Activation of Symbiosis Signaling by Arbuscular Mycorrhizal Fungi in Legumes and Rice

Establishment of arbuscular mycorrhizal interactions involves plant recognition of diffusible signals from the fungus, including lipochitooligosaccharides (LCOs) and chitooligosaccharides (COs). Nitrogen-fixing rhizobial bacteria that associate with leguminous plants also signal to their hosts via LCOs, the so-called Nod factors. Here, we have assessed the induction of symbiotic signaling by the arbuscular mycorrhizal (Myc) fungal-produced LCOs and COs in legumes and rice (Oryza sativa). We show that Myc-LCOs and tetra-acetyl chitotetraose (CO4) activate the common symbiosis signaling pathway, with resultant calcium oscillations in root epidermal cells of Medicago truncatula and Lotus japonicus. The nature of the calcium oscillations is similar for LCOs produced by rhizobial bacteria and by mycorrhizal fungi; however, Myc-LCOs activate distinct gene expression. Calcium oscillations were activated in rice atrichoblasts by CO4, but not the Myc-LCOs, whereas a mix of CO4 and Myc-LCOs activated calcium oscillations in rice trichoblasts. In contrast, stimulation of lateral root emergence occurred following treatment with Myc-LCOs, but not CO4, in M. truncatula, whereas both Myc-LCOs and CO4 were active in rice. Our work indicates that legumes and non-legumes differ in their perception of Myc-LCO and CO signals, suggesting that different plant species respond to different components in the mix of signals produced by arbuscular mycorrhizal fungi.     

丛枝菌根共生的信号转导及其相关基因

大多数植物根系能够与某些真菌形成相互依存、互惠互利的菌根共生关系.植物在提供给丛枝菌根真菌赖以生存的碳源的同时,也通过真菌从土壤中吸取矿质营养.丛枝菌根能够促进植物生长,提高植物抗逆性和抵御外界不良环境,对提高农林业生产效率、增强生态系统稳定性及维护生物多样性具有重要的意义.菌根的形成是一系列信号分子交换传递和共生相关基因表达调控的结果.在信号转导途径中,共生受体样蛋白激酶、离子通道和钙/钙调依赖性蛋白激酶基因的表达对菌根的形成是不可或缺的.就丛枝菌根共生的信号转导机制以及信号途径中3个必需基因的结构、功能及研究现状进行了综述.     

丛枝菌根形成过程及其信号转导途径

丛枝菌根是由一类土壤中古老的丛枝菌根真菌与植物根系形成的互利互惠共生体。通过共生作用丛枝菌根真菌帮助宿主植物提高水和矿质营养（特别是磷）的吸收效率。作为回报,大约20%的光合作用产物被转移到丛枝菌根真菌中,供其完成自身的生活史。丛枝菌根形成的过程中,需要植物与丛枝菌根真菌之间进行一系列信号分子的识别、交换以及信号转导作用,这一过程由一系列植物和菌根真菌的基因控制。首先,植物会分泌一种植物激素——独角金内酯来诱导菌根真菌加速分支,而菌根真菌也会分泌脂质几丁寡糖促进植物与其形成菌根。加速分支的菌根真菌接触到植物根部以后,会附着在植物根的表皮并形成附着胞,通过附着胞穿透植物根的表皮,最后进入维管组织附近的皮层细胞并在其中不断进行二叉分支,形成特有的丛枝结构。通过对模式植物共生现象的研究,已经发现很多植物基因参与到共生形成的信号转导过程中,包括早期植物反应的基因、菌根与根瘤共生共同需要的转导因子以及菌根特异的信号分子等。本文对菌根的形成过程及信号转导途径进行详细的介绍,为人们深入研究菌根关系提供参考。     

Phosphorus and Nitrogen Regulate Arbuscular Mycorrhizal Symbiosis in Petunia hybrida

