under drought stress, disturbs plant water relations, reducing water-use efficiency, disrupts photosynthetic pigments and reduces the gas exchange affecting the plants adversely. In such conditions, the chloroplast, organelle responsible for photosynthesis, is found to counteract the ill effects of drought stress by its critical involvement as a sensor of changes occurring in the environment, as the first process that drought stress affects is photosynthesis. Beside photosynthesis, chloroplasts carry out primary metabolic functions such as the biosynthesis of starch, amino acids, lipids, and tetrapyroles, and play a central role in the assimilation of nitrogen and sulfur. Because the chloroplasts are central organelles where the photosynthetic reactions take place, modifications in their physiology and protein pools are expected in response to the drought stress-induced variations in leaf gas exchanges and the accumulation of ROS. Higher expression levels of various transcription factors and other proteins including heat shock-related protein, LEA proteins seem to be regulating the heat tolerance mechanisms. However, several aspects of plastid alterations, following a water deficit environment are still poorly characterized. Since plants adapt to various stress tolerance mechanisms to respond to drought stress, understanding mechanisms of drought stress tolerance in plants will lead toward the development of drought tolerance in crop plants. This review throws light on major droughts stress-induced molecular/physiological mechanisms in response to severe and prolonged drought stress and addresses the molecular response of chloroplasts in common vegetable crops. It further highlights research gaps, identifying unexplored domains and suggesting recommendations for future investigations.

Zinc Enrichment in two Contrasting Genotypes of Triticum aestivum L. Grains: Interactions between Edaphic Conditions and Foliar Fertilizers

This study aimed to assess the implications of Zn enrichment in wheat grains as a function of contrasting genotypes, edaphic conditions and foliar fertilizers. Triticum aestivum L. varieties Roxo and Paiva were grown in four production fields, and sprayed with ZnSO4 (0, 16.20 and 36.40 kg/ha) Zn-EDTA (0, 6.30 and 12.60 kg/ha) and Tecnifol Zinc (0, 3.90 and 7.80 kg/ha). The heterogeneous edaphic conditions of the wheat fields were chemically characterized, it being found that soil properties determine different Zn accumulation in the grains of both genotypes. Foliar spraying enhanced to different extents Zn content in the grains of both genotypes, but the average of enrichment indexes varied among the wheat fields. Zinc mostly accumulated in the embryo and vascular bundle and to a lesser extent in the endosperm. Grain yield and test weight sprayed by ZnSO4 gave the highest values in both genotypes, but the opposite was found for Zn-EDTA. Considering the color parameters, lightness and red-green transitions were found to be a conjunction of genotype characteristics, fertilization types and edaphic conditions prevailing in each field. It is concluded that the index of Zn enrichment in wheat grains is a docket of edaphic conditions, genotype and type of fertilization.

Standard Thermodynamic Properties, Biosynthesis Rates, and the Driving Force of Growth of Five Agricultural Plants

Elemental composition of Gossypium hirsutum L. (cotton), Oryza sativa L. (Asian rice), Phaseolus vulgaris L. (common bean), Saccharum spp. L. (sugarcane), and Zea mays L. (corn) was used to calculate their empirical formulas (unit carbon formulas) and growth stoichiometry. The empirical formulas were used to find standard enthalpy of formation, standard molar entropy, standard Gibbs energy of formation, and standard molar heat capacity. A comparison was made between thermodynamic properties of live matter of the analyzed plants and other unicellular and multicellular organisms. Moreover, the growth process was analyzed through standard enthalpy, entropy, and Gibbs energy of biosynthesis. The average standard Gibbs energy of biosynthesis was found to be +463.0 kJ/C-mol. Thus, photosynthesis provides energy and carbon for plant growth. The average intercepted photosynthetic energy was found to be 15.5 MJ/C-mol for the analyzed plants. However, due to inefficiency, a great fraction of the intercepted photosynthetic energy cannot be used by plants. The average usable photosynthetic energy was found to be -2.3 MJ/C-mol. The average thermodynamic driving force for growth is -1.9 MJ/C-mol. Driving forces of growth of C3 and C4 plants were compared. It was found that C4 plants have a greater driving force of growth than C3 plants, which reflects the greater efficiency of C4 photosynthesis. The relationship between the driving force and growth rates was analyzed by determining phenomenological L coefficients. The determined phenomenological coefficients span two orders of magnitude, depending on plant species and environmental conditions. The L coefficient of P. vulgaris was found to be lower than that of other plants, due to additional energy requirements of nitrogen fixation.

Protein-driven biomineralization: Comparing silica formation in grass silica cells to other biomineralization processes

Biomineralization is a common strategy adopted by organisms to support their body structure. Plants practice significant silicon and calcium based biomineralization in which silicon is deposited as silica in cell walls and intracellularly in various cell-types, while calcium is deposited mostly as calcium oxalate in vacuoles of specialized cells. In this review, we compare cellular processes leading to protein-dependent mineralization in plants, diatoms and sponges (phylum Porifera). The mechanisms of biomineralization in these organisms are inherently different. The composite silica structure in diatoms forms inside the cytoplasm in a membrane bound vesicle, which after maturation is exocytosed to the cell surface. In sponges, separate vesicles with the mineral precursor (silicic acid), an inorganic template, and organic molecules, fuse together and are extruded to the extracellular space. In plants, calcium oxalate mineral precipitates in vacuolar crystal chambers containing a protein matrix which is never exocytosed. Silica deposition in grass silica cells takes place outside the cell membrane when the cells secrete the mineralizing protein into the apoplasm rich with silicic acid (the mineral precursor molecules). Our review infers that the organism complexity and precursor reactivity (calcium and oxalate versus silicic acid) are main driving forces for the evolution of varied mineralization mechanisms.

Calcium Plays a Double-Edged Role in Modulating Cadmium Uptake and Translocation in Rice

Cadmium (Cd) contamination in soils poses great risks to both agricultural production and human health. Calcium (Ca) is an essential element playing a significant role in protecting plants against Cd toxicity. However, how Ca affects Cd uptake and translocation in rice is still not fully elucidated. In this study, the regulatory role of Ca in Cd uptake and upward translocation was investigated in rice at different growth stages. Our results showed that the supplement of 5 mM Ca significantly reduced Cd uptake by rice roots, because of their competition for Ca-permeable channels as an absorption site and Ca-induced downregulation of OsNRAMP1 and OsNRAMP5. However, Ca application facilitated the upward translocation of Cd by both upregulating OsHMA2 to induce xylem loading of Cd and downregulating OsHMA3 to reduce vacuolar sequestration of Cd. Such contrary results suggested a double-edged role of Ca in regulating root Cd uptake and root-to-shoot Cd translocation in rice. Although it increased Cd content in the aboveground vegetative tissues during the whole growth period, the addition of 5 mM Ca eventually decreased Cd content in rice grains at the ripening stage. All these results suggest that Ca-based amendments possess great potential for the production of low-Cd rice grains.

Manganese in Plants: From Acquisition to Subcellular Allocation

Manganese (Mn) is an important micronutrient for plant growth and development and sustains metabolic roles within different plant cell compartments. The metal is an essential cofactor for the oxygen-evolving complex (OEC) of the photosynthetic machinery, catalyzing the water-splitting reaction in photosystem II (PSII). Despite the importance of Mn for photosynthesis and other processes, the physiological relevance of Mn uptake and compartmentation in plants has been underrated. The subcellular Mn homeostasis to maintain compartmented Mn-dependent metabolic processes like glycosylation, ROS scavenging, and photosynthesis is mediated by a multitude of transport proteins from diverse gene families. However, Mn homeostasis may be disturbed under suboptimal or excessive Mn availability. Mn deficiency is a serious, widespread plant nutritional disorder in dry, well-aerated and calcareous soils, as well as in soils containing high amounts of organic matter, where bio-availability of Mn can decrease far below the level that is required for normal plant growth. By contrast, Mn toxicity occurs on poorly drained and acidic soils in which high amounts of Mn are rendered available. Consequently, plants have evolved mechanisms to tightly regulate Mn uptake, trafficking, and storage. This review provides a comprehensive overview, with a focus on recent advances, on the multiple functions of transporters involved in Mn homeostasis, as well as their regulatory mechanisms in the plant's response to different conditions of Mn availability.

