NENA, a Lotus japonicus homolog of Sec13, is required for rhizodermal infection by arbuscular mycorrhiza fungi and rhizobia but dispensable for cortical endosymbiotic development

Cellular insights into legume root infection by rhizobia

Legume plants establish an endosymbiosis with nitrogen-fixing rhizobia bacteria, which are taken up from the environment anew by each host generation. This requires a dedicated genetic program on the host side to control microbe invasion, involving coordinated reprogramming of host cells to create infection structures that facilitate inward movement of the symbiont. Infection initiates in the epidermis, with different legumes utilizing distinct strategies for crossing this cell layer, either between cells (intercellular infection) or transcellularly (infection thread infection). Recent discoveries on the plant side using fluorescent-based imaging approaches have illuminated the spatiotemporal dynamics of infection, underscoring the importance of investigating this process at the dynamic single-cell level. Extending fluorescence-based live-dynamic approaches to the bacterial partner opens the exciting prospect of learning how individual rhizobia reprogram from rhizospheric to a host-confined state during early root infection.

Identification of QTLs for symbiotic nitrogen fixation and related traits in a soybean recombinant inbred line population

KEY MESSAGE

The genetic architecture of symbiotic N fixation and related traits was investigated in the field. QTLs were identified for percent N derived from the atmosphere, shoot [N] and C to N ratio. Soybean [Glycine max (L.) Merr.] is cultivated worldwide and is the most abundant source of plant-based protein. Symbiotic N2 fixation (SNF) in legumes such as soybean is of great importance; however, yields may still be limited by N in both high yielding and stressful environments. To better understand the genetic architecture of SNF and facilitate the development of high yielding cultivars and sustainable soybean production in stressful environments, a recombinant inbred line population consisting of 190 lines, developed from a cross between PI 442012A and PI 404199, was evaluated for N derived from the atmosphere (Ndfa), N concentration ([N]), and C to N ratio (C/N) in three environments. Significant genotype, environment and genotype × environment effects were observed for all three traits. A linkage map was constructed containing 3309 single nucleotide polymorphism (SNP) markers. QTL analysis was performed for additive effects of QTLs, QTL × environment interactions, and QTL × QTL interactions. Ten unique additive QTLs were identified across all traits and environments. Of these, two QTLs were detected for Ndfa and eight for C/N. Of the eight QTLs for C/N, four were also detected for [N]. Using QTL × environment analysis, six QTLs were detected, of which five were also identified in the additive QTL analysis. The QTL × QTL analysis identified four unique epistatic interactions. The results of this study may be used for genomic selection and introgression of favorable alleles for increased SNF, [N], and C/N via marker-assisted selection.

Cellular and molecular basis of symbiotic nodule development

Root nodule development plays a vital role in establishing the mutualistic relationship between legumes and nitrogen-fixing rhizobia. Two primary processes are involved in nodule development: formative cell divisions in the root cortex and the subsequent differentiation of nodule cells. The first process involves the mitotic reactivation of differentiated root cortex cells to form nodule primordium after perceiving symbiotic signals. The second process enables the nascent nodule primordium cells to develop into various cell types, leading to the creation of a functional nodule capable of supporting nitrogen fixation. Thus, both division and differentiation of nodule cells are crucial for root nodule development. This review provides an overview of the most recent advancements in comprehending the cellular and molecular mechanisms underlying symbiotic nodule development in legumes.

Cytoskeleton as a roadmap navigating rhizobia to establish symbiotic root nodulation in legumes

Legumes enter into symbiotic associations with soil nitrogen-fixing rhizobia, culminating in the creation of new organs, root nodules. This complex process relies on chemical and physical interaction between legumes and rhizobia, including early signalling events informing the host legume plant of a potentially beneficial microbe and triggering the nodulation program. The great significance of this plant-microbe interaction rests upon conversion of atmospheric dinitrogen not accessible to plants into a biologically active form of ammonia available to plants. The plant cytoskeleton consists in a highly dynamic network and undergoes rapid remodelling upon sensing various developmental and environmental cues, including response to attachment, internalization, and accommodation of rhizobia in plant root and nodule cells. This dynamic nature is governed by cytoskeleton-associated proteins that modulate cytoskeletal behaviour depending on signal perception and transduction. Precisely localized cytoskeletal rearrangements are therefore essential for the uptake of rhizobia, their targeted delivery, and establishing beneficial root nodule symbiosis. This review summarizes current knowledge about rhizobia-dependent rearrangements and functions of the cytoskeleton in legume roots and nodules. General patterns and nodule type-, nodule stage-, and species-specific aspects of actin filaments and microtubules remodelling are discussed. Moreover, emerging evidence is provided about fine-tuning the root nodulation process through cytoskeleton-associated proteins. We also consider future perspectives on dynamic localization studies of the cytoskeleton during early symbiosis utilizing state of the art molecular and advanced microscopy approaches. Based on acquired detailed knowledge of the mutualistic interactions with microbes, these approaches could contribute to broader biotechnological crop improvement.

MsSPL9 Modulates Nodulation under Nitrate Sufficiency Condition in Medicago sativa

Nodulation in Leguminous spp. is induced by common environmental cues, such as low nitrogen availability conditions, in the presence of the specific Rhizobium spp. in the rhizosphere. Medicago sativa (alfalfa) is an important nitrogen-fixing forage crop that is widely cultivated around the world and relied upon as a staple source of forage in livestock feed. Although alfalfa's relationship with these bacteria is one of the most efficient between rhizobia and legume plants, breeding for nitrogen-related traits in this crop has received little attention. In this report, we investigate the role of Squamosa-Promoter Binding Protein-Like 9 (SPL9), a target of miR156, in nodulation in alfalfa. Transgenic alfalfa plants with SPL9-silenced (SPL9-RNAi) and overexpressed (35S::SPL9) were compared to wild-type (WT) alfalfa for phenotypic changes in nodulation in the presence and absence of nitrogen. Phenotypic analyses showed that silencing of MsSPL9 in alfalfa caused an increase in the number of nodules. Moreover, the characterization of phenotypic and molecular parameters revealed that MsSPL9 regulates nodulation under a high concentration of nitrate (10 mM KNO₃) by regulating the transcription levels of the nitrate-responsive genes Nitrate Reductase1 (NR1), NR2, Nitrate transporter 2.5 (NRT2.5), and a shoot-controlled autoregulation of nodulation (AON) gene, Super numeric nodules (SUNN). While MsSPL9-overexpressing transgenic plants have dramatically increased transcript levels of SUNN, NR1, NR2, and NRT2.5, reducing MsSPL9 caused downregulation of these genes and displayed a nitrogen-starved phenotype, as downregulation of the MsSPL9 transcript levels caused a nitrate-tolerant nodulation phenotype. Taken together, our results suggest that MsSPL9 regulates nodulation in alfalfa in response to nitrate.

Plant pathogens and symbionts target the plant nucleus

In plant-microbe interactions, symbionts and pathogens live within plants and attempt to avoid triggering plant defense responses. In order to do so, these microbes have evolved multiple mechanisms that target components of the plant cell nucleus. Rhizobia-induced symbiotic signaling requires the function of specific legume nucleoporins within the nuclear pore complex. Symbiont and pathogen effectors harbor nuclear localization sequences that facilitate movement across nuclear pores, allowing these proteins to target transcription factors that function in defense. Oomycete pathogens introduce proteins that interact with plant pre-mRNA splicing components in order to alter host splicing of defense-related transcripts. Together, these functions indicate that the nucleus is an active site of symbiotic and pathogenic functioning in plant-microbe interactions.

RPG interacts with E3-ligase CERBERUS to mediate rhizobial infection in Lotus japonicus

Symbiotic interactions between rhizobia and legumes result in the formation of root nodules, which fix nitrogen that can be used for plant growth. Rhizobia usually invade legume roots through a plant-made tunnel-like structure called an infection thread (IT). RPG (Rhizobium-directed polar growth) encodes a coiled-coil protein that has been identified in Medicago truncatula as required for root nodule infection, but the function of RPG remains poorly understood. In this study, we identified and characterized RPG in Lotus japonicus and determined that it is required for IT formation. RPG was induced by Mesorhizobium loti or purified Nodulation factor and displayed an infection-specific expression pattern. Nodule inception (NIN) bound to the RPG promoter and induced its expression. We showed that RPG displayed punctate subcellular localization in L. japonicus root protoplasts and in root hairs infected by M. loti. The N-terminal predicted C2 lipid-binding domain of RPG was not required for this subcellular localization or for function. CERBERUS, a U-box E3 ligase which is also required for rhizobial infection, was found to be localized similarly in puncta. RPG co-localized and directly interacted with CERBERUS in the early endosome (TGN/EE) compartment and near the nuclei in root hairs after rhizobial inoculation. Our study sheds light on an RPG-CERBERUS protein complex that is involved in an exocytotic pathway mediating IT elongation.

