The action of IAA and kinetin on the mitotic cycle of proliferative and stationary phase excised root meristems

Abstract

The influence of kinetin and IAA on the G1 (presynthetic), S (DNA synthetic) and G2 (postsynthetic) periods of the mitotic cycle of actively dividing (proliferative phase) and initially nondividing (stationary phase) root meristem cells was determined with the use of tritiated thymidine ( 3H-T ) and autoradiography. Proliferative phase meristems were cultured for 72 h in White's medium with 2 per cent sucrose and kinetin or IAA or both. Pulse labeling with 3 H-T was performed at 72 h. The stationary phase meristems were induced by a 24 h deprivation of sucrose. These meristems were almost devoid of dividing and DNA synthesizing cells. After establishment of the stationary phase the roots were transferred to medium containing sucrose, 3H-T, and kinetin or IAA or both. Results obtained with proliferative phase cells suggested that kinetin slightly increased G2 and certainly the (G1 durations. IAA appeared to act primarily on the duration of S. The combination of kinetin and IAA produced all three results. The effect on stationary phase cells, i.e. cells that are retained in either G1 or G2 until provided sucrose, indicated that kinetin and IAA impaired the initiation of DNA synthesis of most but not all of the G1 cells. The combination of kinetin and IAA reduced the initiation of DNA synthesis by G1 cells and further impaired the progress of cells through S; G2 cells also displayed sensitivity to the combination of kinetin and IAA in that they entered mitosis at a low rate. The concentrations of kinetin and IAA used in these experiments were similar to those used to induce cell proliferation in differentiated areas of the pea root; therefore, it is probable that controlling sites or moieties are activated or derepressed in cells of differentiated tissue and inactivated or repressed in meristematic cells by the direct or indirect action of the chemicals.