Seven Lotus japonicus genes required for transcriptional reprogramming of the root during fungal and bacterial symbiosis

Omics approaches to investigate pre-symbiotic responses of the mycorrhizal fungus Tulasnella sp. SV6 to the orchid host Serapias vomeracea

Like other plant-microbe symbioses, the establishment of orchid mycorrhiza (ORM) is likely to require specific communication and metabolic adjustments between the two partners. However, while modulation of plant and fungal metabolism has been investigated in fully established mycorrhizal tissues, the molecular changes occurring during the pre-symbiotic stages of the interaction remain largely unexplored in ORM. In this study, we investigated the pre-symbiotic responses of the ORM fungus Tulasnella sp. SV6 to plantlets of the orchid host Serapias vomeracea in a dual in vitro cultivation system. The fungal mycelium was harvested prior to physical contact with the orchid roots and the fungal transcriptome and metabolome were analyzed using RNA-seq and untargeted metabolomics approaches. The results revealed distinct transcriptomic and metabolomic remodelling of the ORM fungus in the presence of orchid plantlets, as compared to the free-living condition. The ORM fungus responds to the presence of the host plant with a significant up-regulation of genes associated with protein synthesis, amino acid and lipid biosynthesis, indicating increased metabolic activity. Metabolomic analysis supported the RNA-seq data, showing increased levels of amino acids and phospholipids, suggesting a remodelling of cell structure and signalling during the pre-symbiotic interaction. In addition, we identified an increase of transcripts of a small secreted protein that may play a role in early symbiotic signalling. Taken together, our results suggest that Tulasnella sp. SV6 may perceive information from orchid roots, leading to a readjustment of its transcriptomic and metabolomic profiles.

Intricate phytohormonal orchestration mediates mycorrhizal symbiosis and stress tolerance

Arbuscular mycorrhizal fungi (AMF) are an essential symbiotic partner colonizing more than 70% of land plants. In exchange for carbon sources, mycorrhizal association ameliorates plants' growth and yield and enhances stress tolerance and/or resistance. To achieve this symbiosis, plants mediate a series of biomolecular changes, including the regulation of phytohormones. This review focuses on the role of each phytohormone in establishing symbiosis. It encases phytohormone modulation, exogenous application of the hormones, and mutant studies. The review also comments on the plausible phytohormone cross-talk essential for maintaining balanced mycorrhization and preventing fungal parasitism. Finally, we briefly discuss AMF-mediated stress regulation and contribution of phytohormone modulation in plants. We must examine their interplay to understand how phytohormones act species-specific or concentration-dependent manner. The review summarizes the gaps in these studies to improve our understanding of processes underlying plant-AMF symbiosis.

Breaking barriers: improving time and space resolution of arbuscular mycorrhizal symbiosis with single-cell sequencing approaches

The cell and molecular bases of arbuscular mycorrhizal (AM) symbiosis, a crucial plant-fungal interaction for nutrient acquisition, have been extensively investigated by coupling traditional RNA sequencing techniques of roots sampled in bulk, with methods to capture subsets of cells such as laser microdissection. These approaches have revealed central regulators of this complex relationship, yet the requisite level of detail to effectively untangle the intricacies of temporal and spatial development remains elusive.The recent adoption of single-cell RNA sequencing (scRNA-seq) techniques in plant research is revolutionizing our ability to dissect the intricate transcriptional profiles of plant-microbe interactions, offering unparalleled insights into the diversity and dynamics of individual cells during symbiosis. The isolation of plant cells is particularly challenging due to the presence of cell walls, leading plant researchers to widely adopt nuclei isolation methods. Despite the increased resolution that single-cell analyses offer, it also comes at the cost of spatial perspective, hence, it is necessary the integration of these approaches with spatial transcriptomics to obtain a comprehensive overview.To date, few single-cell studies on plant-microbe interactions have been published, most of which provide high-resolution cell atlases that will become crucial for fully deciphering symbiotic interactions and addressing future questions. In AM symbiosis research, key processes such as the mutual recognition of partners during arbuscule development within cortical cells, or arbuscule senescence and degeneration, remain poorly understood, and these advancements are expected to shed light on these processes and contribute to a deeper understanding of this plant-fungal interaction.

Mucoromycotina 'fine root endophytes': a new molecular model for plant-fungal mutualisms?

The most studied plant-fungal symbioses to date are the interactions between plants and arbuscular mycorrhizal (AM) fungi of the Glomeromycotina clade. Advancements in phylogenetics and microbial community profiling have distinguished a group of symbiosis-forming fungi that resemble AM fungi as belonging instead to the Mucoromycotina. These enigmatic fungi are now known as Mucoromycotina 'fine root endophytes' and could provide a means to understand the origins of plant-fungal symbioses. Most of our knowledge of the mechanisms of fungal symbiosis comes from investigations using AM fungi. Here, we argue that inclusion of Mucoromycotina fine root endophytes in future studies will expand our understanding of the mechanisms, evolution, and ecology of plant-fungal symbioses.

Receptor-associated kinases control the lipid provisioning program in plant-fungal symbiosis

The mutualistic association between plants and arbuscular mycorrhizal (AM) fungi requires intracellular accommodation of the fungal symbiont and maintenance by means of lipid provisioning. Symbiosis signaling through lysin motif (LysM) receptor-like kinases and a leucine-rich repeat receptor-like kinase DOES NOT MAKE INFECTIONS 2 (DMI2) activates transcriptional programs that underlie fungal passage through the epidermis and accommodation in cortical cells. We show that two Medicago truncatula cortical cell-specific, membrane-bound proteins of a CYCLIN-DEPENDENT KINASE-LIKE (CKL) family associate with, and are phosphorylation substrates of, DMI2 and a subset of the LysM receptor kinases. CKL1 and CKL2 are required for AM symbiosis and control expression of transcription factors that regulate part of the lipid provisioning program. Onset of lipid provisioning is coupled with arbuscule branching and with the REDUCED ARBUSCULAR MYCORRHIZA 1 (RAM1) regulon for complete endosymbiont accommodation.

Construction of a de novo assembly pipeline using multiple transcriptome data sets from Cypripedium macranthos (Orchidaceae)

The family Orchidaceae comprises the most species of any monocotyledonous family and has interesting characteristics such as seed germination induced by mycorrhizal fungi and flower morphology that co-adapted with pollinators. In orchid species, genomes have been decoded for only a few horticultural species, and there is little genetic information available. Generally, for species lacking sequenced genomes, gene sequences are predicted by de novo assembly of transcriptome data. Here, we devised a de novo assembly pipeline for transcriptome data from the wild orchid Cypripedium (lady slipper orchid) in Japan by mixing multiple data sets and integrating assemblies to create a more complete and less redundant contig set. Among the assemblies generated by combining various assemblers, Trinity and IDBA-Tran yielded good assembly with higher mapping rates and percentages of BLAST hit contigs and complete BUSCO. Using this contig set as a reference, we analyzed differential gene expression between protocorms grown aseptically or with mycorrhizal fungi to detect gene expressions required for mycorrhizal interaction. A pipeline proposed in this study can construct a highly reliable contig set with little redundancy even when multiple transcriptome data are mixed, and can provide a reference that is adaptable to DEG analysis and other downstream analysis in RNA-seq.

