Sugar sensing and signal transduction pathway in Arabidopsis thaliana

Arabidopsis hexokinase 1 (AtHXK1) has been shown to play a critical role in glucose repression of photosynthetic genes and early seedling development. However, it remains unknown if AtHXK1 can modify the expression of diverse sugar-regulated genes involved in other processes. Based on the analysis of expression of various glucose-regulated genes in HXK (hexokinase) transgenic plants, it was suggested that three distinct glucose signal transduction pathways existed in Arabidopsis: an AtHXK1-dependent pathway, a glycolysis-dependent pathway, and an AtHXK1-independent pathway. In addition to affecting gene expression, HXK was shown to play a role in glucose-dependent growth and senescence in plants. Overexpressing Arabidopsis or yeast HXK in plants resulted in the repression of growth and true leaf development, and early senescence, while underexpressing AtHXK1 delayed the senescence process.; A reverse genetic approach was taken to identify novel components that may function in the sugar sensing and signaling pathway in plants. The F-box protein Grr1 (glucose repression resistant) is of central importance in the glucose signaling pathway in yeast. By searching Arabidopsis database and sequence analysis, four novel F-box proteins (AtFBL3--AtFBL6) were found in Arabidopsis, showing high homology to the yeast Grr1. Among these 4 Arabidopsis Grr1 homologues, AtFBL3 has an F-box and 12 LRRs, showing the highest sequence similarity to the yeast Grr1.; To characterize the function of AtFBL3 in plants, genetic, molecular, and physiological approaches were employed. It was shown that AtFBL3 interacted with the Arabidopsis Skp1-like proteins ASK1 and ASK2 in the yeast two-hybrid assay. Transgenic Arabidopsis plants ectopically expressing 35S::AtFBL3 exhibited reduced responses to both sugar and ABA signals that negatively regulate seed germination and early seedling development. In addition, the induction of ADH and AtEM6 by ABA and the repression of CAB and PC by glucose were also diminished in the 35S:: AtFBL3 plants. Finally, genetic analysis suggested that AtFBL3 acted downstream of AtHXK1 in the AtHXK1-dependent glucose signaling pathway. We propose that AtFBL3 acts as a subunit of SCFAtFBL3 in the ubiquitin-mediated degradation of a positive regulator that participates in the converged sugar and ABA signaling network.