Kinetin as trigger for mitosis in mature endomitotic plant cells.

IX root development, cell divisions at the root tip are predominantly localized in the first few millimeters of the typical dicot seedling root. These divisions give rise to the cells which form the mature root structure. By differential enlargement and maturation processes these cells form the distinctive mature tissue systems of the primary root: the epidermis, cortex, endodermis, pericycle, and the xylem and phloem tissues of the central cylinder. Once these tissues are differentiated, their cellular components seldom undergo further mitosis or cytokinesis, but remain as mature differentiated cells of the root. Under appropriate internal stimulus, the pericycle cells may undergo cell division to give rise to lateral roots or at a late stage in root development to vascular cambium or cork cambium. The dividing cells of the root meristem typically possess the 2n or diploid number of chromosomes, the so-called “somatic” chromosome number [9]. This same chromosome number is usually found also in the pericycle even in mature root tissues, so that cells of lateral root meristems derived from the pericycle possess the same diploid chromosome complement as the main axis tip. It is now well established from extensive cytological studies (cf. reviews by D’Amato [7], Geitler [l l]), that in a very large number, if not in most higher plants, both monocots and dicots, the cells of mature root tissues, notably cells of the cortex, endodermis, and metaxylem, undergo chromosome doubling without subsequent mitosis such that the DNA content of individual nuclei may be double or quadruple that in the diploid cell of the meristem. Such chromosome doubling by endomitotic reduplication is a