The chloride channel family gene CLCd negatively regulates pathogen-associated molecular pattern (PAMP)-triggered immunity in Arabidopsis

Dissecting the causal polymorphism of the Lr67res multipathogen resistance gene

Partial resistance to multiple biotrophic fungal pathogens in wheat (Triticum aestivum L.) is conferred by a variant of the Lr67 gene, which encodes a hexose-proton symporter. Two mutations (G144R and V387L) differentiate the resistant and susceptible protein variants (Lr67res and Lr67sus). Lr67res lacks sugar transport capability and was associated with anion transporter-like properties when expressed in Xenopus laevis oocytes. Here, we extended this functional characterization to include yeast and in planta studies. The Lr67res allele, but not Lr67sus, induced sensitivity to ions in yeast (including NaCl, LiCl, and KI), which is consistent with our previous observations that Lr67res expression in oocytes induces novel ion fluxes. We demonstrate that another naturally occurring single amino acid variant in wheat, containing only the Lr67G144R mutation, confers rust resistance. Transgenic barley plants expressing the orthologous HvSTP13 gene carrying the G144R and V387L mutations were also more resistant to Puccinia hordei infection. NaCl treatment of pot-grown adult wheat plants with the Lr67res allele induced leaf tip necrosis and partial leaf rust resistance. An Lr67res-like function can be introduced into orthologous plant hexose transporters via single amino acid mutation, highlighting the strong possibility of generating disease resistance in other crops, especially with gene editing.

Chloride salt enhances plant resistance to biotic stresses

Biotic stresses caused by bacterial and fungal pathogens damage crops; identifying treatments that enhance disease resistance provides important information for understanding plant defenses and sustainable agriculture. Salt stress affects crop yields worldwide; however, studies have focused on the toxic sodium ion, leaving the effects of the chloride ion unclear. In this study, we found that irrigation with a combination of chloride salts (MgCl2, CaCl2, and KCl) suppressed the cell death phenotype of the ceramide kinase mutant acd5. Chloride salt pre-irrigation also significantly limited the cell death caused by Pseudomonas syringae pv maculicola infection and inhibited the multiplication of this bacterial pathogen in a mechanism partially dependent on the salicylic acid pathway. Moreover, chloride salt pre-irrigation improved plant defenses against the fungal pathogen challenge, confining the lesion area caused by Botrytis cinerea infection. Furthermore, the growth of herbivorous larvae of Spodoptera exigua was retarded by feeding on chloride salt irrigated plants. Thus, our data suggest that treatment with Cl- increases broad spectrum resistance to biotic challenges.

The Role of Chloride Channels in Plant Responses to NaCl

Chloride (Cl-) is considered a crucial nutrient for plant growth, but it can be a challenge under saline conditions. Excessive accumulation of Cl- in leaves can cause toxicity. Chloride channels (CLCs) are expressed in the inner membranes of plant cells and function as essential Cl- exchangers or channels. In response to salt stress in plants, CLCs play a crucial role, and CLC proteins assist in maintaining the intracellular Cl- homeostasis by sequestering Cl- into vacuoles. Sodium chloride (NaCl) is the primary substance responsible for causing salt-induced phytotoxicity. However, research on plant responses to Cl- stress is comparatively rare, in contrast to that emphasizing Na+. This review provides a comprehensive overview of the plant response and tolerance to Cl- stress, specifically focusing on comparative analysis of CLC protein structures in different species. Additionally, to further gain insights into the underlying mechanisms, the study summarizes the identified CLC genes that respond to salt stress. This review provides a comprehensive overview of the response of CLCs in terrestrial plants to salt stress and their biological functions, aiming to gain further insights into the mechanisms underlying the response of CLCs in plants to salt stress.

Quantitative trait locus mapping of osmotic stress response in the fungal wheat pathogen Zymoseptoria tritici

Osmotic stress is a ubiquitous and potent stress for all living organisms, but few studies have investigated the genetic basis of salt tolerance in filamentous fungi. The main aim of this study was to identify regions of the genome associated with tolerance to potassium chloride (KCl) in the wheat pathogen Zymoseptoria tritici. A secondary aim was to identify candidate genes affecting salt tolerance within the most promising chromosomal regions. We achieved these aims with a quantitative trait locus (QTL) mapping study using offspring from 2 crosses grown in vitro in the presence or absence of osmotic stress imposed by 0.75 M KCl. We identified significant QTL for most of the traits in both crosses. Several QTLs overlapped with QTL identified in earlier studies for other traits, and some QTL explained trait variation in both the control and salt stress environments. A significant QTL on chromosome 3 explained variation in colony radius at 8-day postinoculation (dpi) in the KCl environment as well as colony radius KCl tolerance at 8 dpi. The QTL peak had a high logarithm of the odds ratio (LOD) and encompassed an interval containing only 36 genes. Six of these genes present promising candidates for functional analyses. A gene ontology (GO) enrichment analysis of QTL unique to the KCl environment found evidence for the enrichment of functions involved in osmotic stress responses.

