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Title: PROTEIN PHOSPHORYLATION PLAYS A ROLE IN SUCROSE-REGULATION OF SUCROSE SYMPORTER EXPRESSION, PHLOEM LOADING AND ASSIMILATION PARTITIONING

Author
item VAUGHN, MATT - PLANT BIOLOGY UOFI URBANA
item RANSOM-HODGKINS, WENDY - PLANT BIOLOGY UOFI URBANA
item BUSH, DANIEL

Submitted to: Proceedings of International Plant Membrane Transports
Publication Type: Proceedings
Publication Acceptance Date: 6/24/2001
Publication Date: N/A
Citation: N/A

Interpretive Summary:

Technical Abstract: A proton-sucrose symporter mediates the key transport step in plants that utilize apoplastic phloem loading. We have described a previously unknown pathway 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 percent 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 also decreased 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 two hour. 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 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".