The plant root-associated environments such as the rhizosphere, rhizoplane, and endosphere are different from the outer soil region (bulk soil). They establish characteristic conditions including microbiota, metabolites, and minerals, and they can directly affect plant growth and development. However, comprehensive insights into those characteristic environments, especially the rhizosphere, and molecular mechanisms of their formation are not well understood. In the present study, we investigated the spatiotemporal dynamics of the root-associated environment in actual field conditions by multi-omics analyses (mineral, microbiome, and transcriptome) of soybean plants. Mineral and microbiome analyses demonstrated a characteristic rhizosphere environment in which most of the minerals were highly accumulated and bacterial communities were distinct from those in the bulk soil. Mantel’s test and co-abundance network analysis revealed that characteristic community structures and dominant bacterial taxa in the rhizosphere significantly interact with mineral contents in the rhizosphere, but not in the bulk soil. Our field multi-omics analysis suggests a rhizosphere-specific close association between the microbiota and mineral environment.

Water containing ultrafine/nano bubbles (UFBs) promoted the growth of tomato (Solanum lycopersicum) in soil damaged by cultivation of tomato in the previous year or bacterial wilt-like disease, and also promoted the growth of lettuce (Lactuca sativa) when lettuce was grown in the soil damaged by repeated cultivation of lettuce. On the other hand, UFB supply did not affect plant growth in rock wool or healthy soil. Furthermore, the growth of lettuce was not affected by UFB water treatment in the soil damaged by the cultivation of tomato. UFB water partly suppressed the growth of the pathogen of bacteria wilt disease, Ralstonia solanacearum in vitro. These data suggest that UFB water is effective to recover the plant growth from soil damage.

L-Canavanine, a conditionally essential non-proteinogenic amino acid analog to L-arginine, plays important roles in cell division, wound healing, immune function, the release of hormones, and a precursor for the synthesis of nitric oxide (NO). In this report, we found that the L-canavanine is released into the soil from the roots of hairy vetch (Vicia villosa) and declines several weeks after growth, while it was absent in bulk proxy. Hairy vetch root was able to exudate L-canavanine in both pots and in vitro conditions in an agar-based medium. The content of the L-canavanine in pots and agar conditions was higher than the field condition. It was also observed that the addition of L-canavanine significantly altered the microbial community composition and diversity in soil. Firmicutes and Actinobacteria became more abundant in the soil after the application of L-canavanine. In contrast, Proteobacteria and Acidobacteria populations were decreased by higher L-canavanine concentration (500 nmol/g soil). Prediction of the soil metabolic pathways using PICRUSt2 estimated that the L-arginine degradation pathway was enriched 1.3-fold when L-canavanine was added to the soil. Results indicated that carbon metabolism-related pathways were altered and the degradation of nitrogen-rich compounds (i.e., amino acids) enriched. The findings of this research showed that secretion of the allelochemical L-canavanine from the root of hairy vetch may alter the soil microbial community and soil metabolite pathways to increase the survival chance of hairy vetch seedlings. This is the first report that L-canavanine acts as an allelochemical that affects the biodiversity of soil microbial community.

In a natural ecosystem, endophytic fungi in the internal tissues of plants help to improve the growth of the host plants and to decrease the negative effects of biotic and abiotic stresses without having adverse effects. In Japan, Vicia villosa (hairy vetch), a legume plant with a high capacity to fix nitrogen, is usually used as a cover crop before soybeans to enhance the fertility and structure of the soil. This study aimed to isolate endophytic fungi associated with different tissues of hairy vetch and to evaluate their potential for growth-promoting and biocontrol effects in plants. Thirty-three fungal endophytes belonging to Ascomycota and Basidiomycota were isolated from the leaf, stem, and root tissues of hairy vetch grown under both greenhouse and field conditions. The highest colonization frequency in both the greenhouse and field-grown hairy vetch plants was obtained from the root tissues. All isolates were checked for indole-3-acetic acid (IAA) and siderophore production. The maximum IAA content in the culture filtrate (4.21 μg mL−1) was produced by the isolate hvef7 (Cladosporium cladosporioides), followed by hvef18 (Penicillium simplicissimum) (3.02 μg mL−1) and hvef1 (Cladosporium pseudocladosporioides) (2.32 μg mL−1). Nineteen isolates among a total of thirty-three isolates produced siderophores. Moreover, some of the isolated strains could solubilize phosphate and potassium. Most of the isolates showed antagonistic potential against Calonectria ilicicola. The results of this study show that endophytic fungi isolated from hairy vetch have the potential for application as plant growth promotion fungi (PGPF) to promote plant growth and control disease in sustainable agriculture.