Characterization of sugar-insensitive mutants and analysis of sugar-regulated gene expression in Arabidopsis thaliana

Sugars serve as signaling molecules in plants, affecting gene expression and a number of developmental processes, but their precise role and the pathways through which they act are not well characterized. In addition, sugar-signaling pathways interact with the vast network of phytohormone-signaling pathways. In order to facilitate an understanding of sugar sensing and signaling pathways in Arabidopsis, sugar-insensitive mutants have been isolated for study and global genome analysis of wild-type and mutant responses to sugars and the phytohormones abscisic acid, ethylene, and gibberellin have been undertaken. Sugar-insensitive mutants were isolated based on their ability to form true leaves and expanded cotyledons in the presence of high concentrations (0.27 M to 0.34 M) of sucrose or glucose. The sis3 and sis6 mutants were chosen for further study. The sis3-1 mutation is recessive and is not linked to the presence of a T-DNA insert carried by the mutant. The sis3 mutation maps to the bottom arm of chromosome 3. The sis3 mutant is slightly insensitive to the effects of exogenously applied abscisic acid (ABA) on seed germination and root elongation but has a wild-type response to all other phytohormones tested. The sis6-1 mutant carries a cDNA encoding the At4g28240 putative wound-inducible gene on a T-DNA insert. The dominant nature of the mutation suggests that the five-fold overexpression of the cDNA in the mutant may be the cause of the sis phenotype. However, disruption of a gene by the T-DNA insertion is also a possible cause of the phenotype. The sis6 mutant is resistant to the inhibition of seed germination and root elongation caused by application of exogenous ABA. It is also resistant to the inhibition of germination caused by application of exogenous paclobutrazol (an inhibitor of gibberellin biosynthesis). This suggests a possible link between sugar and gibberellin signaling pathways. Analysis of gene expression in response to sugars and phytohormones was undertaken using the Affymetrix GeneChip Arabidopsis ATH 1 Genome Array which includes over 24,000 Arabidopsis genes. Both wild-type germinating seeds and adult plants were studied in order to begin determining the impact of the developmental age of the plant on sugar-regulated gene expression