Solute transport in the interaction between wheat and powdery mildew

A series of experiments were undertaken to investigate the transfer of solutes between wheat (Triticum aestivum) leaves and powdery mildew (Erysiphe graminis f. sp. tritici), and in particular whether sucrose was taken up unaltered by the fungal pathogen. Glucose was taken up more rapidly than sucrose into uninfected wheat leaf pieces. Inhibition and competition data suggested that the uptake of glucose and sucrose was carrier-mediated and occurred on separate carriers. Uptake rates for the amino acids glutamine and histidine were similar, and probably carrier-mediated.There was a marked increase in the uptake rate of sugars into leaf pieces following infection with powdery mildew. There was no increase in amino acid uptake rates. Uptake of sugars remained sensitive to inhibitors suggesting that uptake into infected leaves was carrier-mediated. However, glucose was also shown to efflux more rapidly from infected leaves.There was substantial transfer of radiolabel to the fungal mycelium following uptake of glucose and sucrose into infected leaves, but transfer was most rapid following uptake of glucose. There was a very small transfer of amino acids. Chemical assays showed that there was a marked rise in activity of invertases and sucrose synthase in the leaf following infection. Studies of uptake of asymmetrically-labelled sucrose suggested that this sugar was extensively hydrolyzed prior to uptake by leaf cells.Studies of solute uptake with isolated mycelial suspensions showed that sucrose was slowly taken up, whereas glucose uptake was much more rapid, and sensitive to the external pH. Analysis by HPLC of leaf and mycelial extracts following uptake of sugars into the leaf confirmed that sucrose was rapidly hydrolyzed in the leaf. Furthermore, no sucrose or fructose could be detected in mycelial extracts. Glucose was rapidly converted to other substances in the mycelium.It is concluded that infection by powdery mildew causes increased uptake of sugars into the epidermis. Cells in the remainder of the leaf probably become more permeable and sugars efflux into the apoplastic space. Sucrose is hydrolyzed both intracellularly, and in the apoplast due to increased activity of invertases and sucrose synthase. Only glucose is taken up by the pathogen, probably by facilitated diffusion. The concentration gradient is maintained by conversion to other compounds in the mycelium.