A glycan receptor kinase facilitates intracellular accommodation of arbuscular mycorrhiza and symbiotic rhizobia in the legume Lotus japonicus

Receptors that distinguish the multitude of microbes surrounding plants in the environment enable dynamic responses to the biotic and abiotic conditions encountered. In this study, we identify and characterise a glycan receptor kinase, EPR3a, closely related to the exopolysaccharide receptor EPR3. Epr3a is up-regulated in roots colonised by arbuscular mycorrhizal (AM) fungi and is able to bind glucans with a branching pattern characteristic of surface-exposed fungal glucans. Expression studies with cellular resolution show localised activation of the Epr3a promoter in cortical root cells containing arbuscules. Fungal infection and intracellular arbuscule formation are reduced in epr3a mutants. In vitro, the EPR3a ectodomain binds cell wall glucans in affinity gel electrophoresis assays. In microscale thermophoresis (MST) assays, rhizobial exopolysaccharide binding is detected with affinities comparable to those observed for EPR3, and both EPR3a and EPR3 bind a well-defined β-1,3/β-1,6 decasaccharide derived from exopolysaccharides of endophytic and pathogenic fungi. Both EPR3a and EPR3 function in the intracellular accommodation of microbes. However, contrasting expression patterns and divergent ligand affinities result in distinct functions in AM colonisation and rhizobial infection in Lotus japonicus. The presence of Epr3a and Epr3 genes in both eudicot and monocot plant genomes suggest a conserved function of these receptor kinases in glycan perception.

RPG acts as a central determinant for infectosome formation and cellular polarization during intracellular rhizobial infections

Host-controlled intracellular accommodation of nitrogen-fixing bacteria is essential for the establishment of a functional Root Nodule Symbiosis (RNS). In many host plants, this occurs via transcellular tubular structures (infection threads - ITs) that extend across cell layers via polar tip-growth. Comparative phylogenomic studies have identified RPG (RHIZOBIUM-DIRECTED POLAR GROWTH) among the critical genetic determinants for bacterial infection. In Medicago truncatula, RPG is required for effective IT progression within root hairs but the cellular and molecular function of the encoded protein remains elusive. Here, we show that RPG resides in the protein complex formed by the core endosymbiotic components VAPYRIN (VPY) and LUMPY INFECTION (LIN) required for IT polar growth, co-localizes with both VPY and LIN in IT tip- and perinuclear-associated puncta of M. truncatula root hairs undergoing infection and is necessary for VPY recruitment into these structures. Fluorescence Lifetime Imaging Microscopy (FLIM) of phosphoinositide species during bacterial infection revealed that functional RPG is required to sustain strong membrane polarization at the advancing tip of the IT. In addition, loss of RPG functionality alters the cytoskeleton-mediated connectivity between the IT tip and the nucleus and affects the polar secretion of the cell wall modifying enzyme NODULE PECTATE LYASE (NPL). Our results integrate RPG into a core host machinery required to support symbiont accommodation, suggesting that its occurrence in plant host genomes is essential to co-opt a multimeric protein module committed to endosymbiosis to sustain IT-mediated bacterial infection.

Nanobody-driven signaling reveals the core receptor complex in root nodule symbiosis

Understanding the composition and activation of multicomponent receptor complexes is a challenge in biology. To address this, we developed a synthetic approach based on nanobodies to drive assembly and activation of cell surface receptors and apply the concept by manipulating receptors that govern plant symbiosis with nitrogen-fixing bacteria. We show that the Lotus japonicus Nod factor receptors NFR1 and NFR5 constitute the core receptor complex initiating the cortical root nodule organogenesis program as well as the epidermal program controlling infection. We find that organogenesis signaling is mediated by the intracellular kinase domains whereas infection requires functional ectodomains. Finally, we identify evolutionarily distant barley receptors that activate root nodule organogenesis, which could enable engineering of biological nitrogen-fixation into cereals.

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.

Plant-Mycorrhizal Fungi Interactions in Phytoremediation of Geogenic Contaminated Soils

Soil contamination by geogenic contaminants (GCs) represents an imperative environmental problem. Various soil remediation methods have been successfully employed to ameliorate the health risks associated with GCs. Phytoremediation is considered as an eco-friendly and economical approach to revegetate GC-contaminated soils. However, it is a very slow process, as plants take a considerable amount of time to gain biomass. Also, the process is limited only to the depth and surface area of the root. Inoculation of arbuscular mycorrhizal fungi (AMF) with remediating plants has been found to accelerate the phytoremediation process by enhancing plant biomass and their metal accumulation potential while improving the soil physicochemical and biological characteristics. Progress in the field application is hindered by a lack of understanding of complex interactions between host plant and AMF that contribute to metal detoxification/(im)mobilization/accumulation/translocation. Thus, this review is an attempt to reveal the underlying mechanisms of plant-AMF interactions in phytoremediation.

Indigenous Microorganisms Offset Arbuscular Mycorrhizal Fungi-Induced Plant Growth and Nutrient Acquisition Through Negatively Modulating the Genes of Phosphorus Transport and Nitrogen Assimilation

Arbuscular mycorrhizal (AM) fungi that promote plant growth and nutrient acquisition are essential for nutrient-deficient karst areas, while they inevitably regulate host plants jointly with indigenous microorganisms in natural soil. However, how indigenous microorganisms regulate AM-induced benefits on plant growth and nutrient acquisition remains unclear. In this study, the Bidens tripartita as the common plant species in the karst region was cultivated into three soil substrates treated by AM fungi inoculation (AMF), AM fungi inoculation combining with indigenous microorganisms (AMI), and the control without AM fungi and indigenous microorganisms (CK). The plant biomass and concentration of nitrogen (N) and phosphorus (P) were measured, and the transcriptomic analysis was carried out using root tissues. The results showed that AM fungi significantly enhanced the plant biomass, N, and P accumulation with the reduction of plants' N/P ratio; however, the indigenous microorganisms offset the AM-induced benefits in biomass and N and P acquisition. In addition, there are 819 genes in differentially expressed genes (DEGs) of AMF vs. AMI ∩ AMF vs. CK, meaning that AM fungi induced these genes that were simultaneously regulated by indigenous microorganisms. Furthermore, the enrichment analysis suggested that these genes were significantly associated with the metabolic processes of organophosphate, P, sulfur, N, and arginine biosynthesis. Notably, 34 and 17 genes of DEGs were related to P and N metabolism, respectively. Moreover, the indigenous microorganisms significantly downregulated these DEGs, especially those encoding the PHO1 P transporters and the glnA, glutamate dehydrogenase 2 (GDH2), and urease as key enzymes in N assimilation; however, the indigenous microorganisms significantly upregulated genes encoding PHO84 inducing cellular response to phosphate (Pi) starvation. These regulations indicated that indigenous microorganisms restrained the N and P metabolism induced by AM fungi. In conclusion, we suggested that indigenous microorganisms offset nutrient benefits of AM fungi for host plants through regulating these genes related to P transport and N assimilation.

