Macromolecular Trafficking Indicated by Localization and Turnover of Sucrose Transporters in Enucleate Sieve Elements

Abstract

The leaf sucrose transporter SUT1 is essential for phloem loading and long-distance transport of assimilates. Both SUT1 messenger RNA (mRNA) and protein were shown to be diurnally regulated and to have high turnover rates. SUT1 protein was detected by immunolocalization in plasma membranes of enucleate sieve elements (SEs) in tobacco, potato, and tomato. Analysis by in situ hybridization showed that SUT1 mRNA localizes mainly to the SE and is preferentially associated with plasmodesmata. Antisense inhibition of SUT1 expression under control of a companion cell (CC)-specific promoter indicated synthesis of SUT1 mRNA in the CC. These results provide evidence for targeting of plant endogenous mRNA and potentially SUT1 protein through phloem plasmodesmata and for sucrose loading at the plasma membrane of SE.

Sucrose transporter NtSUT1 confers aluminum tolerance on cultured cells of tobacco (Nicotiana tabacumL.)

“…S2). Consistent with the previous reports (Lemoine et al 1996;Kühn et al 1997;Schmitt et al 2008), NtSUT1 seemed to be localized at the plasma membrane, since NtSUT1-GFP fluorescence, but not GFP fluorescence, was observed at the Hechtian strands, a part of the plasma membrane connected to the cell wall, in the plasmolyzed cells (Lang et al 2004;Sasaki et al 2010).…”

“…4). In support of our findings, the presence of auxin-responsive elements was already reported in the promoters of sucrose transporters in tomato (Solanum lycopersicum L.), rice (Oryza sativa L.) and Arabidopsis (Kühn 2012). An OCS element (another type of auxin response element) was also reported in the CaMV 35S promoter (B. Zhang and Singh 1994), possibly explaining the requirement of 2,4-D for the augmented expression of CaMV 35S::NtSUT1 construct in BY 2 cells.…”

Expression of Sucrose Transporter cDNAs Specifically in Companion Cells Enhances Phloem Loading and Long-Distance Transport of Sucrose but Leads to an Inhibition of Growth and the Perception of a Phosphate Limitation

“…Posttranscriptional or posttranslational regulation that limited production, accumulation, or localization of the proteins are also possible explanations for the lack of rescue. For example, StSUT1 and BvSUT1 proteins in potato and sugar beet, respectively, were shown to have half lives in the range of several hours (Kühn et al, 1997;Vaughn et al, 2002), and differential rates of degradation among the proteins encoded by the transgenes tested here may have contributed to the lack of rescue. Notwithstanding, these negative results are worthy of mention because each of these SUTs localizes to the plasma membrane and catalyze membrane transport of Suc in heterologous organisms (yeast and/or X. laevis oocytes), showing that they do not have an obligate need of plant-specific cofactors or protein-protein interactions for activity.…”

An investigation of differences in fruit yield and components contributing to increased fruit yield in Japanese and Dutch tomato cultivars

“…This gene encodes the major isoform of the sucrose transporter in photosynthetic source leaves, which functions in phloem loading of sucrose in tomato plants (Kühn et al 1997;Hackel et al 2006;Yin et al 2010). No significant difference in LeSUT1 transcriptional level was observed between the cultivars at 10, 30, or 50 DAF.…”