Polyphony in the rhizosphere: presymbiotic communication in arbuscular mycorrhizal symbiosis

Root RADAR: how 'rhizocrine' signals allow roots to detect and respond to their soil environment and stresses

Agricultural intensification coupled with changing climate are causing soils to become increasingly vulnerable to stresses such as drought, soil erosion, and compaction. The mechanisms by which roots detect and respond to soil stresses remain poorly understood. Recent breakthroughs show that roots release volatile and soluble hormone signals into the surrounding soil, then monitor their levels to sense soil stresses. Our review discusses how hormones can act 'outside the plant' as 'rhizocrine' signals that function to improve plant resilience to different soil stresses. We also propose a novel signalling paradigm which we term 'root RADAR' where 'rhizocrine' levels change in soil in response to environmental stresses, feeding back to roots and triggering adaptive responses.

Arbuscular Mycorrhizal Fungus Alleviates Charged Nanoplastic Stress in Host Plants via Enhanced Defense-Related Gene Expressions and Hyphal Capture

Contamination of small-sized plastics is recognized as a factor of global change. Nanoplastics (NPs) can readily enter organisms and pose significant ecological risks. Arbuscular mycorrhizal (AM) fungi are the most ubiquitous and impactful plant symbiotic fungi, regulating essential ecological functions. Here, we first found that an AM fungus, Rhizophagus irregularis, increased lettuce shoot biomass by 25-100% when exposed to positively and negatively charged NPs vs control, although it did not increase that grown without NPs. The stress alleviation was attributed to the upregulation of gene expressions involving phytohormone signaling, cell wall metabolism, and oxidant scavenging. Using a root organ-fungus axenic growth system treated with fluorescence-labeled NPs, we subsequently revealed that the hyphae captured NPs and further delivered them to roots. NPs were observed at the hyphal cell walls, membranes, and spore walls. NPs mediated by the hyphae were localized at the root epidermis, cortex, and stele. Hyphal exudates aggregated positively charged NPs, thereby reducing their uptake due to NP aggregate formation (up to 5000 nm). This work demonstrates the critical roles of AM fungus in regulating NP behaviors and provides a potential strategy for NP risk mitigation in terrestrial ecosystems. Consequent NP-induced ecological impacts due to the affected AM fungi require further attention.

Integration of rice apocarotenoid profile and expression pattern of Carotenoid Cleavage Dioxygenases reveals a positive effect of β-ionone on mycorrhization

Carotenoids are susceptible to degrading processes initiated by oxidative cleavage reactions mediated by Carotenoid Cleavage Dioxygenases that break their backbone, leading to products called apocarotenoids. These carotenoid-derived metabolites include the phytohormones abscisic acid and strigolactones, and different signaling molecules and growth regulators, which are utilized by plants to coordinate many aspects of their life. Several apocarotenoids have been recruited for the communication between plants and arbuscular mycorrhizal (AM) fungi and as regulators of the establishment of AM symbiosis. However, our knowledge on their biosynthetic pathways and the regulation of their pattern during AM symbiosis is still limited. In this study, we generated a qualitative and quantitative profile of apocarotenoids in roots and shoots of rice plants exposed to high/low phosphate concentrations, and upon AM symbiosis in a time course experiment covering different stages of growth and AM development. To get deeper insights in the biology of apocarotenoids during this plant-fungal symbiosis, we complemented the metabolic profiles by determining the expression pattern of CCD genes, taking advantage of chemometric tools. This analysis revealed the specific profiles of CCD genes and apocarotenoids across different stages of AM symbiosis and phosphate supply conditions, identifying novel reliable markers at both local and systemic levels and indicating a promoting role of β-ionone in AM symbiosis establishment.

Unveiling the significance of rhizosphere: Implications for plant growth, stress response, and sustainable agriculture

In the rhizosphere, the activities within all processes and functions are primarily influenced by plant roots, microorganisms present in the rhizosphere, and the interactions between roots and microorganisms. The rhizosphere, a dynamic zone surrounding the roots, provides an ideal environment for a diverse microbial community, which significantly shapes plant growth and development. Microbial activity in the rhizosphere can promote plant growth by increasing nutrient availability, influencing plant hormonal signaling, and repelling or outcompeting pathogenic microbial strains. Understanding the associations between plant roots and soil microorganisms has the potential to revolutionize crop yields, improve productivity, minimize reliance on chemical fertilizers, and promote sustainable plant growth technologies. The rhizosphere microbiome could play a vital role in the next green revolution and contribute to sustainable and eco-friendly agriculture. However, there are still knowledge gaps concerning plant root-environment interactions, particularly regarding roots and microorganisms. Advances in metabolomics have helped to understand the chemical communication between plants and soil biota, yet challenges persist. This article provides an overview of the latest advancements in comprehending the communication and interplay between plant roots and microbes, which have been shown to impact crucial factors such as plant growth, gene expression, nutrient absorption, pest and disease resistance, and the alleviation of abiotic stress. By improving these aspects, sustainable agriculture practices can be implemented to increase the overall productivity of plant ecosystems.

Signaling in Legume-Rhizobia Symbiosis

Legumes represent an important source of food protein for human nutrition and animal feed. Therefore, sustainable production of legume crops is an issue of global importance. It is well-known that legume-rhizobia symbiosis allows an increase in the productivity and resilience of legume crops. The efficiency of this mutualistic association strongly depends on precise regulation of the complex interactions between plant and rhizobia. Their molecular dialogue represents a complex multi-staged process, each step of which is critically important for the overall success of the symbiosis. In particular, understanding the details of the molecular mechanisms behind the nodule formation and functioning might give access to new legume cultivars with improved crop productivity. Therefore, here we provide a comprehensive literature overview on the dynamics of the signaling network underlying the development of the legume-rhizobia symbiosis. Thereby, we pay special attention to the new findings in the field, as well as the principal directions of the current and prospective research. For this, here we comprehensively address the principal signaling events involved in the nodule inception, development, functioning, and senescence.

Population Response of Rhizosphere Microbiota of Garden Pea Genotypes to Inoculation with Arbuscular Mycorrhizal Fungi

This study of a legume's rhizosphere in tripartite symbiosis focused on the relationships between the symbionts and less on the overall rhizosphere microbiome. We used an experimental model with different garden pea genotypes inoculated with AM fungi (Rhizophagus irregularis and with a mix of AM species) to study their influence on the population levels of main trophic groups of soil microorganisms as well as their structure and functional relationships in the rhizosphere microbial community. The experiments were carried out at two phenological cycles of the plants. Analyzes were performed according to classical methods: microbial population density defined as CUF/g a.d.s. and root colonization rate with AMF (%). We found a proven dominant effect of AMF on the densities of micromycetes and actinomycetes in the direction of reduction, suggesting antagonism, and on ammonifying, phosphate-solubilizing and free-living diazotrophic Azotobacter bacteria in the direction of stimulation, an indicator of mutualistic relationships. We determined that the genotype was decisive for the formation of populations of bacteria immobilizing mineral NH4+-N and bacteria Rhizobium. We reported significant two-way relationships between trophic groups related associated with soil nitrogen and phosphorus ions availability. The preserved proportions between trophic groups in the microbial communities were indicative of structural and functional stability.

