Formation of callose from sucrose in cotton fiber microsomal membranes

Callose is formed from exogenous sucrose by cotton fiber microsomal membranes that contain both sucrose synthase and callose synthase activity. Although the coupled reaction between sucrose and callose synthases occurs predominantly to channel glucose from sucrosederived uridine diphosphate (UDP)-glucose into callose in the membranes, there is no difference in the UDP-glucose-forming/sucrose-forming activity ratios between the soluble and membrane-bound forms of sucrose synthase. The consumption of UDP-glucose from sucrose into callose probably leads to UDP-glucose formation rather than sucrose formation despite the lower affinity of sucrose synthase for sucrose than for UDP-glucose. Callose formation is markedly stimulated by the addition of either recombinant Glu11 (S11E) or in vitro phosphorylated Ser11 mung bean sucrose synthase but not by the wild-type nonphosphorylated Ser11 enzyme. We propose that a negative charge (by phosphorylation or mutagenesis) at Ser11 in sucrose synthase causes the enzyme to promote a coupled callose-forming reaction.