Calystegines in Calystegia sepium do not inhibit fungal growth and invertase activity but interact with plant invertase

Calystegines are alkaloidal glycosidase inhibitors. They accumulate predominantly in young and meristemic parts of Calystegia sepium (Convolvulaceae). C sepium, bindweed, infests meadows and cereal fields and is difficult to control chemically. Fungal pathogens against C sepium are established as mycoherbicides. Stagonospora convolvuli LA39 attacks C sepium and does not affect crop plants, but young plants of C sepium are less susceptible to the fungus. The interaction of Stagonospora convolvuli with calystegines was investigated. Further, endophytic fungi of several classes were isolated from wild-grown Calystegia sepium leaves, and selected strains were tested for interaction with calystegines. Fungal growth on agar containing calystegines was not affected considerably. Plants in climate chambers were infected with an endophyte, Phomopsis, and with the fungal pathogen, Stagonospora convolvuli. Calystegine levels were measured in infected and non-infected plant tissues. Accumulation depended on developmental stage of the plant tissue and was not influenced by infection. Acid invertase was measured from fungal mycelia and from infected and non-infected plant tissues. Fungal acid invertase activity was not inhibited by 10 mM calystegine 132, while invertase from C sepium leaves was inhibited. It is concluded that calystegines do not inhibit fungal development and sucrose consumption under the conditions of the present investigation, but may act by redirection of plant carbohydrate metabolism.