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

Abstract

Fungal endophytes of the genus Acremonium infect many grass species. They exist in the intercellular spaces of the aerial plant parts, most abundantly in the leaf sheaths. They therefore must obtain all of their nutrients from the apoplastic spaces in the plant. This report is the first comprehensive, biochemically based study of sugar uptake by Acremonium spp. The endophytes (Acremonium typhinum) were isolated from Festuca rubra plants and were grown in culture. Individual colonies were incubated in the presence of [U-¹⁴C]radiolabeled glucose, fructose, or sucrose for 20 minutes and the uptake quantified by scintillation counting. The uptake rates for all three sugars were biphasic, indicating two distinct mechanisms of transport. At low sugar concentrations (below 10 mM) the uptake was saturable and had the characteristics of a carrier-mediated mechanism. At high sugar concentrations (10–100 mM) the uptake was linear suggesting diffusion as the predominant mechanism 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 sucrose uptake were detected, a sucrose carrier and hexose uptake as a result of cell wall invertase. Cell wall invertase activity of the fungal isolates was inducible by growing the fungi on sucrose containing media.

Key Words:

• endophyte
• Epichloë typhina
• hexose
• invertase
• sucrose