β-furfuryl-β-glucoside: An endogenous activator of higher plant UDP-glucose:(1-3)-β-glucan synthase Biological Activity, Distribution, and in vitro Synthesis

In a recent paper (P Ohana, DP Delmer, JC Steffens, DE Matthews, R Mayer, M Benziman [1991] J Biol Chem 266: 13472-13475), we described the purification and structural characterization of β-furfuryl-β-glucoside (FG), an endogenous activator of plant UDP-glucose:(1→3)-β-glucan (callose) synthase. In the present report, we provide evidence that FG specifically stimulates callose synthase. The effects of FG on the kinetic properties of callose synthase were studied, and we ascertained that FG, or at least a very similar compound, is present in other plant systems. Chemically synthesized α-furfuryl-β-glucoside also stimulates callose synthase, exhibiting a slightly higher K_a of 80 micromolar, compared with 50 micromolar for FG. In addition, we have identified and partially characterized an enzyme that catalyzes the synthesis of FG using β-furfuryl alcohol and UDP-glucose as substrates. A model for the regulation of callose synthesis in vivo, involving changes in intracellular compartmentation of FG and Ca²⁺, is proposed.