Volatile organic compounds shape belowground plant-fungi interactions

Volatile organic compounds (VOCs), a bouquet of chemical compounds released by all life forms, play essential roles in trophic interactions. VOCs can facilitate a large number of interactions with different organisms belowground. VOCs-regulated plant-plant or plant-insect interaction both below and aboveground has been reported extensively. Nevertheless, there is little information about the role of VOCs derived from soilborne pathogenic fungi and beneficial fungi, particularly mycorrhizae, in influencing plant performance. In this review, we show how plant VOCs regulate plant-soilborne pathogenic fungi and beneficial fungi (mycorrhizae) interactions. How fungal VOCs mediate plant-soilborne pathogenic and beneficial fungi interactions are presented and the most common methods to collect and analyze belowground volatiles are evaluated. Furthermore, we suggest a promising method for future research on belowground VOCs.

Brassinosteroids Benefit Plants Performance by Augmenting Arbuscular Mycorrhizal Symbiosis

Arbuscular mycorrhizal (AM) play an important role in improving plant growth and development. The interaction between phytohormones and AM symbiosis is gradually revealed. Here we examined the effect of Brassinosteroids (BR) on AM symbiosis and discussed the synergistic promotion of plant growth by BR and AM symbiosis. The xylophyta Eucalyptus grandis Hill (E. grandis) was inoculated with AM fungi Rhizoglomus irregularis R197198 (R. irregularis) and treated with different concentrations (0, 1, 10, and 100 nM) of 24-epibrassinolide (24-epiBL) for 6 weeks. With the increase of 24-epiBL concentration, E. grandis growth was firstly promoted and then inhibited, but inoculation with AM fungi alleviated this inhibition. 24-epiBL and R. irregularis colonization significantly improved E. grandis growth and antioxidant system response, and the synergistic effect was the best. Compared with the control group, 24-epiBL treatment significantly increased the mycorrhizal colonization and arbuscular abundance of AM fungi R. irregular in E. grandis roots. The expression of AM symbiosis maker genes was significantly increased by 24-epiBL treatment. Both 24-epiBL treatment and AM colonization upregulated gibberellins (GA) synthesis genes, but no inhibition caused by GA levels was found. 24-epiBL is a kind of synthetic highly active BR. Based on the results of 24-epiBL treatment, we hypothesized that BR actively regulates AM symbiosis regulates AM symbiosis without affecting GA-INSENSITIVE DWARF1 (GID1)-DELLA expression. The synergistic treatment of BR and AM symbiosis can significantly promote the growth and development of plants. IMPORTANCE Brassinosteroids (BR) and Arbuscular mycorrhizas (AM) symbiosis play an important role in improving plant growth and development. Previous studies have shown that there is a complex regulatory network between phytohormones and AM symbiosis. However, the interactions of BR-signaling and AM symbiosis are still poorly understood. Our results suggest that BR actively regulates the colonization and development of AM fungi, and AM fungal colonization can alleviate the inhibition of plant growth caused by excessive BR. In addition, BR actively regulates AM symbiosis, but does not primarily mediate gibberellins-DELLA interaction. The synergistic treatment of BR and AM symbiosis can significantly promote the growth and development of plants. The conclusions of this study provide a reference for phytohormones-AM symbiosis interaction.

A roadmap of plant membrane transporters in arbuscular mycorrhizal and legume-rhizobium symbioses

Most land plants live in close contact with beneficial soil microbes: the majority of land plant species establish symbiosis with arbuscular mycorrhizal fungi, while most legumes, the third largest plant family, can form a symbiosis with nitrogen-fixing rhizobia. These microbes contribute to plant nutrition via endosymbiotic processes that require modulating the expression and function of plant transporter systems. The efficient contribution of these symbionts involves precisely controlled integration of transport, which is enabled by the adaptability and plasticity of their transporters. Advances in our understanding of these systems, driven by functional genomics research, are rapidly filling the gap in knowledge about plant membrane transport involved in these plant-microbe interactions. In this review, we synthesize recent findings associated with different stages of these symbioses, from the pre-symbiotic stage to nutrient exchange, and describe the role of host transport systems in both mycorrhizal and legume-rhizobia symbioses.

Quantitative Mapping of Flavor and Pharmacologically Active Compounds in European Licorice Roots (Glycyrrhiza glabra L.) in Response to Growth Conditions and Arbuscular Mycorrhiza Symbiosis

Application of a sensitive UHPLC-MS/MS_MRM method enabled the simultaneous quantitation of 23 sweet-, licorice-, and bitter-tasting saponins in Glycyrrhiza glabra L., Glycyrrhiza uralensis Fisch., different licorice plants and root compartments, processed licorice, as well as different Glycyrrhiza spp. The combination of quantitative data with sweet, licorice, and bitter taste thresholds led to the determination of dose-over-threshold factors to elucidate the sweet, licorice, and bitter impact of the individual saponins with and without mycorrhiza symbiosis to evaluate the licorice root quality. Aside from glycyrrhizin (1), which is the predominant sweet- and licorice-tasting saponin in all licorice samples, 20 out of 22 quantitated saponins contributed to the taste profile of licorice roots. Next to sweet-/licorice-tasting glycyrrhizin (1), 24-hydroxy-glycyrrhizin (9), 30-hydroxy-glycyrrhizin (11), and 11-deoxo-24-hydroxy-glycyrrhizin (14) as well as licorice tasting saponins 20α-galacturonic acid glycyrrhizin (17), 24-hydroxy-20α-glycyrrhizin (21), and 11-deoxo-glycyrrhizin (12) were determined as key contributors to licorice root's unique taste profile. A quantitative comparison of 23 saponins as well as 28 polyphenols between licorice roots inoculated with arbuscular mycorrhiza fungi and controls showed that important taste-mediating saponins were increased in mycorrhizal roots, and these alterations depended on the growth substrate and the level of phosphate fertilization.

Factors affecting plant responsiveness to arbuscular mycorrhiza

Arbuscular mycorrhiza (AM) is an ancient, widespread symbiosis between most land plants and fungi of the Glomeromycotina, which receives increasing interest for agricultural application because it can promote plant growth and yield. The ability of plants to react to AM with changes in morphology and/or performance in terms of yield is called 'AM responsiveness'. Its amplitude depends on the plant- fungal genotype combination and the abiotic and biotic environment. A molecular understanding of AM responsiveness is key for enabling rational application of AM in agriculture, for example through targeted breeding of AM-optimised crops. However, the genetic and mechanistic underpinnings of AM responsiveness variation remain still unknown. Here, we review current knowledge on AM responsiveness, with a focus on agricultural crops, and speculate on mechanisms that may contribute to the variation in AM response.