Phosphorus and nitrogen are essential nutrient elements that are needed by plants in large amounts. The arbuscular mycorrhizal symbiosis between plants and soil fungi improves phosphorus and nitrogen acquisition under limiting conditions. On the other hand, these nutrients influence root colonization by mycorrhizal fungi and symbiotic functioning. This represents a feedback mechanism that allows plants to control the fungal symbiont depending on nutrient requirements and supply. Elevated phosphorus supply has previously been shown to exert strong inhibition of arbuscular mycorrhizal development. Here, we address to what extent inhibition by phosphorus is influenced by other nutritional pathways in the interaction between Petunia hybrida and R. irregularis. We show that phosphorus and nitrogen are the major nutritional determinants of the interaction. Interestingly, the symbiosis-promoting effect of nitrogen starvation dominantly overruled the suppressive effect of high phosphorus nutrition onto arbuscular mycorrhiza, suggesting that plants promote the symbiosis as long as they are limited by one of the two major nutrients. Our results also show that in a given pair of symbiotic partners (Petunia hybrida and R. irregularis), the entire range from mutually symbiotic to parasitic can be observed depending on the nutritional conditions. Taken together, these results reveal complex nutritional feedback mechanisms in the control of root colonization by arbuscular mycorrhizal fungi.     

Identification of Membrane-Associated Proteins Regulated by the Arbuscular Mycorrhizal Symbiosis

A sub-cellular proteomic approach was carried out to monitor membrane-associated protein modifications in response to the arbuscular mycorrhizal (AM) symbiosis. Membrane proteins were extracted from Medicago truncatula roots either inoculated or not with the AM fungus Glomus intraradices. Comparative two-dimensional electrophoresis revealed that 36 spots were differentially displayed in response to the fungal colonization including 15 proteins induced, 3 up-regulated and 18 down-regulated. Among them, seven proteins were found to be commonly down-regulated in AM-colonized and phosphate-fertilized roots. Twenty-five spots out of the 36 of interest could be identified by matrix assisted laser desorption/ionisation-time of flight and/or tandem mass spectrometry analyses. Excepting an acid phosphatase and a lectin, none of them was previously reported as being regulated during AM symbiosis. In addition, this proteomic approach allowed us for the first time to identify AM fungal proteins in planta.     

植物中丛枝菌根形成的信号途径研究进展

丛枝菌根(arbuscular mycorrhizal,AM)共生是丛枝菌根真菌与大多数陆地植物的根系之间形成的一种互利共生关系。植物给菌根真菌提供碳水化合物;作为回报,菌根真菌能够增强植物对矿质营养元素(尤其是磷)的吸收。菌根的形成过程是一系列信号交换和转导的结果,具有严格并且一致的顺序。本文以植物中菌根形成的信号途径为主线,对菌根真菌的形成过程和信号转导途径及其方式进行了分析和讨论。高等植物中菌根形成的信号途径与豆科植物的结瘤信号途径部分共享,并且与钙离子信号途径相关,但前者更为广泛。尽管该途径中很多过程目前还不十分清楚,但是相信在不久的将来就可以揭开菌根形成过程中的众多谜团。     

Nonredundant Regulation of Rice Arbuscular Mycorrhizal Symbiosis by Two Members of the PHOSPHATE TRANSPORTER1 Gene Family

Pi acquisition of crops via arbuscular mycorrhizal (AM) symbiosis is becoming increasingly important due to limited high-grade rock Pi reserves and a demand for environmentally sustainable agriculture. Here, we show that 70% of the overall Pi acquired by rice (Oryza sativa) is delivered via the symbiotic route. To better understand this pathway, we combined genetic, molecular, and physiological approaches to determine the specific functions of two symbiosis-specific members of the PHOSPHATE TRANSPORTER1 (PHT1) gene family from rice, ORYsa;PHT1;11 (PT11) and ORYsa;PHT1;13 (PT13). The PT11 lineage of proteins from mono- and dicotyledons is most closely related to homologs from the ancient moss, indicating an early evolutionary origin. By contrast, PT13 arose in the Poaceae, suggesting that grasses acquired a particular strategy for the acquisition of symbiotic Pi. Surprisingly, mutations in either PT11 or PT13 affected the development of the symbiosis, demonstrating that both genes are important for AM symbiosis. For symbiotic Pi uptake, however, only PT11 is necessary and sufficient. Consequently, our results demonstrate that mycorrhizal rice depends on the AM symbiosis to satisfy its Pi demands, which is mediated by a single functional Pi transporter, PT11.     

