Carbon allocation in underground storage organs
Abstract: By increasing knowledge of carbon allocation in underground storage organs and using the knowledge to improve such crops, the competitiveness of these types of storage organs can be strengthened. Starch is the most common storage compound in tubers and roots, but some crops accumulate compounds other than starch. This thesis examined representative underground storage organs accumulating starch, oil and sugars. These were: the oil-accumulating nutsedge (Cyperus esculentus), a half-grass which possesses the unusual ability to accumulate triacylglycerol in considerable levels in small tubers physiologically resembling those of potato; the sucrose-storing taproot of sugar beet (Beta vulgaris); and the starch- and sugar-storing taproot of parsnip (Pastinaca sativa). The lack of starch formation in sugar beet was examined in relation to expression of starch biosynthesis-related genes and enzymes in the taproot. In parallel studies on potato (Solanum tuberosum), a classical starch accumulator, two different transgenic approaches to alter the metabolism and starch biosynthesis of tubers were tested. A novel finding was that expression of the oil transcription factor WRINKLED1 in potato tubers resulted in tubers accumulating oil and negatively affected starch biosynthesis. The oil-accumulating potato tubers shared structural similarities with young nutsedge. Assays on the transcriptome of sugar beet and parsnip revealed that transcripts of two plastidial genes responsible for energy import and phosphate hydrolysis were lower in sugar beet than in parsnip, indicating potential importance for starch accumulation. In potato, the importance of these two genes was assayed by silencing the genes. The outcome was potato tubers with severely affected starch biosynthesis, granule morphology, tuber yield, tuberisation and starch quality, confirming that the silenced genes play an important role in starch biosynthesis in potato tubers.
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