The Arabidopsis cax1 mutant exhibits impaired ion homeostasis, development, and hormonal responses and reveals interplay among vacuolar transporters

Heterodimerization of Arabidopsis calcium/proton exchangers contributes to regulation of guard cell dynamics and plant defense responses

Arabidopsis thaliana cation exchangers (CAX1 and CAX3) are closely related tonoplast-localized calcium/proton (Ca2+/H+) antiporters that contribute to cellular Ca2+ homeostasis. CAX1 and CAX3 were previously shown to interact in yeast; however, the function of this complex in plants has remained elusive. Here, we demonstrate that expression of CAX1 and CAX3 occurs in guard cells. Additionally, CAX1 and CAX3 are co-expressed in mesophyll tissue in response to wounding or flg22 treatment, due to the induction of CAX3 expression. Having shown that the transporters can be co-expressed in the same cells, we demonstrate that CAX1 and CAX3 can form homomeric and heteromeric complexes in plants. Consistent with the formation of a functional CAX1-CAX3 complex, CAX1 and CAX3 integrated into the yeast genome suppressed a Ca2+-hypersensitive phenotype of mutants defective in vacuolar Ca2+ transport, and demonstrated enzyme kinetics different from those of either CAX protein expressed by itself. We demonstrate that the interactions between CAX proteins contribute to the functioning of stomata, because stomata were more closed in cax1-1, cax3-1, and cax1-1/cax3-1 loss-of-function mutants due to an inability to buffer Ca2+ effectively. We hypothesize that the formation of CAX1-CAX3 complexes may occur in the mesophyll to affect intracellular Ca2+ signaling during defense responses.

Ion Transport at the Vacuole during Stomatal Movements

Recent research on vacuolar ion channels, transporters, and pumps of Arabidopsis highlight their function and roles in stomatal opening and closure.

Molecular Evolution of Grass Stomata

Grasses began to diversify in the late Cretaceous Period and now dominate more than one third of global land area, including three-quarters of agricultural land. We hypothesize that their success is likely attributed to the evolution of highly responsive stomata capable of maximizing productivity in rapidly changing environments. Grass stomata harness the active turgor control mechanisms present in stomata of more ancient plant lineages, maximizing several morphological and developmental features to ensure rapid responses to environmental inputs. The evolutionary development of grass stomata appears to have been a gradual progression. Therefore, understanding the complex structures, developmental events, regulatory networks, and combinations of ion transporters necessary to drive rapid stomatal movement may inform future efforts towards breeding new crop varieties.

The Rice High-Affinity K+ Transporter OsHKT2;4 Mediates Mg2+ Homeostasis under High-Mg2+ Conditions in Transgenic Arabidopsis

Rice (Oryza sativa; background Nipponbare) contains nine HKT (high-affinity K⁺ transport)-like genes encoding membrane proteins belonging to the superfamily of Ktr/TRK/HKT. OsHKTs have been proposed to include four selectivity filter-pore-forming domains homologous to the bacterial K⁺ channel KcsA, and are separated into OsHKT1s with Na⁺-selective activity and OsHKT2s with Na⁺-K⁺ symport activity. As a member of the OsHKT2 subfamily, OsHKT2;4 renders Mg²⁺ and Ca²⁺ permeability for yeast cells and Xenopus laevis oocytes, besides K⁺ and Na⁺. However, physiological functions related to Mg²⁺in planta have not yet been identified. Here we report that OsHKT2;4 from rice (O. sativa; background Nipponbare) functions as a low-affinity Mg²⁺ transporter to mediate Mg²⁺ homeostasis in plants under high-Mg²⁺ environments. Using the functional complementation assay in Mg²⁺-uptake deficient Salmonella typhimurium strains MM281 and electrophysiological analysis in X. laevis oocytes, we found that OsHKT2;4 could rescue the growth of MM281 in Mg²⁺-deficient conditions and induced the Mg²⁺ currents in oocytes at millimolar range of Mg²⁺. Additionally, overexpression of OsHKT2;4 to Arabidopsis mutant lines with a knockout of AtMGT6, a gene encoding the transporter protein necessary for Mg²⁺ adaptation in Arabidopsis, caused the Mg²⁺ toxicity to the leaves under the high-Mg²⁺ stress, but not under low-Mg²⁺ environments. Moreover, this Mg²⁺ toxicity symptom resulted from the excessive Mg²⁺ translocation from roots to shoots, and was relieved by the increase in supplemental Ca²⁺. Together, our results demonstrated that OsHKT2;4 is a low-affinity Mg²⁺ transporter responsible for Mg²⁺ transport to aerials in plants under high-Mg²⁺ conditions.

CCX1, a Putative Cation/Ca2+ Exchanger, Participates in Regulation of Reactive Oxygen Species Homeostasis and Leaf Senescence

The major developmental significance of leaf senescence is the massive recycling of nutrients from senescing leaves to nascent organs, including seeds, to meet the requirement of their rapid development, so-called nutrient remobilization. The efficiency of nutrient remobilization is associated with the activity of diverse transporters. A large number of transporters are up-regulated during leaf senescence in Arabidopsis, many of which participate in regulating leaf senescence via different signaling pathways. Here, we report that a member of the cation/Ca²⁺ exchanger family, CCX1, is highly induced during leaf senescence in Arabidopsis. Although single mutation of CCX1 did not change the senescence phenotype, double mutation of CCX1 and CCX4 resulted in a subtle but significant stay-green phenotype during natural and dark-induced leaf senescence, suggesting that some members of the cation/Ca²⁺ exchanger family act redundantly in mediating leaf senescence. Consistently, overexpression of CCX1 accelerated leaf senescence. Moreover, the ccx1ccx4 double mutant was more tolerant to H₂O₂, whereas CCX1-overexpressing lines showed an elevated response to H₂O₂ treatment, presumably due to an overaccumulation of reactive oxygen species (ROS), indicating that CCX1 may promote leaf senescence via modulating ROS homeostasis. Notably, both ccx1-1 and ccx1ccx4 were sensitive to Ca²⁺ deprivation, implying that CCX1 may also be involved in modulating Ca²⁺ signaling and consequently affecting the initiation of leaf senescence.

A Putative Chloroplast-Localized Ca(2+)/H(+) Antiporter CCHA1 Is Involved in Calcium and pH Homeostasis and Required for PSII Function in Arabidopsis

Calcium is important for chloroplast, not only in its photosynthetic but also nonphotosynthetic functions. Multiple Ca(2+)/H(+) transporters and channels have been described and studied in the plasma membrane and organelle membranes of plant cells; however, the molecular identity and physiological roles of chloroplast Ca(2+)/H(+) antiporters have remained unknown. Here we report the identification and characterization of a member of the UPF0016 family, CCHA1 (a chloroplast-localized potential Ca(2+)/H(+) antiporter), in Arabidopsis thaliana. We observed that the ccha1 mutant plants developed pale green leaves and showed severely stunted growth along with impaired photosystem II (PSII) function. CCHA1 localizes to the chloroplasts, and the levels of the PSII core subunits and the oxygen-evolving complex were significantly decreased in the ccha1 mutants compared with the wild type. In high Ca(2+) concentrations, Arabidopsis CCHA1 partially rescued the growth defect of yeast gdt1Δ null mutant, which is defective in a Ca(2+)/H(+) antiporter. The ccha1 mutant plants also showed significant sensitivity to high concentrations of CaCl2 and MnCl2, as well as variation in pH. Taken these results together, we propose that CCHA1 might encode a putative chloroplast-localized Ca(2+)/H(+) antiporter with critical functions in the regulation of PSII and in chloroplast Ca(2+) and pH homeostasis in Arabidopsis.

