Polar transverse expansion of pericycle cells in auxin-treated adventitious roots of onion (Allium cepa)

Abstract

The exogenous application of an auxin (α-naphthaleneacetic acid) to adventitious roots of Allium cepa L. changes the polarity of growth from longitudinal to transverse. Promotion of transverse growth by this auxin involves cortical and pericycle cells located in the elongation zone of these roots. Transverse auxin-promoted expansion of cortical tissue is very marked. Within the vascular cylinder, pericycle cells react to treatment in different ways, according to their location with respect to the vascular system. In terms of cross-sectional area and radial diameter increases, the most marked response to auxin treatment occurs in pericycle cells adjacent to xylem poles. Auxin treatment also promotes cell divisions in pericycle and cortical tissues. The pattern of divisions in the pericycle is very similar to that occurring during lateral root development. Nevertheless, lateral roots do not develop along the distalmost 3 mm of auxin-treated roots. In this region the pericycle simply becomes a multilayered tissue. The microtubules are proposed as a conceivable means to control changes in the polarity of expansion and promotion of periclinal divisions in this experimental system. Keywords: Allium cepa, growth polarity, naphthaleneacetic acid, pericycle, plane of division, root cortex.