Translocation and Utilization of Fungal Storage Lipid in the Arbuscular Mycorrhizal Symbiosis

The arbuscular mycorrhizal (AM) symbiosis is responsible for huge fluxes of photosynthetically fixed carbon from plants to the soil. Carbon is transferred from the plant to the fungus as hexose, but the main form of carbon stored by the mycobiont at all stages of its life cycle is triacylglycerol. Previous isotopic labeling experiments showed that the fungus exports this storage lipid from the intraradical mycelium (IRM) to the extraradical mycelium (ERM). Here, in vivo multiphoton microscopy was used to observe the movement of lipid bodies through the fungal colony and to determine their sizes, distribution, and velocities. The distribution of lipid bodies along fungal hyphae suggests that they are progressively consumed as they move toward growing tips. We report the isolation and measurements of expression of an AM fungal expressed sequence tag that encodes a putative acyl-coenzyme A dehydrogenase; its deduced amino acid sequence suggests that it may function in the anabolic flux of carbon from lipid to carbohydrate. Time-lapse image sequences show lipid bodies moving in both directions along hyphae and nuclear magnetic resonance analysis of labeling patterns after supplying 13C-labeled glycerol to either extraradical hyphae or colonized roots shows that there is indeed significant bidirectional translocation between IRM and ERM. We conclude that large amounts of lipid are translocated within the AM fungal colony and that, whereas net movement is from the IRM to the ERM, there is also substantial recirculation throughout the fungus.     

Role of Arbuscular Mycorrhizal Symbiosis in Proline Biosynthesis and Metabolism of Cicer arietinum L. (Chickpea) Genotypes Under Salt Stress

Salinity causes osmotic stress and negatively impacts plant growth and productivity. Proline is one of the most important osmoprotectants synthesized under stressed conditions. Accumulation of free proline occurs due to enhanced biosynthesis and repressed degradation, and both processes are controlled by feedback regulatory mechanisms. Arbuscular mycorrhizal (AM) fungi are considered to be bioameliorators of salinity stress due to their wide-ranging presence in contaminated soils and their role in modulation of biochemical processes. Chickpea is considered sensitive to salinity. However, reports on AM-induced osmoprotection through regulation of proline biosynthesis in chickpea genotypes are scant. The present study investigated the influence of AM symbiosis on proline metabolism in two chickpea (Cicer arietinum L.) genotypes (PBG-5 and CSG-9505) under salt stress and correlated the same with sodium (Na⁺) ion uptake. Salinity reduced plant biomass (roots and shoots), with roots being more negatively affected than shoots. Mycorrhizal colonization with Glomus mosseae was much stronger in PBG-5 and was correlated with reduced Na⁺ ion uptake and higher growth when compared with CSG-9505 under stressed and unstressed conditions. Mycorrhizal symbiosis with chickpea roots boosted proline biosynthesis by significantly increasing pyrroline-5-carboxylate synthetase (P-5-CS) and glutamate dehydrogenase (GDH) activities with a concomitant decline in proline dehydrogenase (ProDH) activity under salt stress. The enhancement of the activity of these enzymes was higher in PBG-5 than in CSG-9505 and could be directly correlated with the percent mycorrhizal colonization and Na⁺ uptake. The study indicated a strong role of AM symbiosis in enhancing stress tolerance in chickpea by significantly modulating proline metabolism and Na⁺ uptake.     

Two Medicago truncatula Half-ABC Transporters Are Essential for Arbuscule Development in Arbuscular Mycorrhizal Symbiosis

In the symbiotic association of plants and arbuscular mycorrhizal (AM) fungi, the fungal symbiont resides in the root cortical cells where it delivers mineral nutrients to its plant host through branched hyphae called arbuscules. Here, we report a Medicago truncatula mutant, stunted arbuscule (str), in which arbuscule development is impaired and AM symbiosis fails. In contrast with legume symbiosis mutants reported previously, str shows a wild-type nodulation phenotype. STR was identified by positional cloning and encodes a half-size ATP binding cassette (ABC) transporter of a subfamily (ABCG) whose roles in plants are largely unknown. STR is a representative of a novel clade in the ABCG subfamily, and its orthologs are highly conserved throughout the vascular plants but absent from Arabidopsis thaliana. The STR clade is unusual in that it lacks the taxon-specific diversification that is typical of the ABCG gene family. This distinct phylogenetic profile enabled the identification of a second AM symbiosis-induced half-transporter, STR2. Silencing of STR2 by RNA interference results in a stunted arbuscule phenotype identical to that of str. STR and STR2 are coexpressed constitutively in the vascular tissue, and expression is induced in cortical cells containing arbuscules. STR heterodimerizes with STR2, and the resulting transporter is located in the peri-arbuscular membrane where its activity is required for arbuscule development and consequently a functional AM symbiosis.     

