Network of GRAS transcription factors involved in the control of arbuscule development in Lotus japonicus

Mildew Locus O facilitates colonization by arbuscular mycorrhizal fungi in angiosperms

Loss of barley Mildew Resistance Locus O (MLO) is known to confer durable and robust resistance to powdery mildew (Blumeria graminis), a biotrophic fungal leaf pathogen. Based on the increased expression of MLO in mycorrhizal roots and its presence in a clade of the MLO family that is specific to mycorrhizal-host species, we investigated the potential role of MLO in arbuscular mycorrhizal interactions. Using mutants from barley (Hordeum vulgare), wheat (Triticum aestivum), and Medicago truncatula, we demonstrate a role for MLO in colonization by the arbuscular mycorrhizal fungus Rhizophagus irregularis. Early mycorrhizal colonization was reduced in mlo mutants of barley, wheat, and M. truncatula, and this was accompanied by a pronounced decrease in the expression of many of the key genes required for intracellular accommodation of arbuscular mycorrhizal fungi. These findings show that clade IV MLOs are involved in the establishment of symbiotic associations with beneficial fungi, a role that has been appropriated by powdery mildew.

Genetic variation and evolutionary history of a mycorrhizal fungus regulate the currency of exchange in symbiosis with the food security crop cassava

Most land plants form symbioses with arbuscular mycorrhizal fungi (AMF). Diversity of AMF increases plant community productivity and plant diversity. For decades, it was known that plants trade carbohydrates for phosphate with their fungal symbionts. However, recent studies show that plant-derived lipids probably represent the most essential currency of exchange. Understanding the regulation of plant genes involved in the currency of exchange is crucial to understanding stability of this mutualism. Plants encounter many different AMF genotypes that vary greatly in the benefit they confer to plants. Yet the role that fungal genetic variation plays in the regulation of this currency has not received much attention. We used a high-resolution phylogeny of one AMF species (Rhizophagus irregularis) to show that fungal genetic variation drives the regulation of the plant fatty acid pathway in cassava (Manihot esculenta); a pathway regulating one of the essential currencies of trade in the symbiosis. The regulation of this pathway was explained by clearly defined patterns of fungal genome-wide variation representing the precise fungal evolutionary history. This represents the first demonstrated link between the genetics of AMF and reprogramming of an essential plant pathway regulating the currency of exchange in the symbiosis. The transcription factor RAM1 was also revealed as the dominant gene in the fatty acid plant gene co-expression network. Our study highlights the crucial role of variation in fungal genomes in the trade of resources in this important symbiosis and also opens the door to discovering characteristics of AMF genomes responsible for interactions between AMF and cassava that will lead to optimal cassava growth.

APETALA 2 transcription factor CBX1 is a regulator of mycorrhizal symbiosis and growth of Lotus japonicus

An AP2 family gene CBX1 is involved in mycorrhizal symbiosis and growth of Lotus japonicus. APETALA 2 (AP2) transcriptional regulator is highly conserved in plants. CBX1 from Lotus japonicus is a member of AP2 family. AMF (Arbuscular mycorrhizal fungi) inoculation experiment demonstrated that expression of CBX1 was significantly induced by AMF. Further promoter analysis showed that the - 764 to - 498 bp region of the CBX1 promoter containing CTTC motif is the AMF responsive region. Functional analysis of cbx1 mutant suggested CBX1 is critical for mycorrhizal symbiosis, especially for arbuscule formation. Moreover, under noncolonized condition, overexpression of CBX1 reduced the root length of L. japonicus but increased the size of root system and shoot length, whereas cbx1 mutant reduced the root size and shoot length, but not effect on root length. In addition, cbx1 altered activity of monolignol biosynthetic gene and increased lignin levels. Collectively, these data indicated that CBX1 is a positive regulator of symbiotic activity and plays roles in the growth of L. japonicus.

An ancestral signalling pathway is conserved in intracellular symbioses-forming plant lineages

Plants are the foundation of terrestrial ecosystems, and their colonization of land was probably facilitated by mutualistic associations with arbuscular mycorrhizal fungi. Following this founding event, plant diversification has led to the emergence of a tremendous diversity of mutualistic symbioses with microorganisms, ranging from extracellular associations to the most intimate intracellular associations, where fungal or bacterial symbionts are hosted inside plant cells. Here, through analysis of 271 transcriptomes and 116 plant genomes spanning the entire land-plant diversity, we demonstrate that a common symbiosis signalling pathway co-evolved with intracellular endosymbioses, from the ancestral arbuscular mycorrhiza to the more recent ericoid and orchid mycorrhizae in angiosperms and ericoid-like associations of bryophytes. By contrast, species forming exclusively extracellular symbioses, such as ectomycorrhizae, and those forming associations with cyanobacteria, have lost this signalling pathway. This work unifies intracellular symbioses, revealing conservation in their evolution across 450 million yr of plant diversification.

Transcriptome-Wide Identification, Evolutionary Analysis, and GA Stress Response of the GRAS Gene Family in Panax ginseng C. A. Meyer

GRAS transcription factors are a kind of plant-specific transcription factor that have been found in a variety of plants. According to previous studies, GRAS proteins are widely involved in the physiological processes of plant signal transduction, stress, growth and development. The Jilin ginseng (Panax ginseng C.A. Meyer) is a heterogeneous tetraploid perennial herb of the Araliaceae family, ginseng genus. Important information regarding the GRAS transcription factors has not been reported in ginseng. In this study, 59 Panax ginseng GRAS (PgGRAS) genes were obtained from the Jilin ginseng transcriptome data and divided into 13 sub-families according to the classification of Arabidopsis thaliana. Through systematic evolution, structural variation, function and gene expression analysis, we further reveal GRAS’s potential function in plant growth processes and its stress response. The expression of PgGRAS genes responding to gibberellin acids (GAs) suggests that these genes could be activated after application concentration of GA. The qPCR analysis result shows that four PgGRAS genes belonging to the DELLA sub-family potentially have important roles in the GA stress response of ginseng hairy roots. This study provides not only a preliminary exploration of the potential functions of the GRAS genes in ginseng, but also valuable data for further exploration of the candidate PgGRAS genes of GA signaling in Jilin ginseng, especially their roles in ginseng hairy root development and GA stress response.

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.

