Polyphosphate dynamics in mycorrhizal roots during colonization of an arbuscular mycorrhizal fungus

• Inorganic polyphosphate (poly P) has been considered to be a translocatable form of phosphate (P_i) in arbuscular mycorrhizal fungi (AMF). Here we examined time-course changes in poly P content during the AMF colonization process. • Onion (Allium cepa) plants were cultured with or without inoculation with Gigaspora margarita for 2-8 wk with periodic sampling. Poly P in the extracts, purified through gel filtration, was quantified by the reverse reaction of polyphosphate kinase. • The length of poly P in mycorrhizal roots appeared to be shorter than in extraradical hyphae or in spores of the AMF, indicating that AMF depolymerize poly P before providing P_i to the host. The poly P content increased as colonization proceeded, and was highly correlated with the weight of the colonized roots. • These results support the model that AMF supply P_i to the host through the poly P pool, and that the poly P content of a mycorrhizal root can be a good indicator of the P_i-supplying activity of AMF.