Suppression of LjBAK1-mediated immunity by SymRK promotes rhizobial infection in Lotus japonicus

An important question in biology is how organisms can associate with different microbes that pose no threat (commensals), pose a severe threat (pathogens), and those that are beneficial (symbionts). The root nodule symbiosis serves as an important model system for addressing such questions in the context of plant-microbe interactions. It is now generally accepted that rhizobia can actively suppress host immune responses during the infection process, analogous to the way in which plant pathogens can evade immune recognition. However, much remains to be learned about the mechanisms by which the host recognizes the rhizobia as pathogens and how, subsequently, these pathways are suppressed to allow establishment of the nitrogen-fixing symbiosis. In this study, we found that SymRK (Symbiosis Receptor-like Kinase) is required for rhizobial suppression of plant innate immunity in Lotus japonicus. SymRK associates with LjBAK1 (BRASSINOSTEROID INSENSITIVE 1-Associated receptor Kinase 1), a well-characterized positive regulator of plant innate immunity, and directly inhibits LjBAK1 kinase activity. Rhizobial inoculation enhances the association between SymRK and LjBAK1 in planta. LjBAK1 is required for the regulation of plant innate immunity and plays a negative role in rhizobial infection in L. japonicus. The data indicate that the SymRK-LjBAK1 protein complex serves as an intersection point between rhizobial symbiotic signaling pathways and innate immunity pathways, and support that rhizobia may actively suppress the host's ability to mount a defense response during the legume-rhizobium symbiosis.

Multi-species relationships in legume roots: From pairwise legume-symbiont interactions to the plant - microbiome - soil continuum

Mutualistic relationships of legume plants with, either bacteria (like rhizobia) or fungi (like arbuscular mycorrhizal fungi), have been investigated intensively, usually as bi-partite interactions. However, diverse symbiotic interactions take place simultaneously or sequentially under field conditions. Their collective, but not additive, contribution to plant growth and performance remains hard to predict, and appears to be furthermore affected by crop species and genotype, non-symbiotic microbial interactions and environmental variables. The challenge is: (i) to unravel the complex overlapping mechanisms that operate between the microbial symbionts as well as between them, their hosts and the rhizosphere (ii) to understand the dynamics of the respective mechanisms in evolutionary and ecological terms. The target for agriculture, food security and the environment, is to use this insight as a solid basis for developing new integrated technologies, practices and strategies for the efficient use of beneficial microbes in legumes and other plants. We review recent advances in our understanding of the symbiotic interactions in legumes roots brought about with the aid of molecular and bioinformatics tools. We go through single symbiont-host interactions, proceed to tripartite symbiont-host interactions, appraise interactions of symbiotic and associative microbiomes with plants in the root-rhizoplane-soil continuum of habitats and end up by examining attempts to validate community ecology principles in the legume-microbe-soil biosystem.

Connecting the dots: from nanodomains to physiological functions of REMORINs

REMORINs (REMs) are a plant-specific protein family, proposed regulators of membrane-associated molecular assemblies and well-established markers of plasma membrane nanodomains. REMs play a diverse set of functions in plant interactions with pathogens and symbionts, responses to abiotic stresses, hormone signaling and cell-to-cell communication. In this review, we highlight the established and more putative roles of REMs throughout the literature. We discuss the physiological functions of REMs, the mechanisms underlying their nanodomain-organization and their putative role as regulators of nanodomain-associated molecular assemblies. Furthermore, we discuss how REM phosphorylation may regulate their functional versatility. Overall, through data-mining and comparative analysis of the literature, we suggest how to further study the molecular mechanisms underpinning the functions of REMs.

A Transcriptomic Approach Provides Insights on the Mycorrhizal Symbiosis of the Mediterranean Orchid Limodorum abortivum in Nature

The study of orchid mycorrhizal interactions is particularly complex because of the peculiar life cycle of these plants and their diverse trophic strategies. Here, transcriptomics has been applied to investigate gene expression in the mycorrhizal roots of Limodorum abortivum, a terrestrial mixotrophic orchid that associates with ectomycorrhizal fungi in the genus Russula. Our results provide new insights into the mechanisms underlying plant-fungus interactions in adult orchids in nature and in particular into the plant responses to the mycorrhizal symbiont(s) in the roots of mixotrophic orchids. Our results indicate that amino acids may represent the main nitrogen source in mycorrhizal roots of L. abortivum, as already suggested for orchid protocorms and other orchid species. The upregulation, in mycorrhizal L. abortivum roots, of some symbiotic molecular marker genes identified in mycorrhizal roots from other orchids as well as in arbuscular mycorrhiza, may mirror a common core of plant genes involved in endomycorrhizal symbioses. Further efforts will be required to understand whether the specificities of orchid mycorrhiza depend on fine-tuned regulation of these common components, or whether specific additional genes are involved.

The Impacts of Domestication and Agricultural Practices on Legume Nutrient Acquisition Through Symbiosis With Rhizobia and Arbuscular Mycorrhizal Fungi

Legumes are unique among plants as they can obtain nitrogen through symbiosis with nitrogen-fixing rhizobia that form root nodules in the host plants. Therefore they are valuable crops for sustainable agriculture. Increasing nitrogen fixation efficiency is not only important for achieving better plant growth and yield, but it is also crucial for reducing the use of nitrogen fertilizer. Arbuscular mycorrhizal fungi (AMF) are another group of important beneficial microorganisms that form symbiotic relationships with legumes. AMF can promote host plant growth by providing mineral nutrients and improving the soil ecosystem. The trilateral legume-rhizobia-AMF symbiotic relationships also enhance plant development and tolerance against biotic and abiotic stresses. It is known that domestication and agricultural activities have led to the reduced genetic diversity of cultivated germplasms and higher sensitivity to nutrient deficiencies in crop plants, but how domestication has impacted the capability of legumes to establish beneficial associations with rhizospheric microbes (including rhizobia and fungi) is not well-studied. In this review, we will discuss the impacts of domestication and agricultural practices on the interactions between legumes and soil microbes, focusing on the effects on AMF and rhizobial symbioses and hence nutrient acquisition by host legumes. In addition, we will summarize the genes involved in legume-microbe interactions and studies that have contributed to a better understanding of legume symbiotic associations using metabolic modeling.

Key Genes and Genetic Interactions of Plant-Pathogen Functional Modules in Poplar Infected by Marssonina brunnea

Marssonina brunnea, the causative pathogen of Marssonina leaf spot of poplars (MLSP), devastates poplar plantations by forming black spots on leaves and defoliating trees. Although MLSP has been studied for over 30 years, the key genes that function during M. brunnea infection and their effects on plant growth are poorly understood. Here, we used multigene association studies to investigate the effects of key genes in the plant-pathogen interaction pathway, as revealed by transcriptome analysis, on photosynthesis and growth in a natural population of 435 Populus tomentosa individuals. By analyzing transcriptomic changes during three stages of infection, we detected 628 transcription factor genes among the 7,611 differentially expressed genes that might be associated with basal defense responses. Gene Ontology and Kyoto Encyclopedia of Genes and Genomes enrichment analyses revealed that transcriptomic changes across different stages of infection lead to the reprogramming of metabolic processes possibly related to defense activation. We identified 29,399 common single-nucleotide polymorphisms (SNPs) within 221 full-length genes in plant-pathogen interaction pathways that were significantly associated with photosynthetic and growth traits. We also detected 4,460 significant epistatic pairs associated with stomatal conductance, tree diameter, and tree height. Epistasis analysis uncovered significant interactions between 2,561 SNP-SNP pairs from different functional modules in the plant-pathogen interaction pathway, revealing possible genetic interactions. This analysis revealed many key genes that function during M. brunnea infection and their potential roles in mediating photosynthesis and plant growth, shedding light on genetic interactions between functional modules in the plant-pathogen interaction pathway.