Signaling and Detoxification Strategies in Plant-Microbes Symbiosis under Heavy Metal Stress: A Mechanistic Understanding

Plants typically interact with a variety of microorganisms, including bacteria, mycorrhizal fungi, and other organisms, in their above- and below-ground parts. In the biosphere, the interactions of plants with diverse microbes enable them to acquire a wide range of symbiotic advantages, resulting in enhanced plant growth and development and stress tolerance to toxic metals (TMs). Recent studies have shown that certain microorganisms can reduce the accumulation of TMs in plants through various mechanisms and can reduce the bioavailability of TMs in soil. However, relevant progress is lacking in summarization. This review mechanistically summarizes the common mediating pathways, detoxification strategies, and homeostatic mechanisms based on the research progress of the joint prevention and control of TMs by arbuscular mycorrhizal fungi (AMF)-plant and Rhizobium-plant interactions. Given the importance of tripartite mutualism in the plant-microbe system, it is necessary to further explore key signaling molecules to understand the role of plant-microbe mutualism in improving plant tolerance under heavy metal stress in the contaminated soil environments. It is hoped that our findings will be useful in studying plant stress tolerance under a broad range of environmental conditions and will help in developing new technologies for ensuring crop health and performance in future.

Nod factor perception: an integrative view of molecular communication during legume symbiosis

Compatible interaction between rhizobial Nod factors and host receptors enables initial recognition and signaling events during legume-rhizobia symbiosis. Molecular communication is a new paradigm of information relay, which uses chemical signals or molecules as dialogues for communication and has been witnessed in prokaryotes, plants as well as in animal kingdom. Understanding this fascinating relay of signals between plants and rhizobia during the establishment of a synergistic relationship for biological nitrogen fixation represents one of the hotspots in plant biology research. Predominantly, their interaction is initiated by flavonoids exuding from plant roots, which provokes changes in the expression profile of rhizobial genes. Compatible interactions promote the secretion of Nod factors (NFs) from rhizobia, which are recognised by cognate host receptors. Perception of NFs by host receptors initiates the symbiosis and ultimately leads to the accommodation of rhizobia within root nodules via a series of mutual exchange of signals. This review elucidates the bacterial and plant perspectives during the early stages of symbiosis, explicitly emphasizing the significance of NFs and their cognate NF receptors.

An Efficient Agrobacterium rhizogenes-Mediated Hairy Root Transformation Method in a Soybean Root Biology Study

The stable genetic transformation of soybean is time-consuming and inefficient. As a simple and practical alternative method, hairy root transformation mediated by Agrobacterium rhizogenes is widely applied in studying root-specific processes, nodulation, biochemical and molecular functions of genes of interest, gene editing efficiency of CRISPR/Cas9, and biological reactors and producers. Therefore, many laboratories have developed unique protocols to obtain hairy roots in composite plants composed of transgenic roots and wild-type shoots. However, these protocols still suffer from the shortcomings of low efficiency and time, space, and cost consumption. To address this issue, we developed a new protocol efficient regeneration and transformation of hairy roots (eR&T) in soybean, by integrating and optimizing the main current methods to achieve high efficiency in both hairy root regeneration and transformation within a shorter period and using less space. By this eR&T method, we obtained 100% regeneration of hairy roots for all explants, with an average 63.7% of transformation frequency, which promoted the simultaneous and comparative analysis of the function of several genes. The eR&T was experimentally verified Promoter:GUS reporters, protein subcellular localization, and CRISPR/Cas9 gene editing experiments. Employing this approach, we identified several novel potential regulators of nodulation, and nucleoporins of the Nup107-160 sub-complex, which showed development-dependent and tissue-dependent expression patterns, indicating their important roles in nodulation in soybean. Thus, the new eR&T method is an efficient and economical approach for investigating not only root and nodule biology, but also gene function.

A CCaMK/Cyclops response element in the promoter of Lotus japonicus calcium-binding protein 1 (CBP1) mediates transcriptional activation in root symbioses

Early gene expression in arbuscular mycorrhiza (AM) and the nitrogen-fixing root nodule symbiosis (RNS) is governed by a shared regulatory complex. Yet many symbiosis-induced genes are specifically activated in only one of the two symbioses. The Lotus japonicus T-DNA insertion line T90, carrying a promoterless uidA (GUS) gene in the promoter of Calcium Binding Protein 1 (CBP1) is exceptional as it exhibits GUS activity in both root endosymbioses. To identify the responsible cis- and trans-acting factors, we subjected deletion/modification series of CBP1 promoter : reporter fusions to transactivation and spatio-temporal expression analysis and screened ethyl methanesulphonate (EMS)-mutagenized T90 populations for aberrant GUS expression. We identified one cis-regulatory element required for GUS expression in the epidermis and a second element, necessary and sufficient for transactivation by the calcium and calmodulin-dependent protein kinase (CCaMK) in combination with the transcription factor Cyclops and conferring gene expression during both AM and RNS. Lack of GUS expression in T90 white mutants could be traced to DNA hypermethylation detected in and around this element. We concluded that the CCaMK/Cyclops complex can contribute to at least three distinct gene expression patterns on its direct target promoters NIN (RNS), RAM1 (AM), and CBP1 (AM and RNS), calling for yet-to-be identified specificity-conferring factors.

A Perspective on Developing a Plant ‘Holobiont’ for Future Saline Agriculture

Soil salinity adversely affects plant growth and has become a major limiting factor for agricultural development worldwide. There is a continuing demand for sustainable technology innovation in saline agriculture. Among various bio-techniques being used to reduce the salinity hazard, symbiotic microorganisms such as rhizobia and arbuscular mycorrhizal (AM) fungi have proved to be efficient. These symbiotic associations each deploy an array of well-tuned mechanisms to provide salinity tolerance for the plant. In this review, we first comprehensively cover major research advances in symbiont-induced salinity tolerance in plants. Second, we describe the common signaling process used by legumes to control symbiosis establishment with rhizobia and AM fungi. Multi-omics technologies have enabled us to identify and characterize more genes involved in symbiosis, and eventually, map out the key signaling pathways. These developments have laid the foundation for technological innovations that use symbiotic microorganisms to improve crop salt tolerance on a larger scale. Thus, with the aim of better utilizing symbiotic microorganisms in saline agriculture, we propose the possibility of developing non-legume ‘holobionts’ by taking advantage of newly developed genome editing technology. This will open a new avenue for capitalizing on symbiotic microorganisms to enhance plant saline tolerance for increased sustainability and yields in saline agriculture.

Innovation and appropriation in mycorrhizal and rhizobial Symbioses

Most land plants benefit from endosymbiotic interactions with mycorrhizal fungi, including legumes and some nonlegumes that also interact with endosymbiotic nitrogen (N)-fixing bacteria to form nodules. In addition to these helpful interactions, plants are continuously exposed to would-be pathogenic microbes: discriminating between friends and foes is a major determinant of plant survival. Recent breakthroughs have revealed how some key signals from pathogens and symbionts are distinguished. Once this checkpoint has been passed and a compatible symbiont is recognized, the plant coordinates the sequential development of two types of specialized structures in the host. The first serves to mediate infection, and the second, which appears later, serves as sophisticated intracellular nutrient exchange interfaces. The overlap in both the signaling pathways and downstream infection components of these symbioses reflects their evolutionary relatedness and the common requirements of these two interactions. However, the different outputs of the symbioses, phosphate uptake versus N fixation, require fundamentally different components and physical environments and necessitated the recruitment of different master regulators, NODULE INCEPTION-LIKE PROTEINS, and PHOSPHATE STARVATION RESPONSES, for nodulation and mycorrhization, respectively.

Regulation of the Later Stages of Nodulation Stimulated by IPD3/CYCLOPS Transcription Factor and Cytokinin in Pea Pisum sativum L

The IPD3/CYCLOPS transcription factor was shown to be involved in the regulation of nodule primordia development and subsequent stages of nodule differentiation. In contrast to early stages, the stages related to nodule differentiation remain less studied. Recently, we have shown that the accumulation of cytokinin at later stages may significantly impact nodule development. This conclusion was based on a comparative analysis of cytokinin localization between pea wild type and ipd3/cyclops mutants. However, the role of cytokinin at these later stages of nodulation is still far from understood. To determine a set of genes involved in the regulation of later stages of nodule development connected with infection progress, intracellular accommodation, as well as plant tissue and bacteroid differentiation, the RNA-seq analysis of pea mutant SGEFix--2 (sym33) nodules impaired in these processes compared to wild type SGE nodules was performed. To verify cytokinin's influence on late nodule development stages, the comparative RNA-seq analysis of SGEFix--2 (sym33) mutant plants treated with cytokinin was also conducted. Findings suggest a significant role of cytokinin in the regulation of later stages of nodule development.