Genome-wide survey and expression analyses of the GRAS gene family in Brassica napus reveals their roles in root development and stress response

Genome-wide identification, classification, expression analyses, and functional characterization of GRAS genes in oil crop, Brassica napus, indicate their importance in root development and stress response. GRAS proteins are a plant-specific transcription factor gene family involved in tissues development and stress response. We classified 87 putative GRAS genes in the Brassica napus genome (BnGRASs) into 13 subfamilies by phylogenetic analysis. The C-terminal GRAS domains of Brassica napus (B. napus) proteins were less conserved among subfamilies, but were conserved within each subfamily. A series of analyses revealed that 89.7% of the BnGRASs did not have intron insertions, and 24 specific-motifs were found at the N-terminal. A highly conserved microRNA 171 (miRNA171) target was observed specifically in the HAM subfamily across land plants. A total of 868 pairs of interaction proteins were predicted, the primary of which were transcription factors involved in transcriptional regulation and signal transduction. Integrated comparative analysis of GRAS genes across 26 species of algae, mosses, ferns, gymnosperms, and angiosperms revealed that this gene family originated in early mosses and was classified into 19 subfamilies, 14 of which may have originated prior to bryophyte evolution. RNA-Seq analysis demonstrated that most BnGRASs were widely expressed in different tissues/organs at different stages in B. napus, and 24 BnGRASs were highly/specifically expressed in roots. Results from a qRT-PCR analysis suggested that two BnGRASs belonging to SCR and LISCL subfamilies potentially have important roles in the stress response of roots.

Dysfunction in the arbuscular mycorrhizal symbiosis has consistent but small effects on the establishment of the fungal microbiota in Lotus japonicus

Most land plants establish mutualistic interactions with arbuscular mycorrhizal (AM) fungi. Intracellular accommodation of AM fungal symbionts remodels important host traits like root morphology and nutrient acquisition. How mycorrhizal colonization impacts plant microbiota is unclear. To understand the impact of AM symbiosis on fungal microbiota, ten Lotus japonicus mutants impaired at different stages of AM formation were grown in non-sterile natural soil and their root-associated fungal communities were studied. Plant mutants lacking the capacity to form mature arbuscules (arb^- ) exhibited limited growth performance associated with altered phosphorus (P) acquisition and reduction-oxidation (redox) processes. Furthermore, arb^- plants assembled moderately but consistently different root-associated fungal microbiota, characterized by the depletion of Glomeromycota and the concomitant enrichment of Ascomycota, including Dactylonectria torresensis. Single and co-inoculation experiments showed a strong reduction of root colonization by D. torresensis in the presence of AM fungus Rhizophagus irregularis, particularly in arbuscule-forming plants. Our results suggest that impairment of central symbiotic functions in AM host plants leads to specific changes in root microbiomes and in tripartite interactions between the host plant, AM and non-AM fungi. This lays the foundation for mechanistic studies on microbe-microbe and microbe-host interactions in AM symbiosis of the model L. japonicus.

Genome-wide Identification, Classification, Expression and Duplication Analysis of GRAS Family Genes in Juglans regia L

Fifty-two GRAS genes are identified in walnut genome. Based on the evolutionary relationship and motif analysis, the walnut GRAS gene family was divided into eight subfamilies, and the sequence features analysis of JrGRAS proteins showed that the JrGRAS protein sequences were both conserved and altered during the evolutionary process. Gene duplication analysis indicated that seven GRAS genes in walnut have orthologous genes in other species, and five of them occurred duplicated events in walnut genome. Expression pattern analysis of the GRAS family genes in walnut showed that two JrGRAS genes (JrCIGRa-b and JrSCL28a) were differentially expressed between flower bud and leaf bud (p < 0.01), and two JrGRAS genes (JrCIGRa-b and JrSCL13b-d) were differentially expressed between the different development stages of flower buds transition (p < 0.01), besides, three hub genes (JrGAIa, JrSCL3f and JrSHRc) were identified by co-expression analysis, which suggested these GRAS genes may play an important role in regulating the development of apical meristem in walnut. This study laid a foundation for further understanding of the function of GRAS family genes in walnut.

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.

Insights into the complex role of GRAS transcription factors in the arbuscular mycorrhiza symbiosis

To improve access to limiting nutrients, the vast majority of land plants forms arbuscular mycorrhizal (AM) symbioses with Glomeromycota fungi. We show here that AM-related GRAS transcription factors from different subgroups are upregulated during a time course of mycorrhization. Based on expression studies in mutants defective in arbuscule branching (ram1-1, with a deleted MtRam1 GRAS transcription factor gene) or in the formation of functional arbuscules (pt4-2, mutated in the phosphate transporter gene MtPt4), we demonstrate that the five AM-related GRAS transcription factor genes MtGras1, MtGras4, MtGras6, MtGras7, and MtRad1 can be differentiated by their dependency on MtRAM1 and MtPT4, indicating that the network of AM-related GRAS transcription factors consists of at least two regulatory modules. One module involves the MtRAM1- and MtPT4-independent transcription factor MtGRAS4 that activates MtGras7. Another module is controlled by the MtRAM1- and MtPT4-dependent transcription factor MtGRAS1. Genome-wide expression profiles of mycorrhized MtGras1 knockdown and ram1-1 roots differ substantially, indicating different targets. Although an MtGras1 knockdown reduces transcription of AM-related GRAS transcription factor genes including MtRam1 and MtGras7, MtGras1 overexpression alone is not sufficient to activate MtGras genes. MtGras1 knockdown roots display normal fungal colonization, with a trend towards the formation of smaller arbuscules.

Plant-mediated effects of soil phosphorus on the root-associated fungal microbiota in Arabidopsis thaliana

Plants respond to phosphorus (P) limitation through an array of morphological, physiological and metabolic changes which are part of the phosphate (Pi) starvation response (PSR). This response influences the establishment of the arbuscular mycorrhizal (AM) symbiosis in most land plants. It is, however, unknown to what extent available P and the PSR redefine plant interactions with the fungal microbiota in soil. Using amplicon sequencing of the fungal taxonomic marker ITS2, we examined the changes in root-associated fungal communities in the AM nonhost species Arabidopsis thaliana in response to soil amendment with P and to genetic perturbations in the plant PSR. We observed robust shifts in root-associated fungal communities of P-replete plants in comparison with their P-deprived counterparts, while bulk soil communities remained unaltered. Moreover, plants carrying mutations in the phosphate signaling network genes, phr1, phl1 and pho2, exhibited similarly altered root fungal communities characterized by the depletion of the chytridiomycete taxon Olpidium brassicae specifically under P-replete conditions. This study highlights the nutritional status and the underlying nutrient signaling network of an AM nonhost plant as previously unrecognized factors influencing the assembly of the plant fungal microbiota in response to P in nonsterile soil.

