Asymbiotic mass production of the arbuscular mycorrhizal fungus Rhizophagus clarus

Arbuscular mycorrhizal (AM) symbiosis is a mutually beneficial interaction between fungi and land plants and promotes global phosphate cycling in terrestrial ecosystems. AM fungi are recognised as obligate symbionts that require root colonisation to complete a life cycle involving the production of propagules, asexual spores. Recently, it has been shown that Rhizophagus irregularis can produce infection-competent secondary spores asymbiotically by adding a fatty acid, palmitoleic acid. Furthermore, asymbiotic growth can be supported using myristate as a carbon and energy source for their asymbiotic growth to increase fungal biomass. However, the spore production and the ability of these spores to colonise host roots were still limited compared to the co-culture of the fungus with plant roots. Here we show that a combination of two plant hormones, strigolactone and jasmonate, induces the production of a large number of infection-competent spores in asymbiotic cultures of Rhizophagus clarus HR1 in the presence of myristate and organic nitrogen. Inoculation of asymbiotically-generated spores promoted the growth of host plants, as observed for spores produced by symbiotic culture system. Our findings provide a foundation for the elucidation of hormonal control of the fungal life cycle and the development of inoculum production schemes.

Elucidating the Mechanisms Underlying Enhanced Drought Tolerance in Plants Mediated by Arbuscular Mycorrhizal Fungi

Plants are often subjected to various environmental stresses during their life cycle, among which drought stress is perhaps the most significant abiotic stress limiting plant growth and development. Arbuscular mycorrhizal (AM) fungi, a group of beneficial soil fungi, can enhance the adaptability and tolerance of their host plants to drought stress after infecting plant roots and establishing a symbiotic association with their host plant. Therefore, AM fungi represent an eco-friendly strategy in sustainable agricultural systems. There is still a need, however, to better understand the complex mechanisms underlying AM fungi-mediated enhancement of plant drought tolerance to ensure their effective use. AM fungi establish well-developed, extraradical hyphae on root surfaces, and function in water absorption and the uptake and transfer of nutrients into host cells. Thus, they participate in the physiology of host plants through the function of specific genes encoded in their genome. AM fungi also modulate morphological adaptations and various physiological processes in host plants, that help to mitigate drought-induced injury and enhance drought tolerance. Several AM-specific host genes have been identified and reported to be responsible for conferring enhanced drought tolerance. This review provides an overview of the effect of drought stress on the diversity and activity of AM fungi, the symbiotic relationship that exists between AM fungi and host plants under drought stress conditions, elucidates the morphological, physiological, and molecular mechanisms underlying AM fungi-mediated enhanced drought tolerance in plants, and provides an outlook for future research.

Colonization of mutualistic mycorrhizal and parasitic blast fungi requires OsRAM2-regulated fatty acid biosynthesis in rice

Arbuscular mycorrhizal (AM) fungi form a mutual association with the majority of land plants, including most angiosperms of the dicotyledon and monocotyledon lineages. The symbiosis is based upon bidirectional nutrient exchange between the host and symbiont that occurs between inner cortical cells of the root and branched AM hyphae called arbuscules that develop within these cells. Lipid transport and its regulation during the symbiosis have been intensively investigated in dicotyledon plants, especially legumes. Here, we characterize OsRAM2 and OsRAM2L, homologs of M. truncatula RAM2, and found that plants defective in OsRAM2 were unable to be colonized by AM fungi and showed impaired colonization by Magnaporthe oryzae. The induction of OsRAM2 and OsRAM2L is dependent on OsRAM1 and the CSSP pathway genes CCaMK and CYCLOPS, while overexpression of OsRAM1 results in increased expression of OsRAM2 and OsRAM2L. Collectively, our data show that the function and regulation of OsRAM2 is conserved in monocot and dicot plants and reveals that, similar to mutualistic fungi, pathogenic fungi have recruited RAM2-mediated fatty acid biosynthesis to facilitate invasion.

