ROOT HISTOGENESIS FROM TOBACCO THIN CELL-LAYERS

Internode stem explants of Nicotiana tabacum cv. Samsun, consisting of eight cell layers: epidermis, subepidermal chlorenchyma, collenchyma and cortical parenchyma (i.e., thin cell layers), were cultured under conditions inducing rhizogenesis. The aim was to investigate the histological sequence of adventitious root formation in this system. The earliest cytological events in culture (12 h) were nucleolar extrusions and amitotic nuclear divisions. Though not restricted to a specific cell layer, the two phenomena were more frequent in the subepidermal chlorenchyma, and characterized the first phases (12-96 h) of cell proliferation mainly occurring in this layer. Amitoses were followed by the formation of thin walls within the original cells, resulting in the formation of intracellular clusters. These subepidermal clusters were separated by enlarged cells of the parent tissue, whose nuclei showed nucleolar extrusion. At day 3 the first mitoses were observed in cells having abundant starch inclusions. Amitotic divisions also continued, but less frequently. The increasing frequency of mitoses in the subepidermal chlorenchyma (day 4), as well as in the two underlying collenchymatous layers, contributed to the growth of the superficial clusters, in which small clumps of meristematic cells were formed; these, later (day 9), gave rise to root domes. The 5th cell layer remained undivided for a relatively long time (two weeks). The 6th and 7th layers proliferated mitotically later (from day 8 onwards) than the superficial layers and formed root domes following the same histological sequence. Wound callus, generated by the innermost layer, increased markedly in the last two weeks of culture and concomitantly formed vascular clumps surrounded by meristematic layers; these produced root primordia which were frequently anomalous (day 26-27). Regardless of its origin (i.e., superficial or deep layers of the explant, or wound callus cells), root tip formation was always preceded by the differentiation of a sheath of starch-containing cells, from which the root cap developed.