Role of ethylene in effective establishment of the peanut-bradyrhizobia symbiotic interaction

Ethylene has been implicated in nitrogen fixing symbioses in legumes, where rhizobial invasion occurs via infection threads (IT). In the symbiosis between peanut (Arachis hypogaea L.) and bradyrhizobia, the bacteria penetrate the root cortex intercellularly and IT are not formed. Little attention has been paid to the function of ethylene in the establishment of this symbiosis. The aim of this article is to evaluate whether ethylene plays a role in the development of this symbiotic interaction and the participation of Nod Factors (NF) in the regulation of ethylene signalling. Manipulation of ethylene in peanut was accomplished by application of 1-aminocyclopropane-1-carboxylic acid (ACC), which mimics applied ethylene, or AgNO_3, which blocks ethylene responses. To elucidate the participation of NF in the regulation of ethylene signalling, we inoculated plants with a mutant isogenic rhizobial strain unable to produce NF and evaluated the effect of AgNO₃ on gene expression of NF and ethylene responsive signalling pathways. Data revealed that ethylene perception is required for the formation of nitrogen-fixing nodules, while addition of ACC does not affect peanut symbiotic performance. This phenotypic evidence is in agreement with transcriptomic data from genes involved in symbiotic and ethylene signalling pathways. NF seem to modulate the expression of ethylene signalling genes. Unlike legumes infected through IT formation, ACC addition to peanut does not adversely affect nodulation, but ethylene perception is required for establishment of this symbiosis. Evidence for the contribution of NF to the modulation of ethylene-inducible defence gene expression is provided.

Nod factor receptor complex phosphorylates GmGEF2 to stimulate ROP signaling during nodulation

The establishment of the symbiotic interaction between rhizobia and legumes involves the Nod factor signaling pathway. Nod factor recognition occurs through two plant receptors, NFR1 and NFR5. However, the signal transduction mechanisms downstream of NFR1-NFR5-mediated Nod factor perception remain largely unknown. Here, we report that a small guanosine triphosphatase (GTPase), GmROP9, and a guanine nucleotide exchange factor, GmGEF2, are involved in the soybean-rhizobium symbiosis. We show that GmNFR1α phosphorylates GmGEF2a at its N-terminal S86, which stimulates guanosine diphosphate (GDP)-to-GTP exchange to activate GmROP9 and that the active form of GmROP9 can associate with both GmNFR1α and GmNFR5α. We further show that a scaffold protein, GmRACK1, interacts with active GmROP9 and contributes to root nodule symbiosis. Collectively, our results highlight the symbiotic role of GmROP9-GmRACK1 and support the hypothesis that rhizobial signals promote the formation of a protein complex comprising GmNFR1, GmNFR5, GmROP9, and GmRACK1 for symbiotic signal transduction in soybean.

Natural variation identifies a Pxy gene controlling vascular organisation and formation of nodules and lateral roots in Lotus japonicus

Forward and reverse genetics using the model legumes Lotus japonicus and Medicago truncatula have been instrumental in identifying the essential genes governing legume-rhizobia symbiosis. However, little information is known about the effects of intraspecific variation on symbiotic signalling. Here, we use quantitative trait locus sequencing (QTL-seq) to investigate the genetic basis of the differentiated phenotypic responses shown by the Lotus accessions Gifu and MG20 to inoculation with the Mesorhizobium loti exoU mutant that produces truncated exopolysaccharides. We identified through genetic complementation the Pxy gene as a component of this differential exoU response. Lotus Pxy encodes a leucine-rich repeat receptor-like kinase similar to Arabidopsis thaliana PXY, which regulates stem vascular development. We show that Lotus pxy insertion mutants displayed defects in root and stem vascular organisation, as well as lateral root and nodule formation. Our work links Pxy to de novo organogenesis in the root, highlights the genetic overlap between regulation of lateral root and nodule formation, and demonstrates that natural variation in Pxy affects nodulation signalling.

Novel insights into host receptors and receptor-mediated signaling that regulate arbuscular mycorrhizal symbiosis

More than 80% of land plant species benefit from symbiotic partnerships with arbuscular mycorrhizal (AM) fungi, which assist in nutrient acquisition and enhance the ability of host plants to adapt to environmental constraints. Host-generated plasma membrane-residing receptor-like kinases and the intracellular α/β-hydrolase DWARF14-LIKE, a putative karrikin receptor, detect the presence of AM fungi before physical contact between the host and fungus. Detection induces appropriate symbiotic responses, which subsequently enables a favorable environment for AM symbiosis to occur. To prevent hyper-colonization and maintain a mutually beneficial association, the host plant precisely monitors and controls AM colonization by receptor-like kinases, such as SUPER NUMERIC NODULES. Previous studies have elucidated how host plant receptors and receptor-mediated signaling regulate AM symbiosis, but the underlying molecular mechanisms remain poorly understood. The identification of a rice CHITIN ELICITOR RECEPTOR KINASE 1 interaction partner, MYC FACTOR RECEPTOR 1, and new insights into DWARF14-LIKE receptor- and SUPER NUMERIC NODULES receptor-mediated signaling have expanded our understanding of how host plant receptors and their corresponding signals regulate AM symbiosis. This review summarizes these and other recent relevant findings. The identified receptors and/or their signaling components could be manipulated to engineer crops with improved agronomic traits by conferring the ability to precisely control AM colonization.

Nucleocytoplasmic Communication in Healthy and Diseased Plant Tissues

The double membrane of the nuclear envelope (NE) constitutes a selective compartment barrier that separates nuclear from cytoplasmic processes. Plant viability and responses to a changing environment depend on the spatial communication between both compartments. This communication is based on the bidirectional exchange of proteins and RNAs and is regulated by a sophisticated transport machinery. Macromolecular traffic across the NE depends on nuclear transport receptors (NTRs) that mediate nuclear import (i.e. importins) or export (i.e. exportins), as well as on nuclear pore complexes (NPCs) that are composed of nucleoporin proteins (NUPs) and span the NE. In this review, we provide an overview of plant NPC- and NTR-directed cargo transport and we consider transport independent functions of NPCs and NE-associated proteins in regulating plant developmental processes and responses to environmental stresses.

Calcium spikes, waves and oscillations in plant development and biotic interactions

The calcium ion (Ca²⁺) is a universal signal in all eukaryotic cells. A fundamental question is how Ca²⁺, a simple cation, encodes complex information with high specificity. Extensive research has established a two-step process (encoding and decoding) that governs the specificity of Ca²⁺ signals. While the encoding mechanism entails a complex array of channels and transporters, the decoding process features a number of Ca²⁺ sensors and effectors that convert Ca²⁺ signals into cellular effects. Along this general paradigm, some signalling components may be highly conserved, but others are divergent among different organisms. In plant cells, Ca²⁺ participates in numerous signalling processes, and here we focus on the latest discoveries on Ca²⁺-encoding mechanisms in development and biotic interactions. In particular, we use examples such as polarized cell growth of pollen tube and root hair in which tip-focused Ca²⁺ oscillations specify the signalling events for rapid cell elongation. In plant-microbe interactions, Ca²⁺ spiking and oscillations hold the key to signalling specificity: while pathogens elicit cytoplasmic spiking, symbiotic microorganisms trigger nuclear Ca²⁺ oscillations. Herbivore attacks or mechanical wounding can trigger Ca²⁺ waves traveling a long distance to transmit and convert the local signal to a systemic defence program in the whole plant. What channels and transporters work together to carve out the spatial and temporal patterns of the Ca²⁺ fluctuations? This question has remained enigmatic for decades until recent studies uncovered Ca²⁺ channels that orchestrate specific Ca²⁺ signatures in each of these processes. Future work will further expand the toolkit for Ca²⁺-encoding mechanisms and place Ca²⁺ signalling steps into larger signalling networks.

Nucleoporin 160 Regulates Flowering through Anchoring HOS1 for Destabilizing CO in Arabidopsis

Nuclear pore complexes (NPCs), which comprise multiple copies of nucleoporins (Nups), are large protein assemblies embedded in the nuclear envelope connecting the nucleus and cytoplasm. Although it has been known that Nups affect flowering in Arabidopsis, the underlying mechanisms are poorly understood. Here, we show that loss of function of Nucleoporin 160 (Nup160) leads to increased abundance of CONSTANS (CO) protein and the resulting upregulation of FLOWERING LOCUS T (FT) specifically in the morning. We demonstrate that Nup160 regulates CO protein stability through affecting NPC localization of an E3-ubiquitin ligase, HIGH EXPRESSION OF OSMOTICALLY RESPONSIVE GENES1 (HOS1), which destabilizes CO protein in the morning period. Taken together, these results provide a mechanistic understanding of Nup function in the transition from vegetative to reproductive growth, suggesting that deposition of HOS1 at NPCs by Nup160 is essential for preventing precocious flowering in response to photoperiod in Arabidopsis.

