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

104 Mustárdy L. The parent lamellar sheet, interrupted in the vicinity of each nascent granum, constitutes a fret attachment at different levels to the stack. This arrangement maintains a continuity and consequently connects all grana that have been initiated on the same sheet. The „insertion" of the stacked discs into a lamellar sheet introduces a twist in the lamellae surrounding the developing granum, an orientation that would be the primary helix around the granum. Three-dimensional reconstitution of developing grana often reveal incomplete disc at the end of the stacks that have only a single fret connection. This shows that the granum can be increased by „overtopping" of the overgrown fret (Fig. 8b). As the number of granum compartments increases, the fretwork is built into multiple layers. There are two types of configurations contributing to the fretwork increase. Firstly, the primary helix divide and directly give rise to a secondary helix (Fig. 8c). Secondly, numerous thylakoid „splittings" can be reconstituted during this stage (Fig. 8d). After ramifications the cleaved thylakoids extend and link into the existing helical system of neighboring granum or give an additional helix to the stack. This can happen not only in the same lamellar sheet but the growing thylakoid can also extend towards the lower (or upper) plane (Fig. 8a, arrows). From this developmental stage the parallel „parent" lamellae have been replaced by a network which interconnects at different levels. These multiple mechanisms - lamellar expansion, perforation, overgrowth; thylakoid splitting, branching, and bridging via fusion - guide the morphogenesis of the intrachloroplastic network formation and lead to the mature chloroplast. Obsolete models still in use The first model, proposed by Wilhelm Menke still used in some books and homepages, postulates that every second granum thylakoid is continued in sheets that intersect several grana (Fig. 2a). This oversimplified scheme does not satisfy the requirement of membrane continuity, and is inconsistent with the single intrathylakoidal (lumenal) space. This model was modified by Lexa and Crang (1997) (Virtual Cell http:/www.life.uiuc.edu/plantbio/cell/). In a more frequently used, dangerously attractive model the grana are interconnected by tubes, which form a fret (Fig. 2b). This model is based on electron micrographs that suffer from a serious fixation artifact. Although tubular frets would in principle be consistent with most functions, they simply do not exist. Hence, this model should not be used, even for purposes of illustration, it is still found in many textbooks. The folded membrane model (Fig. 2c) is most commonly used in research papers as a close-up view of the granum-stroma connection. This was