Plant Physiology Preview
Published on November 10, 2006; 10.1104/pp.106.089003
Received August 29, 2006
Accepted November 1, 2006
Arabidopsis Sucrose Transporter AtSUC9. High Affinity Transport Activity, Intragenic Control of Expression and Early-flowering Mutant Phenotype
Alicia B. Sivitz , Anke Reinders , Meghan E. Johnson , Anthony D. Krentz , Christopher P. L. Grof , Jai M. Perroux , and John M. Ward *
Department of Plant Biology, University of Minnesota Twin Cities, 1445 Gortner Ave., 250 Biological Sciences Center, St. Paul, MN 55108
CSIRO Plant Industry, Queensland Bioscience Precinct, 306 Carmody Road, QLD, 4067 St. Lucia, Australia; Cooperative Research Centre for Sugar Industry Innovation through Biotechnology, The University of Queensland, St Lucia, QLD, 4072
* Corresponding author; email: jward{at}tc.umn.edu.
AtSUC9 (At5g06170), a sucrose transporter from Arabidopsis thaliana (L. Heynh.), was expressed in Xenopus oocytes and transport activity was analyzed. Compared to all other sucrose transporters, AtSUC9 had an ultra-high affinity for sucrose (K0.5 = 0.066 ± 0.025 mM). AtSUC9 showed low substrate specificity, similar to AtSUC2 (At1g22710), and transported a wide range of glucosides including helicin, salicin, arbutin, maltose, fraxin, esculin, turanose, and  -methyl D glucose. The ability of AtSUC9 to transport ten glucosides was compared directly with that of AtSUC2, HvSUT1 (from barley) and ShSUT1 (from sugarcane) and results indicate that Type I and Type II sucrose transporters have different substrate specificities. AtSUC9 protein was localized to the plasma membrane by transient expression in onion epidermis. Using a whole-gene translational fusion to GUS, AtSUC9 expression was found in sink tissues throughout the shoots and in flowers. AtSUC9 expression in Arabidopsis was dependent on intragenic sequence and this was found to also be the true for AtSUC1 (At1g71880) but not AtSUC2. Plants containing mutations in sucrose transporter gene AtSUC9 were found to have an early-flowering phenotype under short-day conditions. The transport properties of AtSUC9 indicate that it is uniquely suited to provide cellular uptake of sucrose at very low extracellular sucrose concentrations. The mutant phenotype of atsuc9 alleles indicates that AtSUC9 activity leads to a delay in floral transition.
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