Project description:Sucrose synthase (SuSy) is considered the first key enzyme for secondary growth because it is a highly regulated cytosolic enzyme that catalyzes the reversible conversion of sucrose and UDP into UDP-glucose and fructose. Although SuSy enzymes preferentially functions in the direction of sucrose cleavage at most cellular condition, they also catalyze the synthetic reaction. We isolated a gene that encodes a SuSy from Populus simonii×Populus nigra and named it PsnSuSy2 because it shares high similarity to SuSy2 in Populus trichocarpa. RT-PCR revealed that PsnSuSy2 was highly expressed in xylem, but lowly expressed in young leaves. To characterize its functions in secondary growth, multiple tobacco overexpression transgenic lines of PnsSuSy2 were generated via Agrobacterium-mediated transformation. The PsnSuSy2 expression levels and altered wood properties in stem segments from the different transgenic lines were carefully characterized. The results demonstrated that the levels of PsnSuSy2 enzyme activity, chlorophyll content, total soluble sugars, fructose and glucose increased significantly, while the sucrose level decreased significantly. Consequently, the cellulose content and fiber length increased, whereas the lignin content decreased, suggesting that PsnSuSy2 plays a significant role in cleaving sucrose into UDP-glucose and fructose to facilitate cellulose biosynthesis and that promotion of cellulose biosynthesis suppresses lignin biosynthesis. Additionally, the noticeable increase in the lodging resistance in transgenic tobacco stem suggested that the cell wall characteristics were altered by PsnSuSy2 overexpression. Scanning electron microscopy was performed to study the cell wall morphology of stem, and surprisingly, we found that the secondary cell wall was significantly thicker in transgenic tobacco. However, the thickened secondary cell wall did not negatively affect the height of the plants because the PsnSuSy2- overexpressing lines grew taller than the wildtype plants. This systematic analysis demonstrated that PsnSuSy2 plays an important role in cleaving sucrose coupled with cellulose biosynthesis in wood tissue.

Project description:The within-tree variation in wood properties constitutes an exceptional model to study the mechanisms that adjust the different biosynthetic pathways providing substrates with the massive and variable demands of different biosynthetic reactions of cell wall polymers. Although a few genes have been reported as differentially expressed in differentiating compression wood compared to normal or opposite wood, the expression of a larger set of genes is expected to change due the broad range of features that distinguish this reaction wood. By combining the construction of different cDNA libraries with microarray analyses, using samples from different Pinus pinaster provenances collected in different years and geographic locations, we have identified a total of 496 genes that change their expression during differentiation of compression wood (331 up-regulated and 165 down-regulated compared to opposite wood). Consistent with the well-known structural and chemical characteristics of compression wood, a large number of genes involved in the biosynthesis of cell wall components were shown to be up-regulated during compression wood differentiation, including genes involved in synthesis of cellulose, hemicellulose, lignin and lignans. In particular, further analysis of a set of these genes involved in providing S-adenosylmethionine, ammonium recycling, lignin and lignans biosynthesis showed parallel expression profiles to levels of lignin accumulation in cells undergoing xylogenesis in vivo and in vitro. The comparative transcriptomic analysis of compression and opposite wood formation in this work have revealed a broad spectrum of coordinated transcriptional modulation of biosynthetic reactions for different cell wall polymers associated to within-tree variations in softwood structure and composition. In particular, it suggest the occurrence of a mechanism that modulates at transcriptional level genes encoding enzymes involved in S-adenosylmethionine synthesis and ammonium assimilation with coniferyl alcohol demand for lignin and lignan synthesis, as a key metabolic requirement in cells undergoing lignification.

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Overexpression of a developing xylem cDNA library in transgenic poplar generates high mutation rate specific to wood formation.

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