Vcx1-D1 (M383I), the Vcx1 mutant with a calcineurin-independent vacuolar Ca(2+)/H(+) exchanger activity, confers calcineurin-independent Mn(2+) tolerance in Saccharomyces cerevisiae

The Vcx1-M1 mutant is known to confer calcineurin-dependent Mn(2+) tolerance in budding yeast. Here, we demonstrate that another Vcx1 mutant, Vcx1-D1 with calcineurin-independent vacuolar Ca(2+)/H(+) exchanger activity, confers calcineurin-independent Mn(2+) tolerance. Unlike Vcx1-M1, the Mn(2+) tolerance conferred by Vcx1-D1 is dependent on the presence of Pmr1 or Pmc1. The Pmr1-dependent Mn(2+) tolerance of Vcx1-D1 requires the presence of calcineurin but not the functioning of the Ca(2+)/calcineurin signaling pathway. Similar to the wild-type Vcx1, C-terminally green fluorescent protein tagged Vcx1-D1 and Vcx1-M1 mutants localize to the endoplasmic reticulum instead of its normal vacuolar destination, but they remain functional in Ca(2+) sensitivity and Mn(2+) tolerance.

Ca(2+) -regulated and diurnal rhythm-regulated Na(+) /Ca(2+) exchanger AtNCL affects flowering time and auxin signalling in Arabidopsis

Calcium (Ca(2+) ) is vital for plant growth, development, hormone response and adaptation to environmental stresses, yet the mechanisms regulating plant cytosolic Ca(2+) homeostasis are not fully understood. Here, we characterize an Arabidopsis Ca(2+) -regulated Na(+) /Ca(2+) exchanger AtNCL that regulates Ca(2+) and multiple physiological processes. AtNCL was localized to the tonoplast in yeast and plant cells. AtNCL appeared to mediate sodium (Na(+) ) vacuolar sequestration and meanwhile Ca(2+) release. The EF-hand domains within AtNCL regulated Ca(2+) binding and transport of Ca(2+) and Na(+) . Plants with diminished AtNCL expression were more tolerant to high CaCl2 but more sensitive to both NaCl and auxin; heightened expression of AtNCL rendered plants more sensitive to CaCl2 but tolerant to NaCl. AtNCL expression appeared to be regulated by the diurnal rhythm and suppressed by auxin. DR5::GUS expression and root responses to auxin were altered in AtNCL mutants. The auxin-induced suppression of AtNCL was attenuated in SLR/IAA14 and ARF6/8 mutants. The mutants with altered AtNCL expression also altered flowering time and FT and CO expression; FT may mediate AtNCL-regulated flowering time change. Therefore, AtNCL is a vacuolar Ca(2+) -regulated Na(+) /Ca(2+) exchanger that regulates auxin responses and flowering time.

Cloning and characterization of a Ca(2+)/H(+) exchanger from the halophyte Salicornia europaea L

The calcium ion (Ca(2+)), which functions as a second messenger, plays an important role in plants' responses to various abiotic stresses, and Ca(2+)/H(+) exchangers (CAXs) are an important part of this process. In this study, we isolated and characterized a putative Ca(2+)/H(+) exchanger gene (SeCAX3) from Salicornia europaea L., a succulent, leafless euhalophyte. The SeCAX3 open reading frame was 1368 bp long and encoded a 455-amino-acid polypeptide that showed 67.9% similarity to AtCAX3. SeCAX3 was expressed in the shoots and roots of S. europaea. Expression of SeCAX3 was up-regulated by Ca(2+), Na(+), sorbitol, Li(+), abscisic acid, and cold treatments in shoots, but down-regulated by Ca(2+), sorbitol, abscisic acid, and cold treatments in roots. When SeCAX3 was transformed into a Ca-sensitive yeast strain, the transformed cells were able to grow in the presence of 200 mM Ca(2+). Furthermore, SeCAX3 conferred drought, salt, and cold tolerance in yeast. Compared with the control strains, the yeast transformants expressing SeCAX3 were able to grow well in the presence of 30 mM Li(+), 150 mM Mg(2+), or 6 mM Ba(2+). These results showed that the expression of SeCAX3 in yeast suppressed its Ca(2+) hypersensitivity and conferred tolerance to Mg(2+) and Ba(2+). Together, these findings suggest that SeCAX3 might be a Ca(2+) transporter that plays a role in regulating cation tolerance and the responses of S. europaea to various abiotic stresses.

Low temperature tolerance in plants: Changes at the protein level

Low temperature (LT) is one of several important environmental stresses influencing plant performance and distribution. Adaptation to LT is a highly dynamic stress-response phenomenon and involves complex cross-talk between different regulatory levels. Although plants differ in their sensitivity to LT, in temperate species low nonfreezing temperatures cause noticeable alterations in various biochemical and physiological processes that can potentially improve freezing tolerance. This adaptation is associated with changes in the expression pattern of genes and their protein products. Proteins are the major players in most cellular events and are directly involved in plant LT responses, thereby proteome analysis could help uncover additional novel proteins associated with LT tolerance. Proteomics is recommended as an appropriate strategy for complementing transcriptome level changes and characterizing translational and post-translational regulations. In this review, we considered alterations in the expression and accumulation of proteins in response to LT stress in the three major cereal crops produced worldwide (wheat, barley, and rice). LT stress down-regulates many photosynthesis-related proteins. On the contrary, pathways/protein sets that are up-regulated by LT include carbohydrate metabolism (ATP formation), ROS scavenging, redox adjustment, cell wall remodelling, cytoskeletal rearrangements, cryoprotection, defence/detoxification. These modifications are common adaptation reactions also observed in the plant model Arabidopsis, thus representing key potential biomarkers and critical intervention points for improving LT tolerance of crop plants in cold regions with short summers. We believe that an assessment of the proteome within a broad time frame and during the different phenological stages may disclose the molecular mechanisms related to the developmental regulation of LT tolerance and facilitate the progress of genetically engineered stress-resistant plant varieties.

CATION EXCHANGER1 Cosegregates with Cadmium Tolerance in the Metal Hyperaccumulator Arabidopsis halleri and Plays a Role in Limiting Oxidative Stress in Arabidopsis Spp

Arabidopsis halleri is a model species for the study of plant adaptation to extreme metallic conditions. In this species, cadmium (Cd) tolerance seems to be constitutive, and the mechanisms underlying the trait are still poorly understood. A previous quantitative trait loci (QTL) analysis performed on A. halleri × Arabidopsis lyrata backcross population1 identified the metal-pump gene Heavy Metal ATPase4 as the major genetic determinant for Cd tolerance. However, although necessary, Heavy Metal ATPase4 alone is not sufficient for determining this trait. After fine mapping, a gene encoding a calcium(2+)/hydrogen(+) antiporter, cation/hydrogen(+) exchanger1 (CAX1), was identified as a candidate gene for the second QTL of Cd tolerance in A. halleri. Backcross population1 individuals displaying the A. halleri allele for the CAX1 locus exhibited significantly higher CAX1 expression levels compared with the ones with the A. lyrata allele, and a positive correlation between CAX1 expression and Cd tolerance was observed. Here, we show that this QTL is conditional and that it is only detectable at low external Ca concentration. CAX1 expression in both roots and shoots was higher in A. halleri than in the close Cd-sensitive relative species A. lyrata and Arabidopsis thaliana. Moreover, CAX1 loss of function in A. thaliana led to higher Cd sensitivity at low concentration of Ca, higher sensitivity to methylviologen, and stronger accumulation of reactive oxygen species after Cd treatment. Overall, this study identifies a unique genetic determinant of Cd tolerance in the metal hyperaccumulator A. halleri and offers a new twist for the function of CAX1 in plants.

