Arabidopsis Sucrose Transporter AtSUC9. High-Affinity Transport Activity, Intragenic Control of Expression, and Early Flowering Mutant Phenotype

Abstract

(C.P.L.G., J.M.P.) AtSUC9 (At5g06170), a sucrose (Suc) transporter from Arabidopsis (Arabidopsis thaliana) L. Heynh., was expressed in Xenopus (Xenopus laevis) oocytes, and transport activity was analyzed. Compared to all other Suc transporters, AtSUC9 had an ultrahigh affinity for Suc (K 0.5 5 0.066 6 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 a-methyl-Dglucose. The ability of AtSUC9 to transport 10 glucosides was compared directly with that of AtSUC2, HvSUT1 (from barley [Hordeum vulgare]), and ShSUT1 (from sugarcane [Saccharum hybrid]), and results indicate that type I and type II Suc transporters have different substrate specificities. AtSUC9 protein was localized to the plasma membrane by transient expression in onion (Allium cepa) epidermis. Using a whole-gene translational fusion to b-glucuronidase, 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 true for AtSUC1 (At1g71880) but not AtSUC2. Plants containing mutations in Suc 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 Suc at very low extracellular Suc concentrations. The mutant phenotype of atsuc9 alleles indicates that AtSUC9 activity leads to a delay in floral transition.

Proton-Dependent Coniferin Transport, a Common Major Transport Event in Differentiating Xylem Tissue of Woody Plants

“…S6, A and B). The possibility that a Suc transporter would be involved in coniferin transport (Chandran et al, 2003;Sivitz et al, 2007) was tested, but coniferin transport was barely influenced in the presence of Suc (Supplemental Fig. S6C).…”

Genetic Control of Carbon Partitioning in Grasses: Roles of Sucrose Transporters and Tie-dyed Loci in Phloem Loading

“…As has been observed previously in dicots, in all the grass genomes available, there is only a single group 3 or 4 SUT gene (the two group 3 rice genes reported by Sauer [2007] are alleles from different cultivars). In dicots, the group 2 SUT genes underwent duplication and neofunctionalization (Baud et al, 2005;Sivitz et al, 2007Sivitz et al, , 2008, and in monocots, the group 1 SUTs similarly expanded (Fig. 2).…”

A Cascade of Sequentially Expressed Sucrose Transporters in the Seed Coat and Endosperm Provides Nutrition for the Arabidopsis Embryo

“…SUC5 could also act at the interface between the endosperm and embryo, since it is expressed in the endosperm and specifically the epidermis of the outer surface of the cotyledons (Supplemental Figure 7B) (Pommerrenig et al, 2013). Data from a SUC9 translational fusion to GUS indicated that SUC9 protein was also present in the embryo (Sivitz et al, 2007), although no transcriptional GUS activity was detected in seeds (Sauer et al, 2004). All these reports are in agreement with the observation that in vitro dissected embryos from the globular stage can reach the mature stage when grown in media supplemented with sucrose.…”