Accumulation of phosphoinositides in distinct regions of the periarbuscular membrane

Phosphoinositides and phosphatidic acid are small anionic lipids that comprise a minor proportion of total membrane lipids in eukaryotic cells but influence a broad range of cellular processes including endomembrane trafficking, signaling, exocytosis and endocytosis. To investigate the spatial distribution of phosphoinositides during arbuscular mycorrhizal symbiosis, we generated fluorescent reporters of PI(4,5)P₂ and PI4P, as well as phosphatidic acid and diacylglycerol and used them to monitor lipid distribution on the cytoplasmic side of membrane bilayers in colonized cortical cells. The PI4P reporter accumulated strongly on the periarbuscular membrane (PAM) and transiently labeled Golgi bodies, while the PA reporter showed differential labeling of endomembranes and the PAM. Surprisingly, the PI(4,5)P₂ reporter accumulated in small, discrete regions of the PAM on the arbuscule trunks, frequently in two regions on opposing sides of the hypha. A mutant reporter with reduced PI(4,5)P₂ binding capacity did not show these accumulations. The PI(4,5)P₂ -rich regions were detected at all phases of arbuscule development following branching, co-localized with membrane marker proteins potentially indicating high membrane bilayer content, and were associated with an alteration in morphology of the hypha. A possible analogy to the biotrophic interfacial membrane complex formed in rice infected with Magnaporthe orzyae is discussed.

Identification and Expression Analysis of GRAS Transcription Factor Genes Involved in the Control of Arbuscular Mycorrhizal Development in Tomato

The formation and functioning of arbuscular mycorrhizal (AM) symbiosis are complex and tightly regulated processes. Transcriptional regulation mechanisms have been reported to mediate gene expression changes closely associated with arbuscule formation, where nutrients move between the plant and fungus. Numerous genes encoding transcription factors (TFs), with those belonging to the GRAS family being of particular importance, are induced upon mycorrhization. In this study, a screening for candidate transcription factor genes differentially regulated in AM tomato roots showed that more than 30% of known GRAS tomato genes are upregulated upon mycorrhization. Some AM-upregulated GRAS genes were identified as encoding for transcription factors which are putative orthologs of previously identified regulators of mycorrhization in other plant species. The symbiotic role played by other newly identified AM-upregulated GRAS genes remains unknown. Preliminary results on the involvement of tomato SlGRAS18, SlGRAS38, and SlGRAS43 from the SCL3, SCL32, and SCR clades, respectively, in mycorrhization are presented. All three showed high transcript levels in the late stages of mycorrhization, and the analysis of promoter activity demonstrated that SlGRAS18 and SlGRAS43 are significantly induced in cells containing arbuscules. When SlGRAS18 and SlGRAS38 genes were silenced using RNA interference in hairy root composite tomato plants, a delay in mycorrhizal infection was observed, while an increase in mycorrhizal colonization was observed in SlGRAS43 RNAi roots. The possible mode of action of these TFs during mycorrhization is discussed, with a particular emphasis on the potential involvement of the SHR/SCR/SCL3 module of GRAS TFs in the regulation of gibberellin signaling during mycorrhization, which is analogous to other plant developmental processes.

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.

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 mycorrhiza-induced maize ZmPt9 gene affects root development and phosphate availability in nonmycorrhizal plant

The arbuscular mycorrhizal (AM)-induced ZmPt9 gene is an orthologous to some AM-inducible phosphate (Pi) transporter genes involved in Pi-starvation responses. Promoter GFP assay confirmed its transcript was localized surrounding arbuscule in arbuscule-containing cells. But this gene was not an AM fungi-specific gene. Its function in nonmycorrhizal seedlings was verified through phenotypic analysis of ZmPt9-overexpression Arabidopsis. Overexpression of ZmPt9 in Arabidopsis exhibited increased primary root length and lateral root formation. Furthermore, ZmPt9-overexpression Arabidopsis plants contained more phosphorus (P) than that of wild type. The affection of ZmPt9 in nonmycorrhizal Arabidopsis leads to the hypothesis that symbiosis-inducible genes are also involved in root development and Pi accumulation in AM-independent manner.

AP2 transcription factor CBX1 with a specific function in symbiotic exchange of nutrients in mycorrhizal Lotus japonicus

Arbuscular mycorrhizal (AM) fungi promote phosphorus uptake into host plants in exchange for organic carbon. Physiological tracer experiments showed that up to 100% of acquired phosphate can be delivered to plants via the mycorrhizal phosphate uptake pathway (MPU). Previous studies revealed that the CTTC cis-regulatory element (CRE) is required for promoter activation of mycorrhiza-specific phosphate transporter and H⁺-ATPase genes. However, the precise transcriptional mechanism directly controlling MPU is unknown. Here, we show that CBX1 binds CTTC and AW-box CREs and coregulates mycorrhizal phosphate transporter and H⁺-ATPase genes. Interestingly, genes involved in lipid biosynthesis are also regulated by CBX1 through binding to AW box, including RAM2. Our work suggests a common regulatory mechanism underlying complex trait control of symbiotic exchange of nutrients.

The arbuscular mycorrhizal (AM) symbiosis, a widespread mutualistic association between land plants and fungi, depends on reciprocal exchange of phosphorus driven by proton-coupled phosphate uptake into host plants and carbon supplied to AM fungi by host-dependent sugar and lipid biosynthesis. The molecular mechanisms and cis-regulatory modules underlying the control of phosphate uptake and de novo fatty acid synthesis in AM symbiosis are poorly understood. Here, we show that the AP2 family transcription factor CTTC MOTIF-BINDING TRANSCRIPTION FACTOR1 (CBX1), a WRINKLED1 (WRI1) homolog, directly binds the evolutionary conserved CTTC motif that is enriched in mycorrhiza-regulated genes and activates Lotus japonicus phosphate transporter 4 (LjPT4) in vivo and in vitro. Moreover, the mycorrhiza-inducible gene encoding H⁺-ATPase (LjHA1), implicated in energizing nutrient uptake at the symbiotic interface across the periarbuscular membrane, is coregulated with LjPT4 by CBX1. Accordingly, CBX1-defective mutants show reduced mycorrhizal colonization. Furthermore, genome-wide–binding profiles, DNA-binding studies, and heterologous expression reveal additional binding of CBX1 to AW box, the consensus DNA-binding motif for WRI1, that is enriched in promoters of glycolysis and fatty acid biosynthesis genes. We show that CBX1 activates expression of lipid metabolic genes including glycerol-3-phosphate acyltransferase RAM2 implicated in acylglycerol biosynthesis. Our finding defines the role of CBX1 as a regulator of host genes involved in phosphate uptake and lipid synthesis through binding to the CTTC/AW molecular module, and supports a model underlying bidirectional exchange of phosphorus and carbon, a fundamental trait in the mutualistic AM symbiosis.