Magnesium stress signaling in plant: just a beginning

Magnesium (Mg) is one of the most important nutrients, involves mainly in plant growth and development. However, the signaling pathways response to magnesium stresses (MgSs) is known little, but several studies lately may improve the research development. Several phytohormones such as abscisic acid (ABA), ethylene, auxins, and their factors were found responding to MgSs, and there may be some signal pathways linking these hormones and downstream reactions together, e.g., carbon fixation and transfer, starch and sugar metabolism, ion uptaking and reactive oxygen species (ROS) increasing. Consequently, Arabidopsis morphogenesis is remodeled. In this review, we mainly discussed recent literatures involving in plant response to MgSs (Mg deficiency (MgD) and Mg toxicity (MgT)), including plant morphogenesis remodeling, magnesium transporters and signaling transductions. Moreover, the future study challenges related to the responding signalings to MgSs in plants are also presented. Regardless, the iceberg of signal transduction of MgSs in plants is appeared.

Genetical and comparative genomics of Brassica under altered Ca supply identifies Arabidopsis Ca-transporter orthologs

Although Ca transport in plants is highly complex, the overexpression of vacuolar Ca(2+) transporters in crops is a promising new technology to improve dietary Ca supplies through biofortification. Here, we sought to identify novel targets for increasing plant Ca accumulation using genetical and comparative genomics. Expression quantitative trait locus (eQTL) mapping to 1895 cis- and 8015 trans-loci were identified in shoots of an inbred mapping population of Brassica rapa (IMB211 × R500); 23 cis- and 948 trans-eQTLs responded specifically to altered Ca supply. eQTLs were screened for functional significance using a large database of shoot Ca concentration phenotypes of Arabidopsis thaliana. From 31 Arabidopsis gene identifiers tagged to robust shoot Ca concentration phenotypes, 21 mapped to 27 B. rapa eQTLs, including orthologs of the Ca(2+) transporters At-CAX1 and At-ACA8. Two of three independent missense mutants of BraA.cax1a, isolated previously by targeting induced local lesions in genomes, have allele-specific shoot Ca concentration phenotypes compared with their segregating wild types. BraA.CAX1a is a promising target for altering the Ca composition of Brassica, consistent with prior knowledge from Arabidopsis. We conclude that multiple-environment eQTL analysis of complex crop genomes combined with comparative genomics is a powerful technique for novel gene identification/prioritization.

Genome-wide expressional and functional analysis of calcium transport elements during abiotic stress and development in rice

Ca²⁺ homeostasis is required to maintain a delicate balance of cytosolic Ca²⁺ during normal and adverse growth conditions. Various Ca²⁺ transporters actively participate to maintain this delicate balance especially during abiotic stresses and developmental events in plants. In this study, we present a genome-wide account, detailing expression profiles, subcellular localization and functional analysis of rice Ca²⁺ transport elements. Exhaustive in silico data mining and analysis resulted in the identification of 81 Ca²⁺ transport element genes, which belong to various groups such as Ca²⁺-ATPases (pumps), exchangers, channels, glutamate receptor homologs and annexins. Phylogenetic analysis revealed that different Ca²⁺ transporters are evolutionarily conserved across different plant species. Comprehensive expression analysis by gene chip microarray and quantitative RT-PCR revealed that a substantial proportion of Ca²⁺ transporter genes were expressed differentially under abiotic stresses (salt, cold and drought) and reproductive developmental stages (panicle and seed) in rice. These findings suggest a possible role of rice Ca²⁺ transporters in abiotic stress and development triggered signaling pathways. Subcellular localization of Ca²⁺ transporters from different groups in Nicotiana benthamiana revealed their variable localization to different compartments, which could be their possible sites of action. Complementation of Ca²⁺ transport activity of K616 yeast mutant by Ca²⁺-ATPase OsACA7 and involvement in salt tolerance verified its functional behavior. This study will encourage detailed characterization of potential candidate Ca²⁺ transporters for their functional role in planta.

The K+/H+ antiporter LeNHX2 increases salt tolerance by improving K+ homeostasis in transgenic tomato

The endosomal LeNHX2 ion transporter exchanges H(+) with K(+) and, to lesser extent, Na(+) . Here, we investigated the response to NaCl supply and K(+) deprivation in transgenic tomato (Solanum lycopersicum L.) overexpressing LeNHX2 and show that transformed tomato plants grew better in saline conditions than untransformed controls, whereas in the absence of K(+) the opposite was found. Analysis of mineral composition showed a higher K(+) content in roots, shoots and xylem sap of transgenic plants and no differences in Na(+) content between transgenic and untransformed plants grown either in the presence or the absence of 120 mm NaCl. Transgenic plants showed higher Na(+)/H(+) and, above all, K(+)/H(+) transport activity in root intracellular membrane vesicles. Under K(+) limiting conditions, transgenic plants enhanced root expression of the high-affinity K(+) uptake system HAK5 compared to untransformed controls. Furthermore, tomato overexpressing LeNHX2 showed twofold higher K(+) depletion rates and half cytosolic K(+) activity than untransformed controls. Under NaCl stress, transgenic plants showed higher uptake velocity for K(+) and lower cytosolic K(+) activity than untransformed plants. These results indicate the fundamental role of K(+) homeostasis in the better performance of LeNHX2 overexpressing tomato under NaCl stress.

Calcium: The Missing Link in Auxin Action

Due to their sessile lifestyles, plants need to deal with the limitations and stresses imposed by the changing environment. Plants cope with these by a remarkable developmental flexibility, which is embedded in their strategy to survive. Plants can adjust their size, shape and number of organs, bend according to gravity and light, and regenerate tissues that were damaged, utilizing a coordinating, intercellular signal, the plant hormone, auxin. Another versatile signal is the cation, Ca(2+), which is a crucial second messenger for many rapid cellular processes during responses to a wide range of endogenous and environmental signals, such as hormones, light, drought stress and others. Auxin is a good candidate for one of these Ca(2+)-activating signals. However, the role of auxin-induced Ca(2+) signaling is poorly understood. Here, we will provide an overview of possible developmental and physiological roles, as well as mechanisms underlying the interconnection of Ca(2+) and auxin signaling.

