The uptake of glucose, fructose and sucrose into pea powdery mildew (Erysiphe pisi DC) from the apoplast of pea leaves

Abstract

SUMMARYPatterns of sugar uptake into the mycelium of Erysiphe pisi DC have been investigated. Discs of mildewed pea (Pisum sativum L. cv. Onward) leaves were floated on radiolabelled sugar solutions. The mycelium was then removed and its total radioactive content was measured. The uptake from 1 mol m−3 fructose and 3‐O‐methylglucose (3‐OMG) was linear for at least 3 h, whereas the rate of uptake of glucose and sucrose increased with time. The uptake of glucose was markedly more rapid than that of the other three sugars, while 3‐OMG was the least well transported to the mycelium. Over a range of sugar concentrations, uptake of glucose and fructose was biphasic; however, whereas uptake of fructose showed a rapid linear increase over a higher concentration range (> 20 mol m −3), uptake of glucose and sucrose tended to plateau with sucrose hardly changing in rate of uptake at concentrations above 10 mol m−3. Increasing the pH of the uptake solution from 5 to 8 caused a slight but steady decrease in the uptake of glucose, fructose and sucrose. Decreasing the temperature from 25 to 4 °C decreased the uptake of all 3 sugars by 75–85 %. The effects of erythrosin B, parachloromercuribenzene sulphonic acid and carbonyl cyanide m‐chlorophenylhydrazone on sugar uptake were investigated. It appears that much of glucose uptake into the mycelium may be by facilitated diffusion driven by a concentration gradient. This gradient could be maintained by rapid metabolism of glucose in the fungal cytoplasm. It is concluded that there are probably carriers for all three sugars operating at lower concentrations (< 10 mol m −3).