Influence of Arbuscular Mycorrhizal (AM) Symbiosis on Phosphorus Leaching through Soil Cores

Two experiments with soil cores were carried out to investigate the effects of arbuscular mycorrhizal (AM) fungal colonization on mobility of phosphorus (P) during leaching of repacked columns of a soil with a loamy sand texture. Trifolium subterraneum plants inoculated with an AM fungus or not inoculated were grown in cores with low or high P concentrations for 8 or 10 weeks in the glasshouse. Cores were then irrigated with 2500 mL water and the leachate collected. Plant growth and the amounts of P removed by plants, remaining in soil as available P and removed dissolved in leachate were measured. Mycorrhizal fungal colonization and development of external hyphae were also determined. Inoculation and/or P application significantly increased plant growth and plant P removal and decreased P leaching. In low P soils AM fungal colonization significantly increased plant P uptake and decreased soil available P and total dissolved P in leachates. Lower P leaching from cores with AM plants under low P conditions was related to enhancement of plant growth and to scavenging and removal of P from the soil by roots and/or external hyphae. When P was applied AM effects were not observed and available P remaining in the soil after leaching was much higher, regardless of AM fungal colonization.     

Community Dynamics of Arbuscular Mycorrhizal Fungi in High-Input and Intensively Irrigated Rice Cultivation Systems

Application of a mycorrhizal inoculum could be one way to increase the yield of rice plants and reduce the application of fertilizer. We therefore studied arbuscular mycorrhizal fungi (AMF) in the roots of wetland rice (Oryza sativa L.) collected at the seedling, tillering, heading, and ripening stages in four paddy wetlands that had been under a high-input and intensively irrigated rice cultivation system for more than 20 years. It was found that AMF colonization was mainly established in the heading and ripening stages. The AMF community structure was characterized in rhizosphere soils and roots from two of the studied paddy wetlands. A fragment covering the partial small subunit (SSU), the whole internal transcribed spacer (ITS), and the partial large subunit (LSU) rRNA operon regions of AMF was amplified, cloned, and sequenced from roots and soils. A total of 639 AMF sequences were obtained, and these were finally assigned to 16 phylotypes based on a phylogenetic analysis, including 12 phylotypes from Glomeraceae, one phylotype from Claroideoglomeraceae, two phylotypes from Paraglomeraceae, and one unidentified phylotype. The AMF phylotype compositions in the soils were similar between the two surveyed sites, but there was a clear discrepancy between the communities obtained from root and soil. The relatively high number of AMF phylotypes at the surveyed sites suggests that the conditions are suitable for some species of AMF and that they may have an important function in conventional rice cultivation systems. The species richness of root-colonizing AMF increased with the growth of rice, and future studies should consider the developmental stages of this crop in the exploration of AMF function in paddy wetlands.     

The Ecology of Arbuscular Mycorrhizal Fungi

Arbuscular mycorrhiza is a mutually beneficial biological association between species in the fungal phylum Glomeromycota and higher plants roots. The symbiosis is thought to have afforded green plants the opportunity to invade dry land ca 450 Ma ago and the vast majority of extant terrestrial plants retain this association. Arbuscular mycorrhizal (AM) fungi perform various ecological functions in exchange for host photosynthetic carbon that almost always contribute to the fitness of hosts from an individual to community level. Recent AM fungal research, increasingly delving into the ‘Black Box’, suggests that species in this phylum may play a key facilitative role in below-ground micro- and meso-organism community dynamics, even more perhaps, that of a bioengineer. The ubiquitous nature of the symbiosis in extant flora and the fact that variations from the AM symbiosis are recent events suggest that Glomeromycota and plant roots coevolved. This review considers aspects of AM fungal ecology emphasizing past and present importance of the phylum in niche to global ecosystem function. Nutrient exchange, evolution, taxonomy, phenology, below-ground microbial interaction, propagule dissemination, invasive plants interactions, the potential role in phytoremediation and some of the factors affecting AM fungal biology are discussed. We conclude that it is essential to include AM association in any study of higher plants in natural environments in order to provide an holistic understanding of ecosystems.     