Molecular mechanism underlying regulation of Arabidopsis CLCa transporter by nucleotides and phospholipids

Chloride channels (CLCs) transport anion across membrane to regulate ion homeostasis and acidification of intracellular organelles, and are divided into anion channels and anion/proton antiporters. Arabidopsis thaliana CLCa (AtCLCa) transporter localizes to the tonoplast which imports NO3- and to a less extent Cl- from cytoplasm. The activity of AtCLCa and many other CLCs is regulated by nucleotides and phospholipids, however, the molecular mechanism remains unclear. Here we determine the cryo-EM structures of AtCLCa bound with NO3- and Cl-, respectively. Both structures are captured in ATP and PI(4,5)P2 bound conformation. Structural and electrophysiological analyses reveal a previously unidentified N-terminal β-hairpin that is stabilized by ATP binding to block the anion transport pathway, thereby inhibiting the AtCLCa activity. While AMP loses the inhibition capacity due to lack of the β/γ- phosphates required for β-hairpin stabilization. This well explains how AtCLCa senses the ATP/AMP status to regulate the physiological nitrogen-carbon balance. Our data further show that PI(4,5)P2 or PI(3,5)P2 binds to the AtCLCa dimer interface and occupies the proton-exit pathway, which may help to understand the inhibition of AtCLCa by phospholipids to facilitate guard cell vacuole acidification and stomatal closure. In a word, our work suggests the regulatory mechanism of AtCLCa by nucleotides and phospholipids under certain physiological scenarios and provides new insights for future study of CLCs.

Balancing nitrate acquisition strategies in symbiotic legumes

MAIN CONCLUSION

Legumes manage both symbiotic (indirect) and non-symbiotic (direct) nitrogen acquisition pathways. Understanding and optimising the direct pathway for nitrate uptake will support greater legume growth and seed yields. Legumes have multiple pathways to acquire reduced nitrogen to grow and set seed. Apart from the symbiotic N₂-fixation pathway involving soil-borne rhizobia bacteria, the acquisition of nitrate and ammonia from the soil can also be an important secondary nitrogen source to meet plant N demand. The balance in N delivery between symbiotic N (indirect) and inorganic N uptake (direct) remains less clear over the growing cycle and with the type of legume under cultivation. In fertile, pH balanced agricultural soils, NO₃^- is often the predominant form of reduced N available to crop plants and will be a major contributor to whole plant N supply if provided at sufficient levels. The transport processes for NO₃^- uptake into legume root cells and its transport between root and shoot tissues involves both high and low-affinity transport systems called HATS and LATS, respectively. These proteins are regulated by external NO₃^- availability and by the N status of the cell. Other proteins also play a role in NO₃^- transport, including the voltage dependent chloride/nitrate channel family (CLC) and the S-type anion channels of the SLAC/SLAH family. CLC's are linked to NO₃^- transport across the tonoplast of vacuoles and the SLAC/SLAH's with NO₃^- efflux across the plasma membrane and out of the cell. An important step in managing the N requirements of a plant are the mechanisms involved in root N uptake and the subsequent cellular distribution within the plant. In this review, we will present the current knowledge of these proteins and what is understood on how they function in key model legumes (Lotus japonicus, Medicago truncatula and Glycine sp.). The review will examine their regulation and role in N signalling, discuss how post-translational modification affects NO₃^- transport in roots and aerial tissues and its translocation to vegetative tissues and storage/remobilization in reproductive tissues. Lastly, we will present how NO₃^-influences the autoregulation of nodulation and nitrogen fixation and its role in mitigating salt and other abiotic stresses.

Expression of the wheat multipathogen resistance hexose transporter Lr67res is associated with anion fluxes

Many disease resistance genes in wheat (Triticum aestivum L.) confer strong resistance to specific pathogen races or strains, and only a small number of genes confer multipathogen resistance. The Leaf rust resistance 67 (Lr67) gene fits into the latter category as it confers partial resistance to multiple biotrophic fungal pathogens in wheat and encodes a Sugar Transport Protein 13 (STP13) family hexose-proton symporter variant. Two mutations (G144R, V387L) in the resistant variant, Lr67res, differentiate it from the susceptible Lr67sus variant. The molecular function of the Lr67res protein is not understood, and this study aimed to broaden our knowledge on this topic. Biophysical analysis of the wheat Lr67sus and Lr67res protein variants was performed using Xenopus laevis oocytes as a heterologous expression system. Oocytes injected with Lr67sus displayed properties typically associated with proton-coupled sugar transport proteins-glucose-dependent inward currents, a Km of 110 ± 10 µM glucose, and a substrate selectivity permitting the transport of pentoses and hexoses. By contrast, Lr67res induced much larger sugar-independent inward currents in oocytes, implicating an alternative function. Since Lr67res is a mutated hexose-proton symporter, the possibility of protons underlying these currents was investigated but rejected. Instead, currents in Lr67res oocytes appeared to be dominated by anions. This conclusion was supported by electrophysiology and 36Cl- uptake studies and the similarities with oocytes expressing the known chloride channel from Torpedo marmorata, TmClC-0. This study provides insights into the function of an important disease resistance gene in wheat, which can be used to determine how this gene variant underpins disease resistance in planta.

Cryo-EM structure of the plant nitrate transporter AtCLCa reveals characteristics of the anion-binding site and the ATP-binding pocket

Nitrate is one of the major nitrogen sources for most plants. Chloride channel (CLC) proteins mediate the transport and vacuole storage of nitrate in plants, but the structural basis of nitrate transport by plant CLC proteins remains unknown. Here, we solved the cryo-EM structure of ATP-bound Arabidopsis thaliana CLCa (AtCLCa) at 2.8 Å resolution. Structural comparison between nitrate-selective AtCLCa and chloride-selective CLC-7 reveals key differences in the central anion-binding site. We observed that the central nitrate is shifted by ∼1.4 Å from chloride, which is likely caused by a weaker interaction between the anion and Pro160; the side chains of aromatic residues around the central binding site are rearranged to accommodate the larger nitrate. Additionally, we identified the ATP-binding pocket of AtCLCa to be located between the cytosolic cystathionine β-synthase domains and the N-terminus. The N-terminus may mediate the ATP inhibition of AtCLCa by interacting with both ATP and the pore-forming transmembrane helix. Together, our studies provide insights into the nitrate selectivity and ATP regulation of plant CLCs.