Arabidopsis cyclic nucleotide-gated channel 6 is negatively modulated by multiple calmodulin isoforms during heat shock

An increased concentration of cytosolic Ca2+ is an early response of plant cells to heat shock. Arabidopsis cyclic nucleotide-gated ion channel 6 (CNGC6) mediates heat-induced Ca2+ influx and is activated by cAMP. However, it remains unclear how the Ca2+ conductivity of CNGC6 is negatively regulated under the elevated cytosolic Ca2+ concentration. In this study, Arabidopsis calmodulin isoforms CaM1/4, CaM2/3/5, CaM6, and CaM7 were found to bind to CNGC6 to varying degrees, and this binding was dependent on the presence of Ca2+ and IQ6, an atypical isoleucine-glutamine motif in CNGC6. Knockout of CaM2, CaM3, CaM5, and CaM7 genes led to a marked increase in plasma membrane inward Ca2+ current under heat shock conditions; however, knockout of CaM1, CaM4, and CaM6 genes had no significant effect on plasma membrane Ca2+ current. Moreover, the deletion of IQ6 from CNGC6 led to a marked increase in plasma membrane Ca2+ current under heat shock conditions. Taken together, the data suggest that CNGC6-mediated Ca2+ influx is likely to be negatively regulated by CaM2/3/5 and CaM7 isoforms under heat shock conditions, and that IQ6 plays an important role in CaM binding and the feedback regulation of the channel.

Advanced Microscopy Reveals Complex Developmental and Subcellular Localization Patterns of ANNEXIN 1 in Arabidopsis

Annexin 1 (ANN1) is the most abundant member of the evolutionary conserved multigene protein superfamily of annexins in plants. Generally, annexins participate in diverse cellular processes, such as cell growth, differentiation, vesicle trafficking, and stress responses. The expression of annexins is developmentally regulated, and it is sensitive to the external environment. ANN1 is expressed in almost all Arabidopsis tissues, while the most abundant is in the root, root hairs, and in the hypocotyl epidermal cells. Annexins were also occasionally proposed to associate with cytoskeleton and vesicles, but they were never developmentally localized at the subcellular level in diverse plant tissues and organs. Using advanced light-sheet fluorescence microscopy (LSFM), we followed the developmental and subcellular localization of GFP-tagged ANN1 in post-embryonic Arabidopsis organs. By contrast to conventional microscopy, LSFM allowed long-term imaging of ANN1-GFP in Arabidopsis plants at near-environmental conditions without affecting plant viability. We studied developmental regulation of ANN1-GFP expression and localization in growing Arabidopsis roots: strong accumulation was found in the root cap and epidermal cells (preferentially in elongating trichoblasts), but it was depleted in dividing cells localized in deeper layers of the root meristem. During root hair development, ANN1-GFP accumulated at the tips of emerging and growing root hairs, which was accompanied by decreased abundance in the trichoblasts. In aerial plant parts, ANN1-GFP was localized mainly in the cortical cytoplasm of trichomes and epidermal cells of hypocotyls, cotyledons, true leaves, and their petioles. At the subcellular level, ANN1-GFP was enriched at the plasma membrane (PM) and vesicles of non-dividing cells and in mitotic and cytokinetic microtubular arrays of dividing cells. Additionally, an independent immunolocalization method confirmed ANN1-GFP association with mitotic and cytokinetic microtubules (PPBs and phragmoplasts) in dividing cells of the lateral root cap. Lattice LSFM revealed subcellular accumulation of ANN1-GFP around the nuclear envelope of elongating trichoblasts. Massive relocation and accumulation of ANN1-GFP at the PM and in Hechtian strands and reticulum in plasmolyzed cells suggest a possible osmoprotective role of ANN1-GFP during plasmolysis/deplasmolysis cycle. This study shows complex developmental and subcellular localization patterns of ANN1 in living Arabidopsis plants.

Boron Deficiency Increases Cytosolic Ca2+ Levels Mainly via Ca2+ Influx from the Apoplast in Arabidopsis thaliana Roots

Boron (B) is a micronutrient for plant development, and its deficiency alters many physiological processes. However, the current knowledge on how plants are able to sense the B-starvation signal is still very limited. Recently, it has been reported that B deprivation induces an increase in cytosolic calcium concentration ([Ca²⁺]_cyt) in Arabidopsis thaliana roots. The aim of this work was to research in Arabidopsis whether [Ca²⁺]_cyt is restored to initial levels when B is resupplied and elucidate whether apoplastic Ca²⁺ is the major source for B-deficiency-induced rise in [Ca²⁺]_cyt. The use of chemical compounds affecting Ca²⁺ homeostasis showed that the rise in root [Ca²⁺]_cyt induced by B deficiency was predominantly owed to Ca²⁺ influx from the apoplast through plasma membrane Ca²⁺ channels in an IP₃-independent manner. Furthermore, B resupply restored the root [Ca²⁺]_cyt. Interestingly, expression levels of genes encoding Ca²⁺ transporters (ACA10, plasma membrane P_IIB-type Ca²⁺-ATPase; and CAX3, vacuolar cation/proton exchanger) were upregulated by ethylene glycol tetraacetic acid (EGTA) and abscisic acid (ABA). The results pointed out that ACA10, and especially CAX3, would play a major role in the restoration of Ca²⁺ homeostasis after 24 h of B deficiency.

A repertoire of cationic and anionic conductances at the plasma membrane of Medicago truncatula root hairs

Root hairs, as lateral extensions of epidermal cells, provide large absorptive surfaces to the root and are major actors in plant hydromineral nutrition. In contact with the soil they also constitute a site of interactions between the plant and rhizospheric microorganisms. In legumes, initiation of symbiotic interactions with N₂ -fixing rhizobia is often triggered at the root hair cell membrane in response to nodulation factors secreted by rhizobia, and involves early signaling events with changes in H⁺ , Ca²⁺ , K⁺ and Cl^- fluxes inducing transient depolarization of the cell membrane. Here, we aimed to build a functional repertoire of the major root hair conductances to cations and anions in the sequenced legume model Medicago truncatula. Five root hair conductances were characterized through patch-clamp experiments on enzymatically recovered root hair protoplasts. These conductances displayed varying properties of voltage dependence, kinetics and ion selectivity. They consisted of hyperpolarization- and depolarization-activated conductances for K⁺ , cations or Cl^- . Among these, one weakly outwardly rectifying cationic conductance and one hyperpolarization-activated slowly inactivating anionic conductance were not known as active in root hairs. All five conductances were detected in apical regions of young growing root hairs using membrane spheroplasts obtained by laser-assisted cell-wall microdissection. Combined with recent root hair transcriptomes of M. truncatula, this functional repertoire of conductances is expected to help the identification of candidate genes for reverse genetics studies to investigate the possible role of each conductance in root hair growth and interaction with the biotic and abiotic environment.

