Abstract
C4 plants enhance photosynthesis efficiency by concentrating CO2 to the site of Rubisco action. Chloroplasts in C4 plants exhibit structural dimorphism because thylakoid architectures vary depending on energy requirements. Advances in electron microscopy imaging capacity and sample preparation technologies allowed characterization of thylakoid structures and their macromolecular arrangements with unprecedented precision mostly in C3 plants