ROOT HISTOGENESIS IN HERBACEOUS AND WOODY EXPLANTS CULTURED IN-VITRO - A CRITICAL-REVIEW

Competence, determination and root meristemoid organization are key st eps in adventitious rooting. A direct and indirect pattern of root for mation may be present in both herbaceous and woody systems. The patter n is indirect when competence is achieved during the culture; it is di rect when competent cells are present in the tissues at explanting tim e. Both processes may be activated in the same (herbaceous or woody) e xplant, and by the same tissue. A coordinated reactivation to division of cell types not directly involved in the rooting process may occur in the woody cuttings. In general, determination occurs before meriste moid formation; however, the ab initio root identity of the meristemoi d has been debated. Examples of herbaceous and woody systems showing o ne or both rooting patterns are described. Studies on mutations affect ing the specification of root meristem and on genes involved in cell c ycle regulation could contribute to understanding adventitious rooting .