Evolution of plant sucrose uptake transporters

Abstract

In angiosperms, sucrose uptake transporters (SUTs) have important functions especially in vascular tissue. Here we explore the evolutionary origins of SUTs by analysis of angiosperm SUTs and homologous transporters in a vascular early land plant, Selaginella moellendorffii, and a non-vascular plant, the bryophyte Physcomitrella patens, the charophyte algae Chlorokybus atmosphyticus, several red algae and fission yeast, Schizosaccharomyces pombe. Plant SUTs cluster into three types by phylogenetic analysis. Previous studies using angiosperms had shown that types I and II are localized to the plasma membrane while type III SUTs are associated with vacuolar membrane. SUT homologs were not found in the chlorophyte algae Chlamydomonas reinhardtii and Volvox carterii. However, the characean algae Chlorokybus atmosphyticus contains a SUT homolog (CaSUT1) and phylogenetic analysis indicated that it is basal to all other streptophyte SUTs analyzed. SUTs are present in both red algae and S. pombe but they are less related to plant SUTs than CaSUT1. Both Selaginella and Physcomitrella encode type II and III SUTs suggesting that both plasma membrane and vacuolar sucrose transporter activities were present in early land plants. It is likely that SUT transporters are important for scavenging sucrose from the environment and intracellular compartments in charophyte and non-vascular plants. Type I SUTs were only found in eudicots and we conclude that they evolved from type III SUTs, possibly through loss of a vacuolar targeting sequence. Eudicots utilize type I SUTs for phloem (vascular tissue) loading while monocots use type II SUTs for phloem loading. We show that HvSUT1 from barley, a type II SUT, reverted the growth defect of the Arabidopsis atsuc2 (type I) mutant. This indicates that type I and II SUTs evolved similar (and interchangeable) phloem loading transporter capabilities independently.

Keywords: SUT; evolution; phylogeny; sucrose transporter.

Figure 1

Phylogenetic analysis of plant sucrose transporters and homologs. Protein alignment was done using Clustal X. Sequences with greater than 90% identity were not used in construction of the tree (they are shown in Table 1). The variable length N- and C-terminal regions were trimmed from the alignment. The maximum likelihood tree was generated using PhyML 3.0. Numbers indicate percent of 100 bootstrap analyses. Asterisk indicates the single charophyte SUT sequence, CaSUT1, from Chlorokybus atmosphyticus.

Figure 2

Substrate specificities of type I (AtSUC2, AtSUC9), type II (ShSUT1, HvSUT1), and type III (LjSUT4) plant sucrose transporters. Transport activity was assayed by expression in Xenopus oocytes and two-electrode voltage clamping. Oocytes were bathed in sodium Ringer solution containing substrates at concentrations between 0.5 and 25 mM (depending on the transporter affinity and substrate solubility). All currents were normalized to sucrose-dependent currents and are presented as mean ± SE with at least three oocytes per mean. *Indicates substrate not tested. Modified with permission from Chandran et al. (2003), Sivitz et al. (2005, 2007), Reinders et al. (2006, 2008).

Figure 3

Complementation of suc2-1 sucrose transporter mutant. (A) Arabidopsis plants heterozygous for the suc2-1 insertion left, plants homozygous for the suc2-1 insertion right. (B) Both AtSUC2 and HvSUT1 complemented the growth defect of the homozygous suc2-1 mutant. The WS wild-type (left) is shown for comparison. All plants shown in (A) and (B) are 8 weeks old.

Figure 4

Putative dileucine-like vacuolar targeting sequence in type III SUTs. A part of the multiple protein alignment of sucrose transporters is shown. All type III SUTs and selected type I and II SUTs are shown for comparison. Numbers indicate the amino acid positions for each protein. Amino acid positions that conform to the dileucine-like motif LXXLL are shown in bold.