Circadian control of sweet potato granule-bound starch synthase I gene in Arabidopsis plants

A starch granule-bound starch synthase I (GBSSI) gene is regulated by a circadian clock in sweet potato leaves. In order to examine whether the promoter region is responsible for controlling a circadian expression of the GBSSI gene, the sweet potato GBSSI promoter was isolated and deleted to different lengths for functional analysis with a GUS reporter gene in transgenic Arabidopsis plants. Nuclear run-on transcriptional assays showed that the circadian control was regulated at the transcriptional rate level, and de novo synthesized proteins were necessary for controlling the rhythm. Promoter assays showed that the GBSSI promoter fragments containing six I-boxes, two putative circadian regulation elements (CAANNNNATC) and four circadian clock-associated 1 protein-binding sites (AATCT) maintained the activity to induce the circadian expression of the GUS gene. Similar to the GBSSI in sweet potato, GBSSI, soluble starch synthase and ADP-glucose pyrophosphorylase genes in Arabidopsis leaves also exhibited a circadian rhythm. These results suggested that common signals may exist in dicotyledonous plants to coordinate the circadian expression of genes involved in the transitory starch synthetic pathway.