Per-Olof Lundquist

Interactions among Glomus irregulare, arbuscular mycorrhizal spore-associated bacteria, and plant pathogens under in vitro conditions

Arbuscular mycorrhizal (AM) fungi interact with bacteria (AM fungi-associated bacteria, AMB) in the mycorrhizosphere. We previously identified a set of AMB that enhance AM fungal colonization, plant growth, and inhibit pathogens. Here, we used transformed carrot root cultures in a two-compartment plate system for further in vitro studies on interactions taking place among Glomus irregulare (syn.Glomus intraradices), AMB, and plant pathogens. We found that exudates of G. irregulare stimulated growth of all ten AMB isolates tested in multi-well plates. AMB growth stimulation was observed also during co-cultivation of three of these AMB with G. irregulare in the hyphal compartment. In addition, co-cultivation stimulated growth of G. irregulare hyphae and spore production, as well as G. irregulare root colonization. GC/MS analysis in a preliminary screening of metabolites revealed differences in concentrations of several identified but also unidentified compounds in G. irregulare hyphal exudates. Exudates in presence of three different AMB isolates co-cultivated with G. irregulare contained several additional compounds that differed in amount compared with G. irregulare alone. The results indicate that G. irregulare exudates contain carbohydrates, amino acids, and unidentified compounds that could serve as a substrate to stimulate AMB growth. With regard to effects on plant pathogens, growth inhibition of Rhizoctonia solani, Verticillium dahliae, and Pectobacterium carotovorum ssp. carotovorum was evident in the presence of the AMB isolates tested together with the G. irregulare exudates. These in vitro studies suggest that G. irregulare and AMB stimulate growth of each other and that they together seem to provide an additive effect against growth of both fungal and bacterial pathogens.

Evidence for specificity of cultivable bacteria associated with arbuscular mycorrhizal fungal spores.

Bacteria associated with arbuscular mycorrhizal (AM) fungal spores may play functional roles in interactions between AM fungi, plant hosts and defence against plant pathogens. To study AM fungal spore-associated bacteria (AMB) with regard to diversity, source effects (AM fungal species, plant host) and antagonistic properties, we isolated AMB from surface-decontaminated spores of Glomus intraradices and Glomus mosseae extracted from field rhizospheres of Festuca ovina and Leucanthemum vulgare. Analysis of 385 AMB was carried out by fatty acid methyl ester (FAME) profile analysis, and some also identified using 16S rRNA gene sequence analysis. The AMB were tested for capacity to inhibit growth in vitro of Rhizoctonia solani and production of fluorescent siderophores. Half of the AMB isolates could be identified to species (similarity index 0.6) within 16 genera and 36 species. AMB were most abundant in the genera Arthrobacter and Pseudomonas and in a cluster of unidentified isolates related to Stenotrophomonas. The AMB composition was affected by AM fungal species and to some extent by plant species. The occurrence of antagonistic isolates depended on AM fungal species, but not plant host, and originated from G. intraradices spores. AM fungal spores appear to host certain sets of AMB, of which some can contribute to resistance by AM fungi against plant pathogens.

Carbon cost of nitrogenase activity in Frankia-Alnus incana root nodules

The carbon cost of nitrogenase activity was investigated to determine symbiotic efficiency of the actinorhizal root nodule symbiosis between the woody perennial Alnus incana and the soil bacterium Frankia. Respiration (CO2 production) and nitrogenase activity (H2 production) by intact nodulated root systems were continuously recorded in short-term assays in an open-flow gas exchange system. The assays were conducted in N2:O2, thus under N2-fixing conditions, in all experiments except for one. This avoided the declines in nitrogenase activity and respiration due to N2 deprivation that occur in acetylene reduction assays and during extended Ar:O2 exposures in H2 assays. Two approaches were used: (i) direct estimation of root and nodule respiration by removing nodules, and (ii) decreasing the partial pressure of O2 from 21 to 15% to use the strong relationship between respiration and nitrogenase activity to calculate CO2/H2. The electron allocation of nitrogenase was determined to be 0.6 and used to convert the results into moles of CO2 produced per 2e− transferred by nitrogenase to reduction of N2. The results ranged from 2.6 to 3.4 mol CO2 produced per 2e−. Carbon cost expressed as g C produced per g N reduced ranged from 4.5 to 5.8. The result for this actinorhizal tree symbiosis is in the low range of estimates for N2-fixing actinorhizal symbioses and crop legumes. Methodology and comparisons of root nodule physiology among actinorhizal and legume plants are discussed.

Role of nutrient-efficient plants for improving crop yields: bridging plant ecology, physiology, and molecular biology

The efficient use of nutrients underpins food security and reduces losses of nutrients to the environment. Different approaches have been applied to improve and evaluate the nutrient use efficiency (NUE) of plants, depending on the purpose to which the concept has been put. The various approaches differ greatly in the scale and target for NUE improvement and assessment. In this chapter, various approaches and conceptions for improving and evaluating NUE across different scales are discussed: (1) the investigation of the physiology and genetics of NUE at single tissue, cell and molecular scales, targeting understanding and improvement of NUE-related traits through, for example, plant breeding; and (2) the assessment of NUE and nutrient balances at whole plant, field, and regional scales for evaluating crops with NUE-improved traits. Appropriate methodologies are available to link the processes acting at molecular, individual plant, farm, and regional scales, but require increased collaborative efforts between the different disciplines.