Controlled Assays for Phenotyping the Effects of Strigolactone-Like Molecules on Arbuscular Mycorrhiza Development

Arbuscular mycorrhiza is an ancient symbiosis between most land plants and fungi of the Glomeromycotina, in which the fungi provide mineral nutrients to the plant in exchange for photosynthetically fixed organic carbon. Strigolactones are important signals promoting this symbiosis, as they are exuded by plant roots into the rhizosphere to stimulate activity of the fungi. In addition, the plant karrikin signaling pathway is required for root colonization. Understanding the molecular mechanisms underpinning root colonization by AM fungi, requires the use of plant mutants as well as treatments with different environmental conditions or signaling compounds in standardized cocultivation systems to allow for reproducible root colonization phenotypes. Here we describe how we set up and quantify arbuscular mycorrhiza in the model plants Lotus japonicus and Brachypodium distachyon under controlled conditions. We illustrate a setup for open pot culture as well as for closed plant tissue culture (PTC) containers, for plant-fungal cocultivation in sterile conditions. Furthermore, we explain how to harvest, store, stain, and image AM roots for phenotyping and quantification of different AM structures.

A Flexible, Low-Cost Hydroponic Co-Cultivation System for Studying Arbuscular Mycorrhiza Symbiosis

Arbuscular mycorrhiza (AM) is a widespread symbiosis between plant roots and fungi of the Glomeromycotina, which improves nutrient uptake by plants. The molecular mechanisms underlying development and function of the symbiosis are subject to increasing research activity. Since AM occurs in the soil, most studies targeting a molecular understanding of AM development and function, use solid substrates for co-cultivating plants and AM fungi. However, for some experiments very clean roots, highly controlled nutrient conditions or applications of defined concentrations of signaling molecules (such as hormones) or other small chemicals (such as synthetic inhibitors or signaling agonists) are needed. To this end, hydroponics has been widely used in research on mechanisms of plant nutrition and some hydroponic systems were developed for AM fungal spore amplification. Here, we present a hydroponics set-up, which can be successfully utilized for experimental root colonization assays. We established a "tip-wick" system based on pipette tips and rock wool wicks for co-cultivation of AM fungi with small model plants such as Lotus japonicus. A larger "Falcon-wick" system using Falcon tubes and rockwool wicks was developed for larger model plants such as rice. The hydroponic system can also be employed for growing L. japonicus hairy roots after transformation by Agrobacterium rhizogenes, thus circumventing the laborious cultivation on agar medium-containing Petri dishes during hairy root development. The tip-wick and Falcon-wick systems are easy to use and can be built with low cost, conventional and reusable lab plastic ware and a simple aquarium pump.

The Full-Size ABCG Transporter of Medicago truncatula Is Involved in Strigolactone Secretion, Affecting Arbuscular Mycorrhiza

Strigolactones (SLs) are plant-derived signaling molecules that stimulate the hyphal branching of arbuscular mycorrhizal fungi (AMF), and consequently promote symbiotic interaction between the fungus and the plant. Currently, our knowledge on the molecular mechanism of SL transport is restricted to the Solanaceae family. In the Solanaceae family, SL translocation toward the rhizosphere occurs through the exodermis via hypodermal passage cells and involves a member of the G subfamily, of the ATP-binding cassette (ABC) membrane transporters. Most Fabaceae species, including those that are agriculturally important, have a different root anatomy compared to most angiosperm plants (i.e., lacking an exodermis). Thus, we have investigated how SL transport occurs in the model legume Medicago truncatula. Here, we show that overexpression of a SL transporter from petunia (PaPDR1) enhances AMF colonization rates in M. truncatula. This result demonstrates the importance of ABCG proteins for the translocation of orobanchol-type molecules to facilitate arbuscular mycorrhiza, regardless of root anatomy and phylogenetic relationships. Moreover, our research has led to the identification of Medicago ABCG59, a close homologue of Petunia PDR1, that exhibits root specific expression and is up-regulated by phosphate starvation as well as in the presence of rac-GR24, a synthetic SL. Its promoter is active in cortical cells, root tips, and the meristematic zone of nodules. The mtabcg59 loss-of-function mutant displayed a reduced level of mycorrhization compared to the WT plants but had no impact on the number of nodules after Sinorhizobium meliloti inoculation. The reduced mycorrhization indicates that less SLs are secreted by the mutant plants, which is in line with the observation that mtabcg59 exudates exhibit a reduced stimulatory effect on the germination of the parasitic plant Phelipanche ramosa compared to the corresponding wild type.

Endophytic fungus Falciphora oryzae promotes lateral root growth by producing indole derivatives after sensing plant signals

The endophytic fungus Falciphora oryzae was initially isolated from wild rice (Oryza granulata) and colonizes many crop species and promotes plant growth. However, the molecular mechanisms underlying F. oryzae-mediated growth promotion are still unknown. We found that F. oryzae was able to colonize Arabidopsis thaliana. The most dramatic change after F. oryzae inoculation was observed in the root architecture, as evidenced by increased lateral root growth but reduced primary root length, similar to the effect of auxin, a significant plant growth hormone. The expression of genes responsible for auxin biosynthesis, transport, and signalling was regulated in Arabidopsis roots after F. oryzae cocultivation. Indole derivatives were detected at significantly higher levels in liquid media after cocultivation compared with separate cultivation of Arabidopsis and F. oryzae. Consistently, the expression of indole biosynthetic genes was highly upregulated in F. oryzae upon treatment with Arabidopsis exudates. Global analysis of Arabidopsis gene expression at the early stage after F. oryzae cocultivation suggested that signals were exchanged to initiate Arabidopsis-F. oryzae interactions. All these results suggest that signalling molecules from Arabidopsis roots are perceived by F. oryzae and induce the biosynthesis of indole derivatives in F. oryzae, consequently stimulating Arabidopsis lateral root growth.

Bridging the Gap Between Single-Strain and Community-Level Plant-Microbe Chemical Interactions

Although the influence of microbiomes on the health of plant hosts is evident, specific mechanisms shaping the structure and dynamics of microbial communities in the phyllosphere and rhizosphere are only beginning to become clear. Traditionally, plant-microbe interactions have been studied using cultured microbial isolates and plant hosts but the rising use of 'omics tools provides novel snapshots of the total complex community in situ. Here, we discuss the recent advances in tools and techniques used to monitor plant-microbe interactions and the chemical signals that influence these relationships in above- and belowground tissues. Particularly, we highlight advances in integrated microscopy that allow observation of the chemical exchange between individual plant and microbial cells, as well as high-throughput, culture-independent approaches to investigate the total genetic and metabolic contribution of the community. The chemicals discussed have been identified as relevant signals across experimental spectrums. However, mechanistic insight into the specific interactions mediated by many of these chemicals requires further testing. Experimental designs that attempt to bridge the gap in biotic complexity between single strains and whole communities will advance our understanding of the chemical signals governing plant-microbe associations in the rhizosphere and phyllosphere.

