Nutrient Transfer and the Fungus-Root Interface

Abstract

Ectomycorrhizas synthesised in growth pouches between Eucalyptus pilularis and Pisolithus tinctorius show low permeability to the tracer Cellufluor in the apoplast of their sheath. This is attributed to the properties of an intracellular matrix, primarily of carbohydrate, in the sheath interhyphal spaces. A similar result has also been obtained with Pisonia grandis mycorrhizas and it is proposed that this may be a general phenomenon in ectomycorrhizas, effectively isolating the symbiont exchange interface from the soil. Materials travelling across the sheath would be required to travel in the fungal symplast, a mechanism which would place uptake under fungal control, and give it a high degree of selectivity. In eucalypt mycorrhizas the cell layer beneath the Hartig net zone is a fully differentiated state II exodermis, with a well developed suberised lamella in the walls of most cells The apoplast below the Hartig net is therefore also likely to be blocked. The effect of these two apoplastlc blockages, in the sheath and in the exodermis, is to produce a distinct apoplastic compartment at the exchange interface of finite volume and sealed off both internally and externally. Exchange between symbionts must involve an apoplastlc step and isolation of thls part of the apoplast would provide an exchange compartment in which the overall composition and the accumulation of molecules could be controlled by membrane transport processes of the adjoining cells, either fungal or higher plant. Isolation of apoplast would in this way enhance the efficiency of transfer. It would also prevent loss of materlal by back diffusion and its assimilation by competing organisms The concept of a sealed apoplast at the symbiont interface is extended to other symbiotic associations including endomycorrhizas and pathogenic associations and it is proposed that it may be a universal phenomenon in symbioses, necessary for the functional efficiency of the transfer system.