Background. The levels of soluble sugars, such as glucose and sucrose, help regulate many plant metabolic, physiological and developmental processes. Genetic screens are helping identify some of the loci involved in plant sugar response and reveal extensive cross-talk between sugar and phytohormone response pathways. Results. A forward genetic screen was performed to identify mutants with increased resistance to the inhibitory effects of high levels of exogenous sugars on early Arabidopsis seedling development. The positional cloning and characterization of two of these sugar insensitive (sis) mutants, both of which are also involved in abscisic acid (ABA) biosynthesis or response, are reported. Plants carrying mutations in SIS7/NCED3/STO1 or SIS10/ABI3 are resistant to the inhibitory effects of high levels of exogenous Glc and Suc. Quantitative RT-PCR analyses indicate transcriptional upregulation of ABA biosynthesis genes by high concentrations of Glc in wild-type germinating seeds. Gene expression profiling revealed that a significant number of genes that are expressed at lower levels in germinating sis7-1/nced3-4/sto1-4 seeds than in wild-type seeds are implicated in auxin biosynthesis or transport, suggesting cross-talk between ABA and auxin response pathways. The degree of sugar insensitivity of different sis10/abi3 mutant seedlings shows a strong positive correlation with their level of ABA insensitivity during seed germination. Conclusion. Mutations in the SIS7/NCED3/STO1 gene, which is primarily required for ABA biosynthesis under drought conditions, confer a sugar-insensitive phenotype, indicating that a constitutive role in ABA biosynthesis is not necessary to confer sugar insensitivity. Findings presented here clearly demonstrate that mutations in ABI3 can confer a sugar-insensitive phenotype and help explain previous, mixed reports on this topic by showing that ABA and sugar insensitivity exhibit a strong positive correlation in different abi3 mutants. Expression profiling revealed a potentially novel regulation of auxin metabolism and transport in an ABA deficient mutant, sis7-1/nced3-4/sto1-4.
Bibliographical noteFunding Information:
We thank Drs. Laurie Host and Mauricio Bustos (University of Maryland, Baltimore County) for seeds homozygous for the abi3-16 (SALK_023411) mutation and Dr. Eiji Nambara (RIKEN Plant Science Center, Japan) for providing abi3-8, abi3-9, abi3-10, abi3-11, abi3-12 and abi3-13 mutants. Dr. Donna Pattison and Melissa Moon are thanked for assistance with early characterization of the sis7-1, sis7-3 and sis10-1 mutants and Jason Daniels and Mary Larson for assistance in cloning SIS10/ABI3. Michael Sean Kincaid is thanked for isolating the sis7-2 and sis9-1 mutants. We thank three anonymous reviewers for their comments and suggestions. We also thank the Arabidopsis Biological Resource Center (ABRC) and SALK Institute for providing T-DNA insertional lines and mutant seed stocks. The Minnesota Supercomputing Institute at the University of Minnesota is thanked for providing computational support. Research was supported by the United States Department of Energy, Energy Biosciences Program grant DE-FG02-03ER15417 and the Consortium for Plant Biotechnology Research grant DE-FG36-02GO12026-216.