A LysM Receptor Heteromer Mediates Perception of Arbuscular Mycorrhizal Symbiotic Signal in Rice

Symbiotic microorganisms improve nutrient uptake by plants. To initiate mutualistic symbiosis with arbuscular mycorrhizal (AM) fungi, plants perceive Myc factors, including lipochitooligosaccharides (LCOs) and short-chain chitooligosaccharides (CO4/CO5), secreted by AM fungi. However, the molecular mechanism of Myc factor perception remains elusive. In this study, we identified a heteromer of LysM receptor-like kinases consisting of OsMYR1/OsLYK2 and OsCERK1 that mediates the perception of AM fungi in rice. CO4 directly binds to OsMYR1, promoting the dimerization and phosphorylation of this receptor complex. Compared with control plants, Osmyr1 and Oscerk1 mutant rice plants are less sensitive to Myc factors and show decreased AM colonization. We engineered transgenic rice by expressing chimeric receptors that respectively replaced the ectodomains of OsMYR1 and OsCERK1 with those from the homologous Nod factor receptors MtNFP and MtLYK3 of Medicago truncatula. Transgenic plants displayed increased calcium oscillations in response to Nod factors compared with control rice. Our study provides significant mechanistic insights into AM symbiotic signal perception in rice. Expression of chimeric Nod/Myc receptors achieves a potentially important step toward generating cereals that host nitrogen-fixing bacteria.

Microfluidics and Metabolomics Reveal Symbiotic Bacterial-Fungal Interactions Between Mortierella elongata and Burkholderia Include Metabolite Exchange

We identified two poplar (Populus sp.)-associated microbes, the fungus, Mortierella elongata strain AG77, and the bacterium, Burkholderia strain BT03, that mutually promote each other’s growth. Using culture assays in concert with a novel microfluidic device to generate time-lapse videos, we found growth specific media differing in pH and pre-conditioned by microbial growth led to increased fungal and bacterial growth rates. Coupling microfluidics and comparative metabolomics data results indicated that observed microbial growth stimulation involves metabolic exchange during two ordered events. The first is an emission of fungal metabolites, including organic acids used or modified by bacteria. A second signal of unknown nature is produced by bacteria which increases fungal growth rates. We find this symbiosis is initiated in part by metabolic exchange involving fungal organic acids.

Molecular dialogue between arbuscular mycorrhizal fungi and the nonhost plant Arabidopsis thaliana switches from initial detection to antagonism

Approximately 29% of all vascular plant species are unable to establish an arbuscular mycorrhizal (AM) symbiosis. Despite this, AM fungi (Rhizophagus spp.) are enriched in the root microbiome of the nonhost Arabidopsis thaliana, and Arabidopsis roots become colonized when AM networks nurtured by host plants are available. Here, we investigated the nonhost-AM fungus interaction by analyzing transcriptional changes in Rhizophagus, Arabidopsis and the host plant Medicago truncatula while growing in the same mycorrhizal network. In early interaction stages, Rhizophagus activated the Arabidopsis strigolactone biosynthesis genes CCD7 and CCD8, suggesting that detection of AM fungi is not completely impaired. However, in colonized Arabidopsis roots, fungal nutrient transporter genes GintPT, GintAMT2, GintMST2 and GintMST4, essential for AM symbiosis, were not activated. RNA-seq transcriptome analysis pointed to activation of costly defenses in colonized Arabidopsis roots. Moreover, Rhizophagus colonization caused a 50% reduction in shoot biomass, but also led to enhanced systemic immunity against Botrytis cinerea. This suggests that early signaling between AM fungi and Arabidopsis is not completely impaired and that incompatibility appears at later interaction stages. Moreover, Rhizophagus-mediated defenses coincide with reduced Arabidopsis growth, but also with systemic disease resistance, highlighting the multifunctional role of AM fungi in host and nonhost interactions.

Stimulation of asymbiotic sporulation in arbuscular mycorrhizal fungi by fatty acids

Arbuscular mycorrhizal (AM) fungi are obligate symbionts that depend on living host plants to complete their life cycle^1,2. This feature, which leads to their unculturability in the absence of plants, strongly hinders basic research and agricultural application of AM fungi. However, at least one AM fungus can grow and develop fertile spores independently of a host plant in co-culture with the bacterium Paenibacillus validus³. The bacteria-derived substances are thought to act as stimulants or nutrients for fungal sporulation, but these molecules have not been identified. Here, we show that (S)-12-methyltetradecanoic acid^4,5, a methyl branched-chain fatty acid isolated from bacterial cultures, stimulates the branching of hyphae germinated from mother spores and the formation of secondary spores in axenic culture of the AM fungus Rhizophagus irregularis. Extensive testing of fatty acids revealed that palmitoleic acid induces more secondary spores than the bacterial fatty acid in R. irregularis. These induced spores have the ability to infect host plant roots and to generate daughter spores. Our work shows that, in addition to a major source of organic carbon^6-9, fatty acids act as stimulants to induce infection-competent secondary spores in the asymbiotic stage and could provide the key to developing the axenic production of AM inoculum.

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.

Unraveling the Initial Plant Hormone Signaling, Metabolic Mechanisms and Plant Defense Triggering the Endomycorrhizal Symbiosis Behavior

Arbuscular mycorrhizal (AM) fungi establish probably one of the oldest mutualistic relationships with the roots of most plants on earth. The wide distribution of these fungi in almost all soil ecotypes and the broad range of host plant species demonstrate their strong plasticity to cope with various environmental conditions. AM fungi elaborate fine-tuned molecular interactions with plants that determine their spread within root cortical tissues. Interactions with endomycorrhizal fungi can bring various benefits to plants, such as improved nutritional status, higher photosynthesis, protection against biotic and abiotic stresses based on regulation of many physiological processes which participate in promoting plant performances. In turn, host plants provide a specific habitat as physical support and a favorable metabolic frame, allowing uptake and assimilation of compounds required for the life cycle completion of these obligate biotrophic fungi. The search for formal and direct evidences of fungal energetic needs raised strong motivated projects since decades, but the impossibility to produce AM fungi under axenic conditions remains a deep enigma and still feeds numerous debates. Here, we review and discuss the initial favorable and non-favorable metabolic plant context that may fate the mycorrhizal behavior, with a focus on hormone interplays and their links with mitochondrial respiration, carbon partitioning and plant defense system, structured according to the action of phosphorus as a main limiting factor for mycorrhizal symbiosis. Then, we provide with models and discuss their significances to propose metabolic targets that could allow to develop innovations for the production and application of AM fungal inocula.

