Cell cycle changes during callus initiation from cultured maize embryos. An autoradiographic study

Cell cycle changes were examined in various scutellar regions of immature maize embryos during a total culture period of 72 h on a modified MS agar medium containing 1 mg I-1 2,4-D and 6% sucrose. At intervals of 8 h embryos were sampled and transferred to the MS medium supplemented with 5 pCi ml-1 [3H]thymidine and incubated for 8 h. The fraction of labelled nuclei was determined for each interval and a comparison was made between the scutellar regions. During the first day, identical changes in the fraction of labelled nuclei and the mitotic index were observed in all scutellar regions. After the first day, however, various regions showed different cell cycles. The areas from which callus originated were mitotically active, showing high numbers of labelled nuclei, up to 60%. Furthermore, a difference was observed between the adaxial and the abaxial regions concerning the sequence of cell cycle changes. It is concluded that, after the initiation of the in-vitro culture, a shock response and a growth response are manifested, as observed by changes in the cell cycle. These responses proved to be different for the various regions of the immature embryos.