Improved growth and tuber quality of transgenic potato plants overexpressing either NHX antiporter, CLC chloride channel, or both

The nutritional enhancement of potato plants (Solanum tuberosum L.,) is highly critical. As it is considered a worldwide basic vegetarian nutrition to maintain health. S. tuberosum is one of the foremost staples and the world's fourth-largest food crop. In advance, its need is increasing because of its high-industrial value and population blast. To improve both potato growth and behavior under harsh environmental conditions, we produced transgenic potato plants overexpressing either VvNHX (a sodium proton antiporter from Vitis vinifera), VvCLC (a chloride channel from Vitis vinifera), or both. Control and transgenic plants were grown in greenhouse and field under non-stressed conditions for 85 days in order to characterize their phenotype and evaluate their agronomical performance. To this aim, the evaluation of plant growth parameters, tuber yields and characteristics (calibers, eye number and color), the chemical composition of tubers, was conducted and compared between the different lines. The obtained results showed that transgenic plants displayed an improved growth (flowering precocity, gain of vigor and better vegetative growth) along with enhanced tuber yields and quality (increased protein and starch contents). Our findings provide then insight into the role played by the VvNHX antiport and the VvCLC channel and a greater understanding of the effect of their overexpression in potato plants.

Involvement of Arabidopsis Acyl Carrier Protein 1 in PAMP-Triggered Immunity

Plant fatty acids (FAs) and lipids are essential in storing energy and act as structural components for cell membranes and signaling molecules for plant growth and stress responses. Acyl carrier proteins (ACPs) are small acidic proteins that covalently bind the fatty acyl intermediates during the elongation of FAs. The Arabidopsis thaliana ACP family has eight members. Through reverse genetic, molecular, and biochemical approaches, we have discovered that ACP1 localizes to the chloroplast and limits the magnitude of pattern-triggered immunity (PTI) against the bacterial pathogen Pseudomonas syringae pv. tomato. Mutant acp1 plants have reduced levels of linolenic acid (18:3), which is the primary precursor for biosynthesis of the phytohormone jasmonic acid (JA), and a corresponding decrease in the abundance of JA. Consistent with the known antagonistic relationship between JA and salicylic acid (SA), acp1 mutant plants also accumulate a higher level of SA and display corresponding shifts in JA- and SA-regulated transcriptional outputs. Moreover, methyl JA and linolenic acid treatments cause an apparently enhanced decrease of resistance against P. syringae pv. tomato in acp1 mutants than that in WT plants. The ability of ACP1 to prevent this hormone imbalance likely underlies its negative impact on PTI in plant defense. Thus, ACP1 links FA metabolism to stress hormone homeostasis to be negatively involved in PTI in Arabidopsis plant defense. [Formula: see text] Copyright © 2022 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

The Mechanosensitive Ion Channel MSL10 Modulates Susceptibility to Pseudomonas syringae in Arabidopsis thaliana

Plants sense and respond to molecular signals associated with the presence of pathogens and their virulence factors. Mechanical signals generated during pathogenic invasion may also be important, but their contributions have rarely been studied. Here, we investigate the potential role of a mechanosensitive ion channel, MscS-like (MSL)10, in defense against the bacterial pathogen Pseudomonas syringae in Arabidopsis thaliana. We previously showed that overexpression of MSL10-GFP, phospho-mimetic versions of MSL10, and the gain-of-function allele msl10-3G all produce dwarfing, spontaneous cell death, and the hyperaccumulation of reactive oxygen species. These phenotypes are shared by many autoimmune mutants and are frequently suppressed by growth at high temperature in those lines. We found that the same was true for all three MSL10 hypermorphs. In addition, we show that the SGT1/RAR1/HSP90 cochaperone complex was required for dwarfing and ectopic cell death, PAD4 and SID2 were partially required, and the immune regulators EDS1 and NDR1 were dispensable. All MSL10 hypermorphs exhibited reduced susceptibility to infection by P. syringae strain Pto DC3000 and Pto DC3000 expressing the avirulence genes avrRpt2 or avrRpm1 but not Pto DC3000 hrpL and showed an accelerated induction of PR1 expression compared with wild-type plants. Null msl10-1 mutants were delayed in PR1 induction and displayed modest susceptibility to infection by coronatine-deficient P. syringae pv. tomato. Finally, stomatal closure was reduced in msl10-1 loss-of-function mutants in response to P. syringae pv. tomato COR^-. These data show that MSL10 modulates pathogen responses and begin to address the possibility that mechanical signals are exploited by the plant for pathogen perception.[Formula: see text] Copyright © 2022 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

Molecular Cloning and Characterization of SaCLCd, SaCLCf, and SaCLCg, Novel Proteins of the Chloride Channel Family (CLC) from the Halophyte Suaeda altissima (L.) Pall