Partner communication and role of nutrients in the arbuscular mycorrhizal symbiosis

Contents Summary 1031 I. Introduction 1031 II. Interkingdom communication enabling symbiosis 1032 III. Nutritional and regulatory roles for key metabolites in the AM symbiosis 1035 IV. The plant-fungus genotype combination determines the outcome of the symbiosis 1039 V. Perspectives 1039 Acknowledgements 1041 References 1041 SUMMARY: The evolutionary and ecological success of the arbuscular mycorrhizal (AM) symbiosis relies on an efficient and multifactorial communication system for partner recognition, and on a fine-tuned and reciprocal metabolic regulation of each symbiont to reach an optimal functional integration. Besides strigolactones, N-acetylglucosamine-derivatives released by the plant were recently suggested to trigger fungal reprogramming at the pre-contact stage. Remarkably, N-acetylglucosamine-based diffusible molecules also are symbiotic signals produced by AM fungi (AMF) and clues on the mechanisms of their perception by the plant are emerging. AMF genomes and transcriptomes contain a battery of putative effector genes that may have conserved and AMF- or host plant-specific functions. Nutrient exchange is the key feature of AM symbiosis. A mechanism of phosphate transport inside fungal hyphae has been suggested, and first insights into the regulatory mechanisms of root colonization in accordance with nutrient transfer and status were obtained. The recent discovery of the dependency of AMF on fatty acid transfer from the host has offered a convincing explanation for their obligate biotrophism. Novel studies highlighted the importance of plant and fungal genotypes for the outcome of the symbiosis. These findings open new perspectives for fundamental research and application of AMF in agriculture.

Vascular architecture in the bacteriogenic light organ of Euprymna tasmanica (Cephalopoda: Sepiolidae)

Symbiosis between southern dumpling squid, Euprymna tasmanica (Cephalopoda: Sepiolidae), and its luminescent symbiont, the bacterium Vibrio fischeri, provides an experimentally tractable system to examine interactions between the eukaryotic host and its bacterial partner. Luminescence emitted by the symbiotic bacteria provides light for the squid in a behavior termed "counter-illumination," which allows the squid to mask its shadow amidst downwelling moonlight. Although this association is beneficial, light generated from the bacteria requires large quantities of oxygen to maintain this energy-consuming reaction. Therefore, we examined the vascular network within the light organ of juveniles of E. tasmanica with and without V. fischeri. Vessel type, diameter, and location of vessels were measured. Although differences between symbiotic and aposymbiotic squid demonstrated that the presence of V. fischeri does not significantly influence the extent of vascular branching at early stages of symbiotic development, these finding do provide an atlas of blood vessel distribution in the organ. Thus, these results provide a framework to understand how beneficial bacteria influence the development of a eukaryotic closed vascular network and provide insight to the evolutionary developmental dynamics that form during mutualistic interactions.

Host- and stage-dependent secretome of the arbuscular mycorrhizal fungus Rhizophagus irregularis

Arbuscular mycorrhizal fungi form the most wide-spread endosymbiosis with plants. There is very little host specificity in this interaction, however host preferences as well as varying symbiotic efficiencies have been observed. We hypothesize that secreted proteins (SPs) may act as fungal effectors to control symbiotic efficiency in a host-dependent manner. Therefore, we studied whether arbuscular mycorrhizal (AM) fungi adjust their secretome in a host- and stage-dependent manner to contribute to their extremely wide host range. We investigated the expression of SP-encoding genes of Rhizophagus irregularis in three evolutionary distantly related plant species, Medicago truncatula, Nicotiana benthamiana and Allium schoenoprasum. In addition we used laser microdissection in combination with RNA-seq to study SP expression at different stages of the interaction in Medicago. Our data indicate that most expressed SPs show roughly equal expression levels in the interaction with all three host plants. In addition, a subset shows significant differential expression depending on the host plant. Furthermore, SP expression is controlled locally in the hyphal network in response to host-dependent cues. Overall, this study presents a comprehensive analysis of the R. irregularis secretome, which now offers a solid basis to direct functional studies on the role of fungal SPs in AM symbiosis.

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.

Strigolactone Biosynthesis Genes of Rice are Required for the Punctual Entry of Arbuscular Mycorrhizal Fungi into the Roots

Arbuscular mycorrhiza (AM) is a mutualistic association between most plant species and the ancient fungal phylum Glomeromycota in roots, and it plays a key role in a plant's nutrient uptake from the soil. Roots synthesize strigolactones (SLs), derivatives of carotenoids, and exude them to induce energy metabolism and hyphal branching of AM fungi. Despite the well-documented roles of SLs in the pre-symbiotic phase, little is known about the role of SLs in the process of root colonization. Here we show that the expansion of root colonization is suppressed in the mutants of rice (Oryza sativa) SL biosynthesis genes, carotenoid cleavage dioxygenase D10 and more severely in D17. Interestingly, most of the colonization process is normal, i.e. AM fungal hyphae approach the roots and cling around them, and epidermal penetration, arbuscule size, arbuscule number per hyphopodium and metabolic activity of the intraradical mycelium are not affected in d10 and d17 mutants. In contrast, hyphopodium formation is severely attenuated. Our observations establish the requirement for SL biosynthesis genes for efficient hyphopodium formation, suggesting that SLs are required in this process. Efficient hyphopodium formation is required for the punctual internalization of hyphae into roots and maintaining the expansion of colonization.

Medicago truncatula ABCG10 is a transporter of 4-coumarate and liquiritigenin in the medicarpin biosynthetic pathway

The ABCG10 protein of the model legume Medicago truncatula is required for efficient de novo production of the phenylpropanoid-derived phytoalexin medicarpin. Silencing the expression of MtABCG10 results, inter alia, in a lower accumulation of medicarpin and its precursors. In this study, we demonstrate that the impairment of medicarpin biosynthesis can be partially averted by the exogenous application of 4-coumarate, an early precursor of the core phenylpropanoid pathway, and the deoxyisoflavonoid formononetin. Experiments conducted using HPLC/MS in a heterologous system as well as in vitro transport assays with labelled substrate revealed that MtABCG10 is responsible for the membrane translocation of 4-coumarate and liquiritigenin, molecules representing key branching points in the phenylpropanoid pathway. The identification of transporters participating in the distribution of precursors is an important step in understanding phenylpropanoid biosynthesis.

An N-acetylglucosamine transporter required for arbuscular mycorrhizal symbioses in rice and maize

Most terrestrial plants, including crops, engage in beneficial interactions with arbuscular mycorrhizal fungi. Vital to the association is mutual recognition involving the release of diffusible signals into the rhizosphere. Previously, we identified the maize no perception 1 (nope1) mutant to be defective in early signalling. Here, we report cloning of ZmNope1 on the basis of synteny with rice. NOPE1 encodes a functional homologue of the Candida albicans N-acetylglucosamine (GlcNAc) transporter NGT1, and represents the first plasma membrane GlcNAc transporter identified from plants. In C. albicans, exposure to GlcNAc activates cell signalling and virulence. Similarly, in Rhizophagus irregularis treatment with rice wild-type but not nope1 root exudates induced transcriptome changes associated with signalling function, suggesting a requirement of NOPE1 function for presymbiotic fungal reprogramming.

Nuclear Ca2+ signalling in arbuscular mycorrhizal and actinorhizal endosymbioses: on the trail of novel underground signals

Contents 533 I. 533 II. 534 III. 536 IV. 536 537 References 537 SUMMARY: Root endosymbioses are beneficial associations formed between terrestrial plants and either bacterial or fungal micro-organisms. A common feature of these intracellular symbioses is the requirement for mutual recognition between the two partners before host-regulated microbial entry. As part of this molecular dialogue, symbiosis-specific microbial factors set in motion a highly conserved plant signal transduction pathway, of which a central component is the activation of sustained nuclear Ca²⁺ oscillations in target cells of the host epidermis. Here, we focus on recent findings concerning this crucial Ca²⁺ -dependent signalling step for endosymbiotic associations involving either arbuscular mycorrhizal fungi or nitrogen-fixing Frankia actinomycetes, and in particular how this knowledge is contributing to the identification of the respective microbial factors.