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.

Identification and Functional Characterization of a Maize Phosphate Transporter Induced by Mycorrhiza Formation

Phosphorus (P) is an essential macronutrient for plant life, although it is frequently not readily available to crops. Arbuscular mycorrhiza fungi (AMF) can improve plant P levels by inducing the expression of some phosphate (Pi) transporters. Symbiotic Pi uptake by Pi transporters is crucial for AMF colonization and arbuscule dynamics. However, the functions of mycorrhiza-inducible maize Pi transporters are largely unclear. We focused on the interaction between the Pi concentration and AMF colonization in maize, and detecting the induction of a Pi transporter. We investigated AMF colonization and arbuscular development in maize under high and low Pi environments. Low Pi increased AMF colonization and promoted arbuscular development. Further measurement of P concentration showed that AMF significantly improved the maize P status under low Pi conditions. Here, we identified the Pi transporter gene, ZmPt9, which was induced by mycorrhiza formation. In addition, ZmPt9-overexpressing roots were difficult to colonize by AMF. Pi response analysis showed that ZmPt9 complements a yeast mutant defective in Pi transporter activity and improves the P concentration in rice. Together, these data indicated that ZmPt9 is a mycorrhiza-inducible Pi transporter gene involved in Pi uptake.

Symbiosis genes for immunity and vice versa

Basic molecular knowledge on plant-pathogen interactions has largely been gained from reverse and forward genetics in Arabidopsis thaliana. However, as this model plant is unable to establish endosymbiosis with mycorrhizal fungi or rhizobia, plant responses to mutualistic symbionts have been studied in parallel in other plant species, mainly legumes. The resulting analyses led to the identification of gene networks involved in various functions, from microbe recognition to signalling and plant responses, thereafter assigned to either mutualistic symbiosis or immunity, according to the nature of the initially inoculated microbe. The increasing development of new pathosystems and genetic resources in model legumes and the implementation of reverse genetics in plants such as rice and tomato that interact with both mycorrhizal fungi and pathogens, have highlighted the dual role of plant genes previously thought to be specific to mutualistic or pathogenic interactions. The next challenges will be to determine whether such genes have similar functions in both types of interaction and if not, whether the perception of microbial compounds or the involvement of specific plant signalling components is responsible for the appropriate plant responses to the encountered microorganisms.

What have we learnt from studying the evolution of the arbuscular mycorrhizal symbiosis?

The arbuscular mycorrhizal (AM) symbiosis is a nearly ubiquitous association formed by most land plants. Numerous insights into the molecular mechanisms governing this symbiosis have been obtained in recent years leading to the identification of a core set of plant genes essential for successful formation of the AM symbiosis by angiosperm hosts. Recent phylogenetic analyses indicate that while the origin of some of these symbiotic genes predated the first land plants, the rest appeared through processes including de novo evolution and gene duplication that occurred specifically in the land plants. Purifying selection on this core gene set has been maintained over millions of years of plant evolution to conserve the AM symbiosis. However, several independent losses of this association have been recorded in numerous embryophyte lineages. In these lineages, potential compensatory mechanisms have been identified that could have helped these plants overcome the adversities imposed by the loss of the AM symbiosis. This review will focus on the processes governing the conservation of the AM symbiosis in the land plant lineage.

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.

Omics approaches revealed how arbuscular mycorrhizal symbiosis enhances yield and resistance to leaf pathogen in wheat

Besides improved mineral nutrition, plants colonised by arbuscular mycorrhizal (AM) fungi often display increased biomass and higher tolerance to biotic and abiotic stresses. Notwithstanding the global importance of wheat as an agricultural crop, its response to AM symbiosis has been poorly investigated. We focused on the role of an AM fungus on mineral nutrition of wheat, and on its potential protective effect against Xanthomonas translucens. To address these issues, phenotypical, molecular and metabolomic approaches were combined. Morphological observations highlighted that AM wheat plants displayed an increased biomass and grain yield, as well as a reduction in lesion area following pathogen infection. To elucidate the molecular mechanisms underlying the mycorrhizal phenotype, we investigated changes of transcripts and proteins in roots and leaves during the double (wheat-AM fungus) and tripartite (wheat-AM fungus-pathogen) interaction. Transcriptomic and proteomic profiling identified the main pathways involved in enhancing plant biomass, mineral nutrition and in promoting the bio-protective effect against the leaf pathogen. Mineral and amino acid contents in roots, leaves and seeds, and protein oxidation profiles in leaves, supported the omics data, providing new insight into the mechanisms exerted by AM symbiosis to confer stronger productivity and enhanced resistance to X. translucens in wheat.

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.

Transcription factors network in root endosymbiosis establishment and development

Root endosymbioses are mutualistic interactions between plants and the soil microorganisms (Fungus, Frankia or Rhizobium) that lead to the formation of nitrogen-fixing root nodules and/or arbuscular mycorrhiza. These interactions enable many species to survive in different marginal lands to overcome the nitrogen-and/or phosphorus deficient environment and can potentially reduce the chemical fertilizers used in agriculture which gives them an economic, social and environmental importance. The formation and the development of these structures require the mediation of specific gene products among which the transcription factors play a key role. Three of these transcription factors, viz., CYCLOPS, NSP1 and NSP2 are well conserved between actinorhizal, legume, non-legume and mycorrhizal symbioses. They interact with DELLA proteins to induce the expression of NIN in nitrogen fixing symbiosis or RAM1 in mycorrhizal symbiosis. Recently, the small non coding RNA including micro RNAs (miRNAs) have emerged as major regulators of root endosymbioses. Among them, miRNA171 targets NSP2, a TF conserved in actinorhizal, legume, non-legume and mycorrhizal symbioses. This review will also focus on the recent advances carried out on the biological function of others transcription factors during the root pre-infection/pre-contact, infection or colonization. Their role in nodule formation and AM development will also be described.

Structural basis of the specific interactions of GRAS family proteins

The plant‐specific GAI‐RGA‐and‐SCR (GRAS) family of proteins function as transcriptional regulators and play critical roles in development and signalling. Recent structural studies have shed light on the molecular functions at the structural level. The conserved GRAS domain comprises an α‐helical cap and α/β core subdomains. The α‐helical cap mediates head‐to‐head heterodimerization between SHR and SCR GRAS domains. This type of dimerization is predicted for the NSP1‐NSP2 heterodimer and DELLA proteins such as RGA and SLR1 homodimers. The α/β core subdomain possesses a hydrophobic groove formed by surface α3‐ and α7‐helices and mediates protein–protein interactions. The groove of the SHR GRAS domain accommodates the zinc fingers of JKD, a BIRD/IDD family transcription factor, while the groove of the SCL7 GRAS domain mediates the SCL7 homodimerization.

