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Melon (Cucumis melo) 

A phosphate deficiency-induced flavonoid glucoside stimulates vesicular-arbuscular mycorrhiza formation in melon roots under low and high phosphate conditions

K. Akiyama, H. Matsuoka and H. Hayashi
Osaka Prefecture University, Osaka, Japan

Melon (Cucumis melo) grown under phosphate-limiting conditions was analyzed for the levels of secondary metabolites in roots during colonization by the vesicular-arbuscular mycorrhizal fungus Glomus caledonium using HPLC. A compound in the methanol extracts from non-inoculated roots showed a significant increase 40 days after sowing. No accumulation was observed in G. caledonium colonized roots or in roots grown under high phosphate conditions. The compound was isolated by column chromatography and identified to be flavone C-glucoside, isovitexin 2”-O-b-glucoside by spectroscopic methods. The effect of the compound on root colonization of melon by G. caledonium was examined   under low (0.05 mM) and high (2 mM) phosphate conditions. Forty-five days after inoculation, the percentage of colonization in control roots grown under low and high phosphate conditions were 22% and 8.8%, respectively. The compound applied to the soil at 20 mM increased root colonization to 36% under low phosphate conditions. Surprisingly, high degree of colonization (25%) was observed in roots grown under high phosphate conditions upon treatment by the compound at 20 mM. Isovitexin 2”-O-b-glucoside might be a endogenous stimulatory factor for mycorrhizae colonization in melon.