THE STARCH PHOSPHORYLASE GENE IS SUBJECTED TO DIFFERENT MODES OF REGULATION IN STARCH-CONTAINING TISSUES OF POTATO

Analysis of the levels of starch phosphorylase mRNA and its product in the various organs of the potato plant indicates that the gene is dif ferentially regulated, leading to a high accumulation of the gene prod uct in tubers. The amount of phosphorylase transcripts synthesized in nuclei isolated from tubers and leaves indicates that the difference i n the steady-state levels of phosphorylase mRNA in these organs can be explained by different rates of initiation of transcription. However, while rates of initiation of transcription are similar in tubers and stems, the steady-state level of phosphorylase mRNA is much lower in t he stem. Transgenic potato plants expressing the beta-glucuronidase (G US) gene under the control of 5'-flanking sequences of the phosphoryla se gene exhibited high levels of GUS activity in petioles, stems, stol ons, tubers and roots, but low levels in leaves. This confirms the res ults of transcription assays observed for leaves, stems and tubers, an d indicates that accumulation of phosphorylase mRNA in stems and tuber s is not controlled solely by transcription initiation. Finally, histo chemical analysis for GUS activity in transgenic potato plants suggest s that transcription of the phosphorylase gene predominantly occurs in starch-containing cells associated to vascular tissues, and suggests a role for starch phosphorylase in the mobilization of starch stored a long the translocation pathway.