Current Research

Despite lacking specialized symbiotic organs such as root nodules, sweet potato exhibits a nitrogen fixation capacity comparable to that of legumes.

Nitrogen fixation by endophytic bacteria is not limited to sweet potato; it also occurs in sugarcane, Miscanthus, and rice. In animals such as termites, nitrogen fixation is also carried out by gut-associated bacteria. Furthermore, free-living nitrogen-fixing bacteria are present in soils.

Conventional approaches based on DNA, RNA, and proteins reflect the potential function of enzymes, but not necessarily their actual activity, which depends on environmental conditions. Functional outcomes must be verified through metabolic products.

Based on functional potential, many candidate nitrogen-fixing bacteria can be identified in complex microbial communities, yet it remains unclear which bacteria are actually active in situ.

By supplying ¹⁵N₂ to sweet potato and extracting bacteria from its tissues, we successfully identified bacteria that incorporated organic ¹⁵N, representing those that were truly functional, using a non-destructive approach.

By linking ¹⁵N₂-derived metabolites (functional outcomes) to specific bacteria, we aim to uncover the true nitrogen-fixing members actively functioning within complex microbial systems.

It has long been believed that root crops such as sweet potato require substantial amounts of potassium. However, increasing potassium fertilization resulted in little change in its nitrogen fixation capacity. Why does sweet potato show no response to potassium fertilization?

Analysis of potassium content in sweet potato tissues revealed that similar levels of potassium were maintained regardless of fertilization rates. Notably, even in the absence of potassium fertilization, sweet potato exhibited high potassium content, indicating that sufficient potassium was acquired from the growth medium rather than from fertilizer inputs.

We are currently working to elucidate the mechanisms underlying this phenomenon.