Effects of Arbuscular Mycorrhiza on Primary Metabolites in Phloem Exudates of Plantago major and Poa annua and on a Generalist Aphid

Arbuscular mycorrhiza (AM), i.e., the interaction of plants with arbuscular mycorrhizal fungi (AMF), often influences plant growth, physiology, and metabolism. Effects of AM on the metabolic composition of plant phloem sap may affect aphids. We investigated the impacts of AM on primary metabolites in phloem exudates of the plant species Plantago major and Poa annua and on the aphid Myzus persicae. Plants were grown without or with a generalist AMF species, leaf phloem exudates were collected, and primary metabolites were measured. Additionally, the performance of M. persicae on control and mycorrhizal plants of both species was assessed. While the plant species differed largely in the relative proportions of primary metabolites in their phloem exudates, metabolic effects of AM were less pronounced. Slightly higher proportions of sucrose and shifts in proportions of some amino acids in mycorrhizal plants indicated changes in phloem upload and resource allocation patterns within the plants. Aphids showed a higher performance on P. annua than on P. major. AM negatively affected the survival of aphids on P. major, whereas positive effects of AM were found on P. annua in a subsequent generation. Next to other factors, the metabolic composition of the phloem exudates may partly explain these findings.

Involvement of MdWRKY40 in the Defense of Mycorrhizal Apple Against Fusarium Solani

Background: Apple (Malus domestica Borkh.) is an important economic crop. The pathological effects of Fusarium spp., a group of soilborne pathogens, on the root systems of apple plants was unknown. It was unclear how mycorrhizal apple seedlings resist infection by F. solani. The transcriptional profiles of mycorrhizal and non-mycorrhizal plants infected by F. solani were compared using RNA-Seq.Results: Infection with F. solani significantly reduced the dry weight of apple roots, and the roots of mycorrhizal apple plants were less damaged when the plants were infected with F. solani. They also had enhanced activity of antioxidant enzymes and a reduction in the oxidation of membrane lipids. A total of 1,839 differentially expressed genes (DEGs) were obtained after mycorrhizal and non-mycorrhizal apple plants were infected with F. solani. A gene ontogeny (GO) analysis showed that most DEGs were involved in the binding of ADP and calcium ions. In addition, the enrichment observed from an analysis with the Kyoto Encyclopedia of Genes and Genomes (KEGG) primarily involved plant-pathogen interaction and Glycolysis/Gluconeogenesis, the mitogen-activated protein kinase (MAPK) signaling pathway, and plant hormone signal transduction. Based on a MapMan analysis, an enormous number of DEGs were involved in the response of mycorrhizal plants to stress. Among them, the overexpressed transcription factor MdWRKY40 significantly improved the resistance of ‘Orin’ to F. solani and the expression of the resistance gene MdGLU by binding the promoter of MdGLU.Conclusion: This paper outlines how the inoculation of apple seedlings by arbuscular mycorrhizal fungi reduces the response to stress at the transcription level of the root system following infection with F. solani, and MdWRKY40 played an important role in the resistance of mycorrhizal apple seedlings to infection with F. solani.

Nutrient status, hydrogen peroxide content and peroxidase activity of arbuscular mycorrhizal plants of Melilotus albus grown in diesel-contaminated substrate

Background: Petroleum hydrocarbons affect plant growth, but little is known about physiological responses of mycorrhizal plants facing diesel contamination. Objective: To evaluate the effects of arbuscular mycorrhizal fungi (AMF) on the nutritional status, peroxidase activity (POX), and hydrogen peroxide content (H2O2) in leaves of Melilotus albus planted under diesel-contaminated sand (7500 mg kg-1). Methods: A 2x2 factorial experiment was set in a completely randomized design, under greenhouse conditions for 35 days. Seedlings were pre-inoculated with AMF and transplanted to sand with or without diesel, including non-AMF plants. Results and conclusions: Diesel contamination impaired plant growth; AMF plants had similar growth than non-AMF plants at diesel-contamination, but low nutrient content. Protein content decreased due to diesel in non-AMF plants, but this content was low in AMF plants regardless diesel contamination. Diesel increased POX; whereas AMF plants with or without diesel had higher POX than non-AMF plants. The H2O2 content in AMF plants with or without diesel was low than non-AMF plants. Diesel contamination diminished AMF-colonization, but AMF dissipate more diesel hydrocarbons (>40%). Overall, AMF alleviated the toxic effects of diesel on plants.

