The Source of "Fairy Rings": 2-Azahypoxanthine and its Metabolite Found in a Novel Purine Metabolic Pathway in Plants

Rings or arcs of fungus‐stimulated plant growth occur worldwide; these are commonly referred to as “fairy rings”. In 2010, we discovered 2‐azahypoxanthine (AHX), a compound responsible for the fairy‐ring phenomenon caused by fungus; AHX stimulated the growth of all the plants tested. Herein, we reveal the isolation and structure determination of a common metabolite of AHX in plants, 2‐aza‐8‐oxohypoxanthine (AOH). AHX is chemically synthesized from 5‐aminoimidazole‐4‐carboxamide (AICA), and AHX can be converted into AOH by xanthine oxidase. AICA is one of the members of the purine metabolic pathway in animals, plants, and microorganisms. However, further metabolism of AICA remains elusive. Based on these results and facts, we hypothesized that plants themselves produce AHX and AOH through a pathway similar to the chemical synthesis. Herein, we demonstrate the existence of endogenous AHX and AOH and a novel purine pathway to produce them in plants. The fairy of the rings: The common metabolite, 2‐aza‐8‐oxohypoxanthine (2), of the “fairy”, 2‐azahypoxanthine (1), was isolated from rice, thus supporting the existence of endogenous 1 and 2 in plants. Compound 1 is synthesized from 3 by treatment with NaNO2 and then NH3; 1 can then be converted into 2 by xanthine oxidase. The new purine pathway to form the compounds occurs in rice and Arabidopsis through a route similar to the chemical synthesis.