Sugar sensing and signal transduction pathway in Arabidopsis Thaliana

Arabidopsis hexokinase 1 (AtHXKI) has been shown to play a critical role in glucose repression of photosynthetic genes and early seedling development. However, it remains unknown if AtHXKI 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 transgenic plants mis-expressing HXK, it was suggested that three distinct glucose signal transduction pathways existed in Arabidopsis: an A tHXKIdependent pathway in which gene expression was correlated with the AtHXKI mediated signaling function; a glycolysis-dependent pathway that was influenced by the catalytic activity of both AtHXKI and the heterologous yeast Hxk2; and an AtHXKI -independent pathway in which gene expression was independent of A tHXKI. 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 HXKm plants resulted in the repression of growth and true leaf development, and early senescence, while underexpressing AtHXKI delayed the senescence process. The Grr1 (glucose repression resistant) protein is of central importance in the glucose signaling pathway in yeast. Grr1 contains an F-box motif which has about 40-50 amino acids and was named after the first such protein identified, the human protein cyclin F. Grri is a component of the E3 ubiquitin protein ligase termed (Skp1, Cdc53, and F-box protein Grr1) complex. The SCF complex can recruit phosphorylated protein substrates to be ubiquitinated and subsequently degraded by the 26S proteasome. A reverse genetic approach was employed to test whether a G rri homolog exists and functions in the sugar signaling pathway in plants. As a result, four novel F-box proteins (AtFBL3-AtFBL6) were found in Arabidopsis, showing high homology to yeast G r r i. They all contain an F-box motif in the N-terminus and LRRs (leucine rich-repeats) in the C-terminus. Among these 4 Arabidopsis G rri homologs, AtFBL3 has an F-box and 12 LRRs, and shows the highest sequence similarity to the yeast Grri protein. To reveal the function of AtFBL3 in plants, genetic, molecular, and physiological approaches were employed. It was shown that AtFBL3 interacted with the Arabidopsis Skpl-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 AtEM 6 by ABA and the repression of CAB and PC by glucose were also diminished in the 35S::A tFBL3 plants. Finally, genetic analysis showed that the double mutant 35S::AtFBL3 35S::AtHXKI plants behaved like the single mutant 35S::AtFBL3 plants in both sugar and ABA responsive assays, indicating that AtFBL3 acts downstream of AtHXKI in the AtHXKI-dependent glucose signaling pathway. Based on the results of functional assays, we propose that AtFBL3 acts as a subunit of in the ubiquitin-mediated degradation of a positive regulator that participates in the converged sugar and ABA signaling network.