Strigolactone-Induced Putative Secreted Protein 1 Is Required for the Establishment of Symbiosis by the Arbuscular Mycorrhizal Fungus Rhizophagus irregularis

Arbuscular mycorrhizal (AM) symbiosis is the most widespread association between plants and fungi. To provide novel insights into the molecular mechanisms of AM symbiosis, we screened and investigated genes of the AM fungus Rhizophagus irregularis that contribute to the infection of host plants. R. irregularis genes involved in the infection were explored by RNA-sequencing (RNA-seq) analysis. One of the identified genes was then characterized by a reverse genetic approach using host-induced gene silencing (HIGS), which causes RNA interference in the fungus via the host plant. The RNA-seq analysis revealed that 19 genes are up-regulated by both treatment with strigolactone (SL) (a plant symbiotic signal) and symbiosis. Eleven of the 19 genes were predicted to encode secreted proteins and, of these, SL-induced putative secreted protein 1 (SIS1) showed the largest induction under both conditions. In hairy roots of Medicago truncatula, SIS1 expression is knocked down by HIGS, resulting in significant suppression of colonization and formation of stunted arbuscules. These results suggest that SIS1 is a putative secreted protein that is induced in a wide spatiotemporal range including both the presymbiotic and symbiotic stages and that SIS1 positively regulates colonization of host plants by R. irregularis.

Signaling in the Rhizosphere

Signaling studies in the rhizosphere have focused on close interactions between plants and symbiotic microorganisms. However, this focus is likely to expand to other microorganisms because the rhizomicrobiome is important for plant health and is able to influence the structure of the microbial community. We discuss here the shaping of the rhizomicrobiome and define which aspects can be considered signaling. We divide signaling in the rhizosphere into three categories: (i) between microbes, (ii) from plants to microorganisms, and (iii) from microorganisms to plants. Signals act on diverse organisms including the plant. Mycorrhizal and rhizobial interkingdom signaling has revealed its pivotal role in establishing associations, and the recent discovery of signaling with non-symbiotic microorganisms indicates the important role of communication in shaping the rhizomicrobiome.

Effect of volatiles versus exudates released by germinating spores of Gigaspora margarita on lateral root formation

Arbuscular mycorrhizal (AM) fungi influence the root system architecture of their hosts; however, the underlying mechanisms have not been fully elucidated. Ectomycorrhizal fungi influence root architecture via volatiles. To determine whether volatiles also play a role in root system changes in response to AM fungi, spores of the AM fungus Gigaspora margarita were inoculated on the same plate as either wild type (WT) Lotus japonicus, the L. japonicus mutant Ljcastor (which lacks the symbiotic cation channel CASTOR, which is required for inducing nuclear calcium spiking, which is necessary for symbiotic partner recognition), or Arabidopsis thaliana, separated by cellophane membranes (fungal exudates experiment), or on different media but with a shared head space (fungal volatiles experiment). Root development was monitored over time. Both germinating spore exudates (GSEs) and geminated-spore-emitted volatile organic compounds (GVCs) significantly promoted lateral root formation (LRF) in WT L. japonicus. LRF in Ljcastor was significantly enhanced in the presence of GVCs. GVCs stimulated LRF in A. thaliana, whereas GSEs showed an inhibitory effect. The expression profile of the genes involved in mycorrhizal establishment and root development were investigated using quantitative reverse transcription-PCR analysis. Only the expression of the LjCCD7 gene, an important component of the strigolactone synthesis pathway, was differentially expressed following exposure to GVCs. We conclude that volatile organic compounds released by the germinating AM fungal spores may stimulate LRF in a symbiosis signaling pathway (SYM)- and host-independent way, whereas GSEs stimulate LRF in a SYM- and host-dependent way.

Pre-announcement of symbiotic guests: transcriptional reprogramming by mycorrhizal lipochitooligosaccharides shows a strict co-dependency on the GRAS transcription factors NSP1 and RAM1

BACKGROUND

More than 80 % of all terrestrial plant species establish an arbuscular mycorrhiza (AM) symbiosis with Glomeromycota fungi. This plant-microbe interaction primarily improves phosphate uptake, but also supports nitrogen, mineral, and water aquisition. During the pre-contact stage, the AM symbiosis is controled by an exchange of diffusible factors from either partner. Amongst others, fungal signals were identified as a mix of sulfated and non-sulfated lipochitooligosaccharides (LCOs), being structurally related to rhizobial nodulation (Nod)-factor LCOs that in legumes induce the formation of nitrogen-fixing root nodules. LCO signals are transduced via a common symbiotic signaling pathway (CSSP) that activates a group of GRAS transcription factors (TFs). Using complex gene expression fingerprints as molecular phenotypes, this study primarily intended to shed light on the importance of the GRAS TFs NSP1 and RAM1 for LCO-activated gene expression during pre-symbiotic signaling.

RESULTS

We investigated the genome-wide transcriptional responses in 5 days old primary roots of the Medicago truncatula wild type and four symbiotic mutants to a 6 h challenge with LCO signals supplied at 10(-7/-8) M. We were able to show that during the pre-symbiotic stage, sulfated Myc-, non-sulfated Myc-, and Nod-LCO-activated gene expression almost exclusively depends on the LysM receptor kinase NFP and is largely controled by the CSSP, although responses independent of this pathway exist. Our results show that downstream of the CSSP, gene expression activation by Myc-LCOs supplied at 10(-7/-8) M strictly required both the GRAS transcription factors RAM1 and NSP1, whereas those genes either co- or specifically activated by Nod-LCOs displayed a preferential NSP1-dependency. RAM1, a central regulator of root colonization by AM fungi, controled genes activated by non-sulfated Myc-LCOs during the pre-symbiotic stage that are also up-regulated in areas with early physical contact, e.g. hyphopodia and infecting hyphae; linking responses to externally applied LCOs with early root colonization.

CONCLUSIONS

Since both RAM1 and NSP1 were essential for the pre-symbiotic transcriptional reprogramming by Myc-LCOs, we propose that downstream of the CSSP, these GRAS transcription factors act synergistically in the transduction of those diffusible signals that pre-announce the presence of symbiotic fungi.

