α-Tomatine gradient across artificial roots recreates the recruitment of tomato root-associated Sphingobium.

α-Tomatine is a major saponin that accumulates in tomatoes (Solanum lycopersicum). We previously reported that α-tomatine secreted from tomato roots modulates root-associated bacterial communities, particularly by enriching the abundance of Sphingobium belonging to the family Sphingomonadaceae. To further characterize the α-tomatine-mediated interactions between tomato plants and soil bacterial microbiota, we first cultivated tomato plants in pots containing different microbial inoculants originating from three field soils. Four bacterial genera, namely, Sphingobium, Bradyrhizobium, Cupriavidus, and Rhizobacter, were found to be commonly enriched in tomato root-associated bacterial communities. We constructed a pseudo-rhizosphere system using a mullite ceramic tube as an artificial root to investigate the influence of α-tomatine in modifying bacterial communities. The addition of α-tomatine from the artificial root resulted in the formation of a concentration gradient of α-tomatine that mimicked the tomato rhizosphere, and distinctive bacterial communities were observed in the soil close to the artificial root. Sphingobium was enriched according to the α-tomatine concentration gradient, whereas Bradyrhizobium, Cupriavidus, and Rhizobacter were not enriched in α-tomatine-treated soil. The tomato root-associated bacterial communities were similar to the soil bacterial communities in the vicinity of artificial root-secreting exudates; however, hierarchical cluster analysis revealed a distinction between root-associated and pseudo-rhizosphere bacterial communities. These results suggest that the pseudo-rhizosphere device at least partially creates a rhizosphere environment in which α-tomatine enhances the abundance of Sphingobium in the vicinity of the root. Enrichment of Sphingobium in the tomato rhizosphere was also apparent in publicly available microbiota data, further supporting the tight association between tomato roots and Sphingobium mediated by α-tomatine.

Tomato root-associated Sphingobium harbors genes for catabolizing toxic steroidal glycoalkaloids.

IMPORTANCE: Saponins are a group of plant specialized metabolites with various bioactive properties, both for human health and soil microorganisms. Our previous works demonstrated that Sphingobium is enriched in both soils treated with a steroid-type saponin, such as tomatine, and in the tomato rhizosphere. Despite the importance of saponins in plant-microbe interactions in the rhizosphere, the genes involved in the catabolism of saponins and their aglycones (sapogenins) remain largely unknown. Here we identified several enzymes that catalyzed the degradation of steroid-type saponins in a Sphingobium isolate from tomato roots, RC1. A comparative genomic analysis of Sphingobium revealed the limited distribution of genes for saponin degradation in our saponin-degrading isolates and several other isolates, suggesting the possible involvement of the saponin degradation pathway in the root colonization of Sphingobium spp. The genes that participate in the catabolism of sapogenins could be applied to the development of new industrially valuable sapogenin molecules.

Plant specialized metabolites in the rhizosphere of tomatoes: secretion and effects on microorganisms.

Plants interact with microorganisms in the phyllosphere and rhizosphere. Here the roots exude plant specialized metabolites (PSMs) that have diverse biological and ecological functions. Recent reports have shown that these PSMs influence the rhizosphere microbiome, which is essential for the plant's growth and health. This review summarizes several specialized metabolites secreted into the rhizosphere of the tomato plant (Solanum lycopersicum), which is an important model species for plant research and a commercial crop. In this review, we focused on the effects of such plant metabolites on plant-microbe interactions. We also reviewed recent studies on improving the growth of tomatoes by analyzing and reconstructing the rhizosphere microbiome and discussed the challenges to be addressed in establishing sustainable agriculture.

Tomato roots secrete tomatine to modulate the bacterial assemblage of the rhizosphere.

Saponins are the group of plant specialized metabolites which are widely distributed in angiosperm plants and have various biological activities. The present study focused on α-tomatine, a major saponin present in tissues of tomato (Solanum lycopersicum) plants. α-Tomatine is responsible for defense against plant pathogens and herbivores, but its biological function in the rhizosphere remains unknown. Secretion of tomatine was higher at the early growth than the green-fruit stage in hydroponically grown plants, and the concentration of tomatine in the rhizosphere of field-grown plants was higher than that of the bulk soil at all growth stages. The effects of tomatine and its aglycone tomatidine on the bacterial communities in the soil were evaluated in vitro, revealing that both compounds influenced the microbiome in a concentration-dependent manner. Numerous bacterial families were influenced in tomatine/tomatidine-treated soil as well as in the tomato rhizosphere. Sphingomonadaceae species, which are commonly observed and enriched in tomato rhizospheres in the fields, were also enriched in tomatine- and tomatidine-treated soils. Moreover, a jasmonate-responsive ETHYLENE RESPONSE FACTOR 4 mutant associated with low tomatine production caused the root-associated bacterial communities to change with a reduced abundance of Sphingomonadaceae. Taken together, our results highlight the role of tomatine in shaping the bacterial communities of the rhizosphere and suggest additional functions of tomatine in belowground biological communication.

[Natori EI project: the early intervention project for psychiatric disorders by the Miyagi Psychiatric Center].

Attempts to promote early intervention (EI) for psychiatric disorders are becoming accepted worldwide. Although several attempts at EI have begun in Japan, this movement is still limited, and the development of concrete EI services suited to individual regions is required. At the Miyagi Psychiatric Center, the "Natori EI project" is being carried out with the aim of improving the mental health of young people. This project involves three activities: consultation and mental health promotion in high schools, specialized outpatient clinics for young people, and psychosocial intervention for first-episode psychosis. There are many difficulties in building a system to support this kind of EI within the framework of the conventional medical care system; it is necessary to sort out issues such as collaboration with government and educational institutions, sharing the basic principle of EI, medical economic problems, ensuring manpower, and staff training system.