Structural implications of mutations in the pea SYM8 symbiosis gene, the DMI1 ortholog, encoding a predicted ion channel

Innovations in functional genomics and molecular breeding of pea: exploring advances and opportunities

The garden pea (Pisum sativum L.) is a significant cool-season legume, serving as crucial food sources, animal feed, and industrial raw materials. The advancement of functional genomics over the past two decades has provided substantial theoretical foundations and progress to pea breeding. Notably, the release of the pea reference genome has enhanced our understanding of plant architecture, symbiotic nitrogen fixation (SNF), flowering time, floral organ development, seed development, and stress resistance. However, a considerable gap remains between pea functional genomics and molecular breeding. This review summarizes the current advancements in pea functional genomics and breeding while highlighting the future challenges in pea molecular breeding.

Constitutive activation of a nuclear-localized calcium channel complex in Medicago truncatula

Nuclear Ca²⁺ oscillations allow symbiosis signaling, facilitating plant recognition of beneficial microsymbionts, nitrogen-fixing rhizobia, and nutrient-capturing arbuscular mycorrhizal fungi. Two classes of channels, DMI1 and CNGC15, in a complex on the nuclear membrane, coordinate symbiotic Ca²⁺ oscillations. However, the mechanism of Ca²⁺ signature generation is unknown. Here, we demonstrate spontaneous activation of this channel complex, through gain-of-function mutations in DMI1, leading to spontaneous nuclear Ca²⁺ oscillations and spontaneous nodulation, in a CNGC15-dependent manner. The mutations destabilize a hydrogen-bond or salt-bridge network between two RCK domains, with the resultant structural changes, alongside DMI1 cation permeability, activating the channel complex. This channel complex was reconstituted in human HEK293T cell lines, with the resultant calcium influx enhanced by autoactivated DMI1 and CNGC15s. Our results demonstrate the mode of activation of this nuclear channel complex, show that DMI1 and CNGC15 are sufficient to create oscillatory Ca²⁺ signals, and provide insights into its native mode of induction.

Glycine max Homologs of DOESN'T MAKE INFECTIONS 1, 2, and 3 Function to Impair Heterodera glycines Parasitism While Also Regulating Mitogen Activated Protein Kinase Expression

Glycine max root cells developing into syncytia through the parasitic activities of the pathogenic nematode Heterodera glycines underwent isolation by laser microdissection (LM). Microarray analyses have identified the expression of a G. max DOESN'T MAKE INFECTIONS3 (DMI3) homolog in syncytia undergoing parasitism but during a defense response. DMI3 encodes part of the common symbiosis pathway (CSP) involving DMI1, DMI2, and other CSP genes. The identified DMI gene expression, and symbiosis role, suggests the possible existence of commonalities between symbiosis and defense. G. max has 3 DMI1, 12 DMI2, and 2 DMI3 paralogs. LM-assisted gene expression experiments of isolated syncytia under further examination here show G. max DMI1-3, DMI2-7, and DMI3-2 expression occurring during the defense response in the H. glycines-resistant genotypes G.max _{[Peking/PI548402]} and G.max _[PI88788] indicating a broad and consistent level of expression of the genes. Transgenic overexpression (OE) of G. max DMI1-3, DMI2-7, and DMI3-2 impairs H. glycines parasitism. RNA interference (RNAi) of G. max DMI1-3, DMI2-7, and DMI3-2 increases H. glycines parasitism. The combined opposite outcomes reveal a defense function for these genes. Prior functional transgenic analyses of the 32-member G. max mitogen activated protein kinase (MAPK) gene family has determined that 9 of them act in the defense response to H. glycines parasitism, referred to as defense MAPKs. RNA-seq analyses of root RNA isolated from the 9 G. max defense MAPKs undergoing OE or RNAi reveal they alter the relative transcript abundances (RTAs) of specific DMI1, DMI2, and DMI3 paralogs. In contrast, transgenically-manipulated DMI1-3, DMI2-7, and DMI3-2 expression influences MAPK3-1 and MAPK3-2 RTAs under certain circumstances. The results show G. max homologs of the CSP, and defense pathway are linked, apparently involving co-regulated gene expression.