Arbuscular Mycorrhizal Fungi Enhance Sorghum Plant Growth under Nitrogen-Deficient Conditions through Activation of Nitrogen and Carbon Metabolism Enzymes

Nitrogen (N), one of the most important elements for plant growth, is needed by plants in large quantities. However, this nutrient has limited supply in the soil. Arbuscular mycorrhizal fungi (AMF) are known for their ability to form symbiotic association with plants and transfer the mineral nutrients to the host plants. To validate this hypothesis on sorghum plants, three ecotypes of this cereal (3p4, 3p9 and 4p11) were cultivated with and without AMF under low nitrogen concentration (0.5 mM NH4+). Growth parameters were determined and key enzymes responsible for nitrogen and carbon metabolisms such as glutamine synthetase (GS), glutamate dehydrogenase (GDH), phosphoenolpyruvate carboxylase (PEPC), isocitrate dehydrogenase (ICDH), malate dehydrogenase (MDH) and asparate aminotransferase (AAT) were measured. For the three sorghum ecotypes, mycorrhizal plants showed a higher plant growth compared to the control plants. The biochemical parameters revealed a significant increase in the nitrogen assimilatory enzymes; GS and GDH in the leaves and roots of mycorrhizal plants. Furthermore, mycorrhizal fungi also appear to have a significant effect on carbon assimilatory enzymes. These enzymes are known to have a cardinal role in the provision of carbon skeletons essential for the assimilation of ammonium and thus, amino acids synthesis. Our study indicates clearly that AMF can be an efficient way to optimize nitrogen uptake and/or assimilation by plants and thus improve the crop yields with lower amount of nitrogen fertilizers. © 2021 Friends Science Publishers

Breeding against mycorrhizal symbiosis: modern cotton (Gossypium hirsutum L.) varieties perform more poorly than older varieties except at very high phosphorus supply

Background and aims Cotton (Gossypium hirsutum L.) is an important cash fiber crop species, but its root traits related to phosphorus (P) acquisition have been poorly understood. Methods Eight cotton varieties that were released between 1950 and 2013 were grown in pots with or without arbuscular mycorrhizal fungi (AMF) at three P supply levels. Eleven root traits were measured and calculated after seven weeks of growth. Results At the lower two P levels mycorrhizal plants acquired more P and produced more biomass than non-mycorrhizal plants. At the highest P level mycorrhizal plants took up more P than non-mycorrhizal plants, but there was no difference in biomass. At the intermediate P level, root diameter was significantly positively correlated with biomass, P concentration and P content of mycorrhizal plants. More recent accessions had smaller root diameters, acquired less P and obtained less biomass, indicating (inadvertent) varietal selection for thinner roots that provided less cortical space for AMF, which increased the need for high P fertilizer. Conclusion Our study provides support for the importance of the outsourcing model of nutrient acquisition in the root economics space framework. Inadvertent varietal selection in the last decades, resulting in thinner roots and a lower benefit from mycorrhizal colonization, has caused a lower productivity of cotton varieties at moderate P supply, indicating the need to rethink cotton breeding efforts in order to achieve agricultural sustainability.

Mycorrhizal symbiosis primes the accumulation of antiherbivore compounds and enhances herbivore mortality in tomato

Plant association with arbuscular mycorrhizal fungi (AMF) can increase their ability to overcome multiple stresses, but their impact on plant interactions with herbivorous insects is controversial. Here we show higher mortality of the leaf-chewer Spodoptera exigua when fed on tomato plants colonized by the AMF Funneliformis mosseae, evidencing Mycorrhiza-Induced Resistance (MIR). In search of the underlying mechanisms, an untargeted metabolomic analysis through UPLC-MS was performed. The results showed that the mycorrhizal symbiosis had a very limited impact on the leaf metabolome in the absence of stress, but significantly modulated the response to herbivory in the damaged area. A cluster of overaccumulated metabolites was identified in those leaflets damaged by S. exigua feeding in mycorrhizal plants, while unwounded distal leaflets responded similarly to those from non-mycorrhizal plants. These primed-compounds were mostly related to alkaloids, fatty acid derivatives and phenylpropanoid-polyamine conjugates. The deleterious effect on larval survival of some of these compounds, including the alkaloid physostigmine, the fatty acid derivatives 4-oxododecanedioic acid and azelaic acid, was confirmed. Thus, our results evidence the AM impact on metabolic reprograming upon herbivory that leads to a primed accumulation of defensive compounds.

