OLIGOGALACTURONIDES STIMULATE PERICYCLE CELL-WALL THICKENING AND CELLDIVISIONS LEADING TO STOMA FORMATION IN TOBACCO LEAF EXPLANTS

Novel developmental events induced by micromolar concentrations of oli gogalacturonides (OGs) in tobacco leaf explants cultured in vitro are described. Oligogalacturonides induced acceleration and synchronizatio n of the mitotic activity of guard-cell precursors in the epidermis. I n explants cultured for 24 h in the presence of OGs, the number of sto matal mitoses was higher than that observed in explants cultured in th e absence of OGs; however, at the end of the culture period the densit y of mature stomata did not vary upon OG treatment. The OG-induced act ivation of stomatal mitosis was reduced by exogenously added indole-3- acetic acid (IAA). Oligogalacturonides also enhanced mean wall thickne ss, mainly due to cellulose deposition, of foliar pericycle cells, as well as the number of extra-thick-walled pericycle cells; the pericycl e thus formed a sheath surrounding phloem and xylem. Indole-3-acetic a cid decreased the number of extra-thick-walled cells forming in the pr esence of OGs but did not influence wall thickness. Moreover: OGs inhi bited the stimulation of mitotic activity of phloem parenchyma cells ( vascular mitoses) induced by auxin, leading to a nearly complete inhib ition of IAA-induced formation of callus and of meristemoids of indire ct origin. Instead, OGs did not influence mitotic activity occurring i n the absence of auxin. All in all, our results provide further eviden ce of the pleiotropic role exerted on plant development by these oligo saccharins, and of the antagonism between auxin and OGs.