Structure and Development of the Legume-Rhizobial Symbiotic Interface in Infection Threads

The intracellular infection thread initiated in a root hair cell is a unique structure associated with Rhizobium-legume symbiosis. It is characterized by inverted tip growth of the plant cell wall, resulting in a tunnel that allows invasion of host cells by bacteria during the formation of the nitrogen-fixing root nodule. Regulation of the plant-microbial interface is essential for infection thread growth. This involves targeted deposition of the cell wall and extracellular matrix and tight control of cell wall remodeling. This review describes the potential role of different actors such as transcription factors, receptors, and enzymes in the rearrangement of the plant-microbial interface and control of polar infection thread growth. It also focuses on the composition of the main polymers of the infection thread wall and matrix and the participation of reactive oxygen species (ROS) in the development of the infection thread. Mutant analysis has helped to gain insight into the development of host defense reactions. The available data raise many new questions about the structure, function, and development of infection threads.

Symbiotic Regulatory Genes Controlling Nodule Development in Pisum sativum L

Analyses of natural variation and the use of mutagenesis and molecular-biological approaches have revealed 50 symbiotic regulatory genes in pea (Pisum sativum L.). Studies of genomic synteny using model legumes, such as Medicago truncatula Gaertn. and Lotus japonicus (Regel) K. Larsen, have identified the sequences of 15 symbiotic regulatory genes in pea. These genes encode receptor kinases, an ion channel, a calcium/calmodulin-dependent protein kinase, transcription factors, a metal transporter, and an enzyme. This review summarizes and describes mutant alleles, their phenotypic manifestations, and the functions of all identified symbiotic regulatory genes in pea. Some examples of gene interactions are also given. In the review, all mutant alleles in genes with identified sequences are designated and still-unidentified symbiotic regulatory genes of great interest are considered. The identification of these genes will help elucidate additional components involved in infection thread growth, nodule primordium development, bacteroid differentiation and maintenance, and the autoregulation of nodulation. The significance of symbiotic mutants of pea as extremely fruitful genetic models for studying nodule development and for comparative cell biology studies of legume nodules is clearly demonstrated. Finally, it is noted that many more sequences of symbiotic regulatory genes remain to be identified. Transcriptomics approaches and genome-wide sequencing could help address this challenge.

Ca2+-regulated Ca2+ channels with an RCK gating ring control plant symbiotic associations

A family of plant nuclear ion channels, including DMI1 (Does not Make Infections 1) and its homologs CASTOR and POLLUX, are required for the establishment of legume-microbe symbioses by generating nuclear and perinuclear Ca²⁺ spiking. Here we show that CASTOR from Lotus japonicus is a highly selective Ca²⁺ channel whose activation requires cytosolic/nucleosolic Ca²⁺, contrary to the previous suggestion of it being a K⁺ channel. Structurally, the cytosolic/nucleosolic ligand-binding soluble region of CASTOR contains two tandem RCK (Regulator of Conductance for K⁺) domains, and four subunits assemble into the gating ring architecture, similar to that of large conductance, Ca²⁺-gated K⁺ (BK) channels despite the lack of sequence similarity. Multiple ion binding sites are clustered at two locations within each subunit, and three of them are identified to be Ca²⁺ sites. Our in vitro and in vivo assays also demonstrate the importance of these gating-ring Ca²⁺ binding sites to the physiological function of CASTOR as well as DMI1.

CASTOR is a Lotus japonicus ion channel required for nuclear Ca²⁺ spiking and establishing rhizobial and mycorrhizal symbioses. Here, via structural and functional analysis, Kim et al. show that CASTOR is a Ca²⁺-selective channel activated via Ca²⁺ binding to a soluble gating ring consisting of tandem RCK domains.

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

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

Genome Sequencing and Analysis of the Peanut B-Genome Progenitor (Arachis ipaensis)

Peanut (Arachis hypogaea L.), an important leguminous crop, is widely cultivated in tropical and subtropical regions. Peanut is an allotetraploid, having A and B subgenomes that maybe have originated in its diploid progenitors Arachis duranensis (A-genome) and Arachis ipaensis (B-genome), respectively. We previously sequenced the former and here present the draft genome of the latter, expanding our knowledge of the unique biology of Arachis. The assembled genome of A. ipaensis is ~1.39 Gb with 39,704 predicted protein-encoding genes. A gene family analysis revealed that the FAR1 family may be involved in regulating peanut special fruit development. Genomic evolutionary analyses estimated that the two progenitors diverged ~3.3 million years ago and suggested that A. ipaensis experienced a whole-genome duplication event after the divergence of Glycine max. We identified a set of disease resistance-related genes and candidate genes for biological nitrogen fixation. In particular, two and four homologous genes that may be involved in the regulation of nodule development were obtained from A. ipaensis and A. duranensis, respectively. We outline a comprehensive network involved in drought adaptation. Additionally, we analyzed the metabolic pathways involved in oil biosynthesis and found genes related to fatty acid and triacylglycerol synthesis. Importantly, three new FAD2 homologous genes were identified from A. ipaensis and one was completely homologous at the amino acid level with FAD2 from A. hypogaea. The availability of the A. ipaensis and A. duranensis genomic assemblies will advance our knowledge of the peanut genome.