Silencing a key gene of the common symbiosis pathway in Nicotiana attenuata specifically impairs arbuscular mycorrhizal infection without influencing the root-associated microbiome or plant growth

While the biochemical function of calcium and calmodulin-dependent protein kinase (CCaMK) is well studied, and plants impaired in the expression of CCaMK are known not to be infected by arbuscular mycorrhizal fungi (AMF) in glasshouse studies, the whole-plant and ecological consequences of CCaMK silencing are not well understood. Here we show that three independently transformed lines of Nicotiana attenuata plants silenced in CCaMK (irCCaMK) are neither infected by Rhizophagus irregularis in the glasshouse nor by native fungal inoculum in the field. The overall fungal community of field-grown roots did not differ significantly among empty vector (EV) and the transgenic lines, and the bacterial communities only showed minor differences, as revealed by the alpha-diversity parameters of bacterial OTUs, which were higher in EV plants compared with two of the three transformed lines, while beta-diversity parameters did not differ. Furthermore, growth and fitness parameters were similar in the glasshouse and field. Herbivory-inducible and basal levels of salicylic acid, jasmonic acid and abscisic acid did not differ among the genotypes, suggesting that activation of the classical defence pathways are not affected by CCaMK silencing. Based on these results, we conclude that silencing of CCaMK has few, if any, non-target effects.

Recent developments in arbuscular mycorrhizal signaling

Plants can establish root endosymbioses with both arbuscular mycorrhizal fungi and rhizobial bacteria to improve their nutrition. Our understanding of the molecular events underlying the establishment of these symbioses has significantly advanced in the last few years. Here I highlight major recent findings in the field of endosymbiosis signaling. Despite the identification of new signaling components and the definition, or in some cases better re-definition of the molecular functions of previously known players, major questions still remain that need to be addressed. Most notably the mechanisms defining signaling specificities within either symbiosis remain unclear.

Full establishment of arbuscular mycorrhizal symbiosis in rice occurs independently of enzymatic jasmonate biosynthesis

Development of the mutualistic arbuscular mycorrhiza (AM) symbiosis between most land plants and fungi of the Glomeromycota is regulated by phytohormones. The role of jasmonate (JA) in AM colonization has been investigated in the dicotyledons Medicago truncatula, tomato and Nicotiana attenuata and contradicting results have been obtained with respect to a neutral, promotive or inhibitory effect of JA on AM colonization. Furthermore, it is currently unknown whether JA plays a role in AM colonization of monocotyledonous roots. Therefore we examined whether JA biosynthesis is required for AM colonization of the monocot rice. To this end we employed the rice mutant constitutive photomorphogenesis 2 (cpm2), which is deficient in JA biosynthesis. Through a time course experiment the amount and morphology of fungal colonization did not differ between wild-type and cpm2 roots. Furthermore, no significant difference in the expression of AM marker genes was detected between wild type and cpm2. However, treatment of wild-type roots with 50 μM JA lead to a decrease of AM colonization and this was correlated with induction of the defense gene PR4. These results indicate that JA is not required for AM colonization of rice but high levels of JA in the roots suppress AM development likely through the induction of defense.

Effects of arbuscular mycorrhizal fungi and maternal plant sex on seed germination and early plant establishment

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PREMISE OF THE STUDY

Arbuscular mycorrhizal fungi usually enhance overall plant performance, yet their effects on seed germination and early plant establishment, crucial steps in plant cycles, are generally overlooked. In gynodioecious species, sexual dimorphism in these traits has been reported, with females producing seeds that germinate at a faster rate than seeds from hermaphrodites.•

METHODS

Using the gynodioecious plant Geranium sylvaticum, I investigated in a greenhouse experiment whether the presence of arbuscular mycorrhizal spores affects seed germination and early plant establishment, examining at the same time whether the sex of the mother producing the seeds also influences these parameters and whether sex-specific interactions between these two factors exist.•

KEY RESULTS

The presence of arbuscular mycorrhizal spores in the soil decreased seed germination, did not affect plant survival, but did increase plant growth. Moreover, no significant differences in seed traits were detected between the sexes of the plants producing the seeds.•

CONCLUSIONS

This study demonstrates that arbuscular mycorrhizal fungi may have contrasting effects for plants during early life stages and that mycorrhizal effects can take place even at the precolonization stage.

Interplant signalling through hyphal networks

Mycorrhizal fungi can form common mycelial networks (CMNs) that interconnect plants. Here, we provide an insight into recent findings demonstrating that CMNs can be conduits for interplant signalling, influencing defence against insect herbivores and foliar necrotrophic fungi. A likely mechanism is direct transfer of signalling molecules within hyphae. However, electrical signals, which can be induced by wounding, may also enable signalling over relatively long distances, because the biophysical constraints imposed by liquid transport in hyphae and interaction with soil are relieved. We do not yet understand the ecological, evolutionary and agronomic implications of interplant signalling via CMNs. Identifying the mechanism of interplant signalling will help to address these gaps.

Arbuscular mycorrhizal dialogues: do you speak 'plantish' or 'fungish'?

Plants rely on their associated microbiota for crucial physiological activities; realization of this interaction drives research to understand inter-domain communication. This opinion article focuses on the arbuscular mycorrhizal (AM) symbiosis, which involves the Glomeromycota, fungi that can form a symbiosis with most plants. Here we propose the hypothesis that the molecules involved in inter-kingdom symbiotic signaling, such as strigolactones, cutin monomers, and chitin-related molecules, also have key roles in development, originally unrelated to symbiosis. Thus, the symbiotic role of these molecules relies on the co-evolved capacity of the AM partners to perceive and interpret them as symbiotic signals.

Lipochitooligosaccharides modulate plant host immunity to enable endosymbioses

Symbiotic nitrogen-fixing rhizobium bacteria and arbuscular mycorrhizal fungi use lipochitooligosaccharide (LCO) signals to communicate with potential host plants. Upon a compatible match, an intimate relation is established during which the microsymbiont is allowed to enter root (-derived) cells. Plants perceive microbial LCO molecules by specific LysM-domain-containing receptor-like kinases. These do not only activate a common symbiosis signaling pathway that is shared in both symbioses but also modulate innate immune responses. Recent studies revealed that symbiotic LCO receptors are closely related to chitin innate immune receptors, and some of these receptors even function in symbiosis as well as immunity. This raises questions about how plants manage to translate structurally very similar microbial signals into different outputs. Here, we describe the current view on chitin and LCO perception in innate immunity and endosymbiosis and question how LCOs might modulate the immune system. Furthermore, we discuss what it takes to become an endosymbiont.

Reprogramming of plant cells by filamentous plant-colonizing microbes

Although phylogenetically unrelated, filamentous oomycetes and fungi establish similar structures to colonize plants and they represent economically the most important microbial threat to crop production. In mutualistic interactions established by root-colonizing fungi, clear differences to pathogens can be seen, but there is mounting evidence that their infection strategies and molecular interactions have certain common features. To infect the host, fungi and oomycetes employ similar strategies to circumvent plant innate immunity. This process involves the suppression of basal defence responses which are triggered by the perception of conserved molecular patterns. To establish biotrophy, effector proteins are secreted from mutualistic and pathogenic microbes to the host tissue, where they play central roles in the modulation of host immunity and metabolic reprogramming of colonized host tissues. This review article discusses key effector mechanisms of filamentous pathogens and mutualists, how they modulate their host targets and the fundamental differences or parallels between these different interactions. The orchestration of effector actions during plant infection and the importance of their localization within host tissues are also discussed.