Coding sequences of the CLC family genes SaCLCd, SaCLCf, and SaCLCg, the putative orthologs of Arabidopsis thaliana AtCLCd, AtCLCf, and AtCLCg genes, were cloned from the euhalophyte Suaeda altissima (L.) Pall. The key conserved motifs and glutamates inherent in proteins of the CLC family were identified in SaCLCd, SaCLCf, and SaCLCg amino acid sequences. SaCLCd and SaCLCg were characterized by higher homology to eukaryotic (human) CLCs, while SaCLCf was closer to prokaryotic CLCs. Ion specificities of the SaCLC proteins were studied in complementation assays by heterologous expression of the SaCLC genes in the Saccharomyces cerevisiae GEF1 disrupted strain Δgef1. GEF1 encoded the only CLC family protein, the Cl^- transporter Gef1p, in undisrupted strains of this organism. Expression of SaCLCd in Δgef1 cells restored their ability to grow on selective media. The complementation test and the presence of both the "gating" and "proton" conservative glutamates in SaCLCd amino acid sequence and serine specific for Cl^- in its selectivity filter suggest that this protein operates as a Cl^-/H⁺ antiporter. By contrast, expression of SaCLCf and SaCLCg did not complement the growth defect phenotype of Δgef1 cells. The selectivity filters of SaCLCf and SaCLCg also contained serine. However, SaCLCf included only the "gating" glutamate, while SaCLCg contained the "proton" glutamate, suggesting that SaCLCf and SaCLCg proteins act as Cl^- channels. The SaCLCd, SaCLCf, and SaCLCg genes were shown to be expressed in the roots and leaves of S. altissima. In response to addition of NaCl to the growth medium, the relative transcript abundances of all three genes of S. altissima increased in the leaves but did not change significantly in the roots. The increase in expression of SaCLCd, SaCLCf, and SaCLCg in the leaves in response to increasing salinity was in line with Cl^- accumulation in the leaf cells, indicating the possible participation of SaCLCd, SaCLCf, and SaCLCg proteins in Cl^- sequestration in cell organelles. Generally, these results suggest the involvement of SaCLC proteins in the response of S. altissima plants to increasing salinity and possible participation in mechanisms underlying salt tolerance.

Plant Trans-Golgi Network/Early Endosome pH regulation requires Cation Chloride Cotransporter (CCC1)

Plant cells maintain a low luminal pH in the trans-Golgi-network/early endosome (TGN/EE), the organelle in which the secretory and endocytic pathways intersect. Impaired TGN/EE pH regulation translates into severe plant growth defects. The identity of the proton pump and proton/ion antiporters that regulate TGN/EE pH have been determined, but an essential component required to complete the TGN/EE membrane transport circuit remains unidentified − a pathway for cation and anion efflux. Here, we have used complementation, genetically encoded fluorescent sensors, and pharmacological treatments to demonstrate that Arabidopsis cation chloride cotransporter (CCC1) is this missing component necessary for regulating TGN/EE pH and function. Loss of CCC1 function leads to alterations in TGN/EE-mediated processes including endocytic trafficking, exocytosis, and response to abiotic stress, consistent with the multitude of phenotypic defects observed in ccc1 knockout plants. This discovery places CCC1 as a central component of plant cellular function.

ClCd and ClCf act redundantly at the trans-Golgi network/early endosome and prevent acidification of the Golgi stack

The trans-Golgi network/early endosome (TGN/EE) serves as the central hub in which exocytic and endocytic trafficking pathways converge and specificity of cargo routing needs to be achieved. Acidification is a hallmark of the TGN/EE and is maintained by the vacuolar H+-ATPase (V-ATPase) with support of proton-coupled antiporters. We show here that ClCd and ClCf, two distantly related members of the Arabidopsis Cl- channel (ClC) family, colocalize in the TGN/EE, where they act redundantly, and are essential for male gametophyte development. Combining an inducible knockdown approach and in vivo pH measurements, we show here that reduced ClC activity does not affect pH in the TGN/EE but causes hyperacidification of trans-Golgi cisternae. Taken together, our results show that ClC-mediated anion transport into the TGN/EE is essential and affects spatiotemporal aspects of TGN/EE maturation as well as its functional separation from the Golgi stack.

The chloride channels: Silently serving the plants

Chloride channels (CLCs), member of anion transporting proteins, are present ubiquitously in all life forms. Diverging from its name, the CLC family includes both channel and exchanger (proton-coupled) proteins; nevertheless, they share conserved structural organization. They are engaged in diverse indispensable functions such as acid and fluoride tolerance in prokaryotes to muscle stabilization, transepithelial transport, and neuronal development in mammals. Mutations in genes encoding CLCs lead to several physiological disorders in different organisms, including severe diseases in humans. Even in plants, loss of CLC protein function severely impairs various cellular processes critical for normal growth and development. These proteins sequester Cl^- into the vacuole, thus, making them an attractive target for improving salinity tolerance in plants caused by high abundance of salts, primarily NaCl. Besides, some CLCs are involved in NO₃ ^- transport and storage function in plants, thus, influencing their nitrogen use efficiency. However, despite their high significance, not many studies have been carried out in plants. Here, we have attempted to concisely highlight the basic structure of CLC proteins and critical residues essential for their function and classification. We also present the diverse functions of CLCs in plants from their first cloning back in 1996 to the knowledge acquired as of now. We stress the need for carrying out more in-depth studies on CLCs in plants, for they may have future applications towards crop improvement.

