Root histogenesis in herbaceous and woody explants cultured in vitro. A critical review

Competence, determination and root meristemoid organization are key steps in adventitious rooting. A direct and indirect pattern of root formation may be present in both herbaceous and woody systems. The pattern is indirect when competence is achieved during the culture; it is direct when competent cells are present in the tissues at explanting time. Both processes may be activated in the same (herbaceous or woody) explant, 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 meristemoid formation; however, the ab initio root identity of the meristemoid has been debated. Examples of herbaceous and woody systems showing one or both rooting patterns are described. Studies on mutations affecting the specification of root meristem and on genes involved in cell cycle regulation could contribute to understanding adventitious rooting.