Field Inoculation of Arbuscular Mycorrhizal Fungi Improves Fruit Quality and Root Physiological Activity of Citrus

Soil arbuscular mycorrhizal (AM) fungi form a mutualistic symbiosis with plant roots and produce many benefits on host plants under potted conditions, while field inoculation of AM fungi on citrus (a woody plant) has been rarely reported. The present study aimed to analyze the changes in mycorrhizal growth, root vitality, and fruit quality of Citrus reticulata Blanco var. Ponkan mandarin cv. Jinshuigan grafted on Poncirus trifoliata L. after inoculation with a mix of AM fungi (Diversispora versiformis, Funneliformis mosseae, and Rhizophagus intraradices) and single F. mosseae. After the second year of AM fungal inoculations, root mycorrhizal colonization (%), root vitality, hyphal length in soil, and easily extractable glomalin-related soil protein content were significantly increased, while difficult-to-extract glomalin-related soil protein content was decreased. Two mycorrhizal fungal inoculation treatments collectively improved fruit quality parameters such as polar diameter, equatorial diameter, the weight of single fruits, fruit peel, and sarcocarp, coloration value, and soluble solids content. Our study, therefore, suggested that field inoculation with AM fungi improved root physiological activities in terms of mycorrhizal growth and root vitality and thus improved fruit quality. The effect of mixed-AM treatment was more significant than that of F. mosseae alone.

Hormonomic Changes Driving the Negative Impact of Broomrape on Plant Host Interactions with Arbuscular Mycorrhizal Fungi

Belowground interactions of plants with other organisms in the rhizosphere rely on extensive small-molecule communication. Chemical signals released from host plant roots ensure the development of beneficial arbuscular mycorrhizal (AM) fungi which in turn modulate host plant growth and stress tolerance. However, parasitic plants have adopted the capacity to sense the same signaling molecules and to trigger their own seed germination in the immediate vicinity of host roots. The contribution of AM fungi and parasitic plants to the regulation of phytohormone levels in host plant roots and root exudates remains largely obscure. Here, we studied the hormonome in the model system comprising tobacco as a host plant, Phelipanche spp. as a holoparasitic plant, and the AM fungus Rhizophagus irregularis. Co-cultivation of tobacco with broomrape and AM fungi alone or in combination led to characteristic changes in the levels of endogenous and exuded abscisic acid, indole-3-acetic acid, cytokinins, salicylic acid, and orobanchol-type strigolactones. The hormonal content in exudates of broomrape-infested mycorrhizal roots resembled that in exudates of infested non-mycorrhizal roots and differed from that observed in exudates of non-infested mycorrhizal roots. Moreover, we observed a significant reduction in AM colonization of infested tobacco plants, pointing to a dominant role of the holoparasite within the tripartite system.

Impact of Organic and Inorganic Sources of Nutrients on Root Architecture, Soil Microbial Biomass and Yield on Low Land Rice Ecosystem

To study the impact of vermicompost, arbuscular mycorrhizae and FYM application on the rice ecosystem at low land, a field experiment was conducted with rice CO(R) 51 at the Department of Soil Science and Agricultural Chemistry, Tamil Nadu Agricultural University in Coimbatore during the winter of 2020. The experiment was framed in Randomized Block Design comprising of 8 treatments viz., Recommended Dose of Fertilizer Soil Test Crop Response approach (T1), RDF 75 % + Farm Yard Manure @ 12.5 t ha-1 (T2), T2 + Seed treatment with Azospirillum and Phosphobacteria + Soil application of AM fungi (T3), RDF 75 % + Vermicompost @ 5 t ha-1 (T4), T4 + Seed treatment with Azospirillum and Phosphobacteria + Soil application of AM fungi (T5), FYM @ 12.5 t ha-1 + Seed treatment with Azospirillum and Phosphobacteria + Soil application of AM fungi (T6), Vermicompost @ 5 t ha-1+ Seed treatment with Azospirillum and Phosphobacteria + Soil application of AM fungi (T7) and Absolute control (T8) and replicated thrice. The maximum microbial population were registered in the plots that received integrated nutrient application of RDF 75 % STCR approach + Vermicompost 5 t ha-1 + seed treatment with Azospirillum and Phosphobacteria + Soil application of AM fungi. Rice root architecture has changed significantly as a result of mycorrhizal inoculation. Mycorrhizal rice plants have more root volume, length, and spread than plants without mycorrhizae. Nutrient retention and availability influenced the presence of microbial-mediated metabolic activities and nutrient transformations during crop growth. Bacteria, fungus, and actinomycetes became less abundant as the crop reached harvest. The population density of mycorrhizospheres that utilize both organic and inorganic fertilizers is higher. The treatments that received Vermicompost or FYM with Vesicular Arbuscular Mycorrhizae and Nitrogen, Phosphorous & Potassium fertilizers obtained the highest yields of rice grain and straw (6740 and 7840 kg ha-1) respectively, and it was clear that the combination of Vermicompost or FYM, VAM and along with NPK fertilizers produced significantly higher yields than their individual applications and absolute control.

