STUDIES ON THE MECHANISM OF DEPOSITION AND EXTENSION OF PRIMARY CELL WALLS

Abstract

Walls of elongating parenchyma and epidermal cells of Avena coleoptiles and onion roots were studied by autoradiography, using C14-labelled sugars, and by electron microscopy of thin transverse sections. The autoradiographs showed that deposition of cellulose takes place over the whole length of the cell, ruling out the possibility of bipolar tip growth. Autoradiographs of cells grown for short times in labelled sugar gave no evidence of localized incorporation of isotope around primary pit-fields, but rather indicated that deposition of microfibrils is finely dispersed over the entire wall surface. The thin sections revealed a marked change in wall structure as elongation proceeds. Walls of young cells contain only transversely oriented microfibrils while older walls include an inner region of predominately transversely oriented microfibrils bounded outside by a region with microfibrils showing an irregular transition to longitudinal orientation. The degree of longitudinal orientation in the outer region increases with cell length. These results for thin walls are consistent with the multi-net model for wall growth deduced by other workers from electron microscope studies on cell fragments. Thin sections have also revealed layering in several types of growing walls which is difficult to interpret on the basis of the multi-net model alone.