In plants, sugars are not merely a carbohydrate metabolite or a photoassimilate of photosynthesis. They also play an important role in the intricate machinery of signal transduction. Sugar signaling is part of an ancient system for cellular adjustment to shifting environments and has been found to be crucial in responses to various stimuli, most importantly to the carbohydrate status of the plant. Gene responses to a changing carbohydrate status can vary greatly between plants and plant tissues. In general, carbohydrate depletion upregulates genes for photosynthesis, reserve mobilization and export processes, while high carbohydrate levels induce genes involved in storage or growth. Starch is the major storage compound in many plants and, hence, regulation of starch synthesis is to a large extent mediated via sugar signaling. Our aim in this project was to investigate regulators of carbohydrate metabolism in barley, sorghum and the model plant Arabidopsis thaliana. In barley, two highly similar and novel transcription factors, SUSIBA1 and SUSIBA2, were isolated and studied for their involvement in the regulation of two fructan synthesis genes, 6-SFT and 1-SST, and two starch synthesis genes ISO1 and SBEIIB. SUSIBA2 was found to bind as an activator to the promoter of the ISO1 and SBEIIB genes. Regulation of of SBEIIB also depended on binding of an unknown transcription factor to an element in the second intron. SUSIBA1 serves as a repressor and binds to the promoter of the 6-SFT and 1-SST genes. By the use of T-DNA insertion mutants we found two new sugar-inducible genes, AtWRKY4 and AtWRKY34, involved in the regulation of three different isoamylase genes in Arabidopsis. The AtWRKY4 and AtWRKY34 genes were also involved in the regulation of a nucleoside diphosphate kinase, NDPK3a. Both the two SUSIBAs, and AtWRKY4 and AtWRKY34, belong to group I of the WRKY family of transcription factors. These transcription factors display sequence similarities and bind to the same promoter element, the W-box. However, despite sequence similarities between the SUSIBAs, AtWRKY4 and AtWRKY34, they show diversity in function, which illustrates the complexity of sugar signaling in plants