Evolution of the Symbiosis-Specific GRAS Regulatory Network in Bryophytes

Arbuscular mycorrhiza is one of the most common plant symbiotic interactions observed today. Due to their nearly ubiquitous occurrence and their beneficial impact on both partners it was suggested that this mutualistic interaction was crucial for plants to colonize the terrestrial habitat approximately 500 Ma ago. On the plant side the association is established via the common symbiotic pathway (CSP). This pathway allows the recognition of the fungal symbiotic partner, subsequent signaling to the nucleus, and initiation of the symbiotic program with respect to specific gene expression and cellular re-organization. The downstream part of the CSP is a regulatory network that coordinates the transcription of genes necessary to establish the symbiosis, comprising multiple GRAS transcription factors (TFs). These regulate their own expression as an intricate transcriptional network. Deduced from non-host genome data the loss of genes encoding CSP components coincides with the loss of the interaction itself. Here, we analyzed bryophyte species with special emphasis on the moss Physcomitrella patens, supposed to be a non-host, for the composition of the GRAS regulatory network components. We show lineage specific losses and expansions of several of these factors in bryophytes, potentially coinciding with the proposed host/non-host status of the lineages. We evaluate losses and expansions and infer clade-specific evolution of GRAS TFs.

Receptor-Like Kinase LYK9 in Pisum sativum L. Is the CERK1-Like Receptor that Controls Both Plant Immunity and AM Symbiosis Development

Plants are able to discriminate and respond to structurally related chitooligosaccharide (CO) signals from pathogenic and symbiotic fungi. In model plants Arabidopsis thaliana and Oryza sativa LysM-receptor like kinases (LysM-RLK) AtCERK1 and OsCERK1 (chitin elicitor receptor kinase 1) were shown to be involved in response to CO signals. Based on phylogenetic analysis, the pea Pisum sativum L. LysM-RLK PsLYK9 was chosen as a possible candidate given its role on the CERK1-like receptor. The knockdown regulation of the PsLyk9 gene by RNA interference led to increased susceptibility to fungal pathogen Fusarium culmorum. Transcript levels of PsPAL2, PsPR10 defense-response genes were significantly reduced in PsLyk9 RNAi roots. PsLYK9's involvement in recognizing short-chain COs as most numerous signals of arbuscular mycorrhizal (AM) fungi, was also evaluated. In transgenic roots with PsLyk9 knockdown treated with short-chain CO5, downregulation of AM symbiosis marker genes (PsDELLA3, PsNSP2, PsDWARF27) was observed. These results clearly indicate that PsLYK9 appears to be involved in the perception of COs and subsequent signal transduction in pea roots. It allows us to conclude that PsLYK9 is the most likely CERK1-like receptor in pea to be involved in the control of plant immunity and AM symbiosis formation.

The Medicago truncatula GRAS protein RAD1 supports arbuscular mycorrhiza symbiosis and Phytophthora palmivora susceptibility

The roots of most land plants are colonized by symbiotic arbuscular mycorrhiza (AM) fungi. To facilitate this symbiosis, plant genomes encode a set of genes required for microbial perception and accommodation. However, the extent to which infection by filamentous root pathogens also relies on some of these genes remains an open question. Here, we used genome-wide association mapping to identify genes contributing to colonization of Medicago truncatula roots by the pathogenic oomycete Phytophthora palmivora. Single-nucleotide polymorphism (SNP) markers most significantly associated with plant colonization response were identified upstream of RAD1, which encodes a GRAS transcription regulator first negatively implicated in root nodule symbiosis and recently identified as a positive regulator of AM symbiosis. RAD1 transcript levels are up-regulated both in response to AM fungus and, to a lower extent, in infected tissues by P. palmivora where its expression is restricted to root cortex cells proximal to pathogen hyphae. Reverse genetics showed that reduction of RAD1 transcript levels as well as a rad1 mutant are impaired in their full colonization by AM fungi as well as by P. palmivora. Thus, the importance of RAD1 extends beyond symbiotic interactions, suggesting a general involvement in M. truncatula microbe-induced root development and interactions with unrelated beneficial and detrimental filamentous microbes.

Plant carbon nourishment of arbuscular mycorrhizal fungi

Reciprocal nutrient exchange between the majority of land plants and arbucular mycorrhizal (AM) fungi is the cornerstone of a stable symbiosis. To date, a dogma in the comprehension of AM fungal nourishment has been delivery of host organic carbon in the form of sugars. More recently a role for lipids as alternative carbon source or as a signalling molecule during AM symbiosis was proposed. Here we review the symbiotic requirement for carbohydrates and lipids across developmental stages of the AM symbiosis. We present a role for carbohydrate metabolism and signalling to maintain intraradical fungal growth, as opposed to lipid uptake at the arbuscule as an indispensible requirement for completion of the AM fungal life cycle.

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.

Molecular cloning and characterization of five SmGRAS genes associated with tanshinone biosynthesis in Salvia miltiorrhiza hairy roots

The gibberellin-responsive element binding factor (GRAS) family of proteins plays an important role in the transcriptional regulation of plant development and hormone signaling. To date, there are no reports on GRAS family proteins expressed in Salvia miltiorrhiza. In this study, 28 ESTs that contained the GRAS domain were identified from a S. miltiorrhiza cDNA library. Of these, full-length sequences of five genes were cloned and sequence analysis indicated that all five proteins contain only one GRAS domain and therefore, belong to the GRAS family. The five genes were designated S. miltiorrhiza GRAS1-5 (SmGRAS1-5), which belong to groups I (SmGRAS2 and SmGRAS4), II (SmGRAS3), III (SmGRAS1), and VIII (SmGRAS5) respectively. Additionally, SmGRAS1-5 have different expression patterns in the reed head, stems, leaves, flowers, and roots of S. miltiorrhiza. In this study, the expression of SmGRAS1-5 was sensitive to Gibberellin (GA) stress and that of SmGRAS1, SmGRAS4 and SmGRAS5 was sensitive to Ethephon (Eth) stress respectively. Moreover, S. miltiorrhiza copalyl diphosphate synthases 1 (SmCPS1) and S. miltiorrhiza kaurene synthase like 1 (SmKSL1), which are two key enzymes gene in the diterpenoid biosynthesis pathway, were also response to GA and Eth stress. In addition, Dihydrotanshinone (DT-I) and Tanshinone I (T-I) content were enhanced by GA and Eth stress, Tanshinone IIA (T-IIA) content was increased by GA stress, and the accumulation of Cryptotanshinone (CT) was insensitive to both GA and Eth stress. Together, these results provide insights into functional conservation and diversification of SmGRASs and are useful information for further elucidating SmGRAS functions.

