The indirect effect of exogenous auxin on initiation of cell divisions in wheat root expiants (Triticum aestivum) during callus induction

Mitotic activity (indices) was measured in apical and nonapical regions of primary root explants excised from 2-day-old germinating seedlings of wheat (Triticum aestivum Thell em L.) and cultured in the presence of the synthetic auxins 3,6-dichloro-O-anisic acid (dicamba) and 2,4-dichlorophenoxyacetic acid (2,4-D). Mitotic activity in the root apical meristem decreased with increasing concentration of auxin in the culture media. Correlated with the decrease of mitotic activity in the root apical meristem was an increase of mitotic activity in nonapical regions of the root. The degree of cell division activity initiated in nonapical regions of the root was (i) inversely proportional to the degree of inhibition of cell divisions in the root apex, but (ii) independent of the concentration of auxin in the culture medium. These data suggest that the primary effect of these synthetic auxins on mitotic activity in wheat root explants is to suppress cell divisions in the primary root apex and not to stimulate cell divisions in the nonapical regions of the root. Cell cycle control in nonapical regions of the root appears to be a function of factor(s) emanating from mitotically active cells of the primary apical meristem.