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

102 Mustárdy L. 1995). It is tempting to speculate that the repair, which is a complex process, can be performed only at certain sites where both the key enzymes and the ribosomes are available, e.g. on the 'top' of the stroma membranes which is fully exposed to the stroma liquid. The spiral organization of these membranes and the multiple connections to the granum can facilitate the diffusion of the components from these special sites to any target. A similar mechanism may apply virtually to all components of the granum membranes, which are inserted in the stroma membranes and then 'slide' to the stacked region. At the same time the openings on the 'sacs' ensure ionic conductance in a lateral direction between the inner aqueous phases of granum and stroma thylakoids. Further, stroma thylakoids by spiralling up and around the stacks ensure that all inner compartments of a granum are electrically connected. Finally, fusions of adjacent stroma thylakoids wound around neighboring stacks interconnect adjacent grana (cf. Mustárdy 1996). By this means the inner aqueous phases of all thylakoids in the chloroplast become electrically connected to each other. The electrochemical gradients are rapidly 'delocalised' for the entire thylakoid system, and thus the transmembrane pH and electric potential gradients become essentially uniform (Junge 1977; Mitchell 1977). Ontogenetic assembly of thylakoids The spatial relationship of grana and stroma thylakoids during chloroplast differentiation can be most successfully studied by serial section analysis at different levels of ontogenesis. This approach was taken with Lolium multiforum seedlings (Mustárdy and Brangeon, 1978; Brangeon and Mustárdy, 1979) in which four developmental stages of leaves were selected to follow the ontogenetic assembly of thylakoids by analysis and reconstitution of serial sections of developing chloroplasts. In meristematic tissues the intrachloroplastic lamellae show a few prothylakoids of varying length distributed apparently at random within the proplastids (Fig. 6a). However, views of reconstituted serial sections reveal a single lamellar entity rather than separate and independent units. Thylakoids can be traced to a „parent entity" which has apparently split and given rise to several branches of different length. In some cases, the pro-thylakoid form is flattened and funlike, although apparently flexible and capable of changing orientation (Fig. 6b). In crossing this lamellar sheet, sharp cross-sectional views deteriorate into fuzzy tangentional views. Some of the pro-thylakoids show a balloon-or barrel-like form with several marginal splits in the reconstituted serial sections (Fig. 6c). In cross section, these figures appear as roughly circular thylakoids with interruptions and/or as short separate lamellae, depending upon the section plane. A few rudimentary double