Analysis of the first Taraxacum kok-saghyz transcriptome reveals potential rubber yield related SNPs

Taraxacum kok-saghyz (TK) is a potential alternative crop for natural rubber (NR) production, due to its high molecular weight rubber, short breeding cycle, and diverse environmental adaptation. However, improvements in rubber yield and agronomically relevant traits are still required before it can become a commercially-viable crop. An RNA-Seq based transcriptome was developed from a pool of roots from genotypes with high and low rubber yield. A total of 55,532 transcripts with lengths over 200 bp were de novo assembled. As many as 472 transcripts were significantly homologous to 49 out of 50 known plant putative rubber biosynthesis related genes. 158 transcripts were significantly differentially expressed between high rubber and low rubber genotypes. 21,036 SNPs were different in high and low rubber TK genotypes. Among these, 50 SNPs were found within 39 transcripts highly homologous to 49 publically-searched rubber biosynthesis related genes. 117 SNPs were located within 36 of the differentially expressed gene sequences. This comprehensive TK transcriptomic reference, and large set of SNPs including putative exonic markers associated with rubber related gene homologues and differentially expressed genes, provides a solid foundation for further genetic dissection of rubber related traits, comparative genomics and marker-assisted selection for the breeding of TK.

Role of the GRAS transcription factor ATA/RAM1 in the transcriptional reprogramming of arbuscular mycorrhiza in Petunia hybrida

BACKGROUND

Development of arbuscular mycorrhiza (AM) requires a fundamental reprogramming of root cells for symbiosis. This involves the induction of hundreds of genes in the host. A recently identified GRAS-type transcription factor in Petunia hybrida, ATA/RAM1, is required for the induction of host genes during AM, and for morphogenesis of the fungal endosymbiont. To better understand the role of RAM1 in symbiosis, we set out to identify all genes that depend on activation by RAM1 in mycorrhizal roots.

RESULTS

We have carried out a transcript profiling experiment by RNAseq of mycorrhizal plants vs. non-mycorrhizal controls in wild type and ram1 mutants. The results show that the expression of early genes required for AM, such as the strigolactone biosynthetic genes and the common symbiosis signalling genes, is independent of RAM1. In contrast, genes that are involved at later stages of symbiosis, for example for nutrient exchange in cortex cells, require RAM1 for induction. RAM1 itself is highly induced in mycorrhizal roots together with many other transcription factors, in particular GRAS proteins.

CONCLUSION

Since RAM1 has previously been shown to be directly activated by the common symbiosis signalling pathway through CYCLOPS, we conclude that it acts as an early transcriptional switch that induces many AM-related genes, among them genes that are essential for the development of arbuscules, such as STR, STR2, RAM2, and PT4, besides hundreds of additional RAM1-dependent genes the role of which in symbiosis remains to be explored. Taken together, these results indicate that the defect in the morphogenesis of the fungal arbuscules in ram1 mutants may be an indirect consequence of functional defects in the host, which interfere with nutrient exchange and possibly other functions on which the fungus depends.

Differentially expressed genes in mycorrhized and nodulated roots of common bean are associated with defense, cell wall architecture, N metabolism, and P metabolism

Legumes participate in two important endosymbiotic associations, with phosphorus-acquiring arbuscular mycorrhiza (AM, soil fungi) and with nitrogen-fixing bacterial rhizobia. These divergent symbionts share a common symbiotic signal transduction pathway that facilitates the establishment of mycorrhization and nodulation in legumes. However, the unique and shared downstream genes essential for AM and nodule development have not been identified in crop legumes. Here, we used ion torrent next-generation sequencing to perform comparative transcriptomics of common bean (Phaseolus vulgaris) roots colonized by AM or rhizobia. We analyzed global gene expression profiles to identify unique and shared differentially expressed genes (DEGs) that regulate these two symbiotic interactions, and quantitatively compared DEG profiles. We identified 3,219 (1,959 upregulated and 1,260 downregulated) and 2,645 (1,247 upregulated and 1,398 downregulated) unigenes that were differentially expressed in response to mycorrhizal or rhizobial colonization, respectively, compared with uninoculated roots. We obtained quantitative expression profiles of unique and shared genes involved in processes related to defense, cell wall structure, N metabolism, and P metabolism in mycorrhized and nodulated roots. KEGG pathway analysis indicated that most genes involved in jasmonic acid and salicylic acid signaling, N metabolism, and inositol phosphate metabolism are variably expressed during symbiotic interactions. These combined data provide valuable information on symbiotic gene signaling networks that respond to mycorrhizal and rhizobial colonization, and serve as a guide for future genetic strategies to enhance P uptake and N-fixing capacity to increase the net yield of this valuable grain legume.

Lipid transfer from plants to arbuscular mycorrhiza fungi

Arbuscular mycorrhiza (AM) symbioses contribute to global carbon cycles as plant hosts divert up to 20% of photosynthate to the obligate biotrophic fungi. Previous studies suggested carbohydrates as the only form of carbon transferred to the fungi. However, de novo fatty acid (FA) synthesis has not been observed in AM fungi in absence of the plant. In a forward genetic approach, we identified two Lotus japonicus mutants defective in AM-specific paralogs of lipid biosynthesis genes (KASI and GPAT6). These mutants perturb fungal development and accumulation of emblematic fungal 16:1ω5 FAs. Using isotopolog profiling we demonstrate that ¹³C patterns of fungal FAs recapitulate those of wild-type hosts, indicating cross-kingdom lipid transfer from plants to fungi. This transfer of labelled FAs was not observed for the AM-specific lipid biosynthesis mutants. Thus, growth and development of beneficial AM fungi is not only fueled by sugars but depends on lipid transfer from plant hosts.

