Characterization of Source- and Sink-Specific Sucrose/H+ Symporters from Carrot

Abstract

To understand how sucrose (Suc) is transported from source leaves to developing tap roots of carrot (Daucus carota L.), we cloned two cDNAs (DcSUT1 and DcSUT2) for proteins with homologies to plant Suc/H ؉ symporters. The deduced polypeptide sequences are 52% identical and have 12 predicted membranespanning domains each. Transport activities were confirmed by expression of the clones in yeast cells. Both transporters had optimal activity below pH 5.0 and Michaelis constant values of 0.5 mM. Suc uptake was inhibited by protonophores, suggesting that Suc transport is linked to the proton electrochemical potential across the plasma membrane. DcSUT1 and DcSUT2 had markedly different expression patterns. Transcripts of DcSUT1 were found only in the green parts of plants, with highest levels in the lamina of source leaves, indicating that DcSUT1 is required for the loading of Suc into the phloem. In leaf lamina expression was diurnally regulated, suggesting that Suc export from the leaves is higher during the day than during the night. The mRNA of DcSUT2 was found mainly in sink organs, and no diurnal expression pattern was detected in the storage root. Here, expression was not restricted to the phloem but was much higher in storage parenchyma tissues of phloem and xylem. The close relationship of DcSUT2 with a Suc/H ؉ symporter from fava bean, which facilitates Suc uptake into the cotyledons of developing seeds, indicates that this carrot Suc transporter may be involved in loading Suc into storage parenchyma cells.

JrSUT1, a putative xylem sucrose transporter, could mediate sucrose influx into xylem parenchyma cells and be up‐regulated by freeze–thaw cycles over the autumn–winter period in walnut tree (Juglans regia L.)

“…3). Other sink-specific sucrose transporters have been reported such as PmSUC1, the flower-specific sucrose transporter from P. major (Garthz et al 1996), VfSUC1, the seed-specific transporter from V. faba (Weber et al 1997) and DcSUT2, the sink-specific transporter from Daucus carota (Shakya & Sturm 1998). Moreover, JrSUT1 is seemingly not the sole walnut sucrose transporter since another cDNA was detected in the organs when the RT-PCR experiments were carried out with (Sut1A/Sut1B), a couple of primers designed from conserved regions.…”

A New Subfamily of Sucrose Transporters, SUT4, with Low Affinity/High Capacity Localized in Enucleate Sieve Elements of Plants

“…Despite having highly similar transport activities, members of the SUT family have diverse functional roles, including the loading of sucrose into phloem at the sieve elements (Kühn et al, 1997) or in companion cells (Stadler and Sauer, 1996), as well as sucrose uptake into sink cells such as seeds (Weber et al, 1997;Tegeder et al, 1998) and pollen (Lemoine et al, 1999;Stadler et al, 1999). All members of the SUT family characterized thus far catalyze high-affinity sucrose uptake with measured K m values for sucrose between 0.3 and 2 mM (Riesmeier et al, 1992(Riesmeier et al, , 1993Gahrtz et al, 1994Gahrtz et al, , 1996Sauer and Stolz, 1994;Boorer et al, 1996;Weig and Komor, 1996;Weber et al, 1997;Zhou et al, 1997;Shakya and Sturm, 1998). Thus, SUT1 corresponds to the HALC sucrose transport system described in leaves of V. faba (Delrot and Bonnemain, 1981) and B. vulgaris (Maynard and Lucas, 1982).…”

“…The first SUT cDNA clones were isolated from spinach and potato by expression cloning in yeast (Riesmeier et al, 1992(Riesmeier et al, , 1993. SUT1 and closely related homologs from various plants encode high-affinity proton-coupled sucrose transporters with a K m in the range of 0.3 to 2 mM (Riesmeier et al, 1992(Riesmeier et al, , 1993Gahrtz et al, 1994;Sauer and Stolz, 1994;Boorer et al, 1996;Weig and Komor, 1996;Hirose et al, 1997;Zhou et al, 1997;Shakya and Sturm, 1998). Its expression in Xenopus oocytes showed StSUT1 to have a 1:1 transport stoichiometry for sucrose:H ϩ (Boorer et al, 1996) and a slow transport rate with a rate-limiting step of ‫ف‬ 5 sec Ϫ 1 (Boorer et al, 1996).…”

Correction

“…The effect of redox reagents, under constant pH conditions, was much weaker than indicated in our published article. It cannot be excluded that the stimulus reported by Krü gel et al (2008) is partially due to targeting effects and/or secondary effects on the membrane potential.Previous measurements of sucrose uptake performed at lower pH values (pH ,4.5) gave rise to a decrease in uptake (AtSUT4 , DcSUT1, DcSUT2 [Shakya and Sturm, 1998], and AtSUT2 [Schulze et al, 2000]). However, in the case of the St SUT1, sucrose uptake was observed to increase steadily upon lowering the pH value ( Figure 9C).…”

“…Previous measurements of sucrose uptake performed at lower pH values (pH ,4.5) gave rise to a decrease in uptake (AtSUT4 , DcSUT1, DcSUT2 [Shakya and Sturm, 1998], and AtSUT2 [Schulze et al, 2000]). However, in the case of the St SUT1, sucrose uptake was observed to increase steadily upon lowering the pH value ( Figure 9C).…”