An endophytic Fusarium-legume association is partially dependent on the common symbiotic signalling pathway

Legumes interact with a wide range of microbes in their root systems, ranging from beneficial symbionts to pathogens. Symbiotic rhizobia and arbuscular mycorrhizal glomeromycetes trigger a so-called common symbiotic signalling pathway (CSSP), including the induction of nuclear calcium spiking in the root epidermis. By combining gene expression analysis, mutant phenotypic screening and analysis of nuclear calcium elevations, we demonstrate that recognition of an endophytic Fusarium solani strain K (FsK) in model legumes is initiated via perception of chitooligosaccharidic molecules and is, at least partially, CSSP-dependent. FsK induced the expression of Lysin-motif receptors for chitin-based molecules, CSSP members and CSSP-dependent genes in Lotus japonicus. In LysM and CSSP mutant/RNAi lines, root penetration and fungal intraradical progression was either stimulated or limited, whereas FsK exudates triggered CSSP-dependent nuclear calcium spiking, in epidermal cells of Medicago truncatula root organ cultures. Our results corroborate CSSP being involved in the perception of signals from other microbes beyond the restricted group of symbiotic interactions sensu stricto.

Receptor-Like Kinases Sustain Symbiotic Scrutiny

Plant receptor-like kinases (RLKs) control the initiation, development, and maintenance of symbioses with beneficial mycorrhizal fungi and nitrogen-fixing bacteria. Carbohydrate perception activates symbiosis signaling via Lysin-motif RLKs and subsequently the common symbiosis signaling pathway. As the receptors activated are often also immune receptors in multiple species, exactly how carbohydrate identities avoid immune activation and drive symbiotic outcome is still not fully understood. This may involve the coincident detection of additional signaling molecules that provide specificity. Because of the metabolic costs of supporting symbionts, the level of symbiosis development is fine-tuned by a range of local and mobile signals that are activated by various RLKs. Beyond early, precontact symbiotic signaling, signal exchanges ensue throughout infection, nutrient exchange, and turnover of symbiosis. Here, we review the latest understanding of plant symbiosis signaling from the perspective of RLK-mediated pathways.

Anthoceros genomes illuminate the origin of land plants and the unique biology of hornworts

Hornworts comprise a bryophyte lineage that diverged from other extant land plants >400 million years ago and bears unique biological features, including a distinct sporophyte architecture, cyanobacterial symbiosis and a pyrenoid-based carbon-concentrating mechanism (CCM). Here, we provide three high-quality genomes of Anthoceros hornworts. Phylogenomic analyses place hornworts as a sister clade to liverworts plus mosses with high support. The Anthoceros genomes lack repeat-dense centromeres as well as whole-genome duplication, and contain a limited transcription factor repertoire. Several genes involved in angiosperm meristem and stomatal function are conserved in Anthoceros and upregulated during sporophyte development, suggesting possible homologies at the genetic level. We identified candidate genes involved in cyanobacterial symbiosis and found that LCIB, a Chlamydomonas CCM gene, is present in hornworts but absent in other plant lineages, implying a possible conserved role in CCM function. We anticipate that these hornwort genomes will serve as essential references for future hornwort research and comparative studies across land plants.

The intensity of ethylene release by soybean plants under the influence of fungicides in the early stages of legume-rhizobial symbiosis

The effect of pre-sowing treatment of soybean seeds with fungicides on the intensity of ethylene release, the processes of nodulation and nitrogen fixation in different symbiotic systems in the early stages of ontogenesis were investigated. The objects of the study were selected symbiotic systems formed with the participation of soybean (Glycine max (L.) Merr.) Diamond variety, strains Bradyrhizobium japonicum 634b (active, virulent) and 604k (inactive, highly virulent) and fungicides Maxim XL 035 PS (fludioxonil, 25 g/L, metalaxyl, 10 g/L), and Standak Top (fipronil, 250 g/L, thiophanate methyl, 225 g/L, piraclostrobin, 25 g/L). Before sowing, the seeds of soybean were treated with solutions of fungicides, calculated on the basis of one rate of expenditure of the active substance of each preparation indicated by the producer per ton of seed. One part of the seeds treated with fungicides was inoculated with rhizobium culture for 1 h (the titre of bacteria was 107 cells/mL). To conduct the research we used microbiological, physiological, biochemical methods, gas chromatography and spectrophotometry. It is found that, regardless of the effectiveness of soybean rhizobial symbiosis, the highest level of ethylene release by plants was observed in the stages of primordial leaf and first true leaf. This is due to the initial processes of nodulation – the laying of nodule primordia and the active formation of nodules on the roots of soybeans. The results show that with the participation of fungicides in different symbiotic systems, there are characteristic changes in phytohormone synthesis in the primordial leaf stage, when the nodule primordia are planted on the root system of plants. In particular, in the ineffective symbiotic system, the intensity of phytohormone release decreases, while in the effective symbiotic system it increases. At the same time, a decrease in the number of nodules on soybean roots inoculated with an inactive highly virulent rhizobia 604k strain due to the action of fungicides and an increase in their number in variants with co-treatment of fungicides and active virulent strain 634b into the stage of the second true leaf were revealed. It was shown that despite a decrease in the mass of root nodules, there is an increase in their nitrogen-fixing activity in an effective symbiotic system with the participation of fungicides in the stage of the second true leaf. The highest intensity of ethylene release in both symbiotic systems was recorded in the stage of the first true leaf, which decreased in the stage of the second true leaf and was independent of the nature of the action of the active substances of fungicides. The obtained data prove that the action of fungicides changes the synthesis of ethylene by soybean plants, as well as the processes of nodulation and nitrogen fixation, which depend on the efficiency of the formed soybean-rhizobial systems and their ability to realize their symbiotic potential under appropriate growing conditions.

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.

Characterizing standard genetic parts and establishing common principles for engineering legume and cereal roots

Plant synthetic biology and cereal engineering depend on the controlled expression of transgenes of interest. Most engineering in plant species to date has relied heavily on the use of a few, well-established constitutive promoters to achieve high levels of expression; however, the levels of transgene expression can also be influenced by the use of codon optimization, intron-mediated enhancement and varying terminator sequences. Most of these alternative approaches for regulating transgene expression have only been tested in small-scale experiments, typically testing a single gene of interest. It is therefore difficult to interpret the relative importance of these approaches and to design engineering strategies that are likely to succeed in different plant species, particularly if engineering multigenic traits where the expression of each transgene needs to be precisely regulated. Here, we present data on the characterization of 46 promoters and 10 terminators in Medicago truncatula, Lotus japonicus, Nicotiana benthamiana and Hordeum vulgare, as well as the effects of codon optimization and intron-mediated enhancement on the expression of two transgenes in H. vulgare. We have identified a core set of promoters and terminators of relevance to researchers engineering novel traits in plant roots. In addition, we have shown that combining codon optimization and intron-mediated enhancement increases transgene expression and protein levels in barley. Based on our study, we recommend a core set of promoters and terminators for broad use and also propose a general set of principles and guidelines for those engineering cereal species.

