Comparison of the metabolic properties of plastids isolated from developing leaves or embryos of Brassica napus L.

Abstract

Plastids isolated from developing leaves and embryos of oilseed rape (Brassica napus L.) were incubated with substrates in the light or the dark, with or without exogenous ATP. Incorporation of HCO3-, and carbon from a range of substrates into fatty acids and/or starch by leaf chloroplasts was absolutely light-dependent and was unaffected by provision of ATP. Incorporation of HCO3- into fatty acids and/or starch by embryo plastids was also light-dependent. However, the light-dependent rates attained, when expressed on a comparable basis, were less than 32% of those from Glc6P (plus ATP), which was the most effective substrate for starch and fatty acid synthesis. In the light alone the rates of carbon incorporation from Glc6P, pyruvate and acetate into fatty acids, and from Glc6P into starch by embryo plastids were less than 27% of the respective ATP-dependent (dark) rates. Light had no effect on these ATP-dependent rates of synthesis by embryo plastids. While transporter activities for both glucose and Glc6P were present in embryo plastids, leaf chloroplasts did not have the latter activity. It is concluded that light at in vivo levels can contribute energy to carbon metabolism in embryo plastids. However, this contribution is likely to be small and these plastids are therefore largely dependent upon interaction with the cytosol for the ATP, reducing power and carbon precursors that are required for maximal rates of starch and fatty acid synthesis.