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

Plant cells are assumed to embark on rapid elongation immediately afte r the cessation of their mitotic divisions at the proximal end of the meristem. However, a comprehensive appraisal of most of the data previ ously published on this matter, as well as several recently obtained f indings, 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 t he 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 si milar to that of the meristem in steady-state growing roots. We believ e 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 w ith the rapidly elongating cells, although many of the metabolic prope rties of cells in the PIG region are rather unique. In this regard, ce rtain 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 differen tial cell growth patterns essential for the graviresponse of maize roo ts. Other data indicate that the PIG region is of considerable signifi cance for the overall development of roots because of its characterist ic plasticity under unfavourable external conditions.