Protein phosphorylation plays a key role in sucrose-mediated transcriptional regulation of a phloem-specific proton-sucrose symporter.

Assimilate partitioning refers to the systemic distribution of sugars and amino acids 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. Recent results from our laboratory showed that sucrose transport activity is directly proportional to the transcription rate of the phloem-specific proton-sucrose symporter BvSUT1 in Beta vulgaris L. Moreover, symporter gene transcription is regulated by sucrose levels in the leaf. Here we show that sucrose-dependent regulation of BvSUT1 transcription is mediated, at least in part, by a protein phosphorylation relay pathway. Protein phosphatase inhibitors decreased sucrose transport activity, symporter protein and mRNA abundance, and the relative transcription rate of the symporter gene. In contrast, protein kinase inhibitors had no effect or increased sucrose transport, protein and mRNA abundance, and transcription. Furthermore, pre-treating leaves with kinase inhibitors before feeding with sucrose blocked the sucrose-dependent decrease in symporter transcription and transport activity. The latter observation provides direct evidence for a protein phosphorylation cascade operating between the sucrose-sensor and the transcriptional regulator that controls BvSUT1 expression and, ultimately, phloem loading.