A Phosphate-Dependent Requirement for Transcription Factors IPD3 and IPD3L During Arbuscular Mycorrhizal Symbiosis in Medicago truncatula

During arbuscular mycorrhizal (AM) symbiosis, activation of a symbiosis signaling pathway induces gene expression necessary for accommodation of AM fungi. Here, we focus on pathway components Medicago truncatula INTERACTING PROTEIN OF DOES NOT MAKE INFECTIONS 3 (IPD3) and IPD3 LIKE (IPD3L), which are potential orthologs of Lotus japonicus CYCLOPS, a transcriptional regulator essential for AM symbiosis. In the double mutant ipd3 ipd3l, hyphal entry through the epidermis and overall colonization levels are reduced relative to the wild type but fully developed arbuscules are present in the cortex. In comparison with the wild type, colonization of ipd3 ipd3l is acutely sensitive to higher phosphate levels in the growth medium, with a disproportionate decrease in epidermal penetration, overall colonization, and symbiotic gene expression. When constitutively expressed in ipd3 ipd3l, an autoactive DOES NOT MAKE INFECTIONS 3 induces the expression of transcriptional regulators REDUCED ARBUSCULAR MYCORRHIZA 1 and REQUIRED for ARBUSCULE DEVELOPMENT 1, providing a possible avenue for arbuscule development in the absence of IPD3 and IPD3L. An increased sensitivity of ipd3 ipd3l to GA₃ suggests an involvement of DELLA. The data reveal partial redundancy in the symbiosis signaling pathway, which may ensure robust signaling in low-phosphorus environments, while IPD3 and IPD3L maintain signaling in higher-phosphorus environments. The latter may buffer the pathway from short-term variation in phosphorus levels encountered by roots during growth in heterogeneous soil environments.

Transcriptome analysis of soybean (Glycine max) root genes differentially expressed in rhizobial, arbuscular mycorrhizal, and dual symbiosis

Soybean (Glycine max) roots establish associations with nodule-inducing rhizobia and arbuscular mycorrhizal (AM) fungi. Both rhizobia and AM fungi have been shown to affect the activity of and colonization by the other, and their interactions can be detected within host plants. Here, we report the transcription profiles of genes differentially expressed in soybean roots in the presence of rhizobial, AM, or rhizobial-AM dual symbiosis, compared with those in control (uninoculated) roots. Following inoculation, soybean plants were grown in a glasshouse for 6 weeks; thereafter their root transcriptomes were analyzed using an oligo DNA microarray. Among the four treatments, the root nodule number and host plant growth were highest in plants with dual symbiosis. We observed that the expression of 187, 441, and 548 host genes was up-regulated and 119, 1,439, and 1,298 host genes were down-regulated during rhizobial, AM, and dual symbiosis, respectively. The expression of 34 host genes was up-regulated in each of the three symbioses. These 34 genes encoded several membrane transporters, type 1 metallothionein, and transcription factors in the MYB and bHLH families. We identified 56 host genes that were specifically up-regulated during dual symbiosis. These genes encoded several nodulin proteins, phenylpropanoid metabolism-related proteins, and carbonic anhydrase. The nodulin genes up-regulated by the AM fungal colonization probably led to the observed increases in root nodule number and host plant growth. Some other nodulin genes were down-regulated specifically during AM symbiosis. Based on the results above, we suggest that the contribution of AM fungal colonization is crucial to biological N2-fixation and host growth in soybean with rhizobial-AM dual symbiosis.

Mechanisms and Impact of Symbiotic Phosphate Acquisition

Phosphorous is important for life but often limiting for plants. The symbiotic pathway of phosphate uptake via arbuscular mycorrhizal fungi (AMF) is evolutionarily ancient and today occurs in natural and agricultural ecosystems alike. Plants capable of this symbiosis can obtain up to all of the phosphate from symbiotic fungi, and this offers potential means to develop crops less dependent on unsustainable P fertilizers. Here, we review the mechanisms and insights gleaned from the fine-tuned signal exchanges that orchestrate the intimate mutualistic symbiosis between plants and AMF. As the currency of trade, nutrients have signaling functions beyond being the nutritional goal of mutualism. We propose that such signaling roles and metabolic reprogramming may represent commitments for a mutualistic symbiosis that act across the stages of symbiosis development.

Spatial Effects and GWA Mapping of Root Colonization Assessed in the Interaction Between the Rice Diversity Panel 1 and an Arbuscular Mycorrhizal Fungus

If water saving methods of rice management are to be adopted, the interaction between rice plants and arbuscular mycorrhizal (AM) fungi will grow in agronomic significance. As yet there are very few studies on the interaction between rice and AM fungi and none on host genetics. A subset 334 cultivars from the Rice Diversity Panel 1 were grown in 250 L boxes filled with phosphorus (P) deficient aerobic soil without addition, with added rock phosphate and with rock phosphate and the AM fungus Rhizophagus irregularis. Statistical analysis of position of plants revealed a positive effect of their neighbors on their dry weight which was stronger in the presence of rock phosphate and even stronger with rock phosphate and AM fungi. A weak but significant difference in the response of cultivars to AM fungus treatment in terms of shoot dry weight (SDW) was revealed. Neighbor hyphal colonization was positively related to a plant's hyphal colonization, providing insights into the way a network of AM fungi interact with multiple hosts. Hyphal colonization ranged from 21 to 89%, and 42% of the variation was explained by rice genotype. Colonization was slightly lower in aus cultivars than other rice subgroups and high in cultivars from the Philippines. Genome wide association (GWA) mapping for hyphal colonization revealed 23 putative quantitative trait loci (QTLs) indicating there is an opportunity to investigate the impact of allelic variation in rice on AM fungal colonization. Using published transcriptomics data for AM response in rice, some promising candidate genes are revealed under these QTLs being a calcium/calmodulin serine/threonine protein kinase at 4.9 Mbp on chromosome 1, two ammonium transporters genes at 24.6 Mbp on chromosome 2 and a cluster of subtilisin genes at 1.2 Mbp on chromosome 4. Future studies should concentrate on the biological significance of genetic variation in rice for AM colonization.

Nutrient demand and fungal access to resources control the carbon allocation to the symbiotic partners in tripartite interactions of Medicago truncatula

Legumes form tripartite interactions with arbuscular mycorrhizal fungi and rhizobia, and both root symbionts exchange nutrients against carbon from their host. The carbon costs of these interactions are substantial, but our current understanding of how the host controls its carbon allocation to individual root symbionts is limited. We examined nutrient uptake and carbon allocation in tripartite interactions of Medicago truncatula under different nutrient supply conditions, and when the fungal partner had access to nitrogen, and followed the gene expression of several plant transporters of the Sucrose Uptake Transporter (SUT) and Sugars Will Eventually be Exported Transporter (SWEET) family. Tripartite interactions led to synergistic growth responses and stimulated the phosphate and nitrogen uptake of the plant. Plant nutrient demand but also fungal access to nutrients played an important role for the carbon transport to different root symbionts, and the plant allocated more carbon to rhizobia under nitrogen demand, but more carbon to the fungal partner when nitrogen was available. These changes in carbon allocation were consistent with changes in the SUT and SWEET expression. Our study provides important insights into how the host plant controls its carbon allocation under different nutrient supply conditions and changes its carbon allocation to different root symbionts to maximize its symbiotic benefits.