Strontium in the environment: Review about reactions of plants towards stable and radioactive strontium isotopes

Radiostrontium is released to the environment from routine and accidental discharge and acts on living organisms either from external sources or after absorption. When incorporated by plants, it enters the food chain and causes primary threat to human health and the environment. Understanding the mechanisms of plants for strontium uptake and retention is therefore essential for decision making concerning agriculture: are uptake rates low enough so that plants can serve as food? Or is radiostrontium accumulated so that plants should not be eaten but could be probably used for extracting strontium from water and soil in hot spots of pollution? The review presents a summary of studies about the origin of stable and radioactive strontium in the environment and effects coming from both internal and external exposure of plants. Mobility and availability of strontium to plant roots in soil are controlled by external factors such as chemical composition of the soil and pH, temperature and agricultural soil cultivation as well as soil biological networks built by microbial communities. Plant surfaces may receive input of strontium from deposition induced by atmospheric pollution or by acquisition from water through the whole immersed surface. Cells have entry mechanisms for strontium such as plasma membrane transporters for calcium and potassium. Part of absorbed strontium can be lost via processes discussed in this review. We give examples on strontium transfer factors for 149 plants to estimate plant absorption capacity for strontium from soil, water and air. Uptake efficiency of terrestrial and aquatic plants is deciding about their remediation potential to either remove radiostrontium by accumulation and rhizofiltration or to retain it in roots or aerial parts. Data of strontium content in soils after fallout and edible plants from long-term monitoring support the evaluation of the potential hazards posed by strontium input to the food chain.

Maize annexin genes ZmANN33 and ZmANN35 encode proteins that function in cell membrane recovery during seed germination

Plasma membrane (PM) recovery from the impaired dry state is essential for seed germination, but its underlying mechanism remains unclear. In this study, we found that ZmANN33 and ZmANN35, two annexin genes in maize, encode proteins that participate in PM recovery during seed germination. The expression of both genes was up-regulated during seed germination and strongly repressed by chilling (either 15 or 5 °C) as compared with the normal temperature (25 °C). In addition, the increased membrane damage caused by chilling imbibition was correlated with suppressed expression of ZmANN33 and ZmANN35, while rapid recovery of their expression levels accompanied the rescue of the damaged membrane. Arabidopsis seedlings ectopically expressing ZmANN33 or ZmANN35 had longer seedling length than wild-type (WT) plants during the recovery period after 3 d of chilling stress, indicating the positive roles of these two gene products in the plant's recovery from chilling injury. Moreover, these transgenic seedlings had lower lipid peroxidation and higher peroxidase activities than WT during the recovery period. Consistently, root cells of these transgenic seedlings had more intact PM after chilling stress, supporting the proposition that ZmANN33 and ZmANN35 contribute to the maintenance of PM integrity. The enhanced PM integrity is likely due to the accelerated exocytotic process after chilling stress. We also showed that both ZmANN33 and ZmANN35 localized in the cytosol near the plasma membrane. Thus, we conclude that ZmANN33 and ZmANN35 play essential roles in membrane recovery during maize seed germination.

Calcium-Nutrient and Messenger

Calcium is an essential element needed for growth and development of plants under both non-stressed and stress conditions. It thereby fulfills a dual function, being not only an important factor for cell wall and membrane stability, but also serving as a second messenger in many developmental and physiological processes, including the response of plants to biotic stress. The perception of non-self hereby induces an influx of calcium ions (Ca²⁺) into the cytosol, which is decoded into downstream responses ultimately leading to defense. Maintaining intracellular Ca²⁺ homeostasis is crucial for the ability to generate this signal. This review will describe the current knowledge of the mechanisms involved in uptake and transport of calcium as well as cellular homeostasis and signal generation, describing known genes involved and discussing possible implications the plant's nutritional status with regard to calcium might have on immunity.

The pathway of transmembrane cadmium influx via calcium-permeable channels and its spatial characteristics along rice root

To develop elite crops with low cadmium (Cd), a fundamental understanding of the mechanism of Cd uptake by crop roots is necessary. Here, a new mechanism for Cd2+ entry into rice root cells was investigated. The results showed that Cd2+ influx in rice roots exhibited spatially and temporally dynamic patterns. There was a clear longitudinal variation in Cd uptake along rice roots, with the root tip showing much higher Cd2+ influx and concentration than the root mature zone, which might be due to the much higher expression of the well-known Cd transporter genes OsIRT1, OsNRAMP1, OsNRAMP5, and OsZIP1 in the root tip. Both the net Cd2+ influx and the uptake of Cd in rice roots were highly inhibited by ion channel blockers Gd3+ and TEA+, supplementation of Ca2+ and K+, and the plasma membrane H+-ATPase inhibitor vanadate, with Gd3+ and Ca2+ showing the most inhibitory effects. Furthermore, Ca2+- or Gd3+-induced reduction in Cd2+ influx and Cd uptake did not coincide with the expression of Cd transporter genes, but with that of two Ca channel genes, OsAAN4 and OsGLR3.4. These results indicate that Cd transporters are in part responsible for Cd2+ entry into rice root, and provide a new perspective that the Ca channels OsAAN4 and OsGLR3.4 might play an important role in rice root Cd uptake.

Radiostrontium transport in plants and phytoremediation

Radiostrontium is a common product of nuclear fission and was emitted into the environment in the course of nuclear weapon tests as well as from nuclear reactor accidents. The release of ⁹⁰Sr and ⁸⁹Sr into the environment can pose health threats due to their characteristics such as high specific activities and easy access in human body due to its chemical analogy to calcium. Radiostrontium enters the human food chain by the consumption of plants grown on sites comprising fission-derived radionuclides. For humans, Sr is not an essential element, but, due to solubility in water and homology with calcium, once interred in the body, it gets deposited in bones and in teeth. This concern has drawn the attention of researchers throughout the globe to develop sustainable treatment processes to remediate soil and water resources. Nowadays, phytoremediation has become a promising approach for the remediation of large extents of toxic heavy metals. Some of the plants have been reported to accumulate Sr inside their biomass but detailed mechanisms at genetic level are still to be uncovered. However, there is inadequate information offered to assess the possibility of this remediation approach. This review highlights phytoremediation approach for Sr and explains in detail the uptake mechanism inside plants.

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.

Roles of Chloroplast Retrograde Signals and Ion Transport in Plant Drought Tolerance

Worldwide, drought affects crop yields; therefore, understanding plants' strategies to adapt to drought is critical. Chloroplasts are key regulators of plant responses, and signals from chloroplasts also regulate nuclear gene expression during drought. However, the interactions between chloroplast-initiated retrograde signals and ion channels under stress are still not clear. In this review, we summarise the retrograde signals that participate in regulating plant stress tolerance. We compare chloroplastic transporters that modulate retrograde signalling through retrograde biosynthesis or as critical components in retrograde signalling. We also discuss the roles of important plasma membrane and tonoplast ion transporters that are involved in regulating stomatal movement. We propose how retrograde signals interact with ion transporters under stress.

Stable and radioactive cesium: A review about distribution in the environment, uptake and translocation in plants, plant reactions and plants' potential for bioremediation

Radiocesium in water, soil, and air represents a severe threat to human health and the environment. It either acts directly on living organisms from external sources, or it becomes incorporated through the food chain, or both. Plants are at the base of the food chain; it is therefore essential to understand the mechanisms of plants for cesium retention and uptake. In this review we summarize investigations about sources of stable and radioactive cesium in the environment and harmful effects caused by internal and external exposure of plants to radiocesium. Uptake of cesium into cells occurs through molecular mechanisms such as potassium and calcium transporters in the plasma membrane. In soil, bioavailability of cesium depends on the chemical composition of the soil and physical factors such as pH, temperature and tilling as well as on environmental factors such as soil microorganisms. Uptake of cesium occurs also from air through interception and absorption on leaves and from water through the whole submerged surface. We reviewed information about reducing cesium in the vegetation by loss processes, and we extracted transfer factors from the available literature and give an overview over the uptake capacities of 72 plants for cesium from the substratum to the biomass. Plants with high uptake potential could be used to remediate soil and water from radiocesium by accumulation and rhizofiltration. Inside plants, cesium distributes fast between the different plant organs and cells, but cesium in soil is extremely stable and remains for decades in the rhizosphere. Monitoring of contaminated soil therefore has to continue for many decades, and edible plants grown on such soil must continuously be monitored.

