Abstract
Bienertia sinuspersici is a single-cell C4 plant species of which chlorenchyma cells have two distinct groups of chloroplasts spatially segregated in the cytoplasm. The central vacuole encloses most chloroplasts at the cell center and confines the rest of the chloroplasts near the plasma membrane. Young chlorenchyma cells, however, do not have large vacuoles and their chloroplasts are homogenous. Therefore, maturing Bienertia chlorenchyma cells provide a unique opportunity to investigate chloroplast proliferation in the central cluster and the remodeling of chloroplasts that have been displaced by the vacuole to the cell periphery. Chloroplast numbers and sizes increased, more notably, during later stages of maturation than the early stages. Electron tomography analyses indicated that chloroplast enlargement is sustained by thylakoid growth and that invaginations from the inner envelope membrane contributed to thylakoid assembly. Grana stacks acquired more layers, differentiating them from stroma thylakoids as central chloroplasts matured. In peripheral chloroplasts, however, grana stacks stretched out to a degree that the distinction between grana stacks and stroma thylakoids was obscured. In central chloroplasts undergoing division, thylakoids inside the cleavage furrow were kinked and severed. Grana stacks in the division zone were disrupted, and large complexes in their membranes were dislocated, suggesting the existence of a thylakoid fission machinery.
Highlights
C4 photosynthesis is an adaptation in plants that suppresses photorespiration and increases efficiency of water and nitrogen use[1]
Vacuole development was discerned from 2nd stage, separating some chloroplasts to the cell periphery, and by the 3rd stage, chlorenchyma cells had elongated to take on elliptical shapes and vacuoles segregated peripheral chloroplasts (PCs) from central chloroplasts (CCs)
In transmission electron microscopy (TEM) micrographs, PCs appear oppressed in the thin cytosol between the vacuole and the plasma membrane (Fig. 1M)
Summary
C4 photosynthesis is an adaptation in plants that suppresses photorespiration and increases efficiency of water and nitrogen use[1]. An NADP-ME type C4 species, chloroplasts in mesophyll cells have higher concentrations of grana stacks because they contain photosystem II (PSII) associated with thylakoid stacking. In Bienertia-type SCC4 plants, chlorenchyma cells have central chloroplasts (CCs) and peripheral chloroplasts (PCs). In mature Bienertia SCC4 cells, CCs and PCs contain distinct sets of proteins for C4 photosynthesis, and they exchange small molecules through channels composed of cytoplasmic strands. The N-terminal signal peptides of proteins destined for PCs have two components, one that facilitates general entry into the chloroplast and a second one that inhibits import into CCs9,10 In agreement with their macromolecular compositions, CCs and PCs exhibit differential ultrastructural characteristics, but there have been no detailed morphometric analyses to show how CCs and PCs diverge from a homogenous pool of chloroplasts in young in Bienertia chlorenchyma cells
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