Chloroplast Development: Fine Structure and Chlorophyll Synthesis

Abstract

The light-induced development of chloroplast membranes has been widely studied and models describing the course of this development have been proposed by various investigators. The development of the chloroplanst in higher plants as described by von Wettstein is briefly reviewed. Revisions of this scheme are suggested, based on more recent data, especially work using improved fixation methods for electron microscopy. Transformation of the prolamellar body takes place via a direct conversion of the prolamellar body into thylakoids, without an intervening stage of vesicle formation. The conversion is through the extension of membranes from the prolamellar body, non through and alignment of discrete vesicles which have broken off from the prolamellar body. The development of the etioplast and formation of the prolamellar body have been shown by recent work to involve growth of continuous membranes which may be attached to the inner membrane of the chloroplast envelope as well as the prolamellar body. Thus, considerable evidence has accumulated that the plastid membranes maintain their continuity to a great extent-in the mature chloroplast, during formation of prolamellar bodies in the dark, and during light-induced thylakoid formation. Discrete vesicles which break apart and re-fuse under different conditions are probably the result of fixation artifacts, particularly with potassium permanganate. That continuous membranes rather than discrete vesicles are present can be seen particularly well with glutaraldehyde fixation. Glutaraldehyde fixation also reveals that the membranes contain perforations (perforated or reticulate thylakoids) at particular times during the development of the chloroplast. Their presence has now been well documented during prolamellar body formation and the initial stages of thylakoid formation from the prolamellar body. During subsequent stages of thylakoid development, they disappear. Some earlier workers concluded that thylakoid synthesis involved the inner membrane of the plastid envelope as a source of vesicles which budded off and then refused after they had migrated to the center of the plastid. That the inner membrane does have a role in thylakoid synthesis has been substantiated by more recent work. Invaginations of the inner membrane have been reported to take place through both the formation of tubules and the elaboration of continuous sheets. There are also various types of inclusions found in the developing plastids, some of them tubular in nature, whose role is not clear. Althoug earlier work established that the prolamellar body was a thylakoid precursor when etiolated seedlings were exposed to light, its role in plants developing normally in the light was uncertain. Recent reports have greatly expanded our information about the prolamellar body. It appears under a variety of conditions, and in many species of light-grown plants. It has also been reported to occur in several species of algae, although its role in membrane formation in these cases is less certain. The control of prolamellar body structure and formation by light and its role as a thylakoide precursor is discussed. Intraplastid membrane-bound bodies have been examined by a number of authors in several types of tissues and some of them appear to have a role in supplying precursors dirung membrane synthesis. The possible role of chlorophyll synthesis in the development of the membranes is also examined. Certain stages of chlorophyll synthesis and particular changes in the membrane seem to be associated, in that they occur simultaneously. Unfortunately, the evidence pertaining to these relationships is indirect and often contradictory. Although new evidence has failed to demonstrate conclusively what role chlorophyll and its precursors might play in membrane development, it has had the result of casting doubt on some of the old assumptions. Prolamellar bodies are found under conditions where chlorophyll is accumulating, suggesting that accumulation of the chlorophyll precursor, protochlorophyllide, is not always essential for prolamellar body formation. In contrast to previous reports, prolamellar body transformation and protochlorophyllidae photoreduction can be separated and are not necessarily simultaneous events. The photoreceptor is not known for any of the stages in membrane development which are regulated by light.