Microorganisms form diverse communities in virtually every environment on Earth, playing essential roles in ecosystem maintenance, plant growth promotion, nutrient cycling, and pathogen suppression. Among these interactions, bacterial–fungal interactions (BFIs) are particularly important in shaping the composition and function of microbial communities. BFIs encompass a wide range of relationships, from mutualistic and commensal associations to antagonistic interactions, and their significance has been recognized across agriculture, ecology, and medicine. Until now, studies on BFIs have often focused on individual pairings or specific model systems. In contrast, cross-comparative analyses involving phylogenetically diverse bacteria and filamentous fungi remain scarce, and the underlying mechanisms of these interactions are still not fully understood. In this article, we first provide an overview of BFIs, introduce our own research examples, and finally discuss future directions in the field.

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[in Japanese]

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[in Japanese]

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Lettuce black root rot disease caused by Berkeleyomyces rouxiae occurred in Japan recently. B. rouxiae had been reported in Japan as a causal agent of various plant species before the occurrence of lettuce black root rot. However, virulence of Japanese isolates from non-lettuce plants on lettuce was slight. These isolates never acquired high virulence even after repetitive inoculation and reisolation on lettuce plants. Moreover, phylogenetic analysis using microsatellite sequences revealed that isolates of lettuce black root rot pathogen are distinct not only from Japanese non-lettuce isolates, but also from American isolates that are pathogenic on lettuce. Therefore, the infection source of lettuce black root rot occurring in Japan remains unknown. Highly sensitive lettuce cultivars have been cultivated in Japan for several decades. We infer that some change in the pathogens induced the occurrence of lettuce black root rot in Japan. Salinas-type lettuce cultivars with high sensitivity have been increasing since approximately 2000. This increase might be involved with the disease occurrence. Additional research must be conducted to elucidate the occurrence of lettuce black root rot in Japan.

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Studies have demonstrated a large dependence of sweet potatoes on arbuscular mycorrhizal fungi (AMF), highlighting the growth-enhancing effects of AMF on sweet potato plants. However, compared to other annual upland crops, such as soybean, maize, and Welsh onion, there are limited studies on mycorrhizal associations with sweet potatoes. Therefore, further accumulation of fundamental ecological data is necessary. In studies on endophytic diazotrophic bacteria (EDB) in sweet potato plants, a considerable number of isolates and genetical analysis data have been obtained. Inoculation studies using these EDB isolates on sweet potato plants have shown some growth-promoting and yield-enhancing effects. Additionally, endophytic bacterial antagonistic functions have been observed, increasing the disease amelioration evidence. Further EDB inoculation experiments are required to develop technologies that utilize EDB. We conducted a co-inoculation pot experiment using AMF and EDB during the early growth stages of sweet potato plants. AMF inoculation clearly promoted the early growth of sweet potato plants. However, no significant differences between plants with and without EDB inoculation were observed. We found that the effect of EDB inoculation, following initial AMF inoculation, on root dry weight tended to differ among strains, warranting further study. We propose that sweet potato plants are unique materials for soil microbiological research, and we expect to accelerate further studies to develop and deepen our understanding of both AMF and EDB in the ecology of sweet potatoes.

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[in Japanese]

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