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Examining the roles of sucrose synthase isoforms in Arabidopsis growth and development Yao, Danyu

Abstract

Sucrose synthase (SuSy) is one of the two enzyme families catalyzing the first step of sucrose utilization. It has been reported to serve different functional roles during plant growth and development, including supplying carbon for plant respiration, modulating sink strength and phloem loading, and facilitating the biosynthesis of starch and cell wall polymers. In a widely accepted model of cellulose biosynthesis, sucrose synthase is proposed to be tightly associated with the plasma membrane-localized cellulose synthase (CesA) complex and serves to channel carbon from photoassimilate (sucrose) directly to cellulose biosynthesis. Although many studies support this model, direct evidence of true interaction between SuSy and CesA is still lacking. As such, the primary objective of this thesis was to investigate the proposed model of cellulose biosynthesis using Arabidopsis thaliana as a model. The spatiotemporal localization of each of the six Arabidopsis SuSy proteins was investigated via live-cell imaging in a series of tissues including stems, roots, petioles, and siliques. Surprisingly, no single isoform of SuSy was detected in xylem, the major site of cellulose deposition in Arabidopsis plants. In contrast, SuSy1 and SuSy4 were expressed exclusively in phloem companion cells, SuSy5 and SuSy6 were confined to sieve elements, while SuSy2 and SuSy3 showed elevated expression in developing seeds. Thus, a new sus1/sus4/sus5/sus6 quadruple mutant was generated and examined for a putative cellulose/cell wall phenotype, however, the plants showed no obvious growth defect. This can be explained by the activity of invertase (INV) which may compensate for the lack of SuSy activity in phloem tissue. Furthermore, the effect of phosphorylation on SuSy4 activity and membrane association was examined. Results showed that both phosphomimetic and phosphoresistant SuSy4 were largely localized to the cytoplasm of companion cells, similar to that of the native SuSy4. When subjected to flooding, only SuSy4 phosphomimetic transgenic lines exhibited obvious reductions in soluble sugar and starch content. Collectively, these findings suggest a need to reconsider the established and largely accepted model of cellulose biosynthesis in Arabidopsis, and implicate SuSy in biological events related to phloem loading and carbon allocation.

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Attribution-NonCommercial-NoDerivatives 4.0 International