The role of arbuscular mycorrhiza and organosulfur mobilizing bacteria in plant sulphur supply

Abstract

AM fungi are enhancing growth and health of many land plants but only some of these beneficial mechanisms are well understood. This study aimed to uncover the role of bacteria colonising AM fungi in organically-bound sulfur (S) mobilisation, the dominant S pools in soil that are not directly available to plants. The effect of an intact AM symbiosis with access to stable isotope organo-34S enriched soils encased in 35 µm mesh cores was tested in microcosms with Agrostis stolonifera and Plantago lanceolata. At 3 month intervals, the plant shoots were analysed for 34S uptake. After 9 months, hyphae and associated soil was picked from static (mycorrhizal) and rotating (severed hyphae) mesh cores and corresponding rhizosphere soil was sampled for bacterial analysis. AM symbiosis increased uptake of 34S from organo-34S enriched soil at early stages of plant growth when S demand appeared to be high. The static (mycorrhizal) treatments were shown to harbour larger populations of cultivable heterotrophs and sulfonate mobilising bacteria. Microbial communities were significantly different in the hyphosphere of mycorrhizal hyphae and hyphae not associated to plant hosts. Sulfate ester (arylsulfatase enzyme assay, atsA gene) and sulfonate mobilising activity (asfA gene) was altered by an intact AM symbiotic partnership which stimulated the genera Azospirillum, Burkholderia and Polaromonas. Illumina sequencing revealed that AM symbiosis led to community shifts, reduced diversity and dominance of the Planctomycetes and Proteobacteria. This study demonstrated that AM symbioses can promote organo-S mobilization and plant uptake through interaction with hyphospheric bacteria. Research highlights AM hyphae enhanced uptake of organically bound 34S at early stages of growth. AM hyphosphere harboured a large population of organo-S desulfurizing bacteria. Microbial communities significantly differed in rotating and static mesh cores. AM hyphae influenced bacterial sulfate ester and sulfonate mobilising activity. AM hyphae reduced bacterial diversity, increased Planctomycetes and Proteobacteria abundance.