Light-controlled growth of the maize seedling mesocotyl: Mechanical cell-wall changes in the elongation zone and related changes in lignification

Cell extension in the mesocotyl elongation zone (MEZ) of maize (Zea mays L. ) seedlings is inhibited by light, The growth inhibition by blue light in t he MEZ was reversible upon transfer to darkness, This experimental system w as used for investigating the modification of mechanical cell-wall properti es and the role of cell-wall lignification in cell elongation. The occurren ce of lignin in the cortex and vascular bundle tissues of the MEZ was demon strated by the isolation of diagnostic monomers released after thioacidolys is of the cell walls. Concomitantly with the inhibition of growth, blue lig ht induces an increase in cell-wall stiffness (tensile modulus) as well as an increase in extractable lignin in the outer MEZ tissues (cortex + epider mis). Both effects are reversed when growth is resumed in the ME7, in darkn ess after a period of growth inhibition induced by 3 h light. In the vascul ar bundle light produces no comparable change in lignin content. Appearance and disappearance of phenylpropanoid material in MEZ cell walls in the lig ht, or in darkness following a brief light treatment, respectively, can be visualized under the fluorescence microscope by characteristic changes in a utofluorescenee of tissue sections upon excitation with UV radiation, It is concluded from these results that light-induced lignification of primary w alls is involved in cell-wall stiffening and thus inhibition of elongation growth in the MEZ of maize seedlings. Resumption of growth upon redarkening may be initiated by wall loosening in the uppermost MEZ region which displ aces the lignified cell walls towards the lower mesocotyl region.