DNA synthesis in relation to polyploid mitoses in excised pea root segments cultured in vitro.

MITOSES can be stimulated in mature cells of both plant and animal tissues by a variety of procedures including wounding and hormone treatments. The initial response to these treatments may or may not involve DNA synthesis. Thus, Gelfant [lo] observed that mouse ear epidermis contains populations of cells both in Gl and in G2; the latter cells can be induced to enter mitosis without any preceding DNA synthesis by wounding the ear. The initiation of mitoses by auxin and kinetin treatment of mature pea root segments cultured in vitro provides a convenient system in which to determine whether there are cells in a mature root in G2 which can be induced to divide and whether the action of auxin and kinetin in inducing mitoses involves the stimulation of DNA synthesis. Both diploid and polyploid mitoses are initiated in l-mm root segments when they are cut from the mature portion of a seedling pea root and grown on a sterile nutrient medium which contains auxin and a cytokinin such as kinetin [27]. There is a lag period between the time of the excision of the segment and the observed increase in mitotic frequency. For diploid mitoses this lag is about 24 hr; for polyploid mitoses it is about 60 hr [16]. When the segments were cultured on a medium lacking auxin and/or kinetin, no polyploid mitoses are observed; when the missing hormones are added to the culture medium at specific times after excision, one finds that auxin is required during the first 24 hr in culture and that kinetin and possibly auxin are required after the first 24 hr in order to obtain a normal frequency of polyploid mitoses at 74 hr [ 161.