Screening and identification of key genes regulating fall dormancy in alfalfa leaves

Fall dormancy (FD) determines the adaptation of an alfalfa variety and affects alfalfa production and quality. However, the molecular mechanism underlying FD remains poorly understood. Here, 44 genes regulating FD were identified by comparison of the transcriptomes from leaves of Maverick (fall-dormant alfalfa) and CUF101(non-fall-dormant), during FD and non-FD and were classified them depending on their function. The transcription of IAA-amino acid hydrolase ILR1-like 1, abscisic acid receptor PYL8, and monogalactosyldiacylglycerol synthase-3 in Maverick leaves was regulated by daylength and temperature, and the transcription of the abscisic acid receptor PYL8 was mainly affected by daylength. The changes in the expression of these genes and the abundance of their messenger RNA (mRNA) in Maverick leaves differed from those in CUF101 leaves, as evidenced by the correlation analysis of their mRNA abundance profiles obtained from April to October. The present findings suggested that these genes are involved in regulating FD in alfalfa.

Comprehensive Comparative Genomic and Transcriptomic Analyses of the Legume Genes Controlling the Nodulation Process

Nitrogen is one of the most essential plant nutrients and one of the major factors limiting crop productivity. Having the goal to perform a more sustainable agriculture, there is a need to maximize biological nitrogen fixation, a feature of legumes. To enhance our understanding of the molecular mechanisms controlling the interaction between legumes and rhizobia, the symbiotic partner fixing and assimilating the atmospheric nitrogen for the plant, researchers took advantage of genetic and genomic resources developed across different legume models (e.g., Medicago truncatula, Lotus japonicus, Glycine max, and Phaseolus vulgaris) to identify key regulatory protein coding genes of the nodulation process. In this study, we are presenting the results of a comprehensive comparative genomic analysis to highlight orthologous and paralogous relationships between the legume genes controlling nodulation. Mining large transcriptomic datasets, we also identified several orthologous and paralogous genes characterized by the induction of their expression during nodulation across legume plant species. This comprehensive study prompts new insights into the evolution of the nodulation process in legume plant and will benefit the scientific community interested in the transfer of functional genomic information between species.

Full-length de novo assembly of RNA-seq data in pea (Pisum sativum L.) provides a gene expression atlas and gives insights into root nodulation in this species

Next-generation sequencing technologies allow an almost exhaustive survey of the transcriptome, even in species with no available genome sequence. To produce a Unigene set representing most of the expressed genes of pea, 20 cDNA libraries produced from various plant tissues harvested at various developmental stages from plants grown under contrasting nitrogen conditions were sequenced. Around one billion reads and 100 Gb of sequence were de novo assembled. Following several steps of redundancy reduction, 46 099 contigs with N50 length of 1667 nt were identified. These constitute the 'Caméor' Unigene set. The high depth of sequencing allowed identification of rare transcripts and detected expression for approximately 80% of contigs in each library. The Unigene set is now available online (http://bios.dijon.inra.fr/FATAL/cgi/pscam.cgi), allowing (i) searches for pea orthologs of candidate genes based on gene sequences from other species, or based on annotation, (ii) determination of transcript expression patterns using various metrics, (iii) identification of uncharacterized genes with interesting patterns of expression, and (iv) comparison of gene ontology pathways between tissues. This resource has allowed identification of the pea orthologs of major nodulation genes characterized in recent years in model species, as a major step towards deciphering unresolved pea nodulation phenotypes. In addition to a remarkable conservation of the early transcriptome nodulation apparatus between pea and Medicago truncatula, some specific features were highlighted. The resource provides a reference for the pea exome, and will facilitate transcriptome and proteome approaches as well as SNP discovery in pea.

Leguminous plants: inventors of root nodules to accommodate symbiotic bacteria

Legumes and a few other plant species can establish a symbiotic relationship with nitrogen-fixing rhizobia, which enables them to survive in a nitrogen-deficient environment. During the course of nodulation, infection with rhizobia induces the dedifferentiation of host cells to form primordia of a symbiotic organ, the nodule, which prepares plants to accommodate rhizobia in host cells. While these nodulation processes are known to be genetically controlled by both plants and rhizobia, recent advances in studies on two model legumes, Lotus japonicus and Medicago truncatula, have provided great insight into the underlying plant-side molecular mechanism. In this chapter, we review such knowledge, with particular emphasis on two key processes of nodulation, nodule development and rhizobial invasion.

