RNA synthesis in the embryo suspensor ofPhaseolus coccineus at two stages of embryogenesis, and the effect of supplied gibberellic acid

Summary

RNA synthesis in giant cells containing polytene chromosomes in the embryo suspensor ofPhaseolus coccineus was analyzed by autoradiography after [³H]-uridine treatment. Embryos at the heart-shaped stage of development and at a cotyledonary stage were studied. Discontinuous labelling of the polytene chromosomes was always observed. The chromosomes were subdivided into segments (chromosome regions) which behaved as functional units, since discontinuous labelling was never seen within any of the regions. It was found that most chromosome regions were engaged in RNA synthesis to different degrees at the two embryo developmental stages. Regions showing identical labelling patterns tended to lie close together in the chromosome arms and to keep their functional activity coordinated at both stages of embryo development. The chromosome regions bearing 18 S+25 S ribosomal genes were never simultaneously active in RNA synthesis and different regions were preferentially transcribed at each stage of embryo development. However, at both stages, all the chromosome regions bearing 5 S ribosomal genes showed comparable labelling frequencies. The effect on transcription of gibberellic acid (GA₃) treatments was also studied. At both embryo developmental stages, GA₃ enhanced the rate of RNA synthesis in the polytene suspensor cells. The frequency with which certain chromosome regions were transcribed was also increased significantly (P⩽0.001) and this stimulatory effect was greater in embryos at the cotyledonary stage than in heart-shaped embryos. At the latter developmental stage, RNA synthesis was repressed by GA₃ in a few chromosome regions. These results are discussed briefly in relation to previous findings using different methods of studying the organization of polytene chromosomes and the functional activity of the embryo suspensor ofPhaseolus coccineus.