INITIATION OF LATERAL ROOT PRIMORDIA WITHOUT COMPLETION OF MITOSIS AND WITHOUT CYTOKINESIS IN UNISERIATE PERICYCLE

Abstract

In wheat, lateral (branch) roots originate always in the pericycle of the parent root between protoxylem poles of 2 successive xylem ridges and facing a sieve tube. Although the endodermis also contributes to the initial lateral protuberance when lateral root primordia develop close to the parent root apex, the pericycle alone gives rise to the lateral root which subsequently develops. Before initiation of lateral root primordia the pericycle and endodermis are each uniseriate. The first cell divisions that are characteristic of the initiation of lateral root primordia are periclinal divisions in the pericycle. When wheat is germinated and grown on colchicine, mitosis is not completed and hence no nuclear and cell divisions occur (i.e., “c‐mitoses,” but no true mitoses, occur). Within the primary roots of these seedlings appear structures, here called “primordiomorphs,” having the shape of lateral root primordia. Each primordiomorph, which develops without cell division, is a radial protuberance in the pericycle accompanied by an associated protuberance of the adjacent endodermis. Primordiomorphs are found only between protoxylem poles of 2 successive xylem ridges and facing a sieve tube. In primordiomorphs some nuclei repeatedly enter, but none complete, mitosis. The primordiomorphs become large enough to compress the cortical parenchyma and make bulges in the epidermis of the parent root. In typical lateral root primordia comparable in size with primordiomorphs, hundreds of cell divisions have occurred that, are absent in the primordiomorphs. After removal of colchicine the primordiomorphs develop with normal cell divisions into polyploid lateral roots as shown by the following findings: (1) Stages of development are detected that are intermediate between primordiomorphs and polyploid lateral roots with typical histological organization. As in normal lateral root initiation close to the parent root apex, the endodermis contributes to the initial protuberance but the pericycle alone gives rise to the lateral root which subsequently develops. (2) The number of these polyploid lateral roots formed can be accounted for by the number of primordiomorphs which disappear. (3) No “c‐mitoses” occur, i.e., the diploid and polyploid nuclei undergo only normal mitoses. Thus, primordiomorphs have 2 definitive morphological characteristics of typical lateral root primordia, viz.: (1) precise identity of histological site of origin, and (2) potentiality (realized after removal of colchicine) to develop into roots having typical histological organization. It is, therefore, concluded that primordiomorphs are lateral root primordia that developed without completion of mitosis and without cytokinesis. Since the parent root pericycle remains uniseriate, even in that region which comprises the primordiomorph, no periclinal cell divisions within the pericycle could have altered the polarization of growth to set up the apical‐basal axis of the primordiomorph. These results demonstrate that the earliest detectable changes in form involved in the initiation of a lateral root primordium can occur without completion of any mitosis and without any cytokinesis.