INDUCTION OF THE PLASTIDIC STARCH-PHOSPHORYLASE GENE IN POTATO STORAGE SINK TISSUE - EFFECT OF SUCROSE AND EVIDENCE FOR COORDINATED REGULATION OF PHOSPHORYLASE AND STARCH BIOSYNTHETIC GENES

The expression of the gene encoding the plastidic enzyme starch phosph orylase (EC 2.4.1.1) varies according to tissue carbohydrate status. I ncubation of excised potato (Solanum tuberosum L. cv. Desiree) leaves carrying a portion of the stem under a short photoperiod resulted in a drastic accumulation of starch, accompanied by a rapid increase in th e level of phosphorylase mRNA and by a similar change in phosphorylase protein level. However, under the same incubation conditions, the tra nscriptional activity of the phosphorylase promoter in transgenic plan ts did not change markedly. Therefore, the increased expression of the phosphorylase gene in petioles of stem cuttings is not controlled by the level of initiation of transcription. Phosphorylase mRNA accumulat ed to a high level in petioles of detached leaves kept under constant light for 24 h, but not in petioles kept in the dark. The effect of li ght on the accumulation of the mRNA was appreciably reduced if the pet ioles were incubated in ethylendiaminetetraacetic acid (EDTA), a treat ment known to increase phloem exudation in detached leaves. The inhibi tion by EDTA could be partially counteracted by the addition of sucros e to the incubation solution. Furthermore, incubation of petioles in d arkness in solutions with high levels of sucrose resulted in enhanced expression of the gene. These results suggest that sucrose, the main c ompound transported by phloem in potato, is involved in the regulation of the starch phosphorylase gene. This also indicates that conditions favouring starch synthesis lead to increased expression of the phosph orylase gene.