Macromolecular trafficking indicated by localization and turnover of sucrose transporters in enucleated sieve elements

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 plasmamembranes 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 inhi-bition 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. In plants, most cells are separated from each other by cell walls. Despite this sepa-ration, the majority of cells are intercon-nected by plasmodesmata, analogous to gap junctions. The function of these structure