Metabolic Bypass of the Tricarboxylic Acid Cycle during Lipid Mobilization in Germinating Oilseeds1

Abstract

Upon germination of oilseeds, storage triacylglycerols are mobilized by conversion to carbohydrates for transport to the root and shoot axes of the developing seedling. Suc, the major carbohydrate transport form, is used as a substrate for biosynthesis and is respired for energy. Lipid mobilization in postgerminative oilseeds requires the metabolic coordination of four subcellular compartments: the oil body, the glyoxysome, the mitochondrion, and the cytosol (Trelease and Doman, 1984). The elucidation of this complex interaction was dependent on the discovery of the glyoxylate cycle (Kornberg and Krebs, 1957). Fatty acids are cleaved by lipases from their glycerol backbone in the oil body and, after being transported to the glyoxysome, are degraded by ␤-oxidation to acetyl-CoA. The glyoxylate cycle ultimately catalyzes the condensation of two of these acetyl-CoA molecules to form succinate, which is then transported to the mitochondrion and metabolized by a partial TCA cycle. Since the complete TCA cycle catalyzes two decarboxylative reactions, the quantitative conversion of lipid to Suc can occur only if certain TCA cycle reactions are bypassed. During gluconeogenesis, only the TCA cycle activities of succinate dehydrogenase, fumarase, and malate dehydrogenase are required. The activities of the TCA cycle decarboxylative enzymes NAD ϩ -IDH and ␣-ketoglutarate dehydrogenase are avoided.Flux through the TCA cycle in plant tissues can vary, depending on the metabolic requirements of the tissue. For example, a reduction in the activity of the TCA cycle in the light compared with its activity in the dark has been documented (Gemel and Randall, 1992;Hanning and Heldt, 1993). The TCA cycle is regulated by the redox state of the pyridine nucleotide pool (Oliver and McIntosh, 1995). NADH competitively inhibits the activities of NAD ϩ -IDH, ␣-ketoglutarate dehydrogenase, and pyruvate dehydrogenase (although technically not a component of the TCA cycle, the pyruvate dehydrogenase complex is the entry point of glycolytically derived pyruvate into the TCA cycle). Furthermore, NAD ϩ -IDH is noncompetitively inhibited by NADPH (McIntosh and Oliver, 1992). The regulation of the TCA cycle in plants differs from its regulation in animals in that none of the plant enzymes appears to be controlled by ratios of adenine nucleotides, e.g. the ratio of ATP/ADP or of acetyl-CoA/CoA (Voet and Voet, 1990; Oliver and McIntosh, 1995).The proposed TCA cycle bypass was first demonstrated by in vivo labeling studies in castor bean endosperm (Canvin and Beevers, 1961). Radiolabeled acetate was shown to be converted into carbohydrate with an experimental efficiency of 70% for the methyl carbon of acetate and 30% for the carboxyl carbon of acetate. The efficiency of conversion was lower for the carboxyl carbon because it is this carbon that is lost from succinate at the reaction of PEP carboxykinase, the only decarboxylative step of gluconeogenesis. In 1 This research was supported by the National Science Foundation (grant IBN-9696154) and is a...

The sugar-insensitive1 (sis1) Mutant of Arabidopsis Is Allelic to ctr1

“…Although the molecular mechanism by which this inhibition occurs remains to be elucidated, the finding that seedlings lose their susceptibility to high sugar concentrations within approximately 48 h of the start of imbibition aids in developing possible models. For example, the timing with which Arabidopsis seedlings become resistant to high sugar concentrations is similar to the timing with which Brassica napus, a close relative of Arabidopsis, undergoes a dramatic metabolic shift (53). During the first 2 days after the start of imbibition, breakdown of B. napus seed storage lipids proceeds while the CO 2 -evolving reactions of the tricarboxylic acid cycle are bypassed.…”

ArabidopsisABA INSENSITIVE4 Regulates Lipid Mobilization in the Embryo and Reveals Repression of Seed Germination by the Endosperm

“…One possibility is that these apparently unused transcripts have been stored during quiescence, broken down after germination, and actively excluded from postgerminative translation. A study of the postgerminative induction of the TCA cycle enzyme isocitrate dehydrogenase also concluded that transcripts present before or shortly after germination were not translated (Falk et al, 1998), suggesting that this could be a general feature of the regulation of metabolism in germinating seeds. We hypothesize that the unused stored transcripts may themselves be a stored seed reserve that is broken down and the nucleotide components recycled for rapid de novo transcription during early postgerminative growth.…”

Timing the switch to phototrophic growth

“…23 Carbon flow through the full TCA cycle is delayed until the seedling is photosynthetically competent and no longer dependent on stored reserves. In the absence of exogenous sugars, the switch from autotrophic to phototrophic growth occurs in 2-3-day-old seedlings, 23 but in the presence of sucrose, lipid mobilization is retarded 24 and metabolic switching may be delayed. However, the effects of sucrose on lipid mobilization are not seen if sucrose is applied after 3 days.…”