SUCROSE-MEDIATED TRANSCRIPTIONAL REGULATION OF SUCROSE SYMPORTER ACTIVITY IN THE PHLOEM

Authors: VAUGHN, MATT - PLANT BIOLOGY UOFI URBANA; BUSH, DANIEL

Submitted to: Proceedings of the National Academy of Sciences (PNAS)
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/24/2002
Publication Date: 12/20/2002
Citation: VAUGHN, M., BUSH, D.R. SUCROSE-MEDIATED TRANSCRIPTIONAL REGULATION OF SUCROSE SYMPORTER ACTIVITY IN THE PHLOEM. PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES. 2002. v. 99. p. 10876-10880.

Interpretive Summary: Plant leaves capture light energy from the sun and transform that energy into a useful form in the process called photosynthesis. The primary product of photosynthesis is sucrose. Generally, 50 to 80% of the sucrose synthesized is transported from the leaf to supply organic nutrients to many of the edible parts of the plant such as fruits, grains, and tubers. This resource allocation process is called assimilate partitioning and alterations in this system are known to significantly affect crop productivity. The results reported in this paper show that sucrose plays a second vital role in assimilate partitioning by acting as a signal molecule that regulates the activity and gene expression of the transporter that mediates long-distance sucrose transport. This discovery represents a major breakthrough in our understanding of the processes that regulate assimilate partitioning and identifies that signaling pathway as an excellent target for using biotechnology to modify crop productivity. These results will be used by academic and industry scientists to develop new methods to modify the nutritional value and/or yield of harvested tissues.

Technical Abstract: Our laboratory has described a previously unknown pathway in which sucrose acts as a signaling molecule to regulate activity of a sugar beet leaf proton-sucrose symporter. Sucrose transport activity declines in plasma membrane vesicles isolated from leaves fed sucrose via the transpiration stream for 24 hours while alanine transport are unchanged. Northern analysis indicates that symporter mRNA decreases while kinetic analysis of sucrose transport suggests that the observed change in activity is caused by decreased abundance of symporter protein. To further dissect this response, we determined the cellular localization of sucrose symporter expression and examined how the major points of the gene expression pathway responded to sucrose feeding. Symporter expression was limited to phloem companion cells. Symporter transcription was depressed significantly within two hours of feeding 100 mM sucrose while stability of the mRNA was found to be unaffected by sucrose. However, both the symporter mRNA and protein were rapidly turned over, with half-lives on the order of 2 hours. Finally, symporter protein abundance decreased in a sucrose feeding concentration-dependent manner and the decrease was collinear with the change in sucrose transport activity. Taken together, the evidence supports our hypothesis that this response pathway acts as a first step in regulating assimilate partitioning by modulating phloem loading in response to changing levels of sink demand.