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

Three Dimensional Organisation of Thylakoid System. 103 stacks are present at this stage. Within one proplastid, bubble-like invaginations of the inner membrane of the envelope are evident (see inset Fig. 6a) but the large, flattened lamellae are never connected to the inner envelope. Although, the possibility that these vesicles elaborate into thylakoid sheets was proposed by several authors (Hodge et. al., 1956; Mühlethaler and Frey-Wyssling, 1959; Bradbeer et. al., 1974), it has still not been proven. The developing chloroplasts increase in size and become flattened. In the newly formed thylakoids most of the chlorophyll is present in the reaction centers. The light-harvesting complex, which facilitates stacking of membranes (Mullet and Arntzen, 1980), becomes associated only later with the reaction centers. In this stage of chloroplast development the newly synthesized thylakoids are disposed parallel to one another and to the equatorial plane of chloroplasts (Fig. 7a). In the partial serial section the aligned thylakoids are revealed as individual broad sheets often traversing the width of the plastid (Fig. 7b). However, when the sections cross in a right angle, lamellar connections are encountered at the plastid ends (Fig. 7a arrow-heads). During this differentiation step there is an occurrence of multiple perforations dispersed throughout the lamellar sheets. This phenomenon has been sited in a variety of membrane systems, particularly in a rapidly expanding membrane (Dalton and Hagueneau, 1968). The interruptions give a beaded appearance to the thylakoids in the cross-section (Fig. 7a inset) but in the reconstituted view these are revealed as slits or tears (Fig. 7c). In this developmental stage several double or triple stacks can be observed which overlap on the top of and beneath of the lamellar sheet (Fig. 7a, arrows). The three-dimensional reconstitutions show discs of varying diameter in the course of expansion and reveal an overgrowing phenomenon (simultaneous grow and apress). These reconstitutions sometimes disclose remnants of slits alongside the overlaps which suggests that the perforations may be the sites of grana initiation. It is very likely that the edges of the lamellae are the growth points, while at the periphery of the holes, the overlapping growth pattern above and below the single lamellar sheet gives rise to stacking. Very probable, that the appearance of light-harvesting complex at the growth points associated with PSII by its high self-aggregation capability initiates the granum formation. The lateral-aggregation of PSII particles via adhesion of peripheral light-harvesting complexes can lead to the formation of nearly homogenous region which are enriched in PSII and exclude PSI to a large extent. These so-called „macrodomains" through their head-to-head aggregations ensure the granal stacking (Garab et .al., 1991; Barzda et. al., 1994) and cause the separation of the two photosystems.