The Lotus japonicus nucleoporin GLE1 is involved in symbiotic association with rhizobia

Nucleoporins are components of the nuclear pore complexes, channels that regulate the transport of macromolecules between the nucleus and cytoplasm. The nucleoporin GLE1 (GLFG lethal1) functions in the export of messenger RNAs containing poly(A) tails from the nucleus into the cytoplasm. Here we investigated a mutant of the model legume Lotus japonicus that was defective in GLE1, which we designated Ljgle1. The growth of Ljgle1 was retarded under symbiotic association with rhizobia, and the nitrogen-fixation activities of the nodules were around one-third of those in the wild-type plant. The growth of Ljgle1 was not substantialy recovered by supplemention of combined nitrogen. Nodules formed on the Ljgle1 were smaller than those on the wild-type and colored faint pink. The numbers of infected cells of nodules on the Ljgle1 were smaller than on the wild-type plant, and the former cells remained undeveloped. Rhizobia in the cells of the Ljgle1 exhibited disordered forms, and the symbiosome membrane was closely attached to the bacterial membrane. These results indicate that GLE1 plays a distinct role in the symbiotic association between legumes and rhizobia.

Isolation, Characterization, and Complete Genome Sequence of a Bradyrhizobium Strain Lb8 From Nodules of Peanut Utilizing Crack Entry Infection

In many legumes, the colonization of roots by rhizobia is via "root hair entry" and its molecular mechanisms have been extensively studied. However, the nodulation of peanuts (Arachis hypogaea L.) by Bradyrhizobium strains requires an intercellular colonization process called "crack entry," which is understudied. To understand the intercellular crack entry process, it is critical to develop the tools and resources related to the rhizobium in addition to focus on investigating the mechanisms of the plant host. In this study, we isolated a Bradyrhizobium sp. strain, Lb8 from peanut root nodules and sequenced it using PacBio long reads. The complete genome sequence was a circular chromosome of 8,718,147 base-pair (bp) with an average GC content of 63.14%. No plasmid sequence was detected in the sequenced DNA sample. A total of 8,433 potential protein-encoding genes, one rRNA cluster, and 51 tRNA genes were annotated. Fifty-eight percent of the predicted genes showed similarity to genes of known functions and were classified into 27 subsystems representing various biological processes. The genome shared 92% of the gene families with B. diazoefficens USDA 110^T. A presumptive symbiosis island of 778 Kb was detected, which included two clusters of nif and nod genes. A total of 711 putative protein-encoding genes were in this region, among which 455 genes have potential functions related to symbiotic nitrogen fixation and DNA transmission. Of 21 genes annotated as transposase, 16 were located in the symbiosis island. Lb8 possessed both Type III and Type IV protein secretion systems, and our work elucidated the association of flagellar Type III secretion systems in bradyrhizobia. These observations suggested that complex rearrangement, such as horizontal transfer and insertion of different DNA elements, might be responsible for the plasticity of the Bradyrhizobium genome.

CLE-HAR1 Systemic Signaling and NIN-Mediated Local Signaling Suppress the Increased Rhizobial Infection in the daphne Mutant of Lotus japonicus

Legumes survive in nitrogen-limited soil by forming a symbiosis with rhizobial bacteria. During root nodule symbiosis, legumes strictly control the development of their symbiotic organs, the nodules, in a process known as autoregulation of nodulation (AON). The study of hypernodulation mutants has elucidated the molecular basis of AON. Some hypernodulation mutants show an increase in rhizobial infection in addition to developmental alteration. However, the relationship between the AON and the regulation of rhizobial infection has not been clarified. We previously isolated daphne, a nodule inception (nin) allelic mutant, in Lotus japonicus. This mutant displayed dramatically increased rhizobial infection, suggesting the existence of NIN-mediated negative regulation of rhizobial infection. Here, we investigated whether the previously isolated components of AON, especially CLAVATA3/ESR (CLE)-RELATED-ROOT SIGNAL1 (CLE-RS1), CLE-RS2, and their putative receptor HYPERNODULATION AND ABERRANT ROOT FORMATION1 (HAR1), were able to suppress increased infection in the daphne mutant. The constitutive expression of LjCLE-RS1/2 strongly reduced the infection in the daphne mutant in a HAR1-dependent manner. Moreover, reciprocal grafting analysis showed that strong reduction of infection in daphne rootstock constitutively expressing LjCLE-RS1 was canceled by a scion of the har1 or klavier mutant, the genes responsible for encoding putative LjCLE-RS1 receptors. These data indicate that rhizobial infection is also systemically regulated by CLE-HAR1 signaling, a component of AON. In addition, the constitutive expression of NIN in daphne har1 double-mutant roots only partially reduced the rhizobial infection. Our findings indicate that the previously identified NIN-mediated negative regulation of infection involves unknown local signaling, as well as CLE-HAR1 long-distance signaling.

Orchids and their mycorrhizal fungi: an insufficiently explored relationship

Orchids are associated with diverse fungal taxa, including nonmycorrhizal endophytic fungi as well as mycorrhizal fungi. The orchid mycorrhizal (OM) symbiosis is an excellent model for investigating the biological interactions between plants and fungi due to their high dependency on these symbionts for growth and survival. To capture the complexity of OM interactions, significant genomic, numerous transcriptomic, and proteomic studies have been performed, unraveling partly the role of each partner. On the other hand, several papers studied the bioactive metabolites from each partner but rarely interpreted their significance in this symbiotic relationship. In this review, we focus from a biochemical viewpoint on the OM dynamics and its molecular interactions. The ecological functions of OM in plant development and stress resistance are described first, summarizing recent literature. Secondly, because only few studies have specifically looked on OM molecular interactions, the signaling pathways and compounds allowing the establishment/maintenance of mycorrhizal association involved in arbuscular mycorrhiza (AM) are discussed in parallel with OM. Based on mechanistic similarities between OM and AM, and recent findings on orchids' endophytes, a putative model representing the different molecular strategies that OM fungi might employ to establish this association is proposed. It is hypothesized here that (i) orchids would excrete plant molecule signals such as strigolactones and flavonoids but also other secondary metabolites; (ii) in response, OM fungi would secrete mycorrhizal factors (Myc factors) or similar compounds to activate the common symbiosis genes (CSGs); (iii) overcome the defense mechanism by evasion of the pathogen-associated molecular patterns (PAMPs)-triggered immunity and by secretion of effectors such as small inhibitor proteins; and (iv) finally, secrete phytohormones to help the colonization or disrupt the crosstalk of plant defense phytohormones. To challenge this putative model, targeted and untargeted metabolomics studies with special attention to each partner's contribution are finally encouraged and some technical approaches are proposed.

Celebrating 20 Years of Genetic Discoveries in Legume Nodulation and Symbiotic Nitrogen Fixation

Since 1999, various forward- and reverse-genetic approaches have uncovered nearly 200 genes required for symbiotic nitrogen fixation (SNF) in legumes. These discoveries advanced our understanding of the evolution of SNF in plants and its relationship to other beneficial endosymbioses, signaling between plants and microbes, the control of microbial infection of plant cells, the control of plant cell division leading to nodule development, autoregulation of nodulation, intracellular accommodation of bacteria, nodule oxygen homeostasis, the control of bacteroid differentiation, metabolism and transport supporting symbiosis, and the control of nodule senescence. This review catalogs and contextualizes all of the plant genes currently known to be required for SNF in two model legume species, Medicago truncatula and Lotus japonicus, and two crop species, Glycine max (soybean) and Phaseolus vulgaris (common bean). We also briefly consider the future of SNF genetics in the era of pan-genomics and genome editing.

Kinase activity-dependent stability of calcium/calmodulin-dependent protein kinase of Lotus japonicus

Accumulation of calcium/calmodulin-dependent protein kinase (CCaMK) in root cell nucleus depends on its kinase activity but not on nuclear symbiotic components crucial for nodulation. Plant calcium/calmodulin-dependent protein kinase (CCaMK) is a key regulator of symbioses with rhizobia and arbuscular mycorrhizal fungi as it decodes symbiotic calcium signals induced by microsymbionts. CCaMK is expressed mainly in root cells and localizes to the nucleus, where microsymbiont-triggered calcium oscillations occur. The molecular mechanisms that control CCaMK localization are unknown. Here, we analyzed the expression and subcellular localization of mutated CCaMK in the roots of Lotus japonicus and found a clear relation between CCaMK kinase activity and its stability. Kinase-defective CCaMK variants showed lower protein levels than the variants with kinase activity. The levels of transcripts driven by the CaMV 35S promoter were similar among the variants, indicating that stability of CCaMK is regulated post-translationally. We also demonstrated that CCaMK localized to the root cell nucleus in several symbiotic mutants, including cyclops, an interaction partner and phosphorylation target of CCaMK. Our results suggest that kinase activity of CCaMK is required not only for the activation of downstream symbiotic components but also for its stability in root cells.

