Enzymatic studies on the metabolism of vesicular-arbuscular mycorrhiza II. Soluble alkaline phosphatase specific to mycorrhizal infection in onion roots

Abstract

Soluble alkaline phosphatase specific to vesicular-arbuscular mycorrhizal infection has been demonstrated in enzyme extracts from onion roots inoculated with Glomus mosseae. A close correlation existed between the mycorrhiza-specific phosphatase activity and development of both the infection and the host plant. Maximum activity occurred whilst the infection was still young (100% arbuscular) and coincided with the start of the mycorrhizal growth response, declining afterwards as plant development and infection continued. Enzyme activity was optimal in the presence of 10 −5 m orthomosphate but was markedly depressed at concentrations above 10 −4 m. The properties of mycorrhiza-specific alkaline phosphatase were characteristic of alkaline phosphomonesterase (EC 3.1.3.1.): optimal activity at alkaline pH; inhibition by metal chelating agents, certain cations and orthophosphate; requirement for Mg 2+ and Mn 2+ ions; hydrolysis of β-glycerol, phenyl and naphthyl phosphates; inability to hydrolyse more complex phosphate esters. The possible implications of these results with respect to mechanisms involved in the assimilation of phosphorus in the VA mycorrhizal system and/or the establishment of VA mycorrhizal infection are discussed.