A KH-domain RNA-binding protein interacts with FIERY2/CTD phosphatase-like 1 and splicing factors and is important for pre-mRNA splicing in Arabidopsis

Eukaryotic genomes encode hundreds of RNA-binding proteins, yet the functions of most of these proteins are unknown. In a genetic study of stress signal transduction in Arabidopsis, we identified a K homology (KH)-domain RNA-binding protein, HOS5 (High Osmotic Stress Gene Expression 5), as required for stress gene regulation and stress tolerance. HOS5 was found to interact with FIERY2/RNA polymerase II (RNAP II) carboxyl terminal domain (CTD) phosphatase-like 1 (FRY2/CPL1) both in vitro and in vivo. This interaction is mediated by the first double-stranded RNA-binding domain of FRY2/CPL1 and the KH domains of HOS5. Interestingly, both HOS5 and FRY2/CPL1 also interact with two novel serine-arginine (SR)-rich splicing factors, RS40 and RS41, in nuclear speckles. Importantly, FRY2/CPL1 is required for the recruitment of HOS5. In fry2 mutants, HOS5 failed to be localized in nuclear speckles but was found mainly in the nucleoplasm. hos5 mutants were impaired in mRNA export and accumulated a significant amount of mRNA in the nuclei, particularly under salt stress conditions. Arabidopsis mutants of all these genes exhibit similar stress-sensitive phenotypes. RNA-seq analyses of these mutants detected significant intron retention in many stress-related genes under salt stress but not under normal conditions. Our study not only identified several novel regulators of pre-mRNA processing as important for plant stress response but also suggested that, in addition to RNAP II CTD that is a well-recognized platform for the recruitment of mRNA processing factors, FRY2/CPL1 may also recruit specific factors to regulate the co-transcriptional processing of certain transcripts to deal with environmental challenges.

Pre-mRNA processing, including 5′ capping, splicing, and 3′ polyadenylation, is critical for gene expression and is closely coupled with transcription. Phosphorylated carboxyl terminal domain (CTD) of RNA Polymerase II (RNAP II) serves as a platform for the recruitment of pre-mRNA processing factors, yet other components involved in the recruitment are less known. In a genetic study of stress signal transduction in Arabidopsis, we isolated a KH-domain RNA-binding protein HOS5 that plays important roles in stress gene regulation and stress tolerance. We found that HOS5 interacts with FIERY2/CTD phosphatase-like 1 (FRY2/CPL1) and they both also interact with two novel splicing factors, RS40 and RS41, in nuclear speckles. In fry2 mutants, HOS5 was unable to be recruited to nuclear speckles but rather was mainly localized in the nucleoplasm. Mutants in these genes have similar stress-sensitive phenotypes. Transcriptome analyses identified significant intron retention in many stress-related genes in these mutants under salt stress conditions. Our study reveals that, in addition to RNAP II, the CTD phosphatase may also recruit specific splicing factors and RNA binding proteins to regulate the co-transcriptional processing of certain transcripts to deal with environmental stresses.

NO₃⁻/H⁺ antiport in the tonoplast of cucumber root cells is stimulated by nitrate supply: evidence for a reversible nitrate-induced phosphorylation of vacuolar NO₃⁻/H⁺ antiport

Studies in the last few years have shed light on the process of nitrate accumulation within plant cells, achieving molecular identification and partial characterization of the genes and proteins involved in this process. However, contrary to the plasma membrane-localized nitrate transport activities, the kinetics of active nitrate influx into the vacuole and its adaptation to external nitrate availability remain poorly understood. In this work, we have investigated the activity and regulation of the tonoplast-localized H⁺/NO₃⁻ antiport in cucumber roots in response to N starvation and NO₃⁻ induction. The time course of nitrate availability strongly influenced H⁺/NO₃⁻ antiport activity at the tonoplast of root cells. However, under N starvation active nitrate accumulation within the vacuole still occurred. Hence, either a constitutive H⁺-coupled transport system specific for nitrate operates at the tonoplast, or nitrate uses another transport protein of broader specificity to different anions to enter the vacuole via a proton-dependent process. H⁺/NO₃⁻ antiport in cucumber was significantly stimulated in NO₃⁻-induced plants that were supplied with nitrate for 24 hours following 6-day-long N starvation. The cytosolic fraction isolated from the roots of NO₃⁻-induced plants significantly stimulated H⁺/NO₃⁻ antiport in tonoplast membranes isolated from cucumbers growing on nitrate. The stimulatory effect of the cytosolic fraction was completely abolished by EGTA and the protein kinase inhibitor staurosporine and slightly enhanced by the phosphatase inhibitors okadaic acid and cantharidin. Hence, we conclude that stimulation of H⁺/NO₃⁻ antiport at the tonoplast of cucumber roots in response to nitrate provision may occur through the phosphorylation of a membrane antiporter involving Ca-dependent, staurosporine-sensitive protein kinase.

Dissecting plant iron homeostasis under short and long-term iron fluctuations

A wealth of information on the different aspects of iron homeostasis in plants has been obtained during the last decade. However, there is no clear road-map integrating the relationships between the various components. The principal aim of the current review is to fill this gap. In this context we discuss the lack of low affinity iron uptake mechanisms in plants, the utilization of a different uptake mechanism by graminaceous plants compared to the others, as well as the roles of riboflavin, ferritin isoforms, nitric oxide, nitrosylation, heme, aconitase, and vacuolar pH. Cross-homeostasis between elements is also considered, with a specific emphasis on the relationship between iron homeostasis and phosphorus and copper deficiencies. As the environment is a crucial parameter for modulating plant responses, we also highlight how diurnal fluctuations govern iron metabolism. Evolutionary aspects of iron homeostasis have so far attracted little attention. Looking into the past can inform us on how long-term oxygen and iron-availability fluctuations have influenced the evolution of iron uptake mechanisms. Finally, we evaluate to what extent this homeostastic road map can be used for the development of novel biofortification strategies in order to alleviate iron deficiency in human.

Boron deficiency increases the levels of cytosolic Ca(2+) and expression of Ca(2+)-related genes in Arabidopsis thaliana roots

Boron (B) deficiency affects the expressions of genes involved in major physiological processes. However, signal transduction pathway through which plants are able to sense and transmit B-deprivation signal to the nucleus is unknown. The aim of this work was to research in Arabidopsis thaliana roots whether the short-term B deficiency affects cytosolic Ca(2+) levels ([Ca(2+)]cyt) as well as expression of genes involved in Ca(2+) signaling. To visualize in vivo changes in root [Ca(2+)]cyt, Arabidopsis seedlings expressing Yellow Cameleon (YC) 3.6 were grown in a nutrient solution supplemented with 2 μM B and then transferred to a B-free medium for 24 h. Root [Ca(2+)]cyt was clearly higher in B-deficient seedlings upon 6 and 24 h of B treatments when compared to controls. Transcriptome analyses showed that transcript levels of Ca(2+) signaling-related genes were affected by B deprivation. Interestingly, Ca(2+) channel (CNGC19, cyclic nucleotide-gated ion channel) gene was strongly upregulated as early as 6 h after B deficiency. Expression levels of Ca(2+) transporter (ACA, autoinhibited Ca(2+)-ATPase; CAX, cation exchanger) genes increased when seedlings were subjected to B deficiency. Gene expressions of calmodulin-like proteins (CMLs) and Ca(2+)-dependent protein kinases (CPKs) were also overexpressed upon exposure to B starvation. Our results suggest that B deficiency causes early responses in the expression of CNGC19 Ca(2+)-influx channel, ACA- and CAX-efflux, and Ca(2+) sensor genes to regulate Ca(2+) homeostasis. It is the first time that changes in the levels of in vivo cytosolic Ca(2+) and expression of Ca(2+) channel/transporter genes are related with short-term B deficiency in Arabidopsis roots.