Lotus japonicus Symbiosis Genes Impact Microbial Interactions between Symbionts and Multikingdom Commensal Communities

Studies on symbiosis genes in plants typically focus on binary interactions between roots and soilborne nitrogen-fixing rhizobia or mycorrhizal fungi in laboratory environments. We utilized wild type and symbiosis mutants of a model legume, grown in natural soil, in which bacterial, fungal, or both symbioses are impaired to examine potential interactions between the symbionts and commensal microorganisms of the root microbiota when grown in natural soil. This revealed microbial interkingdom interactions between the root symbionts and fungal as well as bacterial commensal communities. Nevertheless, the bacterial root microbiota remains largely robust when fungal symbiosis is impaired. Our work implies a broad role for host symbiosis genes in structuring the root microbiota of legumes.

The wild legume Lotus japonicus engages in mutualistic symbiotic relationships with arbuscular mycorrhiza (AM) fungi and nitrogen-fixing rhizobia. Using plants grown in natural soil and community profiling of bacterial 16S rRNA genes and fungal internal transcribed spacers (ITSs), we examined the role of the Lotus symbiosis genes RAM1, NFR5, SYMRK, and CCaMK in structuring bacterial and fungal root-associated communities. We found host genotype-dependent community shifts in the root and rhizosphere compartments that were mainly confined to bacteria in nfr5 or fungi in ram1 mutants, while symrk and ccamk plants displayed major changes across both microbial kingdoms. We observed in all AM mutant roots an almost complete depletion of a large number of Glomeromycota taxa that was accompanied by a concomitant enrichment of Helotiales and Nectriaceae fungi, suggesting compensatory niche replacement within the fungal community. A subset of Glomeromycota whose colonization is strictly dependent on the common symbiosis pathway was retained in ram1 mutants, indicating that RAM1 is dispensable for intraradical colonization by some Glomeromycota fungi. However, intraradical colonization by bacteria belonging to the Burkholderiaceae and Anaeroplasmataceae is dependent on AM root infection, revealing a microbial interkingdom interaction. Despite the overall robustness of the bacterial root microbiota against major changes in the composition of root-associated fungal assemblages, bacterial and fungal cooccurrence network analysis demonstrates that simultaneous disruption of AM and rhizobium symbiosis increases the connectivity among taxa of the bacterial root microbiota. Our findings imply a broad role for Lotus symbiosis genes in structuring the root microbiota and identify unexpected microbial interkingdom interactions between root symbionts and commensal communities.

Identification of Long Non-Coding RNAs and the Regulatory Network Responsive to Arbuscular Mycorrhizal Fungi Colonization in Maize Roots

Recently, long noncoding RNAs (lncRNAs) have emerged as vital regulators of many biological processes in animals and plants. However, to our knowledge no investigations on plant lncRNAs which respond to arbuscular mycorrhizal (AM) fungi have been reported thus far. In this study, maize roots colonized with AM fungus were analyzed by strand-specific RNA-Seq to identify AM fungi-responsive lncRNAs and construct an associated regulatory network. A total of 1837 differentially expressed protein coding genes (DEGs) were identified from maize roots with Rhizophagus irregularis inoculation. Many AM fungi-responsive genes were homologs to MtPt4, STR, STR2, MtFatM, and enriched pathways such as fatty acid biosynthesis, response to phosphate starvation, and nitrogen metabolism are consistent with previous studies. In total, 5941 lncRNAs were identified, of which more than 3000 were new. Of those, 63 lncRNAs were differentially expressed. The putative target genes of differentially expressed lncRNAs (DELs) were mainly related to phosphate ion transmembrane transport, cellular response to potassium ion starvation, and lipid catabolic processes. Regulatory network analysis showed that DELs might be involved in the regulation of bidirectional nutrient exchange between plant and AM fungi as mimicry of microRNA targets. The results of this study can broaden our knowledge on the interaction between plant and AM fungi.

The LYSIN MOTIF-CONTAINING RECEPTOR-LIKE KINASE 1 protein of banana is required for perception of pathogenic and symbiotic signals

How plants can distinguish pathogenic and symbiotic fungi remains largely unknown. Here, we characterized the role of MaLYK1, a lysin motif receptor kinase of banana. Live cell imaging techniques were used in localization studies. RNA interference (RNAi)-silenced transgenic banana plants were generated to analyze the biological role of MaLYK1. The MaLYK1 ectodomain, chitin beads, chitooligosaccharides (COs) and mycorrhizal lipochitooligosaccharides (Myc-LCOs) were used in pulldown assays. Ligand-induced MaLYK1 complex formation was tested in immunoprecipitation experiments. Chimeric receptors were expressed in Lotus japonicus to characterize the function of the MaLYK1 kinase domain. MaLYK1 was localized to the plasma membrane. MaLYK1 expression was induced by Foc4 (Fusarium oxysporum f. sp. cubense race 4) and diverse microbe-associated molecular patterns. MaLYK1-silenced banana lines showed reduced chitin-triggered defense responses, increased Foc4-induced disease symptoms and reduced mycorrhization. The MaLYK1 ectodomain was pulled down by chitin beads and LCOs or COs impaired this process. Ligand treatments induced MaLYK1 complex formation in planta. The kinase domain of MaLYK1 could functionally replace that of the chitin elicitor receptor kinase 1 (AtCERK1) in Arabidopsis thaliana and of a rhizobial LCO (Nod factor) receptor (LjNFR1) in L. japonicus. MaLYK1 represents a central molecular switch that controls defense- and symbiosis-related signaling.

Diverse role of γ-aminobutyric acid in dynamic plant cell responses

Gamma-aminobutyric acid (GABA), a four-carbon non-protein amino acid, is found in most prokaryotic and eukaryotic organisms. Although, ample research into GABA has occurred in mammals as it is a major inhibitory neurotransmitter; in plants, a role for GABA has often been suggested as a metabolite that changes under stress rather than as a signal, as no receptor or motif for GABA binding was identified until recently and many aspects of its biological function (ranging from perception to function) remain to be answered. In this review, flexible properties of GABA in regulation of plant responses to various environmental biotic and abiotic stresses and its integration in plant growth and development either as a metabolite or a signaling molecule are discussed. We have elaborated on the role of GABA in stress adaptation (i.e., salinity, hypoxia/anoxia, drought, temperature, heavy metals, plant-insect interplay and ROS-related responses) and its contribution in non-stress-related biological pathways (i.e., involvement in plant-microbe interaction, contribution to the carbon and nitrogen metabolism and governing of signal transduction pathways). This review aims to represent the multifunctional contribution of GABA in various biological and physiological mechanisms under stress conditions; the objective is to review the current state of knowledge about GABA role beyond stress-related responses. Our effort is to place findings about GABA in an organized and broader context to highlight its shared metabolic and biologic functions in plants under variable conditions. This will provide potential modes of GABA crosstalk in dynamic plant cell responses.

A set of Arabidopsis genes involved in the accommodation of the downy mildew pathogen Hyaloperonospora arabidopsidis

The intracellular accommodation structures formed by plant cells to host arbuscular mycorrhiza fungi and biotrophic hyphal pathogens are cytologically similar. Therefore we investigated whether these interactions build on an overlapping genetic framework. In legumes, the malectin-like domain leucine-rich repeat receptor kinase SYMRK, the cation channel POLLUX and members of the nuclear pore NUP107-160 subcomplex are essential for symbiotic signal transduction and arbuscular mycorrhiza development. We identified members of these three groups in Arabidopsis thaliana and explored their impact on the interaction with the oomycete downy mildew pathogen Hyaloperonospora arabidopsidis (Hpa). We report that mutations in the corresponding genes reduced the reproductive success of Hpa as determined by sporangiophore and spore counts. We discovered that a developmental transition of haustorial shape occurred significantly earlier and at higher frequency in the mutants. Analysis of the multiplication of extracellular bacterial pathogens, Hpa-induced cell death or callose accumulation, as well as Hpa- or flg22-induced defence marker gene expression, did not reveal any traces of constitutive or exacerbated defence responses. These findings point towards an overlap between the plant genetic toolboxes involved in the interaction with biotrophic intracellular hyphal symbionts and pathogens in terms of the gene families involved.

