Since the first scientific article about "fairy rings" in 1675 and subsequent studies reviewed in 1884, this phenomenon has been a mystery attributed to "fairies". [1] The tendency of all fungi to grow outward from the point of germination of the spore results in circular colonies in a widely varying group of fungi. Fairy rings are zones of stimulated grass growth. They appear as more or less continuous, circular bands of turfgrass that are darker green and faster growing than adjacent plants of the same species (Figure 1 A). These belts of greener plants can range from 10 to 30 cm wide, and the diameter of the circles they form is generally between 0.9 and 3.7 m. Fungi are responsible for this growth stimulation; presumably through the saprophytic action of the fungus mycelium, the protein portion of nonliving organic matter in the soil is decomposed to ammonia. The ammonia combines with other compounds, or is used as a substrate by successive bacteria to generate nitrites and nitrates. The resulting accumulation of nitrogen in the soil in a form readily available to higher plants causes the typical growth pattern of conspicuous bands of taller, darker green plants. [1,2] We questioned this accepted notion, even if it is partially true, and thus investigated the possibility of a specific plantgrowth-regulating substance(s) being produced by the fungi. We cultured a fairy-ring-forming fungus, Lepista sordida, examined the effect of the culture on the growth of turfgrass, and found that the culture supernatant promoted plant growth. L. sordida is widespread in northern temperate zones throughout the world, [3] including the campus of our university where this study was conducted ( Figure S1 in the Supporting Information).First, to confirm that our L. sordida strain exhibits growthpromoting activity, the cultivated mycelia were placed under bentgrass seedlings (Agrostis palustris) in a deep petri dish and incubated for three weeks. Grass treated with the fungus grew taller than untreated grass (Figure 1 B). Isolation of the active compound from the fungus was guided by its growth-regulating activity on bentgrass. The liquid-cultured fungus was filtered, the filtrate was fractionated by repeated chromatography, and the fractions were tested for their growth-regulating activity. This lead to the isolation and purification of the active compound, 2-azahypoxanthine (AHX; Figures 2 A, B, S2 and S3). Although AHX has been reported as a photolytic degradation product from an antitumor drug, dacarbazine (5-(3,3-dimethyltriazeno)imidazole-4-carboxamide) [4] and has been synthesized, [5] this is the first reported isolation of AHX from a natural source.AHX elongated the shoots and roots of bentgrass seedlings (Figure 2 B). The effect of AHX on shoots was observed at 0.2 and 1 mm and on roots at 0.05 and 0.2 mm. In order to confirm the presence of AHX in the interaction between the fungus and bentgrass (Figure 1 B and Figure S4)

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We examined the response of the tea plant (Camellia sinensis L.) to aluminum (Al) exposure under sterile conditions, focusing specifically on the secretion of low molecular mass organic compounds from roots. After germination in agar medium, tea seedlings together with medium were placed on agar containing 0.4 mM Al with 0.2% hematoxyline (hematoxylin-Al medium). The purple color of the hematoxylin-Al medium was observed to fade gradually, until none of the color remained 6 days later. The tea seedlings were then treated with simple calcium solution (0.2 mM, at pH 4.2) containing AlCl 3 , which ranged in concentration from 0 to 0.8 mM, for 24 hrs. The amount of oxalate secreted into the medium increased as the external Al concentration increased, while the concentrations of malate and citrate in the medium remained unchanged. Oxalate secretion started within 30 min after Al exposure and increased linearly thereafter. The findings demonstrated that oxalate was a key compound in the Al-tolerance mechanism employed by the tea plant, which detoxifies Al 3þ externally in the rhizosphere. In addition to oxalate, caffeine was also secreted by tea roots in response to Al exposure. It is possible that caffeine excretion from the roots of tea plants may stimulate root growth through the inhibition of callose deposition in root tips.

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