Electron Tomography Analysis of Thylakoid Assembly and Fission in Chloroplasts of a Single-Cell C4 plant, Bienertia sinuspersici

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.

Electron Microscopy Views of Dimorphic Chloroplasts in C4 Plants

“…B. sinuspersici chlorenchyma cells have PPDKs in their peripheral chloroplasts; the specific localization requires a sequence element in PPDK that blocks its entry into central chloroplasts. A recent electron tomography (ET) imaging of B. sinuspersici visualized the process of chloroplast differentiation (Mai et al, 2019). Young chloroplasts have grana stacks and stroma lamellae and they are not morphologically distinguishable before spatially separated by the central vacuole.…”

“…Dozens of chloroplasts are clustered in B. sinuspersici cells and they proliferate by binary fission. In ET slice images of chloroplast undergoing fission, thylakoids at the division plane were severed before the envelopes were squeezed (Mai et al, 2019). Interestingly, PSII complexes are dislocated from the division site probably because they constitute closely packed 2D array that could block thylakoid severing.…”

“…Bars indicate 500 nm. The model is based on data published in Akhani et al (2005); Koteyeva et al (2016), and Mai et al (2019).…”

“…Intermediates of membrane assembly that were not previously documented are discerned in high-pressure frozen plant cell samples because these structures are not lost during chemical fixation that takes minutes to hours ( Staehelin and Kang, 2008 ; Wang et al., 2017 ; Karahara and Kang, 2013 ; Wang et al., 2019 ). For example, the kinked thylakoid membranes at the midplane of dividing chloroplasts have not been reported until Mai et al. (2019) where photosynthetic cells of Arabidopsis and B. sinuspersici were preserved by high pressure freezing.…”

Accelerated remodeling of the mesophyll-bundle sheath interface in the maize C4 cycle mutant leaves

“…Thylakoids in BS chloroplasts consist of unstacked thylakoids because they lack PSII [11][12][13] . Proplastids in immature cells at the maize leaf base are morphologically similar, indicating that the dimorphic chloroplasts arise as the two cell types mature along the leaf developmental gradient toward the tip 14 .…”