Our work reveals genetic commonalities between biotrophic intracellular interactions with symbiotic and pathogenic hyphal microbes. The majority of land plants engages in arbuscular mycorrhiza (AM) symbiosis with phosphate-acquiring arbuscular mycorrhizal fungi to avoid phosphate starvation. Nutrient exchange in this interaction occurs via arbuscules, tree-shaped fungal structures, hosted within plant root cells. A series of plant genes including the Symbiosis Receptor-like kinase (SYMRK), members of the NUP107-160 subcomplex and nuclear envelope localised cation channels are required for a signalling process leading to the development of AM. The model plant Arabidopsis thaliana lost the ability to form AM. Although the ortholog of SYMRK was deleted during evolution, members of the malectin-like domain leucine-rich repeat receptor kinase (MLD-LRR-RK) gene family, components of the NUP107-160 subcomplex, and an ortholog of the nuclear envelope-localized cation channel POLLUX, are still present in the Arabidopsis genome, and Arabidopsis leaf cells retained the ability to accommodate haustoria, presumed feeding structures of the obligate biotrophic downy mildew pathogen Hyaloperonospora arabidopsidis. We discovered that both of these plant-microbe interactions utilize a corresponding set of genes including the ortholog of POLLUX, members of the NUP107-160 subcomplex and members of the MLD-LRR-RK gene family, thus revealing similarities in the plant program for the intracellular accommodation of biotrophic organisms in symbiosis and disease.

A Lotus japonicus cytoplasmic kinase connects Nod factor perception by the NFR5 LysM receptor to nodulation

The establishment of nitrogen-fixing root nodules in legume-rhizobia symbiosis requires an intricate communication between the host plant and its symbiont. We are, however, limited in our understanding of the symbiosis signaling process. In particular, how membrane-localized receptors of legumes activate signal transduction following perception of rhizobial signaling molecules has mostly remained elusive. To address this, we performed a coimmunoprecipitation-based proteomics screen to identify proteins associated with Nod factor receptor 5 (NFR5) in Lotus japonicus. Out of 51 NFR5-associated proteins, we focused on a receptor-like cytoplasmic kinase (RLCK), which we named NFR5-interacting cytoplasmic kinase 4 (NiCK4). NiCK4 associates with heterologously expressed NFR5 in Nicotiana benthamiana, and directly binds and phosphorylates the cytoplasmic domains of NFR5 and NFR1 in vitro. At the cellular level, Nick4 is coexpressed with Nfr5 in root hairs and nodule cells, and the NiCK4 protein relocates to the nucleus in an NFR5/NFR1-dependent manner upon Nod factor treatment. Phenotyping of retrotransposon insertion mutants revealed that NiCK4 promotes nodule organogenesis. Together, these results suggest that the identified RLCK, NiCK4, acts as a component of the Nod factor signaling pathway downstream of NFR5.

Inside out: root cortex-localized LHK1 cytokinin receptor limits epidermal infection of Lotus japonicus roots by Mesorhizobium loti

During Lotus japonicus-Mesorhizobium loti symbiosis, the LOTUS HISTIDINE KINASE1 (LHK1) cytokinin receptor regulates both the initiation of nodule formation and the scope of root infection. However, the exact spatiotemporal mechanism by which this receptor exerts its symbiotic functions has remained elusive. In this study, we performed cell type-specific complementation experiments in the hyperinfected lhk1-1 mutant background, targeting LHK1 to either the root epidermis or the root cortex. We also utilized various genetic backgrounds to characterize expression of several genes regulating symbiotic infection. We show here that expression of LHK1 in the root cortex is required and sufficient to regulate both nodule formation and epidermal infections. The LHK1-dependent signalling that restricts subsequent infection events is triggered before initial cell divisions for nodule primordium formation. We also demonstrate that AHK4, the Arabidopsis orthologue of LHK1, is able to regulate M. loti infection in L. japonicus, suggesting that an endogenous cytokinin receptor could be sufficient for engineering nitrogen-fixing root nodule symbiosis in nonlegumes. Our data provide experimental evidence for the existence of an LHK1-dependent root cortex-to-epidermis feedback mechanism regulating rhizobial infection. This root-localized regulatory module functionally links with the systemic autoregulation of nodulation (AON) to maintain the homeostasis of symbiotic infection.

Functional conservation of CYCLOPS in crack entry legume Arachis hypogaea

Root nodule symbiosis in legumes is established following interaction of compatible rhizobia that activates an array of genes, commonly known as symbiotic-pathway, resulting in nodule development. In model legumes, bacterial entry mainly occurs through infection thread involving the expression of transcription factor CYCLOPS/IPD3. Here we show the functional analysis of AhCYCLOPS in Arachis hypogaea where bacteria invade roots through epidermal cracks. Exploiting significant cross-species domain conservation, trans-complementation experiments involving ectopic expression of AhCYCLOPS in transgenic hairy-roots of Medicago truncatula ipd3 mutants resulted in functional complementation of Medicago nodules. Moreover, native promoter of AhCYCLOPS was sufficient for this cross-species complementation irrespective of the different modes of infection of roots by rhizobia and nodule ontology. To unravel the role of AhCYCLOPS during 'crack-entry' nodulation in A. hypogaea, RNAi of AhCYCLOPS was performed which resulted in delayed nodule inception followed by drastic reduction in nodule number on transgenic hairy-roots. The infection zone of a significant number of RNAi nodules showed presence of infected cells with enlarged nucleus and rod shaped undifferentiated bacteria. Expression analysis showed downregulation of several nodulation responsible effectors endorsing the compromised condition of RNAi roots. Together, the results indicated that AhCYCLOPS plays an important role in A. hypogaea nodule development.

A subcompatible rhizobium strain reveals infection duality in Lotus

Lotus species develop infection threads to guide rhizobia into nodule cells. However, there is evidence that some species have a genetic repertoire to allow other modes of infection. By conducting confocal and electron microscopy, quantification of marker gene expression, and phenotypic analysis of transgenic roots infected with mutant rhizobia, we elucidated the infection mechanism used by Rhizobium leguminosarum Norway to colonize Lotus burttii. Rhizobium leguminosarum Norway induces a distinct host transcriptional response compared with Mesorhizobium loti. It infects L. burttii utilizing an epidermal and transcellular infection thread-independent mechanism at high frequency. The entry into plant cells occurs directly from the apoplast and is primarily mediated by 'peg'-like structures, the formation of which is dependent on the production of Nod factor by the rhizobia. These results demonstrate that Lotus species can exhibit duality in their infection mechanisms depending on the rhizobial strain that they encounter. This is especially relevant in the context of interactions in the rhizosphere where legumes do not encounter single strains, but complex rhizobial communities. Additionally, our findings support a perception mechanism at the nodule cell entry interface, reinforcing the idea that there are successive checkpoints during rhizobial infection.

Nicotiana benthamiana RanBP1-1 Is Involved in the Induction of Disease Resistance via Regulation of Nuclear-Cytoplasmic Transport of Small GTPase Ran

Plant cells enhance the tolerances to abiotic and biotic stresses via recognition of the stress, activation and nuclear import of signaling factors, up-regulation of defense genes, nuclear export of mRNA and translation of defense proteins. Nuclear pore-mediated transports should play critical roles in these processes, however, the regulatory mechanisms of nuclear-cytoplasmic transport during stress responses are largely unknown. In this study, a regulator of nuclear export of RNA and proteins, NbRanBP1-1 (Ran-binding protein1-1), was identified as an essential gene for the resistance of Nicotiana benthamiana to potato blight pathogen Phytophthora infestans. NbRanBP1-1-silenced plants showed delayed accumulation of capsidiol, a sesquiterpenoid phytoalexin, in response to elicitor treatment, and reduced resistance to P. infestans. Abnormal accumulation of mRNA was observed in NbRanBP1-1-silenced plants, indicating that NbRanBP1-1 is involved in the nuclear export of mRNA. In NbRanBP1-1-silenced plants, elicitor-induced expression of defense genes, NbEAS and NbWIPK, was not affected in the early stage of defense induction, but the accumulation of NbWIPK protein was reduced. Nuclear export of the small G-protein NbRan1a was activated during the induction of plant defense, whereas this process was compromised in NbRanBP1-1-silenced plants. Silencing of genes encoding the nuclear pore proteins, Nup75 and Nup160, also caused abnormal nuclear accumulation of mRNA, defects in the nuclear export of NbRan1a, and reduced production of capsidiol, resulting in decreased resistance to P. infestans. These results suggest that nuclear export of NbRan is a key event for defense induction in N. benthamiana, and both RanBP1-1 and nucleoporins play important roles in the process.

