New insights into carbon transport and incorporation to wood
Abstract: Wood formation in trees requires carbon import from the photosynthetic tissues. In several tree species the majority of this carbon is derived from sucrose transported in the phloem. This thesis describes experimental work on the mechanism of radial sucrose transport from phloem to developing wood and subsequent incorporation of carbon into wood polymers. I investigated the role of active sucrose transport during secondary cell wall formation in hybrid aspen (Populus tremula x tremuloides). Reduction of a plasma membrane localised sucrose transporter (PttSUT3) decreased carbon allocation to secondary walls of wood fibers. The results show the importance of active sucrose transport for wood formation in a symplasmic phloem loading tree species, and identify PttSUT3 as a principal transporter for carbon delivery into secondary cell wall forming wood fibers. To investigate the temporal dynamics of carbon transport and wood polymer biosynthesis, I labelled two-month-old hybrid aspen trees with ¹³CO₂ and tracked the flux of ¹³C from leaves to developing wood. Analysis of the cell wall polymer labelling patterns using 2D-NMR revealed temporal differences in the labelling of carbohydrates and lignin subunits. Further analysis showed that ¹³C incorporation into different wood polymers is influenced by the diurnal cycle. Primary metabolism enzymes in the developing wood play an important role in carbon partitioning to wood cell wall polymers. In this part of the study, the activity of eight primary metabolism enzymes linking sucrose to cell wall precursor biosynthesis was determined in phloem, cambium and in different stages of wood development. Comparison of enzyme activity measurements with transcript and metabolite profiles across the developing wood suggested a central role for transcriptional regulation of carbon allocation to wood. Combined, the results of the three projects provide new insights into the mechanism and regulation of carbon allocation to developing wood.
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