PARENT SESSION

Symposium S1C Photosynthate transport and transporters
Monday August 30th, 2004 10:20 AM-12:20 PM Room 510A
Chair: Andreas Weber
Co-Chair: Marinus Pilon

Dynamic regulation of a proton-sucrose symporter in phloem loading and assimilate partitioning. Daniel Bush^{*,1, 2, 3}, Matt Vaughn, Wendy Ransom-Hodgkins², Greg Harrington², ¹ Photosynthesis Research, USDA-ARS, Urbana, IL, USA² Department of Plant Biology, USA³

ABSTRACT- A proton-sucrose symporter mediates the key transport step in plants that utilize apoplastic phloem loading. We have described a previously unknown system in which sucrose acts as a signaling molecule in a pathway that regulates the activity of the sucrose symporter found in companion cells of sugar beet leaves. Symporter transport activity dropped to 35-50% of water controls when leaves were fed 100 mM sucrose. Western analysis has shown that this decrease in transport activity was caused by a decline in the abundance of symporter protein. RNA gel blot analysis of the leaf symporter showed that symporter message decreased in parallel with the loss of transport activity, and nuclear run-on experiments showed that this was the result of decreased transcription. Further analysis showed symporter protein and message are both degraded with half-lives of about 2 hr, suggesting the symporter represents a tightly regulated step in assimilate partitioning. While investigating various secondary messengers as possible contributors to sucrose signaling, we uncovered evidence implicating protein phosphorylation. Phosphatase inhibitors decrease transport activity and block symporter expression. Protein kinase inhibitors have no effect on or, for some classes of inhibitors, stimulate transcription. Significantly, pretreatment with kinase inhibitors completely blocks the sucrose response, suggesting a protein kinase is down-stream of the sucrose sensor. In addition to our observations with sugar beet, we have measured similar changes in symporter expression in sucrose treated soybean, spinach and Arabidopsis, suggesting this may be a widely distributed response pathway. Reduced sink strength is predicted to cause repression of symporter activity, accumulation of photoassimilate in source tissues and, ultimately, reduced photosynthetic activity. Taken together, these results support our hypothesis that this response pathway regulates assimilate partitioning and photosynthetic activity by modulating phloem loading in response to changing levels of "sink demand".

KEY WORDS: assimilate partitioning, sucrose transport