NATURAL AND EXPERIMENTAL EXPRESSION OF RHIZOGENESIS : BEHAVIOUR OF TARGET AND NON TARGET CELLS

The ability of aerial organs (pea epicotyls, tobacco leaves) to produce roots was analysed. Target cells, able to give rise directly to root meristems, are detected naturally, after IAA treatment (pea plantlets with cotyledons) or in in vitro conditions (pea epicotyls or tobacco leaf explants). These cells are associated with vascular bundles : interfascicular cells in pea epicotyls and pseudopericyclic cells in tobacco leaves. They retain a diploid DNA content while the neighbouring cells may become endopolyploid.

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.