The Plasmodium berghei Ca(2+)/H(+) exchanger, PbCAX, is essential for tolerance to environmental Ca(2+) during sexual development

Ca(2+) contributes to a myriad of important cellular processes in all organisms, including the apicomplexans, Plasmodium and Toxoplasma. Due to its varied and essential roles, free Ca(2+) is tightly regulated by complex mechanisms. These mechanisms are therefore of interest as putative drug targets. One pathway in Ca(2+) homeostatic control in apicomplexans uses a Ca(2+)/H(+) exchanger (a member of the cation exchanger family, CAX). The P. falciparum CAX (PfCAX) has recently been characterised in asexual blood stage parasites. To determine the physiological importance of apicomplexan CAXs, tagging and knock-out strategies were undertaken in the genetically tractable T. gondii and P. berghei parasites. In addition, a yeast heterologous expression system was used to study the function of apicomplexan CAXs. Tagging of T. gondii and P. berghei CAXs (TgCAX and PbCAX) under control of their endogenous promoters could not demonstrate measureable expression of either CAX in tachyzoites and asexual blood stages, respectively. These results were consistent with the ability of parasites to tolerate knock-outs of the genes for TgCAX and PbCAX at these developmental stages. In contrast, PbCAX expression was detectable during sexual stages of development in female gametocytes/gametes, zygotes and ookinetes, where it was dispersed in membranous networks within the cytosol (with minimal mitochondrial localisation). Furthermore, genetically disrupted parasites failed to develop further from "round" form zygotes, suggesting that PbCAX is essential for ookinete development and differentiation. This impeded phenotype could be rescued by removal of extracellular Ca(2+). Therefore, PbCAX provides a mechanism for free living parasites to multiply within the ionic microenvironment of the mosquito midgut. Ca(2+) homeostasis mediated by PbCAX is critical and suggests plasmodial CAXs may be targeted in approaches designed to block parasite transmission.

Energization of vacuolar transport in plant cells and its significance under stress

The plant vacuole is of prime importance in buffering environmental perturbations and in coping with abiotic stress caused by, for example, drought, salinity, cold, or UV. The large volume, the efficient integration in anterograde and retrograde vesicular trafficking, and the dynamic equipment with tonoplast transporters enable the vacuole to fulfill indispensible functions in cell biology, for example, transient and permanent storage, detoxification, recycling, pH and redox homeostasis, cell expansion, biotic defence, and cell death. This review first focuses on endomembrane dynamics and then summarizes the functions, assembly, and regulation of secretory and vacuolar proton pumps: (i) the vacuolar H(+)-ATPase (V-ATPase) which represents a multimeric complex of approximately 800 kDa, (ii) the vacuolar H(+)-pyrophosphatase, and (iii) the plasma membrane H(+)-ATPase. These primary proton pumps regulate the cytosolic pH and provide the driving force for secondary active transport. Carriers and ion channels modulate the proton motif force and catalyze uptake and vacuolar compartmentation of solutes and deposition of xenobiotics or secondary compounds such as flavonoids. ABC-type transporters directly energized by MgATP complement the transport portfolio that realizes the multiple functions in stress tolerance of plants.

Compensatory mutations in predicted metal transporters modulate auxin conjugate responsiveness in Arabidopsis

Levels of the phytohormone indole-3-acetic acid (IAA) can be altered by the formation and hydrolysis of IAA conjugates. The isolation and characterization of Arabidopsis thaliana mutants with reduced IAA-conjugate sensitivity and wild-type IAA responses is advancing the understanding of auxin homeostasis by uncovering the factors needed for conjugate metabolism. For example, the discovery that the IAA-Ala-resistant mutant iar1 is defective in a protein in the ZIP family of metal transporters uncovered a link between metal homeostasis and IAA-conjugate sensitivity. To uncover additional factors impacting auxin conjugate metabolism, we conducted a genetic modifier screen and isolated extragenic mutations that restored IAA-amino acid conjugate sensitivity to the iar1 mutant. One of these suppressor mutants is defective in a putative cation diffusion facilitator, MTP5 (At3g12100; formerly known as MTPc2). Loss of MTP5 function restored IAA conjugate sensitivity to iar1 but not to mutants defective in IAA-amino acid conjugate amidohydrolases. Our results are consistent with a model in which MTP5 and IAR1 transport metals in an antagonistic fashion to regulate metal homeostasis within the subcellular compartment in which the IAA-conjugate amidohydrolases reside, and support previous suggestions that the ion composition in this compartment influences hydrolase activity.

Tonoplast calcium sensors CBL2 and CBL3 control plant growth and ion homeostasis through regulating V-ATPase activity in Arabidopsis

Plant responses to developmental and environmental cues are often mediated by calcium (Ca(2+)) signals that are transmitted by diverse calcium sensors. The calcineurin B-like (CBL) protein family represents calcium sensors that decode calcium signals through specific interactions with a group of CBL-interacting protein kinases. We report functional analysis of Arabidopsis CBL2 and CBL3, two closely related CBL members that are localized to the vacuolar membrane through the N-terminal tonoplast-targeting sequence. While cbl2 or cbl3 single mutant did not show any phenotypic difference from the wild type, the cbl2 cbl3 double mutant was stunted with leaf tip necrosis, underdeveloped roots, shorter siliques and fewer seeds. These defects were reminiscent of those in the vha-a2 vha-a3 double mutant deficient in vacuolar H(+)-ATPase (V-ATPase). Indeed, the V-ATPase activity was reduced in the cbl2 cbl3 double mutant, connecting tonoplast CBL-type calcium sensors to the regulation of V-ATPase. Furthermore, cbl2 cbl3 double mutant was compromised in ionic tolerance and micronutrient accumulation, consistent with the defect in V-ATPase activity that has been shown to function in ion compartmentalization. Our results suggest that calcium sensors CBL2 and CBL3 serve as molecular links between calcium signaling and V-ATPase, a central regulator of intracellular ion homeostasis.

Regulation of the major vacuolar Ca²⁺ transporter genes, by intercellular Ca²⁺ concentration and abiotic stresses, in tip-burn resistant Brassica oleracea

Calcium is an essential plant macronutrient that has unique structural and signaling roles related to tip-burn disorder in Brassica spp. crops. For two types of cabbage inbred lines, tip-burn susceptible and resistant, we measured and compared major macronutrient cations, including Ca(2+), in leaves. In both lines, Ca(2+), Mg(2+), Na(+), and K(+), accumulated more in leaf base than in leaf apex. Ca(2+) and K(+) were >2 times more abundant in the tip-burn resistant line, while Na(+) was higher in the susceptible line. Ca(2+) differences between the two lines resulted from differential accumulation of calcium into cell vacuoles. We profiled major vacuolar Ca(2+) transporters, in both cabbage lines, by growth time and intercellular Ca(2+) concentration. Expression pattern of several Ca(2+) transporter genes differed between tip-burn susceptible and resistant lines by growth time points. We also identified promoter regions of the major Ca(2+) vacuole transporter genes, CAX1, ACA4, and ACA11, which displayed hormonal, light and defense-related cis-acting regulatory elements. Finally, transporter genes in the two cabbage lines responded differently to abiotic stresses, demonstrating diversity in gene regulation among orthologous genes.

