MECHANISM AND RATE OF SUGAR UPTAKE BY ACREMONIUM-TYPHINUM, AN ENDOPHYTIC FUNGUS INFECTING FESTUCA-RUBRA - EVIDENCE FOR PRESENCE OF A CELL-WALL INVERTASE IN ENDOPHYTIC FUNGI

Fungal endophytes of the genus Acremonium infect many grass species. T hey exist in the intercellular spaces of the aerial plant parts, most abundantly in the leaf sheaths. They therefore must obtain all of thei r nutrients from the apoplastic spaces in the plant. This report is th e first comprehensive, biochemically based study of sugar uptake by Ac remonium spp. The endophytes (Acremonium typhinum) were isolated from Festuca rubra plants and were grown in culture. Individual colonies we re incubated in the presence of [U-C-14] radiolabeled glucose, fructos e, or sucrose for 20 minutes and the uptake quantified by scintillatio n counting. The uptake rates for all three sugars were biphasic, indic ating two distinct mechanisms of transport. At low sugar concentration s (below 10 mM) the uptake was saturable and had the characteristics o f a carrier-mediated mechanism. At high sugar concentrations (10-100 m M) the uptake was linear suggesting diffusion as the predominant mecha nism of uptake. From competition experiments (uptake of labeled sugar in the presence of a different unlabeled sugar) it appeared that there were separate carriers for glucose and fructose. Two mechanisms for s ucrose uptake were detected, a sucrose carrier and hexose uptake as a result of cell wall invertase. Cell wall invertase activity of the fun gal isolates was inducible by growing the fungi on sucrose containing media.