Identification of quantitative trait loci and candidate genes for specific cellular resistance responses against Didymella pinodes in pea

KEY MESSAGE

Phenotyping of specific cellular resistance responses and improvement of previous genetic map allowed the identification of novel genomic regions controlling cellular mechanisms involved in pea resistance to ascochyta blight and provided candidate genes suitable for MAS. Didymella pinodes, causing ascochyta blight, is a major pathogen of the pea crop and is responsible for serious damage and yield losses. Resistance is inherited polygenically and several quantitative trait loci (QTLs) have been already identified. However, the position of these QTLs should be further refined to identify molecular markers more closely linked to the resistance genes. In previous works, resistance was scored visually estimating the final disease symptoms; in this study, we have conducted a more precise phenotyping of resistance evaluating specific cellular resistance responses at the histological level to perform a more accurate QTL analysis. In addition, P665 × Messire genetic map used to identify the QTLs was improved by adding 117 SNP markers located in genes. This combined approach has allowed the identification, for the first time, of genomic regions controlling cellular mechanisms directly involved in pea resistance to ascochyta blight. Furthermore, the inclusion of the gene-based SNP markers has allowed the identification of candidate genes co-located with QTLs and has provided robust markers for marker-assisted selection.

Nuclear calcium signaling in plants

Plant cell nuclei can generate calcium responses to a variety of inputs, tantamount among them the response to signaling molecules from symbiotic microorganisms .

The role of DMI1 in establishing Ca (2+) oscillations in legume symbioses

Calcium (Ca (2+)) is a key secondary messenger in many plant signaling pathways. One such pathway is the SYM pathway, required in the establishment of both arbuscular mycorrhizal and rhizobial root symbioses with legume host plants. (1) When the host plant has perceived the diffusible signals from the microbial symbionts, one of the earliest physiological responses are Ca (2+) oscillations in and around the nucleus. (2) These oscillations are essential for activating downstream gene expression, but the precise mechanisms of encoding and decoding the Ca (2+) signals are unclear and still under intense investigation. Here we put forward a hypothesis for the mechanism of the cation channel DMI1.

Buffering capacity explains signal variation in symbiotic calcium oscillations

Legumes form symbioses with rhizobial bacteria and arbuscular mycorrhizal fungi that aid plant nutrition. A critical component in the establishment of these symbioses is nuclear-localized calcium (Ca(2+)) oscillations. Different components on the nuclear envelope have been identified as being required for the generation of the Ca(2+) oscillations. Among these an ion channel, Doesn't Make Infections1, is preferentially localized on the inner nuclear envelope and a Ca(2+) ATPase is localized on both the inner and outer nuclear envelopes. Doesn't Make Infections1 is conserved across plants and has a weak but broad similarity to bacterial potassium channels. A possible role for this cation channel could be hyperpolarization of the nuclear envelope to counterbalance the charge caused by the influx of Ca(2+) into the nucleus. Ca(2+) channels and Ca(2+) pumps are needed for the release and reuptake of Ca(2+) from the internal store, which is hypothesized to be the nuclear envelope lumen and endoplasmic reticulum, but the release mechanism of Ca(2+) remains to be identified and characterized. Here, we develop a mathematical model based on these components to describe the observed symbiotic Ca(2+) oscillations. This model can recapitulate Ca(2+) oscillations, and with the inclusion of Ca(2+)-binding proteins it offers a simple explanation for several previously unexplained phenomena. These include long periods of frequency variation, changes in spike shape, and the initiation and termination of oscillations. The model also predicts that an increase in buffering capacity in the nucleoplasm would cause a period of rapid oscillations. This phenomenon was observed experimentally by adding more of the inducing signal.

Reproducible hairy root transformation and spot-inoculation methods to study root symbioses of pea

Pea has lagged behind other model legumes in the molecular study of nodulation and mycorrhizae-formation because of the difficulty to transform its roots and its poor growth on agar plates. Here we describe for pea 1) a transformation technique which permits the complementation of two known non-nodulating pea mutants, 2) a rhizobial inoculation method which allows the study of early cellular events giving rise to nodule primordia, and 3) a targeted fungal inoculation method which allows us to study short segments of mycorrhizal roots assured to be infected. These tools are certain to advance our knowledge of pea root symbioses.

