Ultrastructural and enzymatic research on the role of sucrose in mobilization of storage lipids in germinating yellow lupine seeds

The role of sucrose in the control of storage lipid mobilization was studied in germinating seeds of Lupinus luteus L. Concentrations of soluble sugars in isolated embryo axes, isolated cotyledons and in seedling axes and cotyledons cultured in vitro for 96 h on a medium without sucrose was maintained at a low level. One of the strategies of stabilization of the level of soluble sugars was their conversion into starch. Ultrastructural observations showed that in dry and imbibed seeds, storage lipids concentrated mainly in cotyledons but they were found also in embryo axes. The lack of sucrose in the medium used for in vitro culture of isolated organs and seedlings markedly accelerated the catabolism of storage lipids and other storage compounds (hemicelluloses, starch, and proteins). The deficit of soluble sugars in tissues stimulated the activity of lipase (EC 3.1.1.3) and catalase (EC 1.11.1.6), but inhibited the activity of NADP+-dependent (EC 1.1.1.42) and NAD+-dependent (EC 1.1.1.41) isocitrate dehydrogenase (IDH). An addition of sucrose to the medium elevated the level of soluble sugars in tissues. Transcription inhibitors (actinomycin and cordycepin) added to the medium cancelled the increase in lipase activity caused in isolated embryo axes by carbohydrate starvation. This indicates that lipase activity is controlled by sugar level at the stage of transcription. The stimulating effect of sucrose on the activity of NADP+-dependent IDH was cancelled by cordycepin, while on NAD+-dependent IDH, by actinomycin. Cycloheximide, irrespective of its time course, did not cancel the stimulating effect of sucrose on IDH activity. The results indicate that a low level of soluble sugars in tissues accelerates the mobilization of storage materials, while a high level can stimulate the transfer of carbon from lipids into amino acids.