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.

Adaptive molecular evolution of the two-pore channel 1 gene TPC1 in the karst-adapted genus Primulina (Gesneriaceae)

BACKGROUND AND AIMS

Limestone karst areas possess high floral diversity and endemism. The genus Primulina, which contributes to the unique calcicole flora, has high species richness and exhibit specific soil-based habitat associations that are mainly distributed on calcareous karst soils. The adaptive molecular evolutionary mechanism of the genus to karst calcium-rich environments is still not well understood. The Ca²⁺-permeable channel TPC1 was used in this study to test whether its gene is involved in the local adaptation of Primulina to karst high-calcium soil environments.

METHODS

Specific amplification and sequencing primers were designed and used to amplify the full-length coding sequences of TPC1 from cDNA of 76 Primulina species. The sequence alignment without recombination and the corresponding reconstructed phylogeny tree were used in molecular evolutionary analyses at the nucleic acid level and amino acid level, respectively. Finally, the identified sites under positive selection were labelled on the predicted secondary structure of TPC1.

KEY RESULTS

Seventy-six full-length coding sequences of Primulina TPC1 were obtained. The length of the sequences varied between 2220 and 2286 bp and the insertion/deletion was located at the 5' end of the sequences. No signal of substitution saturation was detected in the sequences, while significant recombination breakpoints were detected. The molecular evolutionary analyses showed that TPC1 was dominated by purifying selection and the selective pressures were not significantly different among species lineages. However, significant signals of positive selection were detected at both TPC1 codon level and amino acid level, and five sites under positive selective pressure were identified by at least three different methods.

CONCLUSIONS

The Ca²⁺-permeable channel TPC1 may be involved in the local adaptation of Primulina to karst Ca²⁺-rich environments. Different species lineages suffered similar selective pressure associated with calcium in karst environments, and episodic diversifying selection at a few sites may play a major role in the molecular evolution of Primulina TPC1.

Cesium and strontium tolerant Arthrobacter sp. strain KMSZP6 isolated from a pristine uranium ore deposit

Arthrobacter sp. KMSZP6 isolated from a pristine uranium ore deposit at Domiasiat located in North-East India exhibited noteworthy tolerance for cesium (Cs) and strontium (Sr). The strain displayed a high minimum inhibitory concentration (MIC) of 400 mM for CsCl and for SrCl2. Flow cytometric analysis employing membrane integrity indicators like propidium iodide (PI) and thiazole orange (TO) indicated a greater sensitivity of Arthrobacter cells to cesium than to strontium. On being challenged with 75 mM of Cs, the cells sequestered 9612 mg Cs g(-1) dry weight of cells in 12 h. On being challenged with 75 mM of Sr, the cells sequestered 9989 mg Sr g(-1) dry weight of cells in 18 h. Heat killed cells exhibited limited Cs and Sr binding as compared to live cells highlighting the importance of cell viability for optimal binding. The association of the metals with Arthrobacter sp. KMSZP6 was further substantiated by Field Emission-Scanning Electron Microscopy (FE-SEM) coupled with Energy dispersive X-ray (EDX) spectroscopy. This organism tolerated up to 1 kGy (60)Co-gamma rays without loss of survival. The present report highlights the superior tolerance and binding capacity of the KMSZP6 strain for cesium and strontium over other earlier reported strains and reveals its potential for bioremediation of nuclear waste.

AtCNGC2 is involved in jasmonic acid-induced calcium mobilization

Calcium (Ca(2+)) mobilization is a central theme in various plant signal transduction pathways. We demonstrate that Arabidopsis thaliana cyclic nucleotide-gated channel 2 (AtCNGC2) is involved in jasmonic acid (JA)-induced apoplastic Ca(2+) influx in Arabidopsis epidermal cells. Ca(2+) imaging results showed that JA can induce an elevation in the cytosolic cAMP concentration ([cAMP]cyt), reaching a maximum within 3 min. Dibutyryl cAMP (db-cAMP), a cell membrane-permeable analogue of cAMP, induced an increase in the cytosolic Ca(2+) concentration ([Ca(2+)]cyt), with a peak at 4 min. This [Ca(2+)]cyt increase was triggered by the JA-induced increase in [cAMP]cyt. W-7[N-(6-aminohexyl)-5-chloro-1-naphthalenesulfonamide], an antagonist of calmodulin, positively modulated the JA-induced increase in [Ca(2+)]cyt, while W-5[N-(6-aminohexyl)-1-naphthalenesulfonamide], an inactive antagonist of calmodulin, had no apparent effect. db-cAMP and JA positively induced the expression of primary (i.e. JAZ1 and MYC2) and secondary (i.e. VSP1) response genes in the JA signalling pathway in wild-type Arabidopsis thaliana, whereas they had no significant effect in the AtCNGC2 mutant 'defense, no death (dnd1) plants. These data provide evidence that JA first induces the elevation of cAMP, and cAMP, as an activating ligand, activates the AtCNGC2 channel, resulting in apoplastic Ca(2+) influx through AtCNGC2.

Radiocesium accumulation properties of Chengiopanax sciadophylloides

Through the assessments of radioactive contamination after the Fukushima Dai-ichi Nuclear Power Plant (F1NPP) accident, it has been reported that some sprouts of Chengiopanax sciadophylloides (Franch. et Sav.) at the site contained radiocesium (((134),)(137)Cs) at higher concentrations than the other plants. To assess the phytoremediation properties of C. sciadophylloides for (137)Cs decontamination, we aimed to quantify the (137)Cs accumulation in C. sciadophylloides. We measured the (137)Cs concentrations in various organs of C. sciadophylloides collected from the forest in the town of Kawamata, Fukushima prefecture, together with the concentrations of other elements [potassium (K), rubidium, (133)Cs, calcium, strontium, and manganese] present. In addition, we compared the foliar concentrations of these elements in C. sciadophylloides with those in four different deciduous tree species. The mean of foliar (137)Cs concentration in C. sciadophylloides was 28.1 kBq kg(-1) DW, one order of magnitude higher than that found in the other species. The (137)Cs concentrations were in the order of leaves > bark > wood. The wood of the treetop, leaf scars, and roots contained higher amounts of (137)Cs than that of the trunk. From the distribution of (137)Cs in C. sciadophylloides, we confirmed that (137)Cs tends to accumulate in the young growing parts. The difference in the distribution of (137)Cs and (133)Cs indicated that surface uptake of (137)Cs occurs. A significant correlation between K and (137)Cs concentrations in each organ was found, which suggested that (137)Cs in the plant body is transferred through the same pathway as K. On the other hand, there was no correlation between foliar K and (137)Cs concentrations, implying that the uptake ratio of K to (137)Cs was different for each individual. To determine the factors driving specific (137)Cs accumulation and/or the variability of the ratio between K and (137)Cs, the distribution of (137)Cs and the root in soil, the difference of the expression of transporter, and the existence of mycorrhizal fungi should be considered. However, further research is required.