Ectopic activation of cortical cell division during the accommodation of arbuscular mycorrhizal fungi

Arbuscular mycorrhizas (AMs) between plants and soil fungi are widespread symbioses with a major role in soil nutrient uptake. In this study we investigated the induction of root cortical cell division during AM colonization by combining morphometric and gene expression analyses with promoter activation and protein localization studies of the cell-plate-associated exocytic marker TPLATE. Our results show that TPLATE promoter is activated in colonized cells of the root cortex where we also observed the appearance of cells that are half the size of the surrounding cells. Furthermore, TPLATE-green fluorescent protein recruitment to developing cell plates highlighted ectopic cell division events in the inner root cortex during early AM colonization. Lastly, transcripts of TPLATE, KNOLLE and Cyclinlike 1 (CYC1) are all upregulated in the same context, alongside endocytic markers Adaptor-Related Protein complex 2 alpha 1 subunit (AP2A1) and Clathrin Heavy Chain 2 (CHC2), known to be active during cell plate formation. This pattern of gene expression was recorded in wild-type Medicago truncatula roots, but not in a common symbiotic signalling pathway mutant where fungal colonization is blocked at the epidermal level. Altogether, these results suggest the activation of cell-division-related mechanisms by AM hosts during the accommodation of the symbiotic fungus.

MLO Differentially Regulates Barley Root Colonization by Beneficial Endophytic and Mycorrhizal Fungi

Loss-of-function alleles of MLO (Mildew Resistance Locus O) confer broad-spectrum resistance to foliar infections by powdery mildew pathogens. Like pathogens, microbes that establish mutually beneficial relationships with their plant hosts, trigger the induction of some defense responses. Initially, barley colonization by the root endophyte Serendipita indica (syn. Piriformospora indica) is associated with enhanced defense gene expression and the formation of papillae at sites of hyphal penetration attempts. This phenotype is reminiscent of mlo-conditioned immunity in barley leaf tissue and raises the question whether MLO plays a regulatory role in the establishment of beneficial interactions. Here we show that S. indica colonization was significantly reduced in plants carrying mlo mutations compared to wild type controls. The reduction in fungal biomass was associated with the enhanced formation of papillae. Moreover, epidermal cells of S. indica-treated mlo plants displayed an early accumulation of iron in the epidermal layer suggesting increased basal defense activation in the barley mutant background. Correspondingly, the induction of host cell death during later colonization stages was impaired in mlo colonized plants, highlighting the importance of the early biotrophic growth phase for S. indica root colonization. In contrast, the arbuscular mycorrhizal fungus Funneliformis mosseae displayed a similar colonization morphology on mutant and wild type plants. However, the frequency of mycorrhization and number of arbuscules was higher in mlo-5 mutants. These findings suggest that MLO differentially regulates root colonization by endophytic and AM fungi.

Complex regulation of microRNAs in roots of competitively-grown isogenic Nicotiana attenuata plants with different capacities to interact with arbuscular mycorrhizal fungi

BACKGROUND

Nicotiana attenuata is an ecological model plant whose 2.57 Gb genome has recently been sequenced and assembled and for which miRNAs and their genomic locations have been identified. To understand how this plant's miRNAs are reconfigured during plant-arbuscular mycorrhizal fungal (AMF) interactions and whether hostplant calcium- and calmodulin dependent protein kinase (CCaMK) expression which regulates the AMF interaction also modulates miRNAs levels and regulation, we performed a large-scale miRNA analysis of this plant-AMF interaction.

RESULTS

Next generation sequencing of miRNAs in roots of empty vector (EV) N. attenuata plants and an isogenic line silenced in CCaMK expression (irCCaMK) impaired in AMF-interactions grown under competitive conditions with and without AMF inoculum revealed a total of 149 unique miRNAs: 67 conserved and 82 novel ones. The majority of the miRNAs had a length of 21 nucleotides. MiRNA abundances were highly variable ranging from 400 to more than 25,000 reads per million. The miRNA profile of irCCaMK plants impaired in AMF colonization was distinct from fully AMF-functional EV plants grown in the same pot. Six conserved miRNAs were present in all conditions and accumulated differentially depending on treatment and genotype; five (miR6153, miR403a-3p, miR7122a, miR167-5p and miR482d, but not miR399a-3p) showed the highest accumulation in AMF inoculated EV plants compared to inoculated irCCaMK plants. Furthermore, the accumulation patterns of sequence variants of selected conserved miRNAs showed a very distinct pattern related to AMF colonization - one variant of miR473-5p specifically accumulated in AMF-inoculated plants. Also abundances of miR403a-3p, miR171a-3p and one of the sequence variants of miR172a-3p increased in AMF-inoculated EV compared to inoculated irCCaMK plants and to non-inoculated EV plants, while miR399a-3p was most strongly enriched in AMF inoculated irCCaMK plants grown in competition with EV. The analysis of putative targets of selected miRNAs revealed an involvement in P starvation (miR399), phytohormone signaling (Nat-R-PN59, miR172, miR393) and defense (e.g. miR482, miR8667, Nat-R-PN-47).

CONCLUSIONS

Our study demonstrates (1) a large-scale reprograming of miRNAs induced by AMF colonization and (2) that the impaired AMF signaling due to CCaMK silencing and the resulting reduced competitive ability of irCCaMK plants play a role in modulating signal-dependent miRNA accumulation.

Symbiosis Specificity of the Preceding Host Plant Can Dominate but Not Obliterate the Association Between Wheat and Its Arbuscular Mycorrhizal Fungal Partners

The symbiosis established between arbuscular mycorrhizal fungi (AMF) and roots of most land plants plays a key role in plant nutrient acquisition and alleviation of environmental stresses. Despite the ubiquity of the symbiosis, AMF and host species display significant specificity in their interactions. To clarify preferential associations between wheat (Triticum aestivum) and AMF, we characterized root AMF communities in the transition from two first host species, ryegrass (Lolium rigidum) and yellow-serradella (Ornithopus compressus), grown separately or together, to a second host (wheat), by sequencing the large subunit ribosomal DNA (LSU rDNA) gene. The response of AMF communities in wheat to prior soil disturbance – and consequently of the mycelial network [intact extraradical mycelium (ERM) vs. disrupted mycelium] established with either of the first hosts – was also investigated. Since the outcome of a specific host–symbiont interaction depends on the molecular responses of the host plant upon microbial colonization, we studied the expression of six key symbiosis-related genes in wheat roots. AMF communities on L. rigidum and O. compressus roots were clearly distinct. Within an undisturbed ERM, wheat AMF communities were similar to that of previous host, and O. compressus-wheat-AMF interactions supported a greater growth of wheat than L. rigidum-wheat-AMF interactions. This effect declined when ERM was disrupted, but generated a greater activation of symbiotic genes in wheat, indicating that plant symbiotic program depends on some extent on the colonizing symbiont propagule type. When a mixture of L. rigidum and O. compressus was planted, the wheat colonization pattern resembled that of O. compressus, although this was not reflected in a greater growth. These results show a lasting effect of previous hosts in shaping wheat AMF communities through an efficient use of the established ERM, although not completely obliterating host–symbiont specificity.