Effects of Funneliformis mosseae on the utilization of organic phosphorus in Camellia oleifera Abel

Arbuscular mycorrhizal (AM) fungi play an important role in the acquisition of phosphorus (P) by plants. The external hyphae of AM fungi function as an extension of plant roots and may downregulate related functions in the roots. It is not clear whether the ability of AM fungi to mineralize organic P affects root phosphatase activities. A pot experiment was conducted to investigate the effect of Funneliformis mosseae on soil organic P mineralization under phytate application and to explore root phosphatase activities, P uptake, and growth in Camellia oleifera Abel. The plants and their growth substrates were harvested 4 and 8 months after planting. The results showed that organic P application had no effect on the total dry mass of nonmycorrhizal plants, but differences in dry mass under P application were observed in mycorrhizal plants in both harvests. Inoculation with F. mosseae increased soil acid phosphatase, phytase, and alkaline phosphatase activities and reduced the soil organic P content. Mycorrhizal plants had higher root activity, shoot and root P contents and root acid phosphatase and phytase activities than nonmycorrhizal plants irrespective of organic P application. In conclusion, AM fungi enhanced the mineralization of soil organic P and positively affect root phosphatase activities.

Physiological responses of mycorrhizal symbiosis to drought stress in white clover

The aim of the present study was to analyze the effects of two arbuscular mycorrhizal fungi (AMF), Funneliformis mosseae and Paraglomus occultum, on leaf water status, root morphology, root sugar accumulation, root abscisic acid (ABA) levels, root malondialdehyde (MDA) content, and root antioxidant enzyme activities in white clover (Trifolium repens L.) exposed to well-watered (WW) and drought stress (DS) conditions. The results showed that root colonization by F. mosseae and P. occultum was significantly decreased by 7-week soil drought treatment. Under drought stress conditions, mycorrhizal fungal treatment considerably stimulated root total length, surface area and volume, as compared with non-mycorrhizal controls. In addition, inoculation with arbuscular mycorrhizal fungi also increased leaf relative water content and accelerated the accumulation of root glucose and fructose under drought stress. Mycorrhizal plants under drought stress registered higher activities of superoxide dismutase (SOD), catalase (CAT), and peroxidase (POD) and ABA levels in roots, while lower MDA contents, relative to non-mycorrhizal plants. As a result, mycorrhiza-inoculated plants represented better physiological activities (e.g. antioxidant defense systems, root morphology, and sugar accumulation) than non-inoculated plants in response to soil drought, whilst P. occultum had superior effects than F. mosseae.

Arbuscular mycorrhiza and nitrification: Competition for free ammonium ions?

<p>Arbuscular mycorrhiza (AM) is ancient and widespread inter-kingdom symbiotic relationship being established by a majority of terrestrial plant species and specialized fungi, which interconnect plant roots with surrounding soil. By doing so, this symbiosis can greatly increase acquisition of multiple mineral nutrients such as phosphorus, nitrogen (N), and copper by the plants from the soil, in exchange for reduced carbon supplied by the plant host. Supposedly, this is mainly due to extending the soil volume accessible for nutrient acquisition by the fungal hyphae compared to roots alone. Both the plants and the AM fungi require N for construction of their bodies. This can potentially result in different effects of AM symbiosis establishment on plant N nutrition ranging from positive to negative. Yet, the demand for and efficiency of mineral N uptake from the soil by a mycorrhizal plant is usually higher than that of a nonmycorrhizal plant. This may exert important feedbacks of AM symbiosis on soil processes in general and N cycling in particular. Here we asked what role does the symbiosis play in acquisition of N by a model plant, Andropogon gerardii, from an organic source (i.e., plant litter labeled with 15N) supplied in a soil zone beyond the direct reach of roots. Further, we tested whether this process of N acquisition by plant from the soil via mycorrhizal hyphae could be affected by supplying various synthetic nitrification inhibitors (DCD, nitrapyrin, or DMPP) along with the litter. We observed efficient acquisition of N to mycorrhizal plants via mycorrhizal pathway irrespective of the nitrification inhibitor supplied or not along with the plant litter. These results were strongly contrasting with 15N uptake (but not total N content of the plants or the plant biomass) of the nonmycorrhizal plants, which generally received much less 15N than the mycorrhizal plants, and this was further suppressed by nitrapyrin or DMPP supplementation of the organic N source as compared to DCD or the control (i.e., no inhibitor) treatment. Quantitative real-time PCR analyses of the microbial communities indicated that microbes involved in the rate-limiting step of nitrification, i.e., the ammonia oxidizers, were suppressed similarly by AM fungi as they were by nitrapyrin or DMPP amendments. These results suggest that mycorrhizal fungi successfully outcompeted the prokaryotic ammonia oxidizers, and this was most likely by accessing and efficiently utilizing/removing free ammonia ion pool in/from the soil via their extensive hyphal networks.</p>