Comparative Proteomic Analysis Reveals the Effects of Exogenous Calcium against Acid Rain Stress in Liquidambar formosana Hance Leaves

Acid rain (AR) impacts forest health by leaching calcium (Ca) away from soils and plants. Ca is an essential element and participates in various plant physiological responses. In the present study, the protective role of exogenous Ca in alleviating AR stress in Liquidambar formosana Hance at the physiological and proteomic levels was examined. Our results showed that low Ca condition resulted in the chlorophyll content and photosynthesis decreasing significantly in L. formosana leaves; however, these effects could be reversed by high Ca supplementation. Further proteomic analyses successfully identified 81 differentially expressed proteins in AR-treated L. formosana under different Ca levels. In particular, some of the proteins are involved in primary metabolism, photosynthesis, energy production, antioxidant defense, transcription, and translation. Moreover, quantitative real time polymerase chain reaction (qRT-PCR) results indicated that low Ca significantly increased the expression level of the investigated Ca-related genes, which can be reversed by high Ca supplementation under AR stress. Further, Western blotting analysis revealed that exogenous Ca supply reduced AR damage by elevating the expression of proteins involved in the Calvin cycle, reactive oxygen species (ROS) scavenging system. These findings allowed us to better understand how woody plants respond to AR stress at various Ca levels and the protective role of exogenous Ca against AR stress in forest tree species.

Nicotiana sylvestris calcineurin B-like protein NsylCBL10 enhances salt tolerance in transgenic Arabidopsis

KEY MESSAGE

Nicotiana sylvestris calcineurin B-like protein NsylCBL10 improves tolerance to high-salt stress through better maintenance of Na (+) balance. The calcineurin B-like (CBL) proteins represent a unique group of plant calcium sensors and play an important role in regulating the response of a plant cell to the stress. Although many studies have been made in Arabidopsis (Arabidopsis thaliana), rice (Oryza sativa) and poplar (Populus trichocarpa), the characterization and elucidation of the functions of CBLs in tobacco have not yet been reported. In this study, NsylCBL10, a CBL gene showing higher similarities to other CBL10 genes, was cloned from Nicotiana sylvestris. NsylCBL10 is expressed in most of the tobacco tissues, and the protein targets to the plasma membrane specifically. Over-expression of NsylCBL10 enhanced the salt tolerance of Arabidopsis wild type plants greatly, and rescued the high-salt-sensitive phenotype of Arabidopsis cbl10 mutant. The analysis of ion content indicated that over-expressing NsylCBL10 in plants is able to maintain a lower Na(+)/K(+) ratio in roots and higher Na(+)/K(+) ratio in shoots, compared with cbl10 mutant. The results suggest that NsylCBL10 might play an important role in response to high salinity stress in N. sylvestris, by keeping a better ionic homeostasis to reduce the damage of toxic ion to the plant cell.

Cation transporters/channels in plants: Tools for nutrient biofortification

Cation transporters/channels are key players in a wide range of physiological functions in plants, including cell signaling, osmoregulation, plant nutrition and metal tolerance. The recent identification of genes encoding some of these transport systems has allowed new studies toward further understanding of their integrated roles in plant. This review summarizes recent discoveries regarding the function and regulation of the multiple systems involved in cation transport in plant cells. The role of membrane transport in the uptake, distribution and accumulation of cations in plant tissues, cell types and subcellular compartments is described. We also discuss how the knowledge of inter- and intra-species variation in cation uptake, transport and accumulation as well as the molecular mechanisms responsible for these processes can be used to increase nutrient phytoavailability and nutrients accumulation in the edible tissues of plants. The main trends for future research in the field of biofortification are proposed.

Calcium-dependent regulation of photosynthesis

The understanding of calcium as a second messenger in plants has been growing intensively over the last decades. Recently, attention has been drawn to the organelles, especially the chloroplast but focused on the stromal Ca2+ transients in response to environmental stresses. Herein we will expand this view and discuss the role of Ca2+ in photosynthesis. Moreover we address of how Ca2+ is delivered to chloroplast stroma and thylakoids. Thereby, new light is shed on the regulation of photosynthetic electron flow and light-dependent metabolism by the interplay of Ca2+, thylakoid acidification and redox status. This article is part of a Special Issue entitled: Chloroplast biogenesis.

Mechanisms and physiological roles of K+ efflux from root cells

Potassium is the most abundant macronutrient, which is involved in a multitude of physiological processes. Potassium uptake in roots is crucial for plants; however, K(+) efflux can also occur and has important functions. Potassium efflux from roots is mainly induced by stresses, such as pathogens, salinity, freezing, oxidants and heavy metals. Reactive oxygen species (ROS) and exogenous purines also cause this reaction. The depolarisation and activation of cation channels are required for K(+) efflux from plant roots. Potassium channels and nonselective cation channels (NSCCs) are involved in this process. Some of them are 'constitutive', while the others require a chemical agent for activation. In Arabidopsis, there are 77 genes that can potentially encode K(+)-permeable channels. Potassium-selective channel genes include 9 Shaker and 6 Tandem-Pore K(+) channels. Genes of NSCCs are more abundant and present by 20 cyclic nucleotide gated channels, 20 ionotropic glutamate receptors, 1 two-pore channel, 10 mechanosensitive-like channels, 2 mechanosensitive 'Mid1-Complementing Activity' channels, 1 mechanosensitive Piezo channel, and 8 annexins. Two Shakers (SKOR and GORK) and several NSCCs are expressed in root cell plasma membranes. SKOR mediates K(+) efflux from xylem parenchyma cells to xylem vessels while GORK is expressed in the epidermis and functions in K(+) release. Both these channels are activated by ROS. The GORK channel activity is stimulated by hydroxyl radicals that are generated in a Ca(2+)-dependent manner in stress conditions, such as salinity or pathogen attack, resulting in dramatic K(+) efflux from root cells. Potassium loss simulates cytosolic proteases and endonucleases, leading to programmed cell death. Other physiological functions of K(+) efflux channels include repolarisation of the plasma membrane during action potentials and the 'hypothetical' function of a metabolic switch, which provides inhibition of energy-consuming biosyntheses and releasing energy for defence and reparation needs.

Stress-induced electrolyte leakage: the role of K+-permeable channels and involvement in programmed cell death and metabolic adjustment

Electrolyte leakage accompanies plant response to stresses, such as salinity, pathogen attack, drought, heavy metals, hyperthermia, and hypothermia; however, the mechanism and physiological role of this phenomenon have only recently been clarified. Accumulating evidence shows that electrolyte leakage is mainly related to K(+) efflux from plant cells, which is mediated by plasma membrane cation conductances. Recent studies have demonstrated that these conductances include components with different kinetics of activation and cation selectivity. Most probably they are encoded by GORK, SKOR, and annexin genes. Hypothetically, cyclic nucleotide-gated channels and ionotropic glutamate receptors can also be involved. The stress-induced electrolyte leakage is usually accompanied by accumulation of reactive oxygen species (ROS) and often results in programmed cell death (PCD). Recent data strongly suggest that these reactions are linked to each other. ROS have been shown to activate GORK, SKOR, and annexins. ROS-activated K(+) efflux through GORK channels results in dramatic K(+) loss from plant cells, which stimulates proteases and endonucleases, and promotes PCD. This mechanism is likely to trigger plant PCD under severe stress. However, in moderate stress conditions, K(+) efflux could play an essential role as a 'metabolic switch' in anabolic reactions, stimulating catabolic processes and saving 'metabolic' energy for adaptation and repair needs.