Through the doors of perception to function in arbuscular mycorrhizal symbioses

The formation of an arbuscular mycorrhizal (AM) symbiosis is initiated by the bidirectional exchange of diffusible molecules. While strigolactone hormones, secreted from plant roots,stimulate hyphal branching and fungal metabolism, fungal short-chain chitin oligomers as well assulfated and nonsulfated lipochitooligosaccharides (s/nsMyc-LCOs) elicit pre-symbiosis responses in the host. Fungal LCO signals are structurally related to rhizobial Nod-factor LCOs. Genome-wide expression studies demonstrated that defined sets of genes were induced by Nod-, sMyc- and nsMyc-LCOs, indicating LCO-specific perception in the pre-symbiosis phase. During hyphopodium formation and the subsequent root colonization, cross-talk between plant roots and AM fungi also involves phytohormones. Notably, gibberellins control arbuscule formation via DELLA proteins, which themselves serve as positive regulators of arbuscule formation. The establishment of arbuscules is accompanied by a substantial transcriptional and post-transcriptional reprogramming of host roots, ultimately defining the unique protein composition of arbuscule-containing cells. Based on cellular expression profiles, key check points of AM development as well as candidate genes encoding transcriptional regulators and regulatory microRNAs were identified. Detailed functional analyses of promoters specified short motifs sufficient for cell-autonomous gene regulation in cells harboring arbuscules, and suggested simultaneous, multi-level regulation of the mycorrhizal phosphate uptake pathway by integrating AM symbiosis and phosphate starvation response signaling.

Differential activity of Striga hermonthica seed germination stimulants and Gigaspora rosea hyphal branching factors in rice and their contribution to underground communication

Strigolactones (SLs) trigger germination of parasitic plant seeds and hyphal branching of symbiotic arbuscular mycorrhizal (AM) fungi. There is extensive structural variation in SLs and plants usually produce blends of different SLs. The structural variation among natural SLs has been shown to impact their biological activity as hyphal branching and parasitic plant seed germination stimulants. In this study, rice root exudates were fractioned by HPLC. The resulting fractions were analyzed by MRM-LC-MS to investigate the presence of SLs and tested using bioassays to assess their Striga hermonthica seed germination and Gigaspora rosea hyphal branching stimulatory activities. A substantial number of active fractions were revealed often with very different effect on seed germination and hyphal branching. Fractions containing (-)-orobanchol and ent-2'-epi-5-deoxystrigol contributed little to the induction of S. hermonthica seed germination but strongly stimulated AM fungal hyphal branching. Three SLs in one fraction, putative methoxy-5-deoxystrigol isomers, had moderate seed germination and hyphal branching inducing activity. Two fractions contained strong germination stimulants but displayed only modest hyphal branching activity. We provide evidence that these stimulants are likely SLs although no SL-representative masses could be detected using MRM-LC-MS. Our results show that seed germination and hyphal branching are induced to very different extents by the various SLs (or other stimulants) present in rice root exudates. We propose that the development of rice varieties with different SL composition is a promising strategy to reduce parasitic plant infestation while maintaining symbiosis with AM fungi.

Phytohormone signaling in arbuscular mycorhiza development

To establish arbuscular mycorhiza (AM) symbiosis glomeromycotan fungi colonize the interior of roots. This process is associated with developmental changes of root cells as well as fungal hyphae. The formation of fungal colonization-structures and the extent of root colonization are largely under plant control, depending on environmental conditions and the resulting physiological state of the host. Phytohormone signaling pathways are currently emerging as important regulators of AM development. Root exuded strigolactones activate AM fungi before colonization and a host strigolactone receptor component is required for AM development. Auxin quantitatively influences AM colonization and might perform an additional cell-autonomous function in the promotion of arbuscule development. Gibberellin signaling inhibits AM and conversely DELLA proteins are required for AM formation. Given the importance of phytohormone signaling in plant developmental responses to the environment it can be predicted that elucidating how phytohormones regulate AM development will provide a lead into understanding how plants orchestrate AM symbiosis with their physiological needs under changing environmental conditions.

Strigolactones inhibit caulonema elongation and cell division in the moss Physcomitrella patens

In vascular plants, strigolactones (SLs) are known for their hormonal role and for their role as signal molecules in the rhizosphere. SLs are also produced by the moss Physcomitrella patens, in which they act as signaling factors for controlling filament extension and possibly interaction with neighboring individuals. To gain a better understanding of SL action at the cellular level, we investigated the effect of exogenously added molecules (SLs or analogs) in moss growth media. We used the previously characterized Ppccd8 mutant that is deficient in SL synthesis and showed that SLs affect moss protonema extension by reducing caulonema cell elongation and mainly cell division rate, both in light and dark conditions. Based on this effect, we set up bioassays to examine chemical structure requirements for SL activity in moss. The results suggest that compounds GR24, GR5, and 5-deoxystrigol are active in moss (as in pea), while other analogs that are highly active in the control of pea branching show little activity in moss. Interestingly, the karrikinolide KAR1, which shares molecular features with SLs, did not have any effect on filament growth, even though the moss genome contains several genes homologous to KAI2 (encoding the KAR1 receptor) and no canonical homologue to D14 (encoding the SL receptor). Further studies should investigate whether SL signaling pathways have been conserved during land plant evolution.

New strigolactone analogs as plant hormones with low activities in the rhizosphere

Strigolactones (SLs) are known not only as plant hormones, but also as rhizosphere signals for establishing symbiotic and parasitic interactions. The design of new specific SL analogs is a challenging goal in understanding the basic plant biology and is also useful to control plant architectures without favoring the development of parasitic plants. Two different molecules (23 (3'-methyl-GR24), 31 (thia-3'-methyl-debranone-like molecule)) already described, and a new one (AR36), for which the synthesis is presented, are biologically compared with the well-known GR24 and the recently identified CISA-1. These different structures emphasize the wide range of parts attached to the D-ring for the bioactivity as a plant hormone. These new compounds possess a common dimethylbutenolide motif but their structure varies in the ABC part of the molecules: 23 has the same ABC part as GR24, while 31 and AR36 carry, respectively, an aromatic ring and an acyclic carbon chain. Detailed information is given for the bioactivity of such derivatives in strigolactone synthesis or in perception mutant plants (pea rms1 and rms4, Arabidopsis max2 and, max4) for different hormonal functions along with their action in the rhizosphere on arbuscular mycorrhizal hyphal growth and parasitic weed germination.

Membrane transporters and drought resistance - a complex issue

Land plants have evolved complex adaptation strategies to survive changes in water status in the environment. Understanding the molecular nature of such adaptive changes allows the development of rapid innovations to improve crop performance. Plant membrane transport systems play a significant role when adjusting to water scarcity. Here we put proteins participating in transmembrane allocations of various molecules in the context of stomatal, cuticular, and root responses, representing a part of the drought resistance strategy. Their role in the transport of signaling molecules, ions or osmolytes is summarized and the challenge of the forthcoming research, resulting from the recent discoveries, is highlighted.