ROLE OF APOPLASTIC AND CELL-WALL PEROXIDASES ON THE STIMULATION OF ROOT ELONGATION BY ASCORBATE

Elongation of onion (Allium cepa L.) roots was highly stimulated by as corbate (ASC) and its natural precursor I-galactone-gamma-lactone (GL) . When incubation media were supplemented with lycorine (Lye), an inhi bitor of the ASC biosynthesis, root growth was negligible even in the presence of ASC or GL. ASC completely inhibited in vitro guaiacol pero xidase activities that were isolated from both the apoplast and the ce ll wall. However, ferulic-acid-dependent peroxidase from the cell wall was partially inhibited by ASC, whereas ferulic acid peroxidase activ ity from the apoplastic fluid was completely inhibited by ASC as long as ASC was present in the assay medium. ASC content in cells was incre ased by preincubations with ASC or Ct, whereas Lye reduced it. On the other hand, ASC or CL treatments decreased both apoplast and cell-wall -bound peroxidase activities, whereas Lye had a slight stimulating eff ect. These results are discussed on the basis of a possible control of root elongation by ASC via its action on peroxidases that are involve d in the regulation of cell-wall extensibility.