Endophytic fungal association in roots of exotic medicinal plants cultivated in the Nilgiris, Western Ghats, Peninsular India

Algunos microbios beneficiosos para el suelo ayudan en el establecimiento y crecimiento de plantas medicinales exóticas. Por lo tanto, evaluamos la presencia y el estado de la asociación de endófitos de raíces [hongos micorrízicos arbusculares (AM) y hongos endofíticos septados oscuros (DSE)] en diez especies de plantas medicinales exóticas cultivadas en Nilgiris de los Ghats occidentales. El alcance de las variables endófitas de hongos y las características del pelo de la raíz difirieron significativamente entre las plantas medicinales. Se identificaron seis morfotipos de esporas de hongos AM en las muestras de suelo. Por lo tanto, este estudio indicó la asociación de plantas medicinales exóticas con hongos nativos AM y DSE que podrían explotarse para promover el crecimiento y aumentar la producción de metabolitos secundarios en estas especies de plantas. Some soil beneficial microbes help in the establishment and growth of exotic medicinal plants. Therefore, we evaluated the presence and status of root endophyte [arbuscular mycorrhizal (AM) fungi and dark septate endophytic (DSE) fungi] association in ten exotic medicinal plant species cultivated in the Nilgiris of the Western Ghats. The AM fungi colonized all the examined plant species and eight plants had the co-occurrence of DSE fungi. The extent of fungal endophyte variables and root hair characteristics significantly differed among the medicinal plants. Six AM fungal spore morphotypes were identified in the soil samples. Thus, this study indicated the association of exotic medicinal plants with native AM and DSE fungi which could be exploited to promote growth and increase secondary metabolite production in these plant species.

Breeding Practice Improves the Mycorrhizal Responsiveness of Cotton (Gossypium spp. L.)

Maximizing the function of indigenous arbuscular mycorrhizal (AM) fungi by choosing specific crop genotypes offers one of the few untapped opportunities to improve the sustainability of agriculture. In this study, the differences in mycorrhizal responsiveness (MR) in plant growth and shoot phosphorus (P) content among cotton (Gossypium spp. L.) genotypes from different release dates were compared and then the relationships between MR and P uptake-related traits were determined. The experimental design in a greenhouse included 24 genotypes released from 1950 to present in Xinjiang Province, inoculation with or without AM fungi, and P levels (15 and 150 mg P kg–1 added as KH2PO4). Results showed that the modern cotton genotypes exhibited a higher degree of mycorrhizal colonization, the hyphal length density (HLD), and mycorrhizae-induced changes in shoot growth than the old genotypes when inoculated with indigenous AM fungi at both the P levels. Moreover, MR was highly correlated with the HLD at low P levels and the HLD may provide useful insights for future cotton breeding aimed at delivering crop genotypes that can benefit more from AM fungi.

AM Fungi Endow Greater Plant Biomass and Soil Nutrients under Interspecific Competition Rather Than Nutrient Releases for Litter

Plant competition affects belowground ecological processes, such as litter decomposition and nutrient release. Arbuscular mycorrhizal (AM) fungi play an essential role in plant growth and litter decomposition potentially. However, how plant competition affects the nutrient release of litter through AM fungi remains unclear especially for juvenile plants. In this study, a competitive potting experiment was conducted using juvenile seedlings of Broussonetia papyrifera and Carpinus pubescens from a karst habitat, including the intraspecific and interspecific competition treatments. The seedlings were inoculated by AM fungus or not inoculated, and the litter mixtures of B. papyrifera and C. pubescens were added into the soil or not added. The results were as follows: Litter addition significantly increased the root mycorrhizal colonization of two species in intraspecific competition. AM fungus significantly increased the biomass of B. papyrifera seedings and nitrogen release and decreased nitrogen concentration and N/P ratio of litter and further improved the total nitrogen and N/P ratio of soil under litter. The interspecific competition interacting with AM fungus was beneficial to the biomass accumulation of B. papyrifera and improvement of soil nutrients under litter. However, intraspecific competition significantly promoted nutrient releases via AM fungus. In conclusion, we suggest that AM fungi endow greater plant biomass and soil nutrients through interspecific competition, while intraspecific competition prefers to release the nutrients of litter.