Identification and characterization of three Arabidopsis sugar insensitive genes

Abstract

Carbohydrates have signaling functions in regulating gene expression, metabolic pathways and developmental processes. Eukaryotic organisms have evolved conserved and novel mechanisms for sensing and responding to sugars. Plant sugar response pathways are complex and exhibit cross-talk with other response pathways. Sugar responses and signaling pathways have been studied via physiological, biochemical and genetic approaches. Genetic screens have identified sugar response mutants with altered seedling growth phenotypes. The Gibson lab has isolated an array of sugar insensitive (sis) mutants by screening mutagenized Arabidopsis seeds on high concentrations of sugars. The identification and characterization of three of the SIS genes, SIS7, SIS3 and SIS8, are presented here. SIS7 is allelic to NCED3/STO1, an abscisic acid (ABA) biosynthetic gene, which is involved in drought and salt stress responses. Lateral root (LR) development of sis7 mutants is resistant to the inhibitory effects of osmotica. Transcriptomic analysis revealed that a set of auxin-related genes are expressed at lower levels in sis7 seeds than in wildtype seeds when incubated with glucose, suggesting that these genes may be involved in controlling LR development by both ABA and auxin. SIS3 encodes a RING finger protein that functions as an E3 ligase in in vitro ubiquitination assays. The sis3 seeds display wild-type germination responses to ABA and GA. However, the root growth of sis3 mutants has slightly reduced sensitivity to ABA. The sis8 mutants have decreased sensitivity to high sugars and hyperosmolarity. Positional cloning of sis8 revealed that the mutation is in a putative mitogen-activated protein kinase kinase kinase gene. Seed germination assays indicate that sis8 mutants have wild-type sensitivity to ABA and GA, whereas overexpression of SIS8 causes slight hypersensitive responses. Potential interaction partners of SIS8 have been identified via yeast two-hybrid screening. A T-DNA insertion in the gene encoding one potential SIS8- interacting protein, UGT72E1, causes a sis phenotype. Further studies of the SIS3 and SIS8 genes will provide more insight into the mechanisms of sugar signaling in plants.