Zur Bedeutung von Saccharose-Transportern in Pflanzen mit offener Phloemanatomie

Plants translocate photoassimilates, e.g. sucrose, through a specialized transport tissue called phloem. This long distance transport between different parts of the plant is essential for its development and yield. Today there are two ways under discussion by which metabolites, mainly sucrose, can get into this tissue. The first possibility is the uptake of sucrose from the apoplast into the phloem via sucrose transport proteins (SUTs) and is therefore called apoplastic phloem loading. The second possible way is through plasmodesmata and is called symplastic phloem loading. While the theory of apoplastic phloem loading is generally accepted, data concerning symplastic phloem loading are still rare. Therefore, two putative symplastically loading plant species belonging to the family of the Scrophulariaceae were characterized in detail. Alonsoa meridionalis translocates mainly stachyose, followed by raffinose and sucrose, while Asarina barclaiana translocates mainly sucrose. The phloem sap concentrations of sugars and amino acids are in the same range as in apoplastic phloem loaders. Although both species possess several features of symplastic phloem loaders, several sucrose transporters were successfully cloned and characterized. These are one sucrose transporter (AmSUT1) from A. meridionalis and two sucrose transporters (AbSUT1, AbSUT2) from A. barclaiana. They function as sucrose-H+-symporters and show only a weak sensitivity against the inhibitor PCMBS. The mRNAs of AmSUT1, AbSUT1 and AbSUT2 are detectable in the phloem sap of both plant species. The AmSUT1 protein is located specifically in the plasma membrane of phloem cells of minor and major veins, midrib and stem. This was shown by means of a polyclonal anti-AmSUT1-peptide-antibody. AmSUT1 is immunolocalized to companion cells as well as sieve elements of the phloem which indicates that AmSUT1 is responsible for sucrose uptake and retrieval into the phloem. The results of this work also indicate that the current theory of symplastic phloem loading has to be re-evaluated and that both investigated plant species belong to the apoplastic or mixed apoplastic-symplastic phloem loading type.