Changes in onion root development induced by the inhibition of peptidyl-prolyl hydroxylase and influence of the ascorbate system on cell division andelongation

Post-translational hydroxylation of peptide-bound proline residues, catalyz ed by peptidyl-prolyl-4 hydroxylase IEC 1.14.11.2) using ascorbate as cosub strate, is a key event in the maturation of a number of cell wall-associate d hydroxyproline-rich glycoproteins (HRGPs), including extensins and arabin ogalactan-proteins, which are involved in the processes of wall stiffening, signalling and cell proliferation. Allium cepa L. roots treated with 3,4-D L-dehydroproline (DP), a specific inhibitor of peptidyl-prolyl hydroxylase, showed a 56% decrease in the hydroxyproline content of HRGP. Administratio n of DP strongly affected the organization of specialized zones of root dev elopment, with a marked reduction of the post-mitotic isodiametric growth z one, early extension of cells leaving the meristematic zone and a huge incr ease in cell size. Electron-microscopy analysis showed dramatic alterations both to the organization of newly formed cell walls and to the adhesion of the plasma membranes to the cell walls. Moreover, DP administration inhibi ted cell cycle progression. Root tips grown in the presence of DP also show ed an increase both in ascorbate content (+ 53%) and ascorbate-specific per oxidase activity in the cytosol (+ 72%), and a decrease in extracellular "s ecretory" peroxidase activity (-73%). The possible interaction between HRGP s and the ascorbate system in the regulation of both cell division and exte nsion is discussed.