A Novel Positive Regulator of the Early Stages of Root Nodule Symbiosis Identified by Phosphoproteomics

Signals and signaling pathways underlying the symbiosis between legumes and rhizobia have been studied extensively over the past decades. In a previous phosphoproteomic study on the Medicago truncatula-Sinorhizobium meliloti symbiosis, we identified plant proteins that are differentially phosphorylated upon the perception of rhizobial signals, called Nod factors. In this study, we provide experimental evidence that one of these proteins, Early Phosphorylated Protein 1 (EPP1), is required for the initiation of this symbiosis. Upon inoculation with rhizobia, MtEPP1 expression was induced in curled root hairs. Down-regulation of MtEPP1 in M. truncatula roots almost abolished calcium spiking, reduced the expression of essential symbiosis-related genes (MtNIN, MtNF-YB1, MtERN1 and MtENOD40) and strongly decreased nodule development. Phylogenetic analyses revealed that orthologs of MtEPP1 are present in legumes and specifically in plant species able to host arbuscular mycorrhizal fungi, suggesting a possible role in this association too. Short chitin oligomers induced the phosphorylation of MtEPP1 like Nod factors. However, the down-regulation of MtEPP1 affected the colonization of M. truncatula roots by arbuscular mycorrhizal fungi only moderately. Altogether, these findings indicate that MtEPP1 is essential for the establishment of the legume-rhizobia symbiosis but might plays a limited role in the arbuscular mycorrhizal symbiosis.

PLENTY, a hydroxyproline O-arabinosyltransferase, negatively regulates root nodule symbiosis in Lotus japonicus

Legumes can survive in nitrogen-deficient environments by forming root-nodule symbioses with rhizobial bacteria; however, forming nodules consumes energy, and nodule numbers must thus be strictly controlled. Previous studies identified major negative regulators of nodulation in Lotus japonicus, including the small peptides CLAVATA3/ESR (CLE)-RELATED-ROOT SIGNAL1 (CLE-RS1), CLE-RS2, and CLE-RS3, and their putative major receptor HYPERNODULATION AND ABERRANT ROOT FORMATION1 (HAR1). CLE-RS2 is known to be expressed in rhizobia-inoculated roots, and is predicted to be post-translationally arabinosylated, a modification essential for its activity. Moreover, all three CLE-RSs suppress nodulation in a HAR1-dependent manner. Here, we identified PLENTY as a gene responsible for the previously isolated hypernodulation mutant plenty. PLENTY encoded a hydroxyproline O-arabinosyltransferase orthologous to ROOT DETERMINED NODULATION1 in Medicago truncatula. PLENTY was localized to the Golgi, and an in vitro analysis of the recombinant protein demonstrated its arabinosylation activity, indicating that CLE-RS1/2/3 may be substrates for PLENTY. The constitutive expression experiments showed that CLE-RS3 was the major candidate substrate for PLENTY, suggesting the substrate preference of PLENTY for individual CLE-RS peptides. Furthermore, a genetic analysis of the plenty har1 double mutant indicated the existence of another PLENTY-dependent and HAR1-independent pathway negatively regulating nodulation.

LACK OF SYMBIONT ACCOMMODATION controls intracellular symbiont accommodation in root nodule and arbuscular mycorrhizal symbiosis in Lotus japonicus

Nitrogen-fixing rhizobia and arbuscular mycorrhizal fungi (AMF) form symbioses with plant roots and these are established by precise regulation of symbiont accommodation within host plant cells. In model legumes such as Lotus japonicus and Medicago truncatula, rhizobia enter into roots through an intracellular invasion system that depends on the formation of a root-hair infection thread (IT). While IT-mediated intracellular rhizobia invasion is thought to be the most evolutionarily derived invasion system, some studies have indicated that a basal intercellular invasion system can replace it when some nodulation-related factors are genetically modified. In addition, intracellular rhizobia accommodation is suggested to have a similar mechanism as AMF accommodation. Nevertheless, our understanding of the underlying genetic mechanisms is incomplete. Here we identify a L. japonicus nodulation-deficient mutant, with a mutation in the LACK OF SYMBIONT ACCOMMODATION (LAN) gene, in which root-hair IT formation is strongly reduced, but intercellular rhizobial invasion eventually results in functional nodule formation. LjLAN encodes a protein that is homologous to Arabidopsis MEDIATOR 2/29/32 possibly acting as a subunit of a Mediator complex, a multiprotein complex required for gene transcription. We also show that LjLAN acts in parallel with a signaling pathway including LjCYCLOPS. In addition, the lan mutation drastically reduces the colonization levels of AMF. Taken together, our data provide a new factor that has a common role in symbiont accommodation process during root nodule and AM symbiosis.

Symbiosis between plants and beneficial microbes such as nitrogen-fixing bacteria and arbuscular mycorrhizal fungi has enabled plant colonization of new environments. Root nodule symbiosis with nitrogen-fixing rhizobia enables sessile plants to survive in a nitrogen-deficient environment. To establish the symbiosis, host plant cells need to accommodate rhizobia during nodule development, a process mediated by a plant-derived intracellular structure called the infection thread (IT). In this study, we show that LACK OF SYMBIONT ACCOMMODATION (LAN) is involved in intracellular rhizobia accommodation in the model leguminous plant Lotus japonicus. LjLAN encodes a putative subunit of Mediator complex, a multiprotein complex that has a fundamental role as an activator of gene transcription. Mutation analysis suggests that LjLAN is required for root hair IT formation, which enables swift and efficient rhizobial accommodation. Moreover, we show that LjLAN is required for symbiosis with arbuscular mycorrhizal fungi. These data add a new component to the molecular mechanism relevant to the establishment of root nodule and arbuscular mycorrhizal symbiosis.

Beyond Transcription: Fine-Tuning of Circadian Timekeeping by Post-Transcriptional Regulation

The circadian clock is an important endogenous timekeeper, helping plants to prepare for the periodic changes of light and darkness in their environment. The clockwork of this molecular timer is made up of clock proteins that regulate transcription of their own genes with a 24 h rhythm. Furthermore, the rhythmically expressed clock proteins regulate time-of-day dependent transcription of downstream genes, causing messenger RNA (mRNA) oscillations of a large part of the transcriptome. On top of the transcriptional regulation by the clock, circadian rhythms in mRNAs rely in large parts on post-transcriptional regulation, including alternative pre-mRNA splicing, mRNA degradation, and translational control. Here, we present recent insights into the contribution of post-transcriptional regulation to core clock function and to regulation of circadian gene expression in Arabidopsis thaliana.

Medicago AP2-Domain Transcription Factor WRI5a Is a Master Regulator of Lipid Biosynthesis and Transfer during Mycorrhizal Symbiosis

Most land plants have evolved a mutualistic symbiosis with arbuscular mycorrhiza (AM) fungi that improve nutrient acquisition from the soil. In return, up to 20% of host plant photosynthate is transferred to the mycorrhizal fungus in the form of lipids and sugar. Nutrient exchange must be regulated by both partners in order to maintain a reliable symbiotic relationship. However, the mechanisms underlying the regulation of lipid transfer from the plant to the AM fungus remain elusive. Here, we show that the Medicago truncatula AP2/EREBP transcription factor WRI5a, and likely its two homologs WRI5b/Erf1 and WRI5c, are master regulators of AM symbiosis controlling lipid transfer and periarbuscular membrane formation. We found that WRI5a binds AW-box cis-regulatory elements in the promoters of M. truncatula STR, which encodes a periarbuscular membrane-localized ABC transporter required for lipid transfer from the plant to the AM fungus, and MtPT4, which encodes a phosphate transporter required for phosphate transfer from the AM fungus to the plant. The hairy roots of the M. truncatula wri5a mutant and RNAi composite plants displayed impaired arbuscule formation, whereas overexpression of WRI5a resulted in enhanced expression of STR and MtPT4, suggesting that WRI5a regulates bidirectional symbiotic nutrient exchange. Moreover, we found that WRI5a and RAM1 (Required for Arbuscular Mycorrhization symbiosis 1), which encodes a GRAS-domain transcription factor, regulate each other at the transcriptional level, forming a positive feedback loop for regulating AM symbiosis. Collectively, our data suggest a role for WRI5a in controlling bidirectional nutrient exchange and periarbuscular membrane formation via the regulation of genes involved in the biosynthesis of fatty acids and phosphate uptake in arbuscule-containing cells.

Phosphate Deficiency Negatively Affects Early Steps of the Symbiosis between Common Bean and Rhizobia

Phosphate (Pi) deficiency reduces nodule formation and development in different legume species including common bean. Despite significant progress in the understanding of the genetic responses underlying the adaptation of nodules to Pi deficiency, it is still unclear whether this nutritional deficiency interferes with the molecular dialogue between legumes and rhizobia. If so, what part of the molecular dialogue is impaired? In this study, we provide evidence demonstrating that Pi deficiency negatively affects critical early molecular and physiological responses that are required for a successful symbiosis between common bean and rhizobia. We demonstrated that the infection thread formation and the expression of PvNSP2, PvNIN, and PvFLOT2, which are genes controlling the nodulation process were significantly reduced in Pi-deficient common bean seedlings. In addition, whole-genome transcriptional analysis revealed that the expression of hormones-related genes is compromised in Pi-deficient seedlings inoculated with rhizobia. Moreover, we showed that regardless of the presence or absence of rhizobia, the expression of PvRIC1 and PvRIC2, two genes participating in the autoregulation of nodule numbers, was higher in Pi-deficient seedlings compared to control seedlings. The data presented in this study provides a mechanistic model to better understand how Pi deficiency impacts the early steps of the symbiosis between common bean and rhizobia.