Genetic dissection of Rhizobium-induced infection and nodule organogenesis in pea based on ENOD12A and ENOD5 expression analysis

In legumes, perception of rhizobial lipochitooligosacharide-based molecules (Nod factors) and subsequent signal transduction triggers transcription of plant symbiosis-specific genes (early nodulins). We present genetic dissection of Nod factor-controlled processes in Pisum sativum using two early nodulin genes PsENOD12a and PsENOD5, that are differentially up-regulated during symbiosis. A novel set of non-nodulating pea mutants in fourteen loci was examined, among which seven loci are not described in Lotus japonicus and Medicago truncatula. Mutants defective in Pssym10, Pssym8, Pssym19, Pssym9 and Pssym7 exhibited no PsENOD12a and PsENOD5 activation in response to Nod factor-producing rhizobia. Thus, a conserved signalling module from the LysM receptor kinase encoded by Pssym10 down to the GRAS transcription factor encoded by Pssym7 is essential for Nod factor-induced gene expression. Of the two investigated genes, PsENOD5 was more strictly regulated; not only requiring the SYM10-SYM7 module, but also SYM35 (NIN transcription factor), SYM14, SYM16 and SYM34. Since Pssym35, Pssym14, Pssym34 and Pssym16 mutants show arrested infection and nodule formation at various stages, PsENOD5 expression seems to be essential for later symbiotic events, when rhizobia enter into plant tissues. Activation of PsENOD12a only requires components involved in early steps of signalling and can be considered as a marker of early symbiotic events preceding infection.

Chitooligosaccharide sensing and downstream signaling: contrasted outcomes in pathogenic and beneficial plant-microbe interactions

In plants, short chitin oligosaccharides and chitosan fragments (collectively referred to as chitooligosaccharides) are well-known elicitors that trigger defense gene expression, synthesis of antimicrobial compounds, and cell wall strengthening. Recent findings have shed new light on chitin-sensing mechanisms and downstream activation of intracellular signaling networks that mediate plant defense responses. Interestingly, chitin receptors possess several lysin motif domains that are also found in several legume Nod factor receptors. Nod factors are chitin-related molecules produced by nitrogen-fixing rhizobia to induce root nodulation. The fact that chitin and Nod factor receptors share structural similarity suggests an evolutionary conserved relationship between mechanisms enabling recognition of both deleterious and beneficial microorganisms. Here, we will present an update on molecular events involved in chitooligosaccharide sensing and downstream signaling pathways in plants and will discuss how structurally related signals may lead to such contrasted outcomes during plant-microbe interactions.

Highly-multiplexed SNP genotyping for genetic mapping and germplasm diversity studies in pea

BACKGROUND

Single Nucleotide Polymorphisms (SNPs) can be used as genetic markers for applications such as genetic diversity studies or genetic mapping. New technologies now allow genotyping hundreds to thousands of SNPs in a single reaction.In order to evaluate the potential of these technologies in pea, we selected a custom 384-SNP set using SNPs discovered in Pisum through the resequencing of gene fragments in different genotypes and by compiling genomic sequence data present in databases. We then designed an Illumina GoldenGate assay to genotype both a Pisum germplasm collection and a genetic mapping population with the SNP set.

RESULTS

We obtained clear allelic data for more than 92% of the SNPs (356 out of 384). Interestingly, the technique was successful for all the genotypes present in the germplasm collection, including those from species or subspecies different from the P. sativum ssp sativum used to generate sequences. By genotyping the mapping population with the SNP set, we obtained a genetic map and map positions for 37 new gene markers.

CONCLUSION

Our results show that the Illumina GoldenGate assay can be used successfully for high-throughput SNP genotyping of diverse germplasm in pea. This genotyping approach will simplify genotyping procedures for association mapping or diversity studies purposes and open new perspectives in legume genomics.

