Trends in Plant Science
Granum revisited. A three-dimensional model – where things fall into place
Section snippets
Obsolete models still in use
The first model, proposed by Wilhelm Menke, was developed from early electron microscopic observations [1]. This model, still used in some books and on 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.
In a more frequently used, dangerously attractive model, the grana are
Reconstituted 3D model
In the generally accepted, although not widely used model, the thylakoid membranes in the chloroplasts of mesophyll cells of C3 and C4 plants display multiple right-handed helices of stroma lamellae that are wound around the cylindrical grana and form a contiguous system. The helical arrangement of stroma thylakoids around grana was proposed by Dominick J. Paolillo [12] in 1970, and confirmed with the aid of a complete series of sections [13]. For the purpose of better visualization, the
Functions
The pioneering work by Jan M. Anderson and Keith N. Boardman >30 years ago has revealed that the differentiation of thylakoids into granum and stroma membrane regions reflects a compositional and functional difference [16]. The two photosystems are spatially separated: photosystem II (PSII) and its main chlorophyll a/b light-harvesting complex, LHCII, are found predominantly in the stacked membranes; this region is largely deficient in photosystem I (PSI) and LHCI, which are enriched in the
Open questions, perspectives
The ontogeny and evolution of this (type of) granum–stroma assembly and its ancestors are not well understood. Several interesting questions can be posed with regard to the 3D arrangement of this assembly. What is the exact role of this peculiar membrane assembly in the regulation of energy transduction? There are indications that grana and the LHCII-containing macrodomains are needed for the efficient regulation of energy dissipation in excess light 39, 40, but the details of such regulation
Acknowledgements
We are indebted to Peter Simon for the computer graphics, and László Kovács for valuable discussions and help in preparing the figures. This work has been supported by grants from the Hungarian Fund for Basic Research (OTKA T30324, T34188 and T42696).
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