Membrane geometry of ?open? prolamellar bodies

Abstract

The "open" type of prolamellar body in etiplasts was examined by electron microscopy to characterise its three-dimensional organisation. As in more compact forms of prolamellar body, its basic geometric unit is a tetrahedrally branched tubule. In the "open" type, these lie smoothly confluent with one another at the vertices of 5- and 6-membered rings which circumscribe the faces of three kinds of polyhedra: pentagonal dodecahedra (with 12 pentagonal faces), 14-hedra (2 opposite hexagonal faces joined by two circlets of six pentagonal faces), and 15-hedra (3 hexagonal and 12 pentagonal faces). These polyhedra join confluently in their turn, sharing faces with one another in at least one recognisable super-structure which accounts for the appearance of "open" prolamellar bodies in many ultrathin sections. In this organisation, columns of pentagonal dodecahedra are arranged at 120 degrees to one another in the x-y-plane of the lattice. They do not fill the plane but intersect so as to delimit voids in the form of hexagonally arranged 14-hedra (with hexagonal rings in the x-y-plane). Strata of this type alternate with strata made of face-sharing 15-hedra (with their hexagonal rings normal to x-y), which also delimit 14-hedra. The 14-hedra thus lie in register in the z-axis in hexagonally arranged columns, normal to the alternating strata. Other possible organisations cannot be excluded and local variations and dislocations certainly occur, but many micrographs that display elements of symmetry in "open" prolamellar bodies can be matched to thin slices through such a model. Its geometry is like that of the cages of water molecules in type IV (sensu Jeffrey = type III sensu O'Keeffe) clathrate-hydrates, point group P6/mmm, but about two orders of magnitude larger.

Differential Coloring Reveals That Plastids Do Not Form Networks for Exchanging Macromolecules

“…Like leucoplasts, the etioplasts are long, flexible plastids that lack the structural rigidity of chloroplasts. Etioplasts are characterized by the presence of amorphous prolamellar bodies (Gunning, 1965;Kahn, 1968;Mackender, 1978;Murakami et al, 1985;Gunning, 2001;Solymosi and Schoefs, 2010). Our electron microscopybased observations performed on plants that were grown under conditions defined in previous reports confirm these differences in internal membrane organization between chloroplasts and etioplasts.…”

Three-Dimensional Visualization of the Tubular-Lamellar Transformation of the Internal Plastid Membrane Network during Runner Bean Chloroplast Biogenesis

“…Two lattice structures can be present in one PLB, resembling polycrystals. In addition, other combinations and configurations of the PLB lattice can be found (Gunning, 2001). The only type of PLB lattice observed in the etiolated bean plastids was of the wurtzite-type hexagonal prism (Figure 1).…”

“…The paracrystalline structure of PLBs can differ depending on the species and the conditions of PLB crystallization. All types of paracrystalline arrangements have an exceptionally high surface-to-volume ratio (Gunning, 2001). The formation of etioplasts in darkness, which have a characteristic paracrystalline PLB, and their transformation upon exposure to light has been documented since the 1960s (Gunning, 1965(Gunning, , 2001Mostowska, 1986aMostowska, , 1986bSolymosi and Schoefs, 2010), but the spatial arrangement of these changes is still not known.…”

Chloroplast biogenesis: The use of mutants to study the etioplast–chloroplast transition

“…2b, compare lanes 3, 5, 7, and 9 with lanes 4, 6, 8, and 10). Furthermore, chloroplasts isolated from cao-1 plantlets were able to import prePORA and pre-PORB with similar yield when compared with wild-type, although, if these plants contained any Pchilde, it was Pchlide a and not Pchlide b. Etioplasts from dark-grown cao-1 plantlets contained both PORA and PORB (SI Table 1); in addition, etioplasts from cao-1 cotyledons exhibited a normal ultrastructure, especially of the prolamellar body, which contains the Pchlide holochrome (see below) (32). We conclude that Pchlide b plays a role in neither etioplast/chloroplast development in vivo nor import of prePORA and prePORB in vitro.…”

“…Chloroplast development in angiosperm plants is lightdependent because of the light-dependent reduction of Pchlide to Chlide by POR (18,19). In the dark, etioplasts develop whose internal membrane structure consists of a few prothylakoids and a large prolamellar body instead of thylakoids (32). The prolamellar body consists, to a large extent, of the Pchlide holochrom, comprising POR, Pchlide, and NADPH (33).…”