The Mycoheterotrophic Symbiosis Between Orchids and Mycorrhizal Fungi Possesses Major Components Shared with Mutualistic Plant-Mycorrhizal Symbioses

Achlorophylous and early developmental stages of chorolophylous orchids are highly dependent on carbon and other nutrients provided by mycorrhizal fungi, in a nutritional mode termed mycoheterotrophy. Previous findings have implied that some common properties at least partially underlie the mycorrhizal symbioses of mycoheterotrophic orchids and that of autotrophic arbuscular mycorrhizal (AM) plants; however, information about the molecular mechanisms of the relationship between orchids and their mycorrhizal fungi is limited. In this study, we characterized the molecular basis of an orchid-mycorrhizal (OM) symbiosis by analyzing the transcriptome of Bletilla striata at an early developmental stage associated with the mycorrhizal fungus Tulasnella sp. The essential components required for the establishment of mutual symbioses with AM fungi or rhizobia in most terrestrial plants were identified from the B. striata gene set. A cross-species gene complementation analysis showed one of the component genes, calcium and calmodulin-dependent protein kinase gene CCaMK in B. striata, retains functional characteristics of that in AM plants. The expression analysis revealed the activation of homologs of AM-related genes during the OM symbiosis. Our results suggest that orchids possess, at least partly, the molecular mechanisms common to AM plants.

A genetic screen for plant mutants with altered nodulation phenotypes in response to rhizobial glycan mutants

Nodule primordia induced by rhizobial glycan mutants often remain uninfected. To identify processes involved in infection and organogenesis we used forward genetics to identify plant genes involved in perception and responses to bacterial glycans. To dissect the mechanisms underlying the negative plant responses to the Mesorhizobium loti R7AexoU and ML001cep mutants, a screen for genetic suppressors of the nodulation phenotypes was performed on a chemically mutagenized Lotus population. Two mutant lines formed infected nitrogen-fixing pink nodules, while five mutant lines developed uninfected large white nodules, presumably altered in processes controlling organogenesis. Genetic mapping identified a mutation in the cytokinin receptor Lhk1 resulting in an alanine to valine substitution adjacent to a coiled-coil motif in the juxta-membrane region of LHK1. This results in a spontaneous nodulation phenotype and increased ethylene production. The allele was renamed snf5, and segregation studies of snf5 together with complementation studies suggest that snf5 is a gain-of-function allele. This forward genetic approach to investigate the role of glycans in the pathway synchronizing infection and organogenesis shows that a combination of plant and bacterial genetics opens new possibilities to study glycan responses in plants as well as identification of mutant alleles affecting nodule organogenesis.

Mechanisms Underlying Establishment of Arbuscular Mycorrhizal Symbioses

Most land plants engage in mutually beneficial interactions with arbuscular mycorrhizal (AM) fungi, the fungus providing phosphate and nitrogen in exchange for fixed carbon. During presymbiosis, both organisms communicate via oligosaccharides and butenolides. The requirement for a rice chitin receptor in symbiosis-induced lateral root development suggests that cell division programs operate in inner root tissues during both AM and nodule symbioses. Furthermore, the identification of transcription factors underpinning arbuscule development and degeneration reemphasized the plant's regulatory dominance in AM symbiosis. Finally, the finding that AM fungi, as lipid auxotrophs, depend on plant fatty acids (FAs) to complete their asexual life cycle revealed the basis for fungal biotrophy. Intriguingly, lipid metabolism is also central for asexual reproduction and interaction of the fungal sister clade, the Mucoromycotina, with endobacteria, indicative of an evolutionarily ancient role for lipids in fungal mutualism.

Uptake of bacteria into living plant cells, the unifying and distinct feature of the nitrogen-fixing root nodule symbiosis

Despite the presence of complex microbiota on the surfaces of all plants, the uptake of bacteria into plant cells and the subsequent accommodation in a membrane-enclosed compartment is restricted to the nitrogen-fixing root nodule and the Gunnera-Nostoc symbiosis. The plant cell wall and the outward-directed turgor pressure are major constraints for bacterial uptake because localised lysis of the cell wall endangers the integrity of the protoplast. Host cell integrity is consistently maintained by turgescent neighbours, connected via apoplastic polymers that seal a bacteria-containing extracellular compartment prior to localized cell wall lysis. Its unifying and almost exclusive phylogenetic distribution pinpoints the ability to take up bacteria into living plant cells as a key step during the evolution of the nitrogen-fixing root nodule symbiosis.

Transcriptome Profiles of Nod Factor-independent Symbiosis in the Tropical Legume Aeschynomene evenia

Nod factors (NF) were assumed to be indispensable for the establishment of a rhizobium-legume symbiosis until the discovery that certain Bradyrhizobium strains interacting with certain Aeschynomene species lack the canonical nodABC genes required for their synthesis. So far, the molecular dialogue between Aeschynomene and its symbionts remains an open question. Here we report a time course transcriptional analysis of Aeschynomene evenia in response to inoculation with Bradyrhizobium ORS278. The NF-independent symbiotic process was monitored at five time points between bacterial infection and nodule maturity. The five time points correspond to three specific events, root infection by crack entry, nodule organogenesis, and the establishment of the nitrogen fixing process. During the third stage, about 80 NCR-like genes and eight symbiotic genes known to be involved in signaling, bacterial infection or nodulation regulation were highly expressed. Comparative gene expression analyses at the five time points also enabled the selection of genes with an expression profile that makes them promising markers to monitor early plant responses to bacteria. Such markers could be used in bioassays to identify the nature of the bacterial signal(s). Our data represent valuable resources for investigation of this Nod factor-independent symbiosis.

Differential proteome analysis of pea roots at the early stages of symbiosis with nodule bacteria

In this paper, we have analyzed changes in the proteomic spectrum of pea Pisum sativum L. roots during inoculation with rhizobial bacteria with the aim of revealing new regulators of symbiosis development. To study the changes in the proteome spectrum of pea roots, a differential twodimensional (2-D) electrophoresis was performed using fluorescent labels Cy2 and Cy5. The images obtained made it possible to identify differences between the control variant (uninoculated roots) and the root variant after inoculation with Rhizobium leguminosarum bv. viciae RCAM 1026 (24 hours after treatment). 20 proteins were revealed and identified, the synthesis of which was enhanced during the inoculation of pea roots by nodule bacteria. To identify the proteins, a mass spectrometric analysis of tryptic peptides was performed on a quadrupole-time-of-flight mass spectrometer combined with a high-performance liquid chromatograph. Among such proteins, the beta-subunit of the G protein and the disulfide isomerase/phospholipase C were first found, whose function can be related to the signal regulation of symbiosis. This indicates that G-proteins and phospholipases can play a key role in the development of early stages of symbiosis in peas. Further experiments are expected to show whether the beta-subunit of the G protein interacts with the receptors to Nod factors, and how this affects the further signaling. Other proteins that might be interesting were annexin D8 and D1, protein kinase interacting with calcinerin B, actin-binding protein profilin, GTP-binding protein Ran1. They may be involved in the regulation of reactions with calcium, the reorganization of the actin cytoskeleton and other important processes in plants. The study of the role of such regulatory proteins will later become the basis for understanding the complex system of signal regulation, which is activated in pea plants by interaction with nodule bacteria.

Transcriptional Regulation of Arbuscular Mycorrhiza Development

Arbuscular mycorrhiza (AM) is an ancient symbiosis between land plants and fungi of the glomeromycotina that is widespread in the plant kingdom. AM improves plant nutrition, stress resistance and general plant performance, and thus represents a promising addition to sustainable agricultural practices. In return for delivering mineral nutrients, the obligate biotrophic AM fungi receive up to 20% of the photosynthetically fixed carbon from the plant. AM fungi colonize the inside of roots and form highly branched tree-shaped structures, called arbuscules, in cortex cells. The pair of the arbuscule and its host cell is considered the central functional unit of the symbiosis as it mediates the bidirectional nutrient exchange between the symbionts. The development and spread of AM fungi within the root is predominantly under the control of the host plant and depends on its developmental and physiological status. Intracellular accommodation of fungal structures is enabled by the remarkable plasticity of plant cells, which undergo drastic subcellular rearrangements. These are promoted and accompanied by cell-autonomous transcriptional reprogramming. AM development can be dissected into distinct stages using plant mutants. Progress in the application of laser dissection technology has allowed the assignment of transcriptional responses to specific stages and cell types. The first transcription factors controlling AM-specific gene expression and AM development have been discovered, and cis-elements required for AM-responsive promoter activity have been identified. An understanding of their connectivity and elucidation of transcriptional networks orchestrating AM development can be expected in the near future.