Lipid transfer from plants to arbuscular mycorrhiza fungi

Arbuscular mycorrhiza (AM) symbioses contribute to global carbon cycles as plant hosts divert up to 20% of photosynthate to the obligate biotrophic fungi. Previous studies suggested carbohydrates as the only form of carbon transferred to the fungi. However, de novo fatty acid (FA) synthesis has not been observed in AM fungi in absence of the plant. In a forward genetic approach, we identified two Lotus japonicus mutants defective in AM-specific paralogs of lipid biosynthesis genes (KASI and GPAT6). These mutants perturb fungal development and accumulation of emblematic fungal 16:1ω5 FAs. Using isotopolog profiling we demonstrate that 13C patterns of fungal FAs recapitulate those of wild-type hosts, indicating cross-kingdom lipid transfer from plants to fungi. This transfer of labelled FAs was not observed for the AM-specific lipid biosynthesis mutants. Thus, growth and development of beneficial AM fungi is not only fueled by sugars but depends on lipid transfer from plant hosts.

A Transcriptional Program for Arbuscule Degeneration during AM Symbiosis Is Regulated by MYB1

During the endosymbiosis formed between plants and arbuscular mycorrhizal (AM) fungi, the root cortical cells are colonized by branched hyphae called arbuscules, which function in nutrient exchange with the plant [1]. Despite their positive function, arbuscules are ephemeral structures, and their development is followed by a degeneration phase, in which the arbuscule and surrounding periarbuscular membrane and matrix gradually disappear from the root cell [2, 3]. Currently, the root cell's role in this process and the underlying regulatory mechanisms are unknown. Here, by using a Medicago truncatula pt4 mutant in which arbuscules degenerate prematurely [4], we identified arbuscule degeneration-associated genes, of which 38% are predicted to encode secreted hydrolases, suggesting a role in disassembly of the arbuscule and interface. Through RNAi and analysis of an insertion mutant, we identified a symbiosis-specific MYB-like transcription factor (MYB1) that suppresses arbuscule degeneration in mtpt4. In myb1, expression of several degeneration-associated genes is reduced. Conversely, in roots constitutively overexpressing MYB1, expression of degeneration-associated genes is increased and subsequent development of symbiosis is impaired. MYB1-regulated gene expression is enhanced by DELLA proteins and is dependent on NSP1 [5], but not NSP2 [6]. Furthermore, MYB1 interacts with DELLA and NSP1. Our data identify a transcriptional program for arbuscule degeneration and reveal that its regulators include MYB1 in association with two transcriptional regulators, NSP1 and DELLA, both of which function in preceding phases of the symbiosis. We propose that the combinatorial use of transcription factors enables the sequential expression of transcriptional programs for arbuscule development and degeneration.

Three cis-Regulatory Motifs, AuxRE, MYCRS1 and MYCRS2, are Required for Modulating the Auxin- and Mycorrhiza-Responsive Expression of a Tomato GH3 Gene

Auxin is well known to be a key regulator that acts in almost all physiological processes during plant growth, and in interactions between plants and microbes. However, to date, the regulatory mechanisms underlying auxin-mediated plant-arbuscular mycorrhizal (AM) fungi symbiosis have not been well deciphered. Previously we identified a GH3 gene, SlGH3.4, strongly responsive to both auxin induction and mycorrhizal symbiosis. Here, we reported a refined dissection of the SlGH3.4 promoter activity using the β-glucuronidase (GUS) reporter. The SlGH3.4 promoter could drive GUS expression strongly in mycorrhizal roots of soybean and rice plants, and in IAA-treated soybean roots, but not in IAA-treated rice roots. A promoter deletion assay revealed three cis-acting motifs, i.e. the auxin-responsive element, AuxRE, and two newly identified motifs named MYCRS1 and MYCRS2, involved in the activation of auxin- and AM-mediated expression of SlGH3.4. Deletion of the AuxRE from the SlGH3.4 promoter caused almost complete abolition of GUS staining in response to external IAA induction. Seven repeats of AuxRE fused to the Cauliflower mosaic virus (CaMV) 35S minimal promoter could direct GUS expression in both IAA-treated and AM fungal-colonized roots of tobacco plants. Four repeats of MYCRS1 or MYCRS2 fused to the CaMV35S minimal promoter was sufficient to drive GUS expression in arbuscule-containing cells, but not in IAA-treated tobacco roots. In summary, our results offer new insights into the molecular mechanisms underlying the potential cross-talk between the auxin and the AM regulatory pathways in modulating the expression of AM-responsive GH3 genes in diverse mycorrhizal plants.

Evolutionary Analyses of GRAS Transcription Factors in Angiosperms

GRAS transcription factors (TFs) play critical roles in plant growth and development such as gibberellin and mycorrhizal signaling. Proteins belonging to this gene family contain a typical GRAS domain in the C-terminal sequence, whereas the N-terminal region is highly variable. Although, GRAS genes have been characterized in a number of plant species, their classification is still not completely resolved. Based on a panel of eight representative species of angiosperms, we identified 29 orthologous groups or orthogroups (OGs) for the GRAS gene family, suggesting that at least 29 ancestor genes were present in the angiosperm lineage before the "Amborella" evolutionary split. Interestingly, some taxonomic groups were missing members of one or more OGs. The gene number expansion usually observed in transcription factors was not observed in GRAS while the genome triplication ancestral to the eudicots (γ hexaploidization event) was detectable in a limited number of GRAS orthogroups. We also found conserved OG-specific motifs in the variable N-terminal region. Finally, we could regroup OGs in 17 subfamilies for which names were homogenized based on a literature review and described 5 new subfamilies (DLT, RAD1, RAM1, SCLA, and SCLB). This study establishes a consistent framework for the classification of GRAS members in angiosperm species, and thereby a tool to correctly establish the orthologous relationships of GRAS genes in most of the food crops in order to facilitate any subsequent functional analyses in the GRAS gene family. The multi-fasta file containing all the sequences used in our study could be used as database to perform diagnostic BLASTp to classify GRAS genes from other non-model species.

User Guide for the LORE1 Insertion Mutant Resource

Lotus japonicus is a model legume used in the study of plant-microbe interactions, especially in the field of biological nitrogen fixation due to its ability to enter into a symbiotic relationship with a soil bacterium, Mesorhizobium loti. The LORE1 mutant population is a valuable resource for reverse genetics in L. japonicus due to its non-transgenic nature, high tagging efficiency, and low copy count. Here, we outline a workflow for identifying, ordering, and establishing homozygous LORE1 mutant lines for a gene of interest, LjFls2, including protocols for growth and genotyping of a segregating LORE1 population.

