Genetic and Molecular Analysis of a New Control Pathway in Assimilate Partitioning. Final Report

摘要

Assimilate partitioning refers to the systemic distribution of photoassimilate from sites of primary assimilation (source tissue) to import-dependent tissues and organs (sinks). One of the defining questions in this area is how plants balance source productivity with sink demand. We discovered a sucrose-sensing signal transduction pathway that controls the activity of BvSUT1, a proton-sucrose symporter in sugar beet leaf tissue. Sucrose symporters are responsible for sucrose accumulation in the phloem of many plants and, therefore, they mediate the pivotal step in the long-distance transport of photoassimilate to non-photosynthetic tissues, such as roots and seed. We previously showed that sucrose transport activity is directly proportional to the transcription rate of BvSUT1 and that symporter mRNA and protein have high rates of turnover with half-lives on the order of 2 h. We further demonstrated that symporter transcription is regulated by sucrose levels in the leaf and that sucrose-dependent regulation of BvSUT1 transcription is mediated, at least in part, by a protein phosphorylation relay pathway. The goal of the experiments during this current grant were to use genetic and molecular approaches to identify essential components of this vital regulatory system. The initial objectives were to: (1) to characterize Arabidopsis mutants we've isolated that are resistant to growth inhibition by sucrose analogues that are recognized by the sucrose-sensor, (2) to screen for loss of function mutants in BvSUT1-promoter:luciferase transgenic plants that no longer respond to sucrose accumulation in the leaf using non-destructive visualization of luciferase activity, (3) to use gel mobility-shift assays and nuclease protection experiments to identify cis elements in the symporter promoter and DNA-binding proteins that are involved in sucrose regulation of symporter expression.