Symbiosis of Arbuscular Mycorrhizal Fungi and Robinia pseudoacacia L. Improves Root Tensile Strength and Soil Aggregate Stability

Robinia pseudoacacia L. (black locust) is a widely planted tree species on Loess Plateau for revegetation. Due to its symbiosis forming capability with arbuscular mycorrhizal (AM) fungi, we explored the influence of arbuscular mycorrhizal fungi on plant biomass, root morphology, root tensile strength and soil aggregate stability in a pot experiment. We inoculated R. pseudoacacia with/without AM fungus (Rhizophagus irregularis or Glomus versiforme), and measured root colonization, plant growth, root morphological characters, root tensile force and tensile strength, and parameters for soil aggregate stability at twelve weeks after inoculation. AM fungi colonized more than 70% plant root, significantly improved plant growth. Meanwhile, AM fungi elevated root morphological parameters, root tensile force, root tensile strength, Glomalin-related soil protein (GRSP) content in soil, and parameters for soil aggregate stability such as water stable aggregate (WSA), mean weight diameter (MWD) and geometric mean diameter (GMD). Root length was highly correlated with WSA, MWD and GMD, while hyphae length was highly correlated with GRSP content. The improved R. pseudoacacia growth, root tensile strength and soil aggregate stability indicated that AM fungi could accelerate soil fixation and stabilization with R. pseudoacacia, and its function in revegetation on Loess Plateau deserves more attention.     

Global Distributions of Arbuscular Mycorrhizal Fungi

We examined potential large-scale controls over the distribution of arbuscular mycorrhizal (AM) fungi and their host plants. Specifically, we tested the hypothesis that AM fungi should be more prevalent in biomes where nutrients are primarily present in mineral, and not organic, forms. Values of percentage root length colonized (%RLC) by AM fungi, AM abundance, and host plant availability were compiled or calculated from published studies to determine biome-level means. Altogether, 151 geographic locations and nine biomes were represented. Percent RLC differed marginally significantly among biomes and was greatest in savannas. AM abundance (defined as total standing root length colonized by AM fungi) varied 63-fold, with lowest values in boreal forests and highest values in temperate grasslands. Biomes did not differ significantly in the percentage of plant species that host AM fungi, averaging 75%. Contrary to the hypothesis, %RLC, AM abundance, and host plant availability were not related to the size, influx, or turnover rate of soil organic matter pools. Instead, AM abundance was positively correlated with standing stocks of fine roots. The global pool of AM biomass within roots might approach 1.4 Pg dry weight. We note that regions harboring the largest stocks of AM fungi are also particularly vulnerable to anthropogenic nitrogen deposition, which could potentially alter global distributions of AM fungi in the near future.     

Uptake Kinetics of Arsenic in Upland Rice Cultivar Zhonghan 221 Inoculated with Arbuscular Mycorrhizal Fungi

Arbuscular mycorrhizal fungi (AMF) appear to be highly associated with arsenic (As) uptake in host plants because arsenate (As(V)) and phosphorus (P) share the same transporter, whereby AMF can enhance P uptake. A short-term experiment was conducted for low- (0 to 0.05 mM As) and high-affinity (0 to 2.5 mM As) uptake systems, to investigate the AMF role on As uptake mechanism in plants, which may explain As uptake kinetics in upland rice cultivar: Zhonghan 221. When concentration of As ranged from 0 to 0.05 mM, Funneliformis geosporum (Fg) significantly decreased arsenite (As(III)) and monomethylarsonicacid (MMA) uptake when (p < 0.05) compared to non-mycorrhizal (NM) treatment, since the major route for (As(III)) in rice roots—rice silicon transporter Lsi1 would be influenced by Fg inoculation at high As concentrations. Fg can also reduce As(V) uptake significantly (p < 0.05) under both uptake systems relative to NM treatment, whereas, Funneliformis mosseae (Fm) increased As(V) and MMA uptake in rice roots, with MMA uptake rate generally lower than As(III) and As(V). Using suitable AMF species inoculation with rice, As uptake and accumulation in rice grains can be reduced and the risk to human health, once consumed, can be minimized.