A rice Serine/Threonine receptor-like kinase regulates arbuscular mycorrhizal symbiosis at the peri-arbuscular membrane

In terrestrial ecosystems most plant species live in mutualistic symbioses with nutrient-delivering arbuscular mycorrhizal (AM) fungi. Establishment of AM symbioses includes transient, intracellular formation of fungal feeding structures, the arbuscules. A plant-derived peri-arbuscular membrane (PAM) surrounds the arbuscules, mediating reciprocal nutrient exchange. Signaling at the PAM must be well coordinated to achieve this dynamic cellular intimacy. Here, we identify the PAM-specific Arbuscular Receptor-like Kinase 1 (ARK1) from maize and rice to condition sustained AM symbiosis. Mutation of rice ARK1 causes a significant reduction in vesicles, the fungal storage structures, and a concomitant reduction in overall root colonization by the AM fungus Rhizophagus irregularis. Arbuscules, although less frequent in the ark1 mutant, are morphologically normal. Co-cultivation with wild-type plants restores vesicle and spore formation, suggesting ARK1 function is required for the completion of the fungal life-cycle, thereby defining a functional stage, post arbuscule development.

The peri-arbuscular membrane (PAM) mediates mutually-beneficial nutrient exchange between plants and arbuscular mycorrhizal (AM) fungi. Here the authors identify ARK1, a PAM-specific receptor-like kinase from rice that sustains AM symbiosis post-arbuscule development.

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.

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.

Understanding the Arbuscule at the Heart of Endomycorrhizal Symbioses in Plants

Arbuscular mycorrhizal fungi form associations with most land plants and facilitate nutrient uptake from the soil, with the plant receiving mineral nutrients from the fungus and in return providing the fungus with fixed carbon. This nutrient exchange takes place through highly branched fungal structures called arbuscules that are formed in cortical cells of the host root. Recent discoveries have highlighted the importance of fatty acids, in addition to sugars, acting as the form of fixed carbon transferred from the plant to the fungus and several studies have begun to elucidate the mechanisms that control the plant processes necessary for fungal colonisation and arbuscule development. In this review, we analyse the mechanisms that allow arbuscule development and the processes necessary for nutrient exchange between the plant and the fungus.

Epidermal auxin biosynthesis facilitates rhizobial infection in Lotus japonicus

Symbiotic nitrogen fixation in legumes requires nodule organogenesis to be coordinated with infection by rhizobia. The plant hormone auxin influences symbiotic infection, but the precise timing of auxin accumulation and the genetic network governing it remain unclear. We used a Lotus japonicus optimised variant of the DII-based auxin accumulation sensor and identified a rapid accumulation of auxin in the epidermis, specifically in the root hair cells. This auxin accumulation occurs in the infected root hairs during rhizobia invasion, while Nod factor application induces this response across a broader range of root hairs. Using the DR5 auxin responsive promoter, we demonstrate that activation of auxin signalling also occurs specifically in infected root hairs. Analysis of root hair transcriptome data identified induction of an auxin biosynthesis gene of the Tryptophan Amino-transferase Related (LjTar1) family following both bacteria inoculation and Nod factor treatment. Genetic analysis showed that both expression of the LjTar1 biosynthesis gene and the auxin response requires Nod factor perception, while common symbiotic pathway transcription factors are only partially required or act redundantly to initiate auxin accumulation. Using a chemical genetics approach, we confirmed that auxin biosynthesis has a functional role in promoting symbiotic infection events in the epidermis.

Comparative genomics of the nonlegume Parasponia reveals insights into evolution of nitrogen-fixing rhizobium symbioses

Nodules harboring nitrogen-fixing rhizobia are a well-known trait of legumes, but nodules also occur in other plant lineages, with rhizobia or the actinomycete Frankia as microsymbiont. It is generally assumed that nodulation evolved independently multiple times. However, molecular-genetic support for this hypothesis is lacking, as the genetic changes underlying nodule evolution remain elusive. We conducted genetic and comparative genomics studies by using Parasponia species (Cannabaceae), the only nonlegumes that can establish nitrogen-fixing nodules with rhizobium. Intergeneric crosses between Parasponia andersonii and its nonnodulating relative Trema tomentosa demonstrated that nodule organogenesis, but not intracellular infection, is a dominant genetic trait. Comparative transcriptomics of P. andersonii and the legume Medicago truncatula revealed utilization of at least 290 orthologous symbiosis genes in nodules. Among these are key genes that, in legumes, are essential for nodulation, including NODULE INCEPTION (NIN) and RHIZOBIUM-DIRECTED POLAR GROWTH (RPG). Comparative analysis of genomes from three Parasponia species and related nonnodulating plant species show evidence of parallel loss in nonnodulating species of putative orthologs of NIN, RPG, and NOD FACTOR PERCEPTION Parallel loss of these symbiosis genes indicates that these nonnodulating lineages lost the potential to nodulate. Taken together, our results challenge the view that nodulation evolved in parallel and raises the possibility that nodulation originated ∼100 Mya in a common ancestor of all nodulating plant species, but was subsequently lost in many descendant lineages. This will have profound implications for translational approaches aimed at engineering nitrogen-fixing nodules in crop plants.

From structure to function - a family portrait of plant subtilases

Contents Summary 901 I. Introduction 901 II. Biochemistry and structure of plant SBTs 902 III. Phylogeny of plant SBTs and family organization 903 IV. Physiological roles of plant SBTs 905 V. Conclusions and outlook 911 Acknowledgements 912 References 912 SUMMARY: Subtilases (SBTs) are serine peptidases that are found in all three domains of life. As compared with homologs in other Eucarya, plant SBTs are more closely related to archaeal and bacterial SBTs, with which they share many biochemical and structural features. However, in the course of evolution, functional diversification led to the acquisition of novel, plant-specific functions, resulting in the present-day complexity of the plant SBT family. SBTs are much more numerous in plants than in any other organism, and include enzymes involved in general proteolysis as well as highly specific processing proteases. Most SBTs are targeted to the cell wall, where they contribute to the control of growth and development by regulating the properties of the cell wall and the activity of extracellular signaling molecules. Plant SBTs affect all stages of the life cycle as they contribute to embryogenesis, seed development and germination, cuticle formation and epidermal patterning, vascular development, programmed cell death, organ abscission, senescence, and plant responses to their biotic and abiotic environments. In this article we provide a comprehensive picture of SBT structure and function in plants.

CEP peptide hormones: key players in orchestrating nitrogen-demand signalling, root nodulation, and lateral root development

Secreted peptide hormones play pivotal roles in plant growth and development. So far, CEPs (C-TERMINALLY ENCODED PEPTIDEs) have been shown to act through CEP receptors (CEPRs) to control nitrogen (N)-demand signalling, nodulation, and lateral root development. Secreted CEP peptides can enter the xylem stream to act as long-distance signals, but evidence also exists for CEPs acting in local circuits. Recently, CEP peptide species varying in sequence, length, and post-translational modifications have been identified. A more comprehensive understanding of CEP biology requires insight into the in planta function of CEP genes, CEP peptide biogenesis, the components of CEP signalling cascades and, finally, how CEP peptide length, amino-acid composition, and post-translational modifications affect biological activity. In this review, we highlight recent studies that have advanced our understanding in these key areas and discuss some future directions.