Vacuolar CAX1 and CAX3 influence auxin transport in guard cells via regulation of apoplastic pH

CATION EXCHANGERs CAX1 and CAX3 are vacuolar ion transporters involved in ion homeostasis in plants. Widely expressed in the plant, they mediate calcium transport from the cytosol to the vacuole lumen using the proton gradient across the tonoplast. Here, we report an unexpected role of CAX1 and CAX3 in regulating apoplastic pH and describe how they contribute to auxin transport using the guard cell's response as readout of hormone signaling and cross talk. We show that indole-3-acetic acid (IAA) inhibition of abscisic acid (ABA)-induced stomatal closure is impaired in cax1, cax3, and cax1/cax3. These mutants exhibited constitutive hypopolarization of the plasma membrane, and time-course analyses of membrane potential revealed that IAA-induced hyperpolarization of the plasma membrane is also altered in these mutants. Both ethylene and 1-naphthalene acetic acid inhibited ABA-triggered stomatal closure in cax1, cax3, and cax1/cax3, suggesting that auxin signaling cascades were functional and that a defect in IAA transport caused the phenotype of the cax mutants. Consistent with this finding, chemical inhibition of AUX1 in wild-type plants phenocopied the cax mutants. We also found that cax1/cax3 mutants have a higher apoplastic pH than the wild type, further supporting the hypothesis that there is a defect in IAA import in the cax mutants. Accordingly, we were able to fully restore IAA inhibition of ABA-induced stomatal closure in cax1, cax3, and cax1/cax3 when stomatal movement assays were carried out at a lower extracellular pH. Our results suggest a network linking the vacuolar cation exchangers to apoplastic pH maintenance that plays a crucial role in cellular processes.

Knockout of multiple Arabidopsis cation/H(+) exchangers suggests isoform-specific roles in metal stress response, germination and seed mineral nutrition

Cation/H⁺ exchangers encoded by CAX genes play an important role in the vacuolar accumulation of metals including Ca²⁺ and Mn²⁺. Arabidopsis thaliana CAX1 and CAX3 have been previously shown to differ phylogenetically from CAX2 but the physiological roles of these different transporters are still unclear. To examine the functions and the potential of redundancy between these three cation transporters, cax1/cax2 and cax2/cax3 double knockout mutants were generated and compared with wild type and cax single knockouts. These double mutants had equivalent metal stress responses to single cax mutants. Both cax1 and cax1/cax2 had increased tolerance to Mg stress, while cax2 and cax2/cax3 both had increased sensitivity to Mn stress. The cax1/cax2 and cax2/cax3 mutants did not exhibit the deleterious developmental phenotypes previously seen with the cax1/cax3 mutant. However, these new double mutants did show alterations in seed germination, specifically a delay in germination time. These alterations correlated with changes in nutrient content within the seeds of the mutants, particularly the cax1/cax2 mutant which had significantly higher seed content of Ca and Mn. This study indicates that the presence of these Arabidopsis CAX transporters is important for normal germination and infers a role for CAX proteins in metal homeostasis within the seed.

Overexpression of SlSOS2 (SlCIPK24) confers salt tolerance to transgenic tomato

The Ca(2+)-dependent SOS pathway has emerged as a key mechanism in the homeostasis of Na(+) and K(+) under saline conditions. We have identified and functionally characterized the gene encoding the calcineurin-interacting protein kinase of the SOS pathway in tomato, SlSOS2. On the basis of protein sequence similarity and complementation studies in yeast and Arabidopsis, it can be concluded that SlSOS2 is the functional tomato homolog of Arabidopsis AtSOS2 and that SlSOS2 operates in a tomato SOS signal transduction pathway. The biotechnological potential of SlSOS2 to provide salt tolerance was evaluated by gene overexpression in tomato (Solanum lycopersicum L. cv. MicroTom). The better salt tolerance of transgenic plants relative to non-transformed tomato was shown by their faster relative growth rate, earlier flowering and higher fruit production when grown with NaCl. The increased salinity tolerance of SlSOS2-overexpressing plants was associated with higher sodium content in stems and leaves and with the induction and up-regulation of the plasma membrane Na(+)/H(+) (SlSOS1) and endosomal-vacuolar K(+), Na(+)/H(+) (LeNHX2 and LeNHX4) antiporters, responsible for Na(+) extrusion out of the root, active loading of Na(+) into the xylem, and Na(+) and K(+) compartmentalization.

Alteration of the bZIP60/IRE1 pathway affects plant response to ER stress in Arabidopsis thaliana

The Unfolded Protein Response (UPR) is elicited under cellular and environmental stress conditions that disrupt protein folding in the endoplasmic reticulum (ER). Through the transcriptional induction of genes encoding ER resident chaperones and proteins involved in folding, the pathway contributes to alleviating ER stress by increasing the folding capacity in the ER. Similarly to other eukaryotic systems, one arm of the UPR in Arabidopsis is set off by a non-conventional splicing event mediated by ribonuclease kinase IRE1b. The enzyme specifically targets mature bZIP60 RNA for cleavage, which results in a novel splice variant encoding a nuclear localized transcription factor. Although it is clear that this molecular switch widely affects the transcriptome, its exact role in overall plant response to stress has not been established and mutant approaches have not provided much insight. In this study, we took a transgenic approach to manipulate the pathway in positive and negative fashions. Our data show that the ER-resident chaperone BiP accumulates differentially depending on the level of activation of the pathway. In addition, phenotypes of the transgenic lines suggest that BiP accumulation is positively correlated with plant tolerance to chronic ER stress.

Arabidopsis sodium dependent and independent phenotypes triggered by H⁺-PPase up-regulation are SOS1 dependent

Coordinate regulation of transporters at both the plasma membrane and vacuole contribute to plant cell's ability to adapt to a changing environment and play a key role in the maintenance of the chemiosmotic circuits required for cellular growth. The plasma membrane (PM) Na⁺/H⁺ antiporter (SOS1) is involved in salt tolerance, presumably in sodium extrusion; the vacuolar type I H⁺-PPase AVP1 is involved in vacuolar sodium sequestration, but its overexpression has also been shown to alter the abundance and activity of the PM H⁺-ATPase. Here we investigate the relationship between these transporters utilizing loss-of-function mutants of SOS1 (sos1) and increased expression of AVP1 (AVP1OX). Heightened expression of AVP1 enhances pyrophosphate-dependent proton pump activity, salt tolerance, ion vacuolar sequestration, K⁺ uptake capacity, root hair development, osmotic responses, and PM ATPase hydrolytic and proton pumping activities. In sos1 lines overexpressing AVP1, these phenotypes are negatively affected demonstrating that sos1 is epistatic to AVP1. Enhanced AVP1 protein levels require SOS1 and this regulation appears to be post-translational.

The role of CAX1 and CAX3 in elemental distribution and abundance in Arabidopsis seed

The ability to alter nutrient partitioning within plants cells is poorly understood. In Arabidopsis (Arabidopsis thaliana), a family of endomembrane cation exchangers (CAXs) transports Ca(2+) and other cations. However, experiments have not focused on how the distribution and partitioning of calcium (Ca) and other elements within seeds are altered by perturbed CAX activity. Here, we investigate Ca distribution and abundance in Arabidopsis seed from cax1 and cax3 loss-of-function lines and lines expressing deregulated CAX1 using synchrotron x-ray fluorescence microscopy. We conducted 7- to 10-μm resolution in vivo x-ray microtomography on dry mature seed and 0.2-μm resolution x-ray microscopy on embryos from lines overexpressing deregulated CAX1 (35S-sCAX1) and cax1cax3 double mutants only. Tomograms showed an increased concentration of Ca in both the seed coat and the embryo in cax1, cax3, and cax1cax3 lines compared with the wild type. High-resolution elemental images of the mutants showed that perturbed CAX activity altered Ca partitioning within cells, reducing Ca partitioning into organelles and/or increasing Ca in the cytosol and abolishing tissue-level Ca gradients. In comparison with traditional volume-averaged metal analysis, which confirmed subtle changes in seed elemental composition, the collection of spatially resolved data at varying resolutions provides insight into the impact of altered CAX activity on seed metal distribution and indicates a cell type-specific function of CAX1 and CAX3 in partitioning Ca into organelles. This work highlights a powerful technology for inferring transport function and quantifying nutrient changes.

