Actin in pumpkin phloem exudate

When analyzing cytoskeletal proteins in Cucurbita pepo phloem exudate by immunoblotting, we detected actin in an amount comparable to that in some plant tissues and a small amount of alpha-tubulin. Electron-microscopic examination of the exudate permitted us to observe filaments that were capable of interacting with the myosin subfragment S1 from rabbit skeletal muscle and with phalloidin conjugated with colloidal gold. The addition of 0.5 mM phalloidin to the exudate in the medium containing 20 mM dithiothreitol (DTT) resulted in an increased number of filaments. Since high DTT concentrations induce a breakdown of filaments of the phloem protein PP1, it seems likely that the produced filaments were composed of actin. The addition of 50 mM MgCl2 to the exudate resulted in the formation of dense bundles and paracrystals, which resembled those produced by muscle actin under similar conditions. Our results demonstrated that actin in phloem sap was capable of polymerization with filament formation.