Abstract
Brassica napus leaves developed at low temperature display rapid in situ desaturation of monogalactosyldiacylglycerol (MGDG) fatty acids leading to the production of hexadecatrienoic/linolenic acid. This was shown by radioactivity-tracer experiments to occur via a sequence of desaturations proceeding from the initially synthesized palmitic/oleic acid molecular species to palmitic/linoleic acid, palmitoleic/linoleic acid, hexadecadienoic/linoleic acid, hexadecadienoic/linolenic acid, and finally to hexadecatrienoic/linolenic acid. The results suggest that there is increased activity in all five desaturation steps in leaves developed at low temperatures. Labeling data also indicate that there is another pool of MGDG which is more slowly desaturated before galactosylation to digalactosyldiacylglycerol (DGDG). Our data further suggest that relative rates of galactosylation of chloroplastic and cytosolic MGDG molecular species may regulate the final amounts of chloroplastic and cytosolic MGDG and DGDG in the leaf. We have proposed a model for chloroplastic biosynthesis and desaturation of galactosyldiacylglycerols in the leaves of Brassica napus, a 16:3 plant.