The diversion of carbon from carbohydrate and other nonlipid molecules to lipid represents another possible strategy for enhancing seed oil content. The relationship of carbohydrate catabolism to lipid biosynthesis in photosynthetic developing seeds of B. napus is outlined in Figure 1.2. Reactions that have been shown to have a substantial effect on seed oil accumulation are identified with an asterisk.
Tomlinson et al. (2004) hypothesized that a significant degree of control over the flow of carbon into seed oil was associated with pathways of sucrose catabolism and entry into metabolism. Invertase and hexokinase represent one of two routes through which sucrose can enter metabolism, with activities of these enzymes also having possible effects on the status of sugar-sensing pathways. Using constructs with seed-specific promoters and yeast genes encoding invertase or hexokinase, the investigators introduced additional invertase and hexokinase activity into the apoplast and cytosol of developing tobacco (N. tabacumL.) seeds, respectively. The yeast enzymes were expressed alone in tobacco seed or in combination. Despite enormous increases in the activities of these enzymes during seed development, there was essentially no effect on seed oil accumulation, indicating that control over oil accumulation is associated with other levels of metabolism or metabolite transport.
Mitochondrial pyruvate dehydrogenase (PDH) complex catalyzes the first committed step in respiratory carbon metabolism and represents a link between glycolytic carbon metabolism and the tricarboxylic acid cycle. Seed-specific antisense repression of the gene encoding mitochondrial pyruvate dehydrogenase kinase (PDHK), a negative regulator of the mitochondrial PDH complex, has been shown to result in increased seed oil content and average seed weight in Arabidopsis (Zou et al. 1999a, Marillia et al. 2003). Identical experiments conducted with B. napus produced similar results, and in preliminary field trials, the seed oil contents of B. napus antisense mitochondrial PDHK transgenics were increased by 3%-5% (Marillia and Taylor, unpublished data). Feeding studies with [3-14C]pyruvate, using siliques of transgenic Arabidopsis, supported the hypothesis that the observed increase in seed oil accumulation was attributable to an increased supply of acetyl-CoA from the mitochondria due to the enhanced action of the PDH complex in the decarboxylation of pyruvate (Marillia et al. 2003). It was suggested that mitochondrial-generated acetyl-CoA was probably hydrolyzed to free acetate in the mitochondria. It was further suggested that the acetate may in turn move into the plastid for conversion into acetyl-CoA by the catalytic action of plastidial acetyl-CoA synthetase. Li et al. (2006) used subtractive
Sucrose Glu6PDH^ OPPP Glc6P
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