NATURAL AND EXPERIMENTAL EXPRESSION OF RHIZOGENESIS : BEHAVIOUR OF TARGET AND NON TARGET CELLS
From such target cells, IAA induces a direct rhizogenesis starting with DNA synthesis from initial cells in G1, with a 4 to 20 hours lag phase according to species. Simultaneously, the G2 cells enter mitosis (pea). High IAA concentrations reactivate all the target cells of pea. After a prerhizogenetic phase (72 hrs), a structuration period (48 hrs) occurs characterized by the individualisation of a primordial root-cap. Cells able to build the roots exhibit a short cell cycle with predominant [S + G2] DNA contents. The last phase of root growth is inhibited by the highest IAA concentration. The non target cells undergo DNA syntheses without mitoses.
An indirect rhizogenesis can occur after culture on media containing both sucrose and NAA (tobacco) or 2,4-D (pea). With 2,4-D, interfascicular zones and cortex of pea epicotyl initiate DNA syntheses followed by binucleolation and amitoses then by a more or less complete cellularization. Amitoses are also induced from nuclei initially in G2. True mitoses are observed later. A meristematic sheath arises from the reactivated perifascicular cells and produces root teratomas without structuration. Cortical cells develop only callus.
Low concentrations of NAA (<10-6M) can induce in tobacco leaf explants some indirect roots which arise from a previous callus set up by means of largely amitotic processes. These roots are chimeric with aneuploid and atypical DNA contents. Higher NAA levels (>5.10-6M) stimulate callogenesis without rhizogenesis.