Chemical signaling involved in plant-microbe interactions

Microorganisms are found everywhere, and they are closely associated with plants. Because the establishment of any plant-microbe association involves chemical communication, understanding crosstalk processes is fundamental to defining the type of relationship. Although several metabolites from plants and microbes have been fully characterized, their roles in the chemical interplay between these partners are not well understood in most cases, and they require further investigation. In this review, we describe different plant-microbe associations from colonization to microbial establishment processes in plants along with future prospects, including agricultural benefits.

Commonalities and Differences in Controlling Multipartite Intracellular Infections of Legume Roots by Symbiotic Microbes

Legumes have the almost unique ability to establish symbiotic associations with rhizobia and arbuscular mycorrhizal fungi. Forward and reverse genetics have identified a large number of genes that are required for either or both interactions. However, and in sharp contrast to natural soils, these interactions have been almost exclusively investigated under laboratory conditions by using separate inoculation systems, whereas both symbionts are simultaneously present in the field. Considering our recent understanding of the individual symbioses, the community is now promisingly positioned to co-inoculate plants with two or more microbes in order to understand mechanistically how legumes efficiently balance, regulate and potentially separate these symbioses and other endophytic microbes within the same root. Here, we discuss a number of key control layers that should be considered when assessing tri- or multipartite beneficial interactions and that may contribute to colonization patterns in legume roots.

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.

Nicotiana attenuata's capacity to interact with arbuscular mycorrhiza alters its competitive ability and elicits major changes in the leaf transcriptome

To study the local and systemic effects of arbuscular mycorrhizal fungal (AMF) colonization, Nicotiana attenuata plants impaired in their interactions with AMF due to silencing of a calcium- and calmodulin dependent protein kinase (inverted repreat (ir)CCaMK) were grown competitively in pairs with empty vector (EV) plants, with and without two different types of inoculum. When inoculated, EV plants strongly outperformed irCCaMK plants. Foliar transcript profiling revealed that AMF colonization significantly changed gene expression of P-starvation and -transporter genes in irCCaMK plants. The Pht1 family phosphate transporter NaPT5 was not only specifically induced in roots after AMF colonization, but also in leaves of AMF-colonized irCCaMK plants, and in plants grown under low Pi conditions in the absence of AMF. The P-starvation signature of inoculated irCCaMK plants corresponded with increases in selected amino acids and phenolic compounds in leaves. We also found a strong AMF-induced increase in amino acids and phenolic metabolites in roots. Plants impaired in their interactions with AMF clearly have a fitness disadvantage when competing for limited soil nutrients with a fully functional isogenic line. The additional role of the AMF-induced Pht1 family transporter NaPT5 in leaves under P-starvation conditions will require further experiments to fully resolve.

Know your enemy, embrace your friend: using omics to understand how plants respond differently to pathogenic and mutualistic microorganisms

Microorganisms, or 'microbes', have formed intimate associations with plants throughout the length of their evolutionary history. In extant plant systems microbes still remain an integral part of the ecological landscape, impacting plant health, productivity and long-term fitness. Therefore, to properly understand the genetic wiring of plants, we must first determine what perception systems plants have evolved to parse beneficial from commensal from pathogenic microbes. In this review, we consider some of the most recent advances in how plants respond at the molecular level to different microbial lifestyles. Further, we cover some of the means by which microbes are able to manipulate plant signaling pathways through altered destructiveness and nutrient sinks, as well as the use of effector proteins and micro-RNAs (miRNAs). We conclude by highlighting some of the major questions still to be answered in the field of plant-microbe research, and suggest some of the key areas that are in greatest need of further research investment. The results of these proposed studies will have impacts in a wide range of plant research disciplines and will, ultimately, translate into stronger agronomic crops and forestry stock, with immune perception and response systems bred to foster beneficial microbial symbioses while repudiating pathogenic symbioses.

IPD3 and IPD3L Function Redundantly in Rhizobial and Mycorrhizal Symbioses

Legume plants form symbiotic associations with either nitrogen-fixing bacteria or arbuscular mycorrhizal (AM) fungi, which are regulated by a set of common symbiotic signaling pathway genes. Central to the signaling pathway is the activation of the DMI3/IPD3 protein complex by Ca²⁺ oscillations, and the initiation of nodule organogenesis and mycorrhizal symbiosis. DMI3 is essential for rhizobial infection and nodule organogenesis; however, ipd3 mutants have been shown to be impaired only in infection thread formation but not in root nodule organogenesis in Medicago truncatula. We identified an IPD3-like (IPD3L) gene in the M. truncatula genome. A single ipd3l mutant exhibits a normal root nodule phenotype. The ipd3l/ipd3-2 double mutant is completely unable to initiate infection threads and nodule primordia. IPD3L can functionally replace IPD3 when expressed under the control of the IPD3 promoter, indicating functional redundancy between these two transcriptional regulators. We constructed a version of IPD3 that was phosphomimetic with respect to two conserved serine residues (IPD3-2D). This was sufficient to trigger root nodule organogenesis, but the increased multisite phosphorylation of IPD3 (IPD3-8D) led to low transcriptional activity, suggesting that the phosphorylation levels of IPD3 fine-tune its transcriptional activity in the root nodule symbiosis. Intriguingly, the phosphomimetic version of IPD3 triggers spontaneous root-like nodules on the roots of dmi3-1 and dmi2-1 (DMI2 is an LRR-containing receptor-like kinase gene which is required for Ca²⁺ spiking), but not on the roots of wild-type or ipd3l ipd3-2 plants. In addition, fully developed arbuscules were formed in the ipd3l ipd3-2 mutants but not the ccamk/dmi3-1 mutants. Collectively, our data indicate that, in addition to IPD3 and IPD3L, another new genetic component or other new phosphorylation sites of IPD3 function downstream of DMI3 in rhizobial and mycorrhizal symbioses.

Plant Signaling and Metabolic Pathways Enabling Arbuscular Mycorrhizal Symbiosis

Plants have lived in close association with arbuscular mycorrhizal (AM) fungi for over 400 million years. Today, this endosymbiosis occurs broadly in the plant kingdom where it has a pronounced impact on plant mineral nutrition. The symbiosis develops deep within the root cortex with minimal alterations in the external appearance of the colonized root; however, the absence of macroscopic alterations belies the extensive signaling, cellular remodeling, and metabolic alterations that occur to enable accommodation of the fungal endosymbiont. Recent research has revealed the involvement of a novel N-acetyl glucosamine transporter and an alpha/beta-fold hydrolase receptor at the earliest stages of AM symbiosis. Calcium channels required for symbiosis signaling have been identified, and connections between the symbiosis signaling pathway and key transcriptional regulators that direct AM-specific gene expression have been established. Phylogenomics has revealed the existence of genes conserved for AM symbiosis, providing clues as to how plant cells fine-tune their biology to enable symbiosis, and an exciting coalescence of genome mining, lipid profiling, and tracer studies collectively has led to the conclusion that AM fungi are fatty acid auxotrophs and that plants provide their fungal endosymbionts with fatty acids. Here, we provide an overview of the molecular program for AM symbiosis and discuss these recent advances.