Az Eszterházy Károly Tanárképző Főiskola Tudományos Közleményei. 2004. Sectio Biologiae. (Acta Academiae Paedagogicae Agriensis : Nova series ; Tom. 25)

Mustárdy, L.: Tree-dimensional Organization of Thylakoid System in Granal Chloroplasts and its Functional Role

100 Mustárdy L. two of them. From this arrangement it was concluded, that the stroma thylakoids could intersect the granum at an angle. In the three-dimensional interpretation by Heslop-Harrison (1962) showed, that one stroma lamella interconnects all the grana compartments. The consequence of this arrangement was that all the intertylakoidal space (loculus or lumen) might be a continuum in the lamellar system. This idea was further persuade by Wehrmeyer (1964), who proposed that a single stroma lamella could exist in a spiral or helical arrangement around the granum being connected a few individual granum compartment. This general idea was elaborated by Paolillo (1970), who proposed that more than one stroma lamella could be helically attached and confirmed with the aid of a complete series of sections (Mustárdy and Brangeon 1978; Brangeon and Mustárdy 1979). The helical organization of the stroma lamellae around the grana can also be seen in other, more conventional (i.e. non-serial-section) electron micrographs. For instance, in thin sections the helices become evident when the section plane crosses the marginal region of the granum (Fig. 3a). In this relatively rare case, the thin section exposes the stroma membranes in their tilt positions and the granum stacks remain masked, or not included in the section. The helical arrangement of the stroma thylakoids can also be seen on freeze etching replicas (Staehelin, 1986) (Fig. 3b) and it is most clear in scanning electron micrographs of isolated grana obtained by sonication and differential centrifugádon (Mustárdy and Jánossy, 1979) (Fig. 3c). This figure shows that a helical strip of the stroma membranes covers the granum 'pillar'. Based on these electronmicroscopical observations a detailed computer model was constructed, showing the striking 3D structure with the stroma membrane helically wound around the granum (Mustárdy and Garab, 2003) (Fig. 4). These helical granum-stroma assemblies are fused together as it can be seen on the serial section electron micrographs, i.e. the cylindrical granum of stacked membranes surrounded by multiple helices of the stroma thylakoids, are interconnected by fusion at the 'outer edges' of stroma membranes (Fig. 5). Evidently, this can be extended to the entire chloroplast, which lends a substantial stability to the thylakoid membrane system. It is shown by sequential thin sections that - as a consequence of the helical organization - the stroma membranes originating from one level of the multilamellar granum run to different levels on the 'opposite side' (i.e. on the interconnected granum) (Fig. 5a-c). The electron micrographs also show that the width of the pillars, i.e. the relative lengths of the granum membranes with respect to the stroma thylakoids, can vary inside a single granum. This probably reflects local variations in the composition during the build-up of the granum structure. Clearly, transient de-stacking of membranes in the marginal sections can also