IMPORTANCE OF THE POSTMITOTIC ISODIAMETRIC GROWTH (PIG) REGION FOR GROWTH AND DEVELOPMENT OF ROOTS

Plant cells are assumed to embark on rapid elongation immediately after the cessation of their mitotic divisions at the proximal end of the meristem. However, a comprehensive appraisal of most of the data previously published on this matter, as well as several recently obtained findings, has convincingly revealed, at least for root cells, that this belief is incorrect. For example, morphometric analysis of the maize root apex has clearly defined a distinct transitional region between the meristem and the zone of rapid cell elongation. This growth region is operationally defined as the post-mitotic isodiametric growth (PIG) region. In the middle of the cortex, this region may have a length similar to that of the meristem in steady-state growing roots. We believe that PIG is a specific phase in root cell ontogeny and that cells of the PIG region have more in common with the meristematic cells than with the rapidly elongating cells, although many of the metabolic properties of cells in the PIG region are rather unique. In this regard, certain properties of cells in the PIG region, such as the distributions of their microtubules and their sensitivity towards auxin and calcium, were found to be of crucial importance for the induction of differential cell growth patterns essential for the graviresponse of maize roots. Other data indicate that the PIG region is of considerable significance for the overall development of roots because of its characteristic plasticity under unfavourable external conditions.