Vacuolar transporters in their physiological context

Vacuoles in vegetative tissues allow the plant surface to expand by accumulating energetically cheap inorganic osmolytes, and thereby optimize the plant for absorption of sunlight and production of energy by photosynthesis. Some specialized cells, such as guard cells and pulvini motor cells, exhibit rapid volume changes. These changes require the rapid release and uptake of ions and water by the vacuole and are a prerequisite for plant survival. Furthermore, seed vacuoles are important storage units for the nutrients required for early plant development. All of these fundamental processes rely on numerous vacuolar transporters. During the past 15 years, the transporters implicated in most aspects of vacuolar function have been identified and characterized. Vacuolar transporters appear to be integrated into a regulatory network that controls plant metabolism. However, little is known about the mode of action of these fundamental processes, and deciphering the underlying mechanisms remains a challenge for the future.

The role of CAX1 and CAX3 in elemental distribution and abundance in Arabidopsis seed

The ability to alter nutrient partitioning within plants cells is poorly understood. In Arabidopsis (Arabidopsis thaliana), a family of endomembrane cation exchangers (CAXs) transports Ca(2+) and other cations. However, experiments have not focused on how the distribution and partitioning of calcium (Ca) and other elements within seeds are altered by perturbed CAX activity. Here, we investigate Ca distribution and abundance in Arabidopsis seed from cax1 and cax3 loss-of-function lines and lines expressing deregulated CAX1 using synchrotron x-ray fluorescence microscopy. We conducted 7- to 10-μm resolution in vivo x-ray microtomography on dry mature seed and 0.2-μm resolution x-ray microscopy on embryos from lines overexpressing deregulated CAX1 (35S-sCAX1) and cax1cax3 double mutants only. Tomograms showed an increased concentration of Ca in both the seed coat and the embryo in cax1, cax3, and cax1cax3 lines compared with the wild type. High-resolution elemental images of the mutants showed that perturbed CAX activity altered Ca partitioning within cells, reducing Ca partitioning into organelles and/or increasing Ca in the cytosol and abolishing tissue-level Ca gradients. In comparison with traditional volume-averaged metal analysis, which confirmed subtle changes in seed elemental composition, the collection of spatially resolved data at varying resolutions provides insight into the impact of altered CAX activity on seed metal distribution and indicates a cell type-specific function of CAX1 and CAX3 in partitioning Ca into organelles. This work highlights a powerful technology for inferring transport function and quantifying nutrient changes.

High Resolution Melt (HRM) analysis is an efficient tool to genotype EMS mutants in complex crop genomes

BACKGROUND

Targeted Induced Loci Lesions IN Genomes (TILLING) is increasingly being used to generate and identify mutations in target genes of crop genomes. TILLING populations of several thousand lines have been generated in a number of crop species including Brassica rapa. Genetic analysis of mutants identified by TILLING requires an efficient, high-throughput and cost effective genotyping method to track the mutations through numerous generations. High resolution melt (HRM) analysis has been used in a number of systems to identify single nucleotide polymorphisms (SNPs) and insertion/deletions (IN/DELs) enabling the genotyping of different types of samples. HRM is ideally suited to high-throughput genotyping of multiple TILLING mutants in complex crop genomes. To date it has been used to identify mutants and genotype single mutations. The aim of this study was to determine if HRM can facilitate downstream analysis of multiple mutant lines identified by TILLING in order to characterise allelic series of EMS induced mutations in target genes across a number of generations in complex crop genomes.

RESULTS

We demonstrate that HRM can be used to genotype allelic series of mutations in two genes, BraA.CAX1a and BraA.MET1.a in Brassica rapa. We analysed 12 mutations in BraA.CAX1.a and five in BraA.MET1.a over two generations including a back-cross to the wild-type. Using a commercially available HRM kit and the Lightscanner™ system we were able to detect mutations in heterozygous and homozygous states for both genes.

CONCLUSIONS

Using HRM genotyping on TILLING derived mutants, it is possible to generate an allelic series of mutations within multiple target genes rapidly. Lines suitable for phenotypic analysis can be isolated approximately 8-9 months (3 generations) from receiving M3 seed of Brassica rapa from the RevGenUK TILLING service.

Characterization of an AtCCX5 gene from Arabidopsis thaliana that involves in high-affinity K⁺ uptake and Na⁺ transport in yeast

The gene for a putative cation calcium exchanger (CCX) from Arabidopsis thaliana, AtCCX5, was cloned and its function was analyzed in yeast. Green fluorescent protein-tagged AtCCX5 expressed in yeast was localized in the plasma membrane and nuclear periphery. The yeast transformants expressing AtCCX5 were created and their growth in the presence of various cations (K(+), Na(+), Ca(2+), Mg(2+), Fe(2+), Cu(2+), Co(2+), Cd(2+), Mn(2+), Ba(2+), Ni(2+), Zn(2+), and Li(+)) were analyzed. AtCCX5 expression was found to affect the response to K(+) and Na(+) in yeast. The AtCCX5 transformant also showed a little better growth to Zn(2+). The yeast mutant 9.3 expressing AtCCX5 restored growth of the mutant on medium with low K(+) (0.5mM), and also suppressed its Na(+) sensitivity. Ion uptake experiments showed that AtCCX5 mediated relatively high-affinity K(+) uptake and was also involved in Na(+) transport in yeast. Taken together, these findings suggest that the AtCCX5 is a novel transport protein involves in mediating high-affinity K(+) uptake and Na(+) transport in yeast.

Vacuolar Ca2+/H+ transport activity is required for systemic phosphate homeostasis involving shoot-to-root signaling in Arabidopsis

Calcium ions (Ca(2+)) and Ca(2+)-related proteins mediate a wide array of downstream processes involved in plant responses to abiotic stresses. In Arabidopsis (Arabidopsis thaliana), disruption of the vacuolar Ca(2+)/H(+) transporters CAX1 and CAX3 causes notable alterations in the shoot ionome, including phosphate (P(i)) content. In this study, we showed that the cax1/cax3 double mutant displays an elevated P(i) level in shoots as a result of increased P(i) uptake in a miR399/PHO2-independent signaling pathway. Microarray analysis of the cax1/cax3 mutant suggests the regulatory function of CAX1 and CAX3 in suppressing the expression of a subset of shoot P(i) starvation-responsive genes, including genes encoding the PHT1;4 P(i) transporter and two SPX domain-containing proteins, SPX1 and SPX3. Moreover, although the expression of several PHT1 genes and PHT1;1/2/3 proteins is not up-regulated in the root of cax1/cax3, results from reciprocal grafting experiments indicate that the cax1/cax3 scion is responsible for high P(i) accumulation in grafted plants and that the pht1;1 rootstock is sufficient to moderately repress such P(i) accumulation. Based on these findings, we propose that CAX1 and CAX3 mediate a shoot-derived signal that modulates the activity of the root P(i) transporter system, likely in part via posttranslational regulation of PHT1;1 P(i) transporters.

