A cultivable endophytic fungus, Piriformospora indica, improves growth and enhances stress tolerance of host plants, but the underlying mechanisms remain unknown. We hypothesized that P. indica enhanced the drought tolerance of the host by regulating the antioxidant defense system and composition of fatty acids. Trifolaite orange (Poncirus trifoliata) seedlings were inoculated with P. indica under ample water and drought stress to analyze the change in plant growth, reactive oxygen species (ROS) levels, antioxidant enzyme activities, non-enzymatic antioxidant concentrations, fatty acid compositions, and expressions of antioxidant enzyme genes and fatty acid desaturase (FAD) genes. Although the 9-week soil water deficit significantly increased the colonization of P. indica to roots, P. indica still promoted the increase of shoot biomass under drought. Soil drought triggered an elevation of hydrogen peroxide in roots, while the inoculated plants had lower levels of ROS (hydrogen peroxide and superoxide anion radicals) and lower degree of membrane lipid peroxidation (based on malondialdehyde levels) under drought. Drought treatment also elevated ascorbic acid and glutathione concentrations, and the elevation was further amplified after P. indica inoculation. Inoculated plants under drought also recorded significantly higher iron-superoxide dismutase (Fe-SOD), manganese-superoxide dismutase (Mn-SOD), peroxidases, catalase, glutathione reductase, and ascorbate peroxidase activities, accompanied by up-regulation of PtFe-SOD and PtCu/Zn-SOD expressions. Inoculation with P. indica significantly increased total saturated fatty acids (e.g., C6:0, C15:0, C16:0, C23:0, and C24:0) concentration and reduced total unsaturated fatty acids (e.g., C18:1N9C, C18:2 N6, C18:3 N3, C18:1 N12, and C19:1N9T) concentration, leading to a decrease in the unsaturation index of fatty acids, which may be associated with the up-regulation of PtFAD2 and PtFAD6 and down-regulation of PtΔ9. It was concluded that the colonization of P. indica can activate enzyme and non-enzyme defense systems and regulate the composition of fatty acids under drought, thus alleviating the oxidative damage to the host caused by drought.

Many terrestrial plants form reciprocal symbioses with beneficial fungi in roots; however, it is not clear whether Vicia villosa, an important forage and green manure crop, can co-exist with these fungi and how such symbiosis affects plant growth and soil properties. The aim of this study is to analyze the effects of inoculation with three arbuscular mycorrhizal fungi (AMF) such as Diversisporaspurca, Funneliformismosseae, and Rhizophagusintraradices and an endophytic fungus Serendipitaindica on plant growth, root morphology, chlorophyll and sugar levels, soil nutrients, and aggregate size distribution and stability in V. villosa plants. After 63 days of inoculation, the beneficial fungi colonized the roots with colonization rates of 12% to 92%, and also improved plant growth performance and root morphology to varying degrees, accompanied by the most significant promoted effects after R.intraradices inoculation. All AMF significantly raised chlorophylls a and b, carotenoids and total chlorophyll concentrations, along with a significant increase in leaf sucrose, which consequently formed a significantly higher accumulation of glucose and fructose in roots providing carbon sources for the symbionts. Root fungal colonization was significantly (p < 0.01) positively correlated with chlorophyll compositions, leaf sucrose, and root glucose. In addition, inoculation with symbiotic fungi appeared to trigger a significant decrease in soil Olsen-P and available K and a significant increase in NH4-N, NO3-N, and glomalin-related soil protein levels, plus a significant increase in the proportion of water-stable aggregates at the size of 0.5–4 mm as well as aggregate stability. This improvement in soil aggregates was significantly (p < 0.01) positively correlated with root fungal colonization rate and glomalin-related soil protein concentrations. The study concludes that symbiotic fungi, especially R. intraradices, improve the growth of V. villosa, which is associated with fungal modulation of sugars, soil fertility and root structural improvement.

Monoculture frequently causes loss of soil nutrients and the emergence of soil-borne diseases in walnut orchards, whereas it is unknown whether sod culture with Vicia villosa (a popular agroforestry system) in walnut orchards impacts the structural composition and diversity of soil fungal communities. Fungal communities in walnut orchards with the cover plant V. villosa were investigated in this work utilizing high-throughput sequencing of ITS, as well as examination of root arbuscular mycorrhizal colonization and hyphal length of soil fungi. The monoculture and interplanted walnut models generated 33,511 and 34,620 effective tags with sequence similarity of 97%, respectively annotating 245 and 236 operational taxonomic units (OTUs). Among these, a total of 158 OTUs were found to be shared across monoculture and interplanted orchards. Walnuts grown in monoculture had a total of 245 species, belonging to 245 genera and 36 phyla, while walnuts with V. villosa as cover crops had 236 species, belonging to 236 genera and 19 phyla. The application of V. villosa as a cover plant significantly increased 1-Simpson and Shannon indices of soil fungi, indicating that interplanting V. villosa promoted soil fungal community diversity. Three dominant fungal phyla were detected in the soil, with Glosseromycota being the most dominant phylum. V. villosa as a cover plant significantly reduced the abundance of Funneliformis and Densospora in the soil, while it significantly increased the colonization of native arbuscular mycorrhizal fungi in roots by 94%, along with a 39% significant decrease in mycorrhizal hyphal length, as compared with the monoculture. Overall, V. villosa as a cover plant alters the composition and diversity of the soil fungal community, with reduced Funneliformis (F. geosporum) and Densospora abundance, and increased mycorrhizal colonization rate in roots, contributing to the sustainable and high-quality development of walnuts.