Lotus Base: An integrated information portal for the model legume Lotus japonicus

Lotus japonicus is a well-characterized model legume widely used in the study of plant-microbe interactions. However, datasets from various Lotus studies are poorly integrated and lack interoperability. We recognize the need for a comprehensive repository that allows comprehensive and dynamic exploration of Lotus genomic and transcriptomic data. Equally important are user-friendly in-browser tools designed for data visualization and interpretation. Here, we present Lotus Base, which opens to the research community a large, established LORE1 insertion mutant population containing an excess of 120,000 lines, and serves the end-user tightly integrated data from Lotus, such as the reference genome, annotated proteins, and expression profiling data. We report the integration of expression data from the L. japonicus gene expression atlas project, and the development of tools to cluster and export such data, allowing users to construct, visualize, and annotate co-expression gene networks. Lotus Base takes advantage of modern advances in browser technology to deliver powerful data interpretation for biologists. Its modular construction and publicly available application programming interface enable developers to tap into the wealth of integrated Lotus data. Lotus Base is freely accessible at: https://lotus.au.dk.

The Mutualistic Interaction between Plants and Arbuscular Mycorrhizal Fungi

Mycorrhizal fungi belong to several taxa and develop mutualistic symbiotic associations with over 90% of all plant species, from liverworts to angiosperms. While descriptive approaches have dominated the initial studies of these fascinating symbioses, the advent of molecular biology, live cell imaging, and “omics” techniques have provided new and powerful tools to decipher the cellular and molecular mechanisms that rule mutualistic plant-fungus interactions. In this article we focus on the most common mycorrhizal association, arbuscular mycorrhiza (AM), which is formed by a group of soil fungi belonging to Glomeromycota. AM fungi are believed to have assisted the conquest of dry lands by early plants around 450 million years ago and are found today in most land ecosystems. AM fungi have several peculiar biological traits, including obligate biotrophy, intracellular development inside the plant tissues, coenocytic multinucleate hyphae, and spores, as well as unique genetics, such as the putative absence of a sexual cycle, and multiple ecological functions. All of these features make the study of AM fungi as intriguing as it is challenging, and their symbiotic association with most crop plants is currently raising a broad interest in agronomic contexts for the potential use of AM fungi in sustainable production under conditions of low chemical input.

NIN Is Involved in the Regulation of Arbuscular Mycorrhizal Symbiosis

Arbuscular mycorrhizal (AM) symbiosis is an intimate and ancient symbiosis found between most of terrestrial plants and fungi from the Glomeromycota family. Later during evolution, the establishment of the nodulation between legume plants and soil bacteria known as rhizobia, involved several genes of the signaling pathway previously implicated for AM symbiosis. For the past years, the identification of the genes belonging to this Common Symbiotic Signaling Pathway have been mostly done on nodulation. Among the different genes already well identified as required for nodulation, we focused our attention on the involvement of Nodule Inception (NIN) in AM symbiosis. We show here that NIN expression is induced during AM symbiosis, and that the Medicago truncatula nin mutant is less colonized than the wild-type M. truncatula strain. Moreover, nin mutant displays a defect in the ability to be infected by the fungus Rhizophagus irregularis. This work brings a new evidence of the common genes involved in overlapping signaling pathways of both nodulation and in AM symbiosis.

Phosphate Treatment Strongly Inhibits New Arbuscule Development But Not the Maintenance of Arbuscule in Mycorrhizal Rice Roots

Phosphorus (P) is a crucial nutrient for plant growth, but its availability to roots is limited in soil. Arbuscular mycorrhizal (AM) symbiosis is a promising strategy for improving plant P acquisition. However, P fertilizer reduces fungal colonization (P inhibition) and compromises mycorrhizal P uptake, warranting studies on the mechanistic basis of P inhibition. In this study, early morphological changes in P inhibition were identified in rice (Oryza sativa) using fungal cell wall staining and live-cell imaging of plant membranes that were associated with arbuscule life cycles. Arbuscule density decreased, and aberrant hyphal branching was observed in roots at 5 h after P treatment. Although new arbuscule development was severely inhibited, preformed arbuscules remained intact and longevity remained constant. P inhibition was accelerated in the rice pt11-1 mutant, which lacks P uptake from arbuscule branches, suggesting that mature arbuscules are stabilized by the symbiotic P transporter under high P condition. Moreover, P treatment led to increases in the number of vesicles, in which lipid droplets accumulated and then decreased within a few days. The development of new arbuscules resumed within by 2 d. Our data established that P strongly and temporarily inhibits new arbuscule development, but not intraradical accommodation of AM fungi.

Crystal Structure of the GRAS Domain of SCARECROW-LIKE7 in Oryza sativa

GRAS proteins belong to a plant-specific protein family with many members and play essential roles in plant growth and development, functioning primarily in transcriptional regulation. Proteins in the family are minimally defined as containing the conserved GRAS domain. Here, we determined the structure of the GRAS domain of Os-SCL7 from rice (Oryza sativa) to 1.82 Å. The structure includes cap and core subdomains and elucidates the features of the conserved GRAS LRI, VHIID, LRII, PFYRE, and SAW motifs. The structure is a dimer, with a clear groove to accommodate double-stranded DNA. Docking a DNA segment into the groove to generate an Os-SCL7/DNA complex provides insight into the DNA binding mechanism of GRAS proteins. Furthermore, the in vitro DNA binding property of Os-SCL7 and model-defined recognition residues are assessed by electrophoretic mobility shift analysis and mutagenesis assays. These studies reveal the structure and preliminary DNA interaction mechanisms of GRAS proteins and open the door to in-depth investigation and understanding of the individual pathways in which they play important roles.

Identification of Arbuscular Mycorrhiza (AM)-Responsive microRNAs in Tomato

A majority of land plants can form symbiosis with arbuscular mycorrhizal (AM) fungi. MicroRNAs (miRNAs) have been implicated to regulate this process in legumes, but their involvement in non-legume species is largely unknown. In this study, by performing deep sequencing of sRNA libraries in tomato roots and comparing with tomato genome, a total of 700 potential miRNAs were predicted, among them, 187 are known plant miRNAs that have been previously deposited in miRBase. Unlike the profiles in other plants such as rice and Arabidopsis, a large proportion of predicted tomato miRNAs was 24 nt in length. A similar pattern was observed in the potato genome but not in tobacco, indicating a Solanum genus-specific expansion of 24-nt miRNAs. About 40% identified tomato miRNAs showed significantly altered expressions upon Rhizophagus irregularis inoculation, suggesting the potential roles of these novel miRNAs in AM symbiosis. The differential expression of five known and six novel miRNAs were further validated using qPCR analysis. Interestingly, three up-regulated known tomato miRNAs belong to a known miR171 family, a member of which has been reported in Medicago truncatula to regulate AM symbiosis. Thus, the miR171 family likely regulates AM symbiosis conservatively across different plant lineages. More than 1000 genes targeted by potential AM-responsive miRNAs were provided and their roles in AM symbiosis are worth further exploring.