Mn-euvering manganese: the role of transporter gene family members in manganese uptake and mobilization in plants

Manganese (Mn), an essential trace element, is important for plant health. In plants, Mn serves as a cofactor in essential processes such as photosynthesis, lipid biosynthesis and oxidative stress. Mn deficient plants exhibit decreased growth and yield and are more susceptible to pathogens and damage at freezing temperatures. Mn deficiency is most prominent on alkaline soils with approximately one third of the world's soils being too alkaline for optimal crop production. Despite the importance of Mn in plant development, relatively little is known about how it traffics between plant tissues and into and out of organelles. Several gene transporter families have been implicated in Mn transport in plants. These transporter families include NRAMP (natural resistance associated macrophage protein), YSL (yellow stripe-like), ZIP (zinc regulated transporter/iron-regulated transporter [ZRT/IRT1]-related protein), CAX (cation exchanger), CCX (calcium cation exchangers), CDF/MTP (cation diffusion facilitator/metal tolerance protein), P-type ATPases and VIT (vacuolar iron transporter). A combination of techniques including mutant analysis and Synchrotron X-ray Fluorescence Spectroscopy can assist in identifying essential transporters of Mn. Such knowledge would vastly improve our understanding of plant Mn homeostasis.

AtrbohD and AtrbohF positively regulate abscisic acid-inhibited primary root growth by affecting Ca2+ signalling and auxin response of roots in Arabidopsis

Reactive oxygen species (ROS) originating from the NADPH oxidases AtrbohD and AtrbohF play an important role in abscisic acid (ABA)-inhibited primary root growth in Arabidopsis. However, the mechanisms underlying this process remain elusive. In this study, the double mutant atrbohD1/F1 and atrbohD2/F2, in which both AtrbohD and AtrbohF were disrupted, were less sensitive to ABA suppression of root cell elongation than wild-type (WT) plants. Furthermore, the double mutants showed impaired ABA responses in roots, including ROS generation, cytosolic Ca(2+) increases, and activation of plasma membrane Ca(2+)-permeable channels compared with WT. Exogenous H2O2 can activate the Ca(2+) currents in roots of atrbohD1/F1. In addition, exogenous application of the auxin transport inhibitor naphthylphthalamic acid effectively promoted ABA inhibition of root growth of the mutants relative to that of WT. The ABA-induced decreases in auxin sensitivity of the root tips were more pronounced in WT than in atrbohD1/F1. These findings suggest that both AtrbohD and AtrbohF are essential for ABA-promoted ROS production in roots. ROS activate Ca(2+) signalling and reduce auxin sensitivity of roots, thus positively regulating ABA-inhibited primary root growth in Arabidopsis.

Occurrence of interactions between individual Sr²⁺- and Ca²⁺-effects on maize root and shoot growth and Sr²⁺, Ca²⁺ and Mg²⁺ contents, and membrane potential: consequences on predicting Sr²⁺-impact

Occurrence of functional interactions between Sr(2+) and Ca(2+) were investigated on maize plants grown under hydroponic conditions in presence of various mixtures of SrCl2 [0-0.01-1-10 mM] and CaCl₂ [0-0.2-2-20 mM]. External [Ca(2+)] modulated the effect of Sr(2+) on the plant dry weight, and on the Sr(2+), Ca(2+) and Mg(2+) contents of roots and shoots. An intermediary functional step between external [Sr(2+)] and [Ca(2+)], and organ ion content, occurred at the plasma membrane of cortical root cells where Sr(2+) and Ca(2+) could influence ion uptake by acting on membrane potential. The decrease of the Sr(2+)-evoked membrane depolarization induced by Ca(2+) could not solely be attributed to the Ca(2+)-effect on the resting membrane potential. Most of the time the individual effects of Sr(2+) and Ca(2+) were not additive, as these two ions clearly interacted with each other to jointly affect the plant physiology. In spite of these interactions, both [Sr(2+)](ext) or [Sr(2+)](ext)/[Ca(2+)](ext) ratio values seemed to enable a correct prediction of the Sr(2+)-effects on the plant. However using the [Sr(2+)](ext)/[Ca(2+)](ext) ratio improved significantly the adequacy of prediction compared to the use of [Sr(2+)](ext) alone, as it increased up to 25% the proportion of variability accounted for by the model.

Interacting glutamate receptor-like proteins in Phloem regulate lateral root initiation in Arabidopsis

Molecular, genetic, and electrophysiological evidence indicates that at least one of the plant Glu receptor-like molecules, GLR3.4, functions as an amino acid-gated Ca²⁺channel at the plasma membrane. The aspect of plant physiology, growth, or development to which GLR3.4 contributes is an open question. Protein localization studies performed here provide important information. In roots, GLR3.4 and the related GLR3.2 protein were present primarily in the phloem, especially in the vicinity of the sieve plates. GLR3.3 was expressed in most cells of the growing primary root but was not enriched in the phloem, including the sieve plate area. GLR3.2 and GLR3.4 physically interacted with each other better than with themselves as evidenced by a biophotonic assay performed in human embryonic kidney cells and Nicotiana benthamiana leaf cells. GLR3.3 interacted poorly with itself or the other two GLRs. Mutations in GLR3.2, GLR3.4, or GLR3.2 and GLR3.4 caused the same and equally severe phenotype, namely, a large overproduction and aberrant placement of lateral root primordia. Loss of GLR3.3 did not affect lateral root primordia. These results support the hypothesis that apoplastic amino acids acting through heteromeric GLR3.2/GLR3.4 channels affect lateral root development via Ca²⁺ signaling in the phloem.

Effect of metal tolerant plant growth promoting bacteria on growth and metal accumulation in Zea mays plants grown in fly ash amended soil

The present study was undertaken to examine the effect of the application of fly ash (FA) into Garden soil (GS), with and without inoculation of plant growth promoting bacteria (PGPB), on the growth and metal uptake by Zea mays plants. Three FA tolerant PGPB strains, Pseudomonas sp. PS5, PS14, and Bacillus sp. BC29 were isolated from FA contaminated soils and assessed for their plant growth promoting features on the Z. mays plants. All three strains were also examined for their ability to solubilize phosphate and to produce Indole Acetic Acid (IAA), siderophores, and hydrogencynide acid (HCN) production. Although inoculation of all strains significantly enhanced the growth of plants at both the concentration of FA but maximum growth was observed in plants inoculated with BC29 and PS14 at low level (25%) of FA concentration. The experimental results explored the plant growth promoting features of selected strains which not only enhanced growth and biomass of plants but also protected them from toxicity of FA.

A Ca(2+)-sensitive system mediates low-affinity K(+) uptake in the absence of AKT1 in Arabidopsis plants

K(+) acquisition by Arabidopsis roots is mainly mediated by the high-affinity K(+) transporter AtHAK5 and the inward-rectifier K(+) channel AtAKT1. This model is probably universal to plants. Mutant plants lacking these two systems (athak5,atakt1) take up K(+) and grow when the external K(+) concentration is above a certain level, indicating that an additional transport system may compensate for the absence of AtHAK5 and AtAKT1. Here we describe that this alternative system is essential for providing sufficient K(+) to sustain growth of athak5,atakt1 plants. This system is especially sensitive to Ca(2+), Mg(2+), Ba(2+) and La(3+), it transports Cs(+) and its activity is reduced by cyclic nucleotides. These results suggest that a Ca(2+)-permeable voltage-independent non-selective cation channel, probably belonging to the cyclic nucleotide gated channel (CNGC) family, may provide the pathway for K(+) uptake in athak5,atakt1 plants. The genes encoding the two members of the CNGC family that have been described as mediating root K(+) uptake, AtCNGC3 and AtCNGC10, are not up-regulated in athak5,atakt1 plants, excluding overexpression of these genes as a compensatory mechanism. On the other hand, an increased driving force for K(+) in athak5,atakt1 plants due to a hyperpolarization of the membrane potential of its root cells is also discarded. The identification of this unknown system may provide tools to improve plant K(+) nutrition in conditions where AtAKT1 functionality is reduced, such as under salinity. In addition, this system may constitute an important pathway for accumulation of toxic cations such as Cs(+) or radiocesium ((137)Cs(+)), and could play a role in phytoremediation.

