Lipids and function of etiochloroplasts after ultraviolet, blue and red light-illumination.

Abstract

The morphogenesis of whole plants and the development of cell organelles are highly affected or regulated by light-treatment. Chloroplasts normally develop from proplastids when illuminated. In complete darkness, proplastids develop into etioplasts with an assembly of characteristic prolamellar bodies. Light-treatment induces the disappearance of the prolamellar bodies and the development of photosynthetically active thylakoids. Besides chlorophyll, nearly all lipids are already present in etioplasts. Some authors divide the thylakoid lipids into more functional moieties as chlorophylls, carotenoids or lipophilic plastidquinones, and into more structural lipids as the glycerolipids, but it must be said that the function of glycoand phospho-lipids has not yet been elucidated. Further, it is not clear what role the glycerolipids play during physiological plastid development and how the lipid synthesis is regulated. Therefore we performed experiments with etiolated tissues and isolated etioplasts from Hordeum vuIgure L. (cv. Breuns Villa) tQ obtain information about the participation of glycerolipids in etiochloroplast development following continuous illumination. The etiochloroplasts were isolated from illuminated tissues by a density-gradient procedure, the fractions were checked for contamination with other organelles by marker-enzyme measurements. The plastid fraction showed contamination with other material of less than 1 %. In further experiments we used short-time irradiation with different light-qualities for studying regulation phenomena for lipid metabolism. As reported earlier, the lipid composition of etiolated barley seedlings changed by continuous irradiation with white and red light (Tevini & Herm, 1974). Especially within the first 8 h of illumination, a special metabolism of glycerolipids and their fatty acids could be seen. We used light with the following intensities: white light, 0.8 W/mZ; red light, 0.5W/mZ; blue light, 0.8W/mZ. In these early stages of development white, red and blue light enhanced the phospholipid synthesis, whereas the glycolipid synthesis showed the opposite effect (Figs. la and lb). The same holds true when lipid values are related to a constant plastid number (lo9). We infer that a part of the galactolipids as the main constituents of the prolammellar system and plastoglobuli in etioplasts must be hydrolysed before use for thylakoid assembly. Phospholipids may play the role of fatty acid carriers for newly formed galactolipids. Long-wave U.V. light (interference filter with Amax. = 366nm, 0.5 W/mZ) on the other hand enhances the galactolipid synthesis during the first hours of illumination. This may be a reflection of partly inhibited prolamellar-body breakdown and small chlorophyll synthesis, so that all newly formed galactolipids can be measured. Protochlorophyll-absorption measurements show no 'Shibata-shift' meaning that no phytylation takes place under these low light-intensity conditions. In contrast with all other light-qualities U.V. light causes inhibition of phospholipid synthesis during the first hours of illumination and later on even the breakdown of phospholipids can be seen by the formation of lysoglycerides. Blue light, given at the same intensity as red light, induces higher chlorophyll amounts and an earlier appearance of normal chlorophyll a/chlorophyll b ratios than red light. The photosynthetic activity of greening etioplasts, isolated from redand blue-light-illuminated tissues by repeated density-gradient centrifugation, develops somewhat earlier under blue light. Even on a chlorophyll basis the dichlorophenolindophenoland ferricyanide-reduction rates of blue-light-illuminated chloroplasts are considerably higher than those of red-light-illuminated chloroplasts (Table 1). The analysis of the ultrastructure of etiochloroplasts (M. Tevini & K. Herm, unpublished work) proves the earlier appearance of multilayered grana stacks ,