Genetic dissection of nitrogen nutrition in pea through a QTL approach of root, nodule, and shoot variability

Pea (Pisum sativum L.) is the third most important grain legume worldwide, and the increasing demand for protein-rich raw material has led to a great interest in this crop as a protein source. Seed yield and protein content in crops are strongly determined by nitrogen (N) nutrition, which in legumes relies on two complementary pathways: absorption by roots of soil mineral nitrogen, and fixation in nodules of atmospheric dinitrogen through the plant-Rhizobium symbiosis. This study assessed the potential of naturally occurring genetic variability of nodulated root structure and functioning traits to improve N nutrition in pea. Glasshouse and field experiments were performed on seven pea genotypes and on the 'Cameor' x 'Ballet' population of recombinant inbred lines selected on the basis of parental contrast for root and nodule traits. Significant variation was observed for most traits, which were obtained from non-destructive kinetic measurements of nodulated root and shoot in pouches, root and shoot image analysis, (15)N quantification, or seed yield and protein content determination. A significant positive relationship was found between nodule establishment and root system growth, both among the seven genotypes and the RIL population. Moreover, several quantitative trait loci for root or nodule traits and seed N accumulation were mapped in similar locations, highlighting the possibility of breeding new pea cultivars with increased root system size, sustained nodule number, and improved N nutrition. The impact on both root or nodule traits and N nutrition of the genomic regions of the major developmental genes Le and Af was also underlined.

Nuclear membrane ion channels mediate root nodule development

Previous work has implicated two predicted ion channels in mediating perinuclear calcium spiking, which is essential for rhizobia-induced root nodule formation in legumes. A new study demonstrates that these ion channels are preferentially permeable to cations, such as potassium, and are located in the nuclear envelope. Here, we consider ways in which the ion channels influence perinuclear calcium spiking and discuss a potentially broader role for nuclear membrane ion channels in signal transduction in plants.

Positioning the nodule, the hormone dictum

The formation of a nitrogen-fixing nodule involves two diverse developmental processes in the legume root: infection thread initiation in epidermal cells and nodule primordia formation in the cortex. Several plant hormones have been reported to positively or negatively regulate nodulation. These hormones function at different stages in the nodulation process and may facilitate the coordinated development of the epidermal and cortical developmental programs that are necessary to allow bacterial infection into the developing nodule. In this paper, we review and discuss how the tissue specific nature of hormonal action dictates where, when and how a nodule is formed.

Antiquity and function of CASTOR and POLLUX, the twin ion channel-encoding genes key to the evolution of root symbioses in plants

Root symbioses with arbuscular mycorrhizal fungi and rhizobial bacteria share a common signaling pathway in legumes. Among the common symbiosis genes are CASTOR and POLLUX, the twin homologous genes in Lotus japonicus that encode putative ion channel proteins. Here, we show that the orthologs of CASTOR and POLLUX are ubiquitously present and highly conserved in both legumes and nonlegumes. Using rice (Oryza sativa) as a study system, we employ reverse genetic tools (knockout mutants and RNA interference) to demonstrate that Os-CASTOR and Os-POLLUX are indispensable for mycorrhizal symbiosis in rice. Furthermore, a cross-species complementation test indicates that Os-POLLUX can restore nodulation, but not rhizobial infection, to a Medicago truncatula dmi1 mutant.

Lotus japonicus CASTOR and POLLUX are ion channels essential for perinuclear calcium spiking in legume root endosymbiosis

The mechanism underlying perinuclear calcium spiking induced during legume root endosymbioses is largely unknown. Lotus japonicus symbiosis-defective castor and pollux mutants are impaired in perinuclear calcium spiking. Homology modeling suggested that the related proteins CASTOR and POLLUX might be ion channels. Here, we show that CASTOR and POLLUX form two independent homocomplexes in planta. CASTOR reconstituted in planar lipid bilayers exhibited ion channel activity, and the channel characteristics were altered in a symbiosis-defective mutant carrying an amino acid replacement close to the selectivity filter. Permeability ratio determination and competition experiments reveled a weak preference of CASTOR for cations such as potassium over anions. POLLUX has an identical selectivity filter region and complemented a potassium transport-deficient yeast mutant, suggesting that POLLUX is also a potassium-permeable channel. Immunogold labeling localized the endogenous CASTOR protein to the nuclear envelope of Lotus root cells. Our data are consistent with a role of CASTOR and POLLUX in modulating the nuclear envelope membrane potential. They could either trigger the opening of calcium release channels or compensate the charge release during the calcium efflux as counter ion channels.

Coordinating nodule morphogenesis with rhizobial infection in legumes

The formation of nitrogen-fixing nodules on legumes requires an integration of infection by rhizobia at the root epidermis and the initiation of cell division in the cortex, several cell layers away from the sites of infection. Several recent developments have added to our understanding of the signaling events in the epidermis associated with the perception of rhizobial nodulation factors and the role of plant hormones in the activation of cell division leading to nodule morphogenesis. This review focuses on the tissue-specific nature of the developmental processes associated with nodulation and the mechanisms by which these processes are coordinated during the formation of a nodule.