Evolutionary adaptation of plant annexins has diversified their molecular structures, interactions and functional roles

Annexins are an homologous, structurally related superfamily of proteins known to associate with membrane lipid and cytoskeletal components. Their involvement in membrane organization, vesicle trafficking and signaling is fundamental to cellular processes such as growth, differentiation, secretion and repair. Annexins exist in some prokaryotes and all eukaryotic phyla within which plant annexins represent a monophyletic clade of homologs descended from green algae. Genomic, proteomic and transcriptomic approaches have provided data on the diversity, cellular localization and expression patterns of different plant annexins. The availability of 35 complete plant genomes has enabled systematic comparative analysis to determine phylogenetic relationships, characterize structures and observe functional specificity between and within individual subfamilies. Short amino termini and selective erosion of the canonical type 2 calcium coordinating sites in domains 2 and 3 are typical of plant annexins. The convergent evolution of alternate functional motifs such as 'KGD', redox-sensitive Cys and hydrophobic Trp/Phe residues argues for their functional relevance and contribution to mechanistic diversity in plant annexins. This review examines recent findings and advances in plant annexin research with special focus on their structural diversity, cellular and molecular interactions and their potential integrated functions in the broader context of physiological responses.

Potassium starvation induces oxidative stress in Solanum lycopersicum L. roots

The relationship between potassium deficiency and the antioxidative defense system has received little study. The aim of this work was to study the induction of oxidative stress in response to K(+) deficiency and the putative role of antioxidants. The tomato plants were grown in hydroponic systems to determine the role of reactive oxygen species (ROS) in the root response to potassium deprivation. Parameters of oxidative stress (malondialdehyde and hydrogen peroxide (H(2)O(2)) concentration), activities of antioxidant enzymes (superoxide dismutase (SOD), catalase (CAT), ascorbate peroxidase (APX), dehydroascorbate reductase (DHAR) and glutathione reductase (GR)) and antioxidant molecules (ascorbate (ASC) and glutathione) were investigated. H(2)O(2) was subcellularly located by laser confocal microscopy after potassium starvation in roots. During the first 24h, H(2)O(2) induced the cascade of the cellular response to low potassium, and ROS accumulation was located mainly in epidermal cells in the elongation zone and meristematic cells of the root tip and the epidermal cells of the mature zones of potassium starved roots. The activity of the antioxidative enzymes SOD, peroxidase and APX in potassium deprivation significantly increased, whereas CAT and DHAR activity was significantly depressed in the potassium starvation treatment compared to controls. GR did not show significant differences between control and potassium starvation treatments. Based on these results, we put forward the hypothesis that antioxidant molecule accumulations probably scavenge H(2)O(2) and might be regenerated by the ASC-glutathione cycle enzymes, such as DHAR and GR.

The rice monovalent cation transporter OsHKT2;4: revisited ionic selectivity

The family of plant membrane transporters named HKT (for high-affinity K(+) transporters) can be subdivided into subfamilies 1 and 2, which, respectively, comprise Na(+)-selective transporters and transporters able to function as Na(+)-K(+) symporters, at least when expressed in yeast (Saccharomyces cerevisiae) or Xenopus oocytes. Surprisingly, a subfamily 2 member from rice (Oryza sativa), OsHKT2;4, has been proposed to form cation/K(+) channels or transporters permeable to Ca(2+) when expressed in Xenopus oocytes. Here, OsHKT2;4 functional properties were reassessed in Xenopus oocytes. A Ca(2+) permeability through OsHKT2;4 was not detected, even at very low external K(+) concentration, as shown by highly negative OsHKT2;4 zero-current potential in high Ca(2+) conditions and lack of sensitivity of OsHKT2;4 zero-current potential and conductance to external Ca(2+). The Ca(2+) permeability previously attributed to OsHKT2;4 probably resulted from activation of an endogenous oocyte conductance. OsHKT2;4 displayed a high permeability to K(+) compared with that to Na(+) (permeability sequence: K(+) > Rb(+) ≈ Cs(+) > Na(+) ≈ Li(+) ≈ NH(4)(+)). Examination of OsHKT2;4 current sensitivity to external pH suggested that H(+) is not significantly permeant through OsHKT2;4 in most physiological ionic conditions. Further analyses in media containing both Na(+) and K(+) indicated that OsHKT2;4 functions as K(+)-selective transporter at low external Na(+), but transports also Na(+) at high (>10 mm) Na(+) concentrations. These data identify OsHKT2;4 as a new functional type in the K(+) and Na(+)-permeable HKT transporter subfamily. Furthermore, the high permeability to K(+) in OsHKT2;4 supports the hypothesis that this system is dedicated to K(+) transport in the plant.

A heat-activated calcium-permeable channel--Arabidopsis cyclic nucleotide-gated ion channel 6--is involved in heat shock responses

An increased concentration of cytosolic calcium ions (Ca²⁺) is an early response by plant cells to heat shock. However, the molecular mechanism underlying the heat-induced initial Ca²⁺ response in plants is unclear. In this study, we identified and characterized a heat-activated Ca²⁺-permeable channel in the plasma membrane of Arabidopsis thaliana root protoplasts using reverse genetic analysis and the whole-cell patch-clamp technique. The results indicated that A. thaliana cyclic nucleotide-gated ion channel 6 (CNGC6) mediates heat-induced Ca²⁺ influx and facilitates expression of heat shock protein (HSP) genes and the acquisition of thermotolerance. GUS and GFP reporter assays showed that CNGC6 expression is ubiquitous in A. thaliana, and the protein is localized to the plasma membrane of cells. Furthermore, it was found that the level of cytosolic cAMP was increased by a mild heat shock, that CNGC6 was activated by cytosolic cAMP, and that exogenous cAMP promoted the expression of HSP genes. The results reveal the role of cAMP in transduction of heat shock signals in plants. The correlation of an increased level of cytosolic cAMP in a heat-shocked plant with activation of the Ca²⁺ channels and downstream expression of HSP genes sheds some light on how plants transduce a heat stimulus into a signal cascade that leads to a heat shock response.

Genome-wide comparative in silico analysis of calcium transporters of rice and sorghum

The mechanism of calcium uptake, translocation and accumulation in Poaceae has not yet been fully understood. To address this issue, we conducted genome-wide comparative in silico analysis of the calcium (Ca²⁺) transporter gene family of two crop species, rice and sorghum. Gene annotation, identification of upstream cis-acting elements, phylogenetic tree construction and syntenic mapping of the gene family were performed using several bioinformatics tools. A total of 31 Ca²⁺ transporters, distributed on 9 out of 12 chromosomes, were predicted from rice genome, while 28 Ca²⁺ transporters predicted from sorghum are distributed on all the chromosomes except chromosome 10 (Chr 10). Interestingly, most of the genes on Chr 1 and Chr 3 show an inverse syntenic relationship between rice and sorghum. Multiple sequence alignment and motif analysis of these transporter proteins revealed high conservation between the two species. Phylogenetic tree could very well identify the subclasses of channels, ATPases and exchangers among the gene family. The in silico cis-regulatory element analysis suggested diverse functions associated with light, stress and hormone responsiveness as well as endosperm- and meristem-specific gene expression. Further experiments are warranted to validate the in silico analysis of the predicted transporter gene family and elucidate the functions of Ca²⁺ transporters in various biological processes.

