The regular ultrastructure of isolated prolamellar bodies depends on the presence of membrane‐bound NADPH‐protochlorophyllide oxidoreductase

Abstract

Light‐induced alterations of isolated prolamellar bodies (PLBs) were studied in flash‐irradiated suspensions of a PLB‐enriched fraction and a mixed membrane fraction isolated from dark‐grown seedlings of wheat (Triticum aestivum L. cv. Walde). The mixed membrane fraction consisted of PLB fragments and membrane vesicles originating from the prothylakoids. Ultrastructural and spectral properties, as well as pigment and protein composition of non‐irradiated and of flash‐irradiated suspensions were studied. The addition of 0.3 mM NADPH prevented spectral shifts towards shorter wavelengths in irradiated as well as in non‐irradiated PLB‐fractions. as measured by fluorescence emission at – 196°C. In non‐irradiated PLB‐fractions the amount of phototransformable protochlorophyllide (PChlide) as compared to nonphototransformable PChlide decreased when NADPH was not added. The emission maximum due to chlorophyll(ide) shifted from 696 nm to 680 um in the flashirradiated fractions where no NADPH was added. The amount of chlorophyllous pigments, as well as the amount of NADPH‐protochlorophyllide oxidoreductase, decreased during the experimental period of 4 h in the suspensions without added NADPH. especially in the irradiated ones. The ultrastructure of the pelletable material in the different suspensions was analyzed by transmission and scanning electron microscopy. The non‐irradiated PLBs appeared as cottonball‐like structures in the scanning electron microscope. Without NADPH added more PLBs with an irregular tubular appearance were seen. After irradiation and storage for 1 h in darkness the surface was covered with vesicles. These vesicles were still present after 4 h. In the presence of NADPH no vesicle‐formation occurred and the regular network of the PLBs was preserved also after an irradiation which caused transformation of PChlide to chlorophyllide. Thus, the regular structure seems to depend on an ample supply of NADPH. which in turn may be necessary to stabilize the pigment‐protein complex in the lipid moiety of the PLB membranes. The formation of vesicles may thus be caused by a loss of this pigment‐protein complex in suspensions with a low level of NADPH. The possible significance of an NADPH‐dependence in vivo is discussed.