Plant cation/H+ exchangers (CAXs): biological functions and genetic manipulations

Inorganic cations play decisive roles in many cellular and physiological processes and are essential components of plant nutrition. Therefore, the uptake of cations and their redistribution must be precisely controlled. Vacuolar antiporters are important elements in mediating the intracellular sequestration of these cations. These antiporters are energized by the proton gradient across the vacuolar membrane and allow the rapid transport of cations into the vacuole. CAXs (for CAtion eXchanger) are members of a multigene family and appear to predominately reside on vacuoles. Defining CAX regulation and substrate specificity have been aided by utilising yeast as an experimental tool. Studies in plants suggest CAXs regulate apoplastic Ca(2+) levels in order to optimise cell wall expansion, photosynthesis, transpiration and plant productivity. CAX studies provide the basis for making designer transporters that have been used to develop nutrient enhanced crops and plants for remediating toxic soils.

The cellular energization state affects peripheral stalk stability of plant vacuolar H+-ATPase and impairs vacuolar acidification

The plant vacuolar H(+)-ATPase takes part in acidifying compartments of the endomembrane system including the secretory pathway and the vacuoles. The structural variability of the V-ATPase complex as well as its presence in different compartments and tissues involves multiple isoforms of V-ATPase subunits. Furthermore, a versatile regulation is essential to allow for organelle- and tissue-specific fine tuning. In this study, results from V-ATPase complex disassembly with a chaotropic reagent, immunodetection and in vivo fluorescence resonance energy transfer (FRET) analyses point to a regulatory mechanism in plants, which depends on energization and involves the stability of the peripheral stalks as well. Lowering of cellular ATP by feeding 2-deoxyglucose resulted in structural alterations within the V-ATPase, as monitored by changes in FRET efficiency between subunits VHA-E and VHA-C. Potassium iodide-mediated disassembly revealed a reduced stability of V-ATPase after 2-deoxyglucose treatment of the cells, but neither the complete V(1)-sector nor VHA-C was released from the membrane in response to 2-deoxyglucose treatment, precluding a reversible dissociation mechanism like in yeast. These data suggest the existence of a regulatory mechanism of plant V-ATPase by modification of the peripheral stator structure that is linked to the cellular energization state. This mechanism is distinct from reversible dissociation as reported for the yeast V-ATPase, but might represent an evolutionary precursor of reversible dissociation.

Cell-specific vacuolar calcium storage mediated by CAX1 regulates apoplastic calcium concentration, gas exchange, and plant productivity in Arabidopsis

The physiological role and mechanism of nutrient storage within vacuoles of specific cell types is poorly understood. Transcript profiles from Arabidopsis thaliana leaf cells differing in calcium concentration ([Ca], epidermis <10 mM versus mesophyll >60 mM) were compared using a microarray screen and single-cell quantitative PCR. Three tonoplast-localized Ca(2+) transporters, CAX1 (Ca(2+)/H(+)-antiporter), ACA4, and ACA11 (Ca(2+)-ATPases), were identified as preferentially expressed in Ca-rich mesophyll. Analysis of respective loss-of-function mutants demonstrated that only a mutant that lacked expression of both CAX1 and CAX3, a gene ectopically expressed in leaves upon knockout of CAX1, had reduced mesophyll [Ca]. Reduced capacity for mesophyll Ca accumulation resulted in reduced cell wall extensibility, stomatal aperture, transpiration, CO(2) assimilation, and leaf growth rate; increased transcript abundance of other Ca(2+) transporter genes; altered expression of cell wall-modifying proteins, including members of the pectinmethylesterase, expansin, cellulose synthase, and polygalacturonase families; and higher pectin concentrations and thicker cell walls. We demonstrate that these phenotypes result from altered apoplastic free [Ca(2+)], which is threefold greater in cax1/cax3 than in wild-type plants. We establish CAX1 as a key regulator of apoplastic [Ca(2+)] through compartmentation into mesophyll vacuoles, a mechanism essential for optimal plant function and productivity.

Calcium storage in plants and the implications for calcium biofortification

Calcium (Ca) is an essential nutrient for plants and animals, with key structural and signalling roles, and its deficiency in plants can result in poor biotic and abiotic stress tolerance, reduced crop quality and yield. Likewise, low Ca intake in humans has been linked to various diseases (e.g. rickets, osteoporosis, hypertension and colorectal cancer) which can threaten quality of life and have major economic costs. Biofortification of various food crops with Ca has been suggested as a good method to enhance human intake of Ca and is advocated as an economically and environmentally advantageous strategy. Efforts to enhance Ca content of crops via transgenic means have had promising results. Overall Ca content of transgenic plants has been increased but in some cases adverse affects on plant function have been observed. This suggests that a better understanding of how Ca ions (Ca(2+)) are stored and transported through plants is required to maximise the effectiveness of future approaches.

Calcium storage in plants and the implications for calcium biofortification

Calcium (Ca) is an essential nutrient for plants and animals, with key structural and signalling roles, and its deficiency in plants can result in poor biotic and abiotic stress tolerance, reduced crop quality and yield. Likewise, low Ca intake in humans has been linked to various diseases (e.g. rickets, osteoporosis, hypertension and colorectal cancer) which can threaten quality of life and have major economic costs. Biofortification of various food crops with Ca has been suggested as a good method to enhance human intake of Ca and is advocated as an economically and environmentally advantageous strategy. Efforts to enhance Ca content of crops via transgenic means have had promising results. Overall Ca content of transgenic plants has been increased but in some cases adverse affects on plant function have been observed. This suggests that a better understanding of how Ca ions (Ca(2+)) are stored and transported through plants is required to maximise the effectiveness of future approaches.

Growth performance and root transcriptome remodeling of Arabidopsis in response to Mars-like levels of magnesium sulfate

Background

Martian regolith (unconsolidated surface material) is a potential medium for plant growth in bioregenerative life support systems during manned missions on Mars. However, hydrated magnesium sulfate mineral levels in the regolith of Mars can reach as high as 10 wt%, and would be expected to be highly inhibitory to plant growth.

Methodology and Principal Findings

Disabling ion transporters AtMRS2-10 and AtSULTR1;2, which are plasma membrane localized in peripheral root cells, is not an effective way to confer tolerance to magnesium sulfate soils. Arabidopsis mrs2-10 and sel1-10 knockout lines do not mitigate the growth inhibiting impacts of high MgSO₄·7H₂O concentrations observed with wildtype plants. A global approach was used to identify novel genes with potential to enhance tolerance to high MgSO₄·7H₂O (magnesium sulfate) stress. The early Arabidopsis root transcriptome response to elevated concentrations of magnesium sulfate was characterized in Col-0, and also between Col-0 and the mutant line cax1-1, which was confirmed to be relatively tolerant of high levels of MgSO₄·7H₂O in soil solution. Differentially expressed genes in Col-0 treated for 45 min. encode enzymes primarily involved in hormone metabolism, transcription factors, calcium-binding proteins, kinases, cell wall related proteins and membrane-based transporters. Over 200 genes encoding transporters were differentially expressed in Col-0 up to 180 min. of exposure, and one of the first down-regulated genes was CAX1. The importance of this early response in wildtype Arabidopsis is exemplified in the fact that only four transcripts were differentially expressed between Col-0 and cax1-1 at 180 min. after initiation of treatment.

Conclusions/Significance

The results provide a solid basis for the understanding of the metabolic response of plants to elevated magnesium sulfate soils; it is the first transcriptome analysis of plants in this environment. The results foster the development of Mars soil-compatible plants by showing that cax1 mutants exhibit partial tolerance to magnesium sulfate, and by elucidating a small subset (500 vs. >10,000) of candidate genes for mutation or metabolic engineering that will enhance tolerance to magnesium sulfate soils.