Plasma membrane protein OsMCA1 is involved in regulation of hypo-osmotic shock-induced Ca2+ influx and modulates generation of reactive oxygen species in cultured rice cells

BACKGROUND

Mechanosensing and its downstream responses are speculated to involve sensory complexes containing Ca2+-permeable mechanosensitive channels. On recognizing osmotic signals, plant cells initiate activation of a widespread signal transduction network that induces second messengers and triggers inducible defense responses. Characteristic early signaling events include Ca2+ influx, protein phosphorylation and generation of reactive oxygen species (ROS). Pharmacological analyses show Ca2+ influx mediated by mechanosensitive Ca2+ channels to influence induction of osmotic signals, including ROS generation. However, molecular bases and regulatory mechanisms for early osmotic signaling events remain poorly elucidated.

RESULTS

We here identified and investigated OsMCA1, the sole rice homolog of putative Ca2+-permeable mechanosensitive channels in Arabidopsis (MCAs). OsMCA1 was specifically localized at the plasma membrane. A promoter-reporter assay suggested that OsMCA1 mRNA is widely expressed in seed embryos, proximal and apical regions of shoots, and mesophyll cells of leaves and roots in rice. Ca2+ uptake was enhanced in OsMCA1-overexpressing suspension-cultured cells, suggesting that OsMCA1 is involved in Ca2+ influx across the plasma membrane. Hypo-osmotic shock-induced ROS generation mediated by NADPH oxidases was also enhanced in OsMCA1-overexpressing cells. We also generated and characterized OsMCA1-RNAi transgenic plants and cultured cells; OsMCA1-suppressed plants showed retarded growth and shortened rachises, while OsMCA1-suppressed cells carrying Ca2+-sensitive photoprotein aequorin showed partially impaired changes in cytosolic free Ca2+ concentration ([Ca2+]cyt) induced by hypo-osmotic shock and trinitrophenol, an activator of mechanosensitive channels.

CONCLUSIONS

We have identified a sole MCA ortholog in the rice genome and developed both overexpression and suppression lines. Analyses of cultured cells with altered levels of this putative Ca2+-permeable mechanosensitive channel indicate that OsMCA1 is involved in regulation of plasma membrane Ca2+ influx and ROS generation induced by hypo-osmotic stress in cultured rice cells. These findings shed light on our understanding of mechanical sensing pathways.

Brassica juncea plant cadmium resistance 1 protein (BjPCR1) facilitates the radial transport of calcium in the root

Calcium (Ca) is an important structural component of plant cell walls and an intracellular messenger in plants and animals. Therefore, plants tightly control the balance of Ca by regulating Ca uptake and its transfer from cell to cell and organ to organ. Here, we propose that Brassica juncea PCR1 (PCR1), a member of the plant cadmium resistance (PCR) protein family in Indian mustard, is a Ca(2+) efflux transporter that is required for the efficient radial transfer of Ca(2+) in the root and is implicated in the translocation of Ca to the shoot. Knock-down lines of BjPCR1 were greatly stunted and translocated less Ca to the shoot than did the corresponding WT. The localization of BjPCR1 to the plasma membrane and the preferential expression of BjPCR1 in the root epidermal cells of WT plants suggest that BjPCR1 antisense plants could not efficiently transfer Ca(2+) from the root epidermis to the cells located inside the root. Protoplasts isolated from BjPCR1 antisense lines had lower Ca(2+) efflux activity than did those of the WT, and membrane vesicles isolated from BjPCR1-expressing yeast exhibited increased Ca(2+) transport activity. Inhibitor studies, together with theoretical considerations, indicate that BjPCR1 exports one Ca(2+) in exchange for three protons. Root hair-specific expression of BjPCR1 in Arabidopsis results in plants that exhibit increased Ca(2+) resistance and translocation. In conclusion, our data support the hypothesis that BjPCR1 is an exporter required for the translocation of Ca(2+) from the root epidermis to the inner cells, and ultimately to the shoot.

Physiological limits to zinc biofortification of edible crops

It has been estimated that one-third of the world's population lack sufficient Zn for adequate nutrition. This can be alleviated by increasing dietary Zn intakes through Zn biofortification of edible crops. Biofortification strategies include the application of Zn-fertilizers and the development of crop genotypes that acquire more Zn from the soil and accumulate it in edible portions. Zinc concentrations in roots, leaves, and stems can be increased through the application of Zn-fertilizers. Root Zn concentrations of up to 500-5000 mg kg(-1) dry matter (DM), and leaf Zn concentrations of up to 100-700 mg kg(-1) DM, can be achieved without loss of yield when Zn-fertilizers are applied to the soil. It is possible that greater Zn concentrations in non-woody shoot tissues can be achieved using foliar Zn-fertilizers. By contrast, Zn concentrations in fruits, seeds, and tubers are severely limited by low Zn mobility in the phloem and Zn concentrations higher than 30-100 mg kg(-1) DM are rarely observed. However, genetically modified plants with improved abilities to translocate Zn in the phloem might be used to biofortify these phloem-fed tissues. In addition, genetically modified plants with increased tolerance to high tissue Zn concentrations could be used to increase Zn concentrations in all edible produce and, thereby, increase dietary Zn intakes.

Fusicoccin counteracts the toxic effect of cadmium on the growth of maize coleoptile segments

The effects of cadmium (Cd; 0.1-1000 μM) and fusicoccin (FC) on growth, Cd(2+) content, and membrane potential (E(m)) in maize coleoptile segments were studied. In addition, the E(m) changes and accumulation of Cd and calcium (Ca) in coleoptile segments treated with Cd(2+) combined with 1 μM FC or 30 mM tetraethylammonium (TEA) chloride (K(+)-channel blocker) were also determined. In this study, the effects of Ca(2+)-channel blockers [lanthanum (La) and verapamil (Ver)] on growth and content of Cd(2+) and Ca(2+) in coleoptile segments were also investigated. It was found that Cd at high concentrations (100 and 1000 μM) significantly inhibited endogenous growth of coleoptile segments and simultaneously measured proton extrusion. FC combined with Cd(2+) counteracted the toxic effect of Cd(2+) on endogenous growth and significantly decreased Cd(2+) content (not the case for Cd(2+) at the highest concentration) in coleoptile segments. Addition of Cd to the control medium caused depolarization of E (m), the extent of which was dependent on Cd concentration and time of treatment with Cd(2+). Hyperpolarization of E(m) induced by FC was suppressed in the presence of Cd(2+) at 1000 μM but not Cd(2+) at 100 μM. It was also found that treatment of maize coleoptile segments with 30 mM TEA chloride caused hyperpolarization of E (m) and decreased Cd(2+) content in coleoptile segments, suggesting that, in the same way as for FC, accumulation of Cd(2+) was dependent on plasma membrane (PM) hyperpolarization. Similar to FC, TEA chloride also decreased Ca(2+) content in coleoptile segments. La and Ver combined with Cd(2+) (100 μM) significantly decreased Cd content in maize coleoptile segments, but only La completely abolished the toxic effect of Cd(2+) on endogenous growth and growth in the presence of FC. Taken together, these results suggest that the mechanism by which FC counteracts the toxic effect of Cd(2+) (except at 1000 μM Cd(2+)) on the growth of maize coleoptile segments involves both stimulation of PM H(+)-ATPase activity by FC as well as Cd(2+)-permeable, voltage-dependent Ca channels, which are blocked by FC and TEA chloride-induced PM hyperpolarization.