In transformed tobacco cells the apoplasmic invertase inhibitor operates as a regulatory switch of cell wall invertase

Summary Agrobacterium tumefaciens-transformed tobacco suspension-cultured cells (TSCC) exhibit no significant quantitative changes of cell wall invertase protein (CWI) during a culture period of 40 days, whereas CWI activity decreases strongly between 10 and 30 days after cell transfer to fresh medium. Western blot analysis revealed that the apoplasmic invertase inhibitor (INH) is equally expressed throughout the entire culture period. When apoplasmic protein fractions from 4 and 28 days old cell cultures are chromatographed on Concanavalin A(ConA)-Sepharose, the non-glycosylated INH always coelutes with the ConA-bound fraction, suggesting that (i) INH and the glycosylated CWI form a complex in the apoplasmic space, and (ii) INH binding is not sufficient for CWI inhibition. The high specificity of INH binding to CWI was confirmed by native cathodic polyacrylamide gel electrophoresis. Expression analysis of CWI and INH indicates that, at least during certain stages of plant development (seedlings, roots of adult plants), CWI activity may be modulated by INH, the latter operating as a regulatory switch.