Genome-wide identification and expression analysis of the CLC gene family in pomegranate (Punica granatum) reveals its roles in salt resistance

BACKGROUNDS

Pomegranate (Punica granatum L.) is an important commercial fruit tree, with moderate tolerance to salinity. The balance of Cl^- and other anions in pomegranate tissues are affected by salinity, however, the accumulation patterns of anions are poorly understood. The chloride channel (CLC) gene family is involved in conducting Cl^-, NO₃^-, HCO₃^- and I^-, but its characteristics have not been reported on pomegranate.

RESULTS

In this study, we identified seven PgCLC genes, consisting of four antiporters and three channels, based on the presence of the gating glutamate (E) and the proton glutamate (E). Phylogenetic analysis revealed that seven PgCLCs were divided into two clades, with clade I containing the typical conserved regions GxGIPE (I), GKxGPxxH (II) and PxxGxLF (III), whereas clade II not. Multiple sequence alignment revealed that PgCLC-B had a P [proline, Pro] residue in region I, which was suspected to be a NO₃^-/H⁺ exchanger, while PgCLC-C1, PgCLC-C2, PgCLC-D and PgCLC-G contained a S [serine, Ser] residue, with a high affinity to Cl^-. We determined the content of Cl^-, NO₃^-, H₂PO₄^-, and SO₄^2- in pomegranate tissues after 18 days of salt treatments (0, 100, 200 and 300 mM NaCl). Compared with control, the Cl^- content increased sharply in pomegranate tissues. Salinity inhibited the uptake of NO₃^- and SO₄^2-, but accelerated H₂PO₄^- uptake. The results of real-time reverse transcription PCR (qRT-PCR) revealed that PgCLC genes had tissue-specific expression patterns. The high expression levels of three antiporters PgCLC-C1, PgCLC-C2 and PgCLC-D in leaves might be contributed to sequestrating Cl^- into the vacuoles. However, the low expression levels of PgCLCs in roots might be associated with the exclusion of Cl^- from root cells. Also, the up-regulated PgCLC-B in leaves indicated that more NO₃^- was transported into leaves to mitigate the nitrogen deficiency.

CONCLUSIONS

Our findings suggested that the PgCLC genes played important roles in balancing of Cl^- and NO₃^- in pomegranate tissues under salt stress. This study established a theoretical foundation for the further functional characterization of the CLC genes in pomegranate.

Identification of MAMP-Responsive Plasma Membrane-Associated Proteins in Arabidopsis thaliana Following Challenge with Different LPS Chemotypes from Xanthomonas campestris

Lipopolysaccharides (LPS) are recognized as microbe-associated molecular patterns (MAMPs) responsible for eliciting defense-related responses and while the effects have been well-documented in mammals, there is a lack of knowledge regarding the mechanism of perception in plant systems and recognized structural moieties within the macromolecular lipoglycan structure. Thus, identification of the LPS plasma membrane (PM) receptor(s)/receptor complex in Arabidopsis thaliana through proteomics will contribute to a deeper understanding of induced defense responses. As such, structurally characterized LPS chemotypes from Xanthomonas campestris pv. campestris (Xcc) wild-type 8004 (prototypical smooth-type LPS) and mutant 8530 (truncated core with no O–chain) strains were utilized to pre-treat A. thaliana plants. The associated proteomic response/changes within the PM were compared over a 24 h period using mass spectrometry-based methodologies following three variants of LPS-immobilized affinity chromatography. This resulted in the identification of proteins from several functional categories, but importantly, those involved in perception and defense. The distinct structural features between wild-type and mutant LPS are likely responsible for the differential changes to the proteome profiles, and many of the significant proteins were identified in response to the wild-type Xcc LPS where it is suggested that the core oligosaccharide and O-chain participate in recognition by receptor-like kinases (RLKs) in a multiprotein complex and, notably, varied from that of the mutant chemotype.

CATION-CHLORIDE CO-TRANSPORTER 1 (CCC1) Mediates Plant Resistance against Pseudomonas syringae

Plasma membrane (PM) depolarization functions as an initial step in plant defense signaling pathways. However, only a few ion channels/transporters have been characterized in the context of plant immunity. Here, we show that the Arabidopsis (Arabidopsis thaliana) Na+:K+:2Cl- (NKCC) cotransporter CCC1 has a dual function in plant immunity. CCC1 functions independently of PM depolarization and negatively regulates pathogen-associated molecular pattern-triggered immunity. However, CCC1 positively regulates plant basal and effector-triggered resistance to Pseudomonas syringae pv. tomato (Pst) DC3000. In line with the compromised immunity to Pst DC3000, ccc1 mutants show reduced expression of genes encoding enzymes involved in the biosynthesis of antimicrobial peptides, camalexin, and 4-OH-ICN, as well as pathogenesis-related proteins. Moreover, genes involved in cell wall and cuticle biosynthesis are constitutively down-regulated in ccc1 mutants, and the cell walls of these mutants exhibit major changes in monosaccharide composition. The role of CCC1 ion transporter activity in the regulation of plant immunity is corroborated by experiments using the specific NKCC inhibitor bumetanide. These results reveal a function for ion transporters in immunity-related cell wall fortification and antimicrobial biosynthesis.

Chloride as a Beneficial Macronutrient in Higher Plants: New Roles and Regulation

Chloride (Cl-) has traditionally been considered a micronutrient largely excluded by plants due to its ubiquity and abundance in nature, its antagonism with nitrate (NO3-), and its toxicity when accumulated at high concentrations. In recent years, there has been a paradigm shift in this regard since Cl- has gone from being considered a harmful ion, accidentally absorbed through NO3- transporters, to being considered a beneficial macronutrient whose transport is finely regulated by plants. As a beneficial macronutrient, Cl- determines increased fresh and dry biomass, greater leaf expansion, increased elongation of leaf and root cells, improved water relations, higher mesophyll diffusion to CO2, and better water- and nitrogen-use efficiency. While optimal growth of plants requires the synchronic supply of both Cl- and NO3- molecules, the NO3-/Cl- plant selectivity varies between species and varieties, and in the same plant it can be modified by environmental cues such as water deficit or salinity. Recently, new genes encoding transporters mediating Cl- influx (ZmNPF6.4 and ZmNPF6.6), Cl- efflux (AtSLAH3 and AtSLAH1), and Cl- compartmentalization (AtDTX33, AtDTX35, AtALMT4, and GsCLC2) have been identified and characterized. These transporters have proven to be highly relevant for nutrition, long-distance transport and compartmentalization of Cl-, as well as for cell turgor regulation and stress tolerance in plants.

Genome-scale characterization of the vacuole nitrate transporter Chloride Channel (CLC) genes and their transcriptional responses to diverse nutrient stresses in allotetraploid rapeseed

The Chloride Channel (CLC) gene family is reported to be involved in vacuolar nitrate (NO₃^-) transport. Nitrate distribution to the cytoplasm is beneficial for enhancing NO₃^- assimilation and plays an important role in the regulation of nitrogen (N) use efficiency (NUE). In this study, genomic information, high-throughput transcriptional profiles, and gene co-expression analysis were integrated to identify the CLCs (BnaCLCs) in Brassica napus. The decreased NO₃^- concentration in the clca-2 mutant up-regulated the activities of nitrate reductase and glutamine synthetase, contributing to increase N assimilation and higher NUE in Arabidopsis thaliana. The genome-wide identification of 22BnaCLC genes experienced strong purifying selection. Segmental duplication was the major driving force in the expansion of the BnaCLC gene family. The most abundant cis-acting regulatory elements in the gene promoters, including DNA-binding One Zinc Finger, W-box, MYB, and GATA-box, might be involved in the transcriptional regulation of BnaCLCs expression. High-throughput transcriptional profiles and quantitative real-time PCR results showed that BnaCLCs responded differentially to distinct NO₃^- regimes. Transcriptomics-assisted gene co-expression network analysis identified BnaA7.CLCa-3 as the core member of the BnaCLC family, and this gene might play a central role in vacuolar NO₃^- transport in crops. The BnaCLC members also showed distinct expression patterns under phosphate depletion and cadmium toxicity. Taken together, our results provide comprehensive insights into the vacuolar BnaCLCs and establish baseline information for future studies on BnaCLCs-mediated vacuolar NO₃^- storage and its effect on NUE.

Plant behaviour under combined stress: tomato responses to combined salinity and pathogen stress

Crop plants are subjected to a variety of stresses during their lifecycle, including abiotic stress factors such as salinity and biotic stress factors such as pathogens. Plants have developed a multitude of defense and adaptation responses to these stress factors. In the field, different stress factors mostly occur concurrently resulting in a new state of stress, the combined stress. There is evidence that plant resistance to pathogens can be attenuated or enhanced by abiotic stress factors. With stress tolerance research being mostly focused on plant responses to individual stresses, the understanding of a plant's ability to adapt to combined stresses is limited. In the last few years, we studied powdery mildew resistance under salt stress conditions in the model crop plant tomato with the aim to understand the requirements to achieve plant resilience to a wider array of combined abiotic and biotic stress combinations. We uncovered specific responses of tomato plants to combined salinity-pathogen stress, which varied with salinity intensity and plant resistance genes. Moreover, hormones, with their complex regulation and cross-talk, were shown to play a key role in the adaptation of tomato plants to the combined stress. In this review, we attempt to understand the complexity of plant responses to abiotic and biotic stress combinations, with a focus on tomato responses (genetic control and cross-talk of signaling pathways) to combined salinity and pathogen stresses. Further, we provide recommendations on how to design novel strategies for breeding crops with a sustained performance under diverse environmental conditions.

Large-Scale Investigation of Soybean Gene Functions by Overexpressing a Full-Length Soybean cDNA Library in Arabidopsis

Molecular breeding has become an important approach for crop improvement, and a prerequisite for molecular breeding is elucidation of the functions of genetic loci or genes. Soybean is one of the most important food and oil crops worldwide. However, due to the difficulty of genetic transformation in soybean, studies of its functional genomics lag far behind those of other crops such as rice, which severely impairs the progress of molecular improvement in soybean. Here, we describe an effective large-scale strategy to investigate the functions of soybean genes via overexpression of a full-length soybean cDNA library in Arabidopsis. The overexpression vector pJL12 was modified for use in the construction of a normalized full-length cDNA library. The constructed cDNA library showed good quality; repetitive clones represented approximately 4%, insertion fragments were approximately 2.2 kb, and the full-length rate was approximately 98%. This cDNA library was then overexpressed in Arabidopsis, and approximately 2000 transgenic lines were preliminarily obtained. Phenotypic analyses of the positive T₁ transgenic plants showed that more than 5% of the T₁ transgenic lines displayed abnormal developmental phenotypes, and approximately 1% of the transgenic lines exhibited potentially favorable traits. We randomly amplified 4 genes with obvious phenotypes (enlarged seeds, yellowish leaves, more branches, and dense siliques) and repeated the transgenic analyses in Arabidopsis. Subsequent phenotypic observation demonstrated that these phenotypes were indeed due to the overexpression of soybean genes. We believe our strategy represents an effective large-scale approach to investigate the functions of soybean genes and further reveal genes favorable for molecular improvement in soybean.

Electrical Signaling, Photosynthesis and Systemic Acquired Acclimation

Electrical signaling in higher plants is required for the appropriate intracellular and intercellular communication, stress responses, growth and development. In this review, we have focus on recent findings regarding the electrical signaling, as a major regulator of the systemic acquired acclimation (SAA) and the systemic acquired resistance (SAR). The electric signaling on its own cannot confer the required specificity of information to trigger SAA and SAR, therefore, we have also discussed a number of other mechanisms and signaling systems that can operate in combination with electric signaling. We have emphasized the interrelation between ionic mechanism of electrical activity and regulation of photosynthesis, which is intrinsic to a proper induction of SAA and SAR. In a special way, we have summarized the role of non-photochemical quenching and its regulator PsbS. Further, redox status of the cell, calcium and hydraulic waves, hormonal circuits and stomatal aperture regulation have been considered as components of the signaling. Finally, a model of light-dependent mechanisms of electrical signaling propagation has been presented together with the systemic regulation of light-responsive genes encoding both, ion channels and proteins involved in regulation of their activity. Due to space limitations, we have not addressed many other important aspects of hormonal and ROS signaling, which were presented in a number of recent excellent reviews.

Genetics of mycorrhizal symbiosis in winter wheat (Triticum aestivum)

Bread wheat (Triticum aestivum) is a major staple food and therefore of prime importance for feeding the Earth's growing population. Mycorrhiza is known to improve plant growth, but although extensive knowledge concerning the interaction between mycorrhizal fungi and plants is available, genotypic differences concerning the ability of wheat to form mycorrhizal symbiosis and quantitative trait loci (QTLs) involved in mycorrhization are largely unknown. Therefore, a diverse set of 94 bread wheat genotypes was evaluated with regard to root colonization by arbuscular mycorrhizal fungi. In order to identify genomic regions involved in mycorrhization, these genotypes were analyzed using the wheat 90k iSelect chip, resulting in 17 823 polymorphic mapped markers, which were used in a genome-wide association study. Significant genotypic differences (P < 0.0001) were detected in the ability to form symbiosis and 30 significant markers associated with root colonization, representing six QTL regions, were detected on chromosomes 3A, 4A and 7A, and candidate genes located in these QTL regions were proposed. The results reported here provide key insights into the genetics of root colonization by mycorrhizal fungi in wheat.

Chloride: not simply a 'cheap osmoticum', but a beneficial plant macronutrient

At macronutrient levels, chloride has positive effects on plant growth, which are distinct from its function in photosynthesis..

Abscisic acid negatively regulates post-penetration resistance of Arabidopsis to the biotrophic powdery mildew fungus

Pytohormone abscisic acid (ABA) plays important roles in defense responses. Nonetheless, how ABA regulates plant resistance to biotrophic fungi remains largely unknown. Arabidopsis ABA-deficient mutants, aba2-1 and aba3-1, displayed enhanced resistance to the biotrophic powdery mildew fungus Golovinomyces cichoracearum. Moreover, exogenously administered ABA increased the susceptibility of Arabidopsis to G. cichoracearum. Arabidopsis ABA perception components mutants, abi1-1 and abi2-1, also displayed similar phenotypes to ABA-deficient mutants in resistance to G. cichoracearum. However, the resistance to G. cichoracearum is not changed in downstream ABA signaling transduction mutants, abi3-1, abi4-1, and abi5-1. Microscopic examination revealed that hyphal growth and conidiophore production of G. cichoracearum were compromised in the ABA deficient mutants, even though pre-penetration and penetration growth of the fungus were not affected. In addition, salicylic acid (SA) and MPK3 are found to be involved in ABA-regulated resistance to G. cichoracearum. Our work demonstrates that ABA negatively regulates post-penetration resistance of Arabidopsis to powdery mildew fungus G. cichoracearum, probably through antagonizing the function of SA.

GmCLC1 Confers Enhanced Salt Tolerance through Regulating Chloride Accumulation in Soybean

The family of chloride channel proteins that mediate Cl(-) transportation play vital roles in plant nutrient supply, cellular action potential and turgor pressure adjustment, stomatal movement, hormone signal recognition and transduction, Cl(-) homeostasis, and abiotic and biotic stress tolerance. The anionic toxicity, mainly caused by chloride ions (Cl(-)), on plants under salt stress remains poorly understood. In this work, we investigated the function of soybean Cl(-)/H(+) antiporter GmCLC1 under salt stress in transgenic Arabidopsis thaliana, soybean, and yeast. We found that GmCLC1 enhanced salt tolerance in transgenic A. thaliana by reducing the Cl(-) accumulation in shoots and hence released the negative impact of salt stress on plant growth. Overexpression of GmCLC1 in the hairy roots of soybean sequestered more Cl(-) in their roots and transferred less Cl(-) to their shoots, leading to lower relative electrolyte leakage values in the roots and leaves. When either the soybean GmCLC1 or the yeast chloride transporter gene, GEF1, was transformed into the yeast gef1 mutant, and then treated with different chloride salts (MnCl2, KCl, NaCl), enhanced survival rate was observed. The result indicates that GmCLC1 and GEF1 exerted similar effects on alleviating the stress of diverse chloride salts on the yeast gef1 mutant. Together, this work suggests a protective function of GmCLC1 under Cl(-) stress.

Characterization of the Chloride Channel-Like, AtCLCg, Involved in Chloride Tolerance in Arabidopsis thaliana

In plant cells, anion channels and transporters are essential for key functions such as nutrition, ion homeostasis and resistance to biotic or abiotic stresses. We characterized AtCLCg, a member of the chloride channel (CLC) family in Arabidopsis localized in the vacuolar membrane. When grown on NaCl or KCl, atclcg knock-out mutants showed a decrease in biomass. In the presence of NaCl, these mutants overaccumulate chloride in shoots. No difference in growth was detected in response to osmotic stress by mannitol. These results suggest a physiological function of AtCLCg in the chloride homeostasis during NaCl stress. AtCLCg shares a high degree of identity (62%) with AtCLCc, another vacuolar CLC essential for NaCl tolerance. However, the atclcc atclccg double mutant is not more sensitive to NaCl than single mutants. As the effects of both mutations are not additive, gene expression analyses were performed and revealed that: (i)AtCLCg is expressed in mesophyll cells, hydathodes and phloem while AtCLCc is expressed in stomata; and (ii)AtCLCg is repressed in the atclcc mutant background, and vice versa. Altogether these results demonstrate that both AtCLCc and AtCLCg are important for tolerance to excess chloride but not redundant, and form part of a regulatory network controlling chloride sensitivity.

The Arabidopsis Thylakoid Chloride Channel AtCLCe Functions in Chloride Homeostasis and Regulation of Photosynthetic Electron Transport

Chloride ions can be translocated across cell membranes through Cl(-) channels or Cl(-)/H(+) exchangers. The thylakoid-located member of the Cl(-) channel CLC family in Arabidopsis thaliana (AtCLCe) was hypothesized to play a role in photosynthetic regulation based on the initial photosynthetic characterization of clce mutant lines. The reduced nitrate content of Arabidopsis clce mutants suggested a role in regulation of plant nitrate homeostasis. In this study, we aimed to further investigate the role of AtCLCe in the regulation of ion homeostasis and photosynthetic processes in the thylakoid membrane. We report that the size and composition of proton motive force were mildly altered in two independent Arabidopsis clce mutant lines. Most pronounced effects in the clce mutants were observed on the photosynthetic electron transport of dark-adapted plants, based on the altered shape and associated parameters of the polyphasic OJIP kinetics of chlorophyll a fluorescence induction. Other alterations were found in the kinetics of state transition and in the macro-organization of photosystem II supercomplexes, as indicated by circular dichroism measurements. Pre-treatment with KCl but not with KNO3 restored the wild-type photosynthetic phenotype. Analyses by transmission electron microscopy revealed a bow-like arrangement of the thylakoid network and a large thylakoid-free stromal region in chloroplast sections from the dark-adapted clce plants. Based on these data, we propose that AtCLCe functions in Cl(-) homeostasis after transition from light to dark, which affects chloroplast ultrastructure and regulation of photosynthetic electron transport.

Transcripts and MicroRNAs Responding to Salt Stress in Musa acuminata Colla (AAA Group) cv. Berangan Roots

Physiological responses to stress are controlled by expression of a large number of genes, many of which are regulated by microRNAs. Since most banana cultivars are salt-sensitive, improved understanding of genetic regulation of salt induced stress responses in banana can support future crop management and improvement in the face of increasing soil salinity related to irrigation and climate change. In this study we focused on determining miRNA and their targets that respond to NaCl exposure and used transcriptome sequencing of RNA and small RNA from control and NaCl-treated banana roots to assemble a cultivar-specific reference transcriptome and identify orthologous and Musa-specific miRNA responding to salinity. We observed that, banana roots responded to salinity stress with changes in expression for a large number of genes (9.5% of 31,390 expressed unigenes) and reduction in levels of many miRNA, including several novel miRNA and banana-specific miRNA-target pairs. Banana roots expressed a unique set of orthologous and Musa-specific miRNAs of which 59 respond to salt stress in a dose-dependent manner. Gene expression patterns of miRNA compared with those of their predicted mRNA targets indicated that a majority of the differentially expressed miRNAs were down-regulated in response to increased salinity, allowing increased expression of targets involved in diverse biological processes including stress signaling, stress defence, transport, cellular homeostasis, metabolism and other stress-related functions. This study may contribute to the understanding of gene regulation and abiotic stress response of roots and the high-throughput sequencing data sets generated may serve as important resources related to salt tolerance traits for functional genomic studies and genetic improvement in banana.

The roles of anion channels in Arabidopsis immunity

Anion efflux is one of the most immediate responses of plant cells to pathogen attacks, suggesting that anion channels may play a role in plant defense. Recently we reported that the chloride channel AtCLCd negatively regulates Arabidopsis pathogen-associated molecular pattern-triggered immunity (PTI), probably by affecting trafficking of the pattern recognition receptors (PRRs). Since AtCLCd is localized to the trans-Golgi network, it is not likely to be directly involved in anion flux across the plasma membrane. Here, we used a pharmacological approach to explore further the function of plasma membrane-localized R-type and S-type anion channels in plant immunity. We found that the R-type and S-type anion channels play opposite roles in Arabidopsis innate immunity. Inhibition of the R-type anion channels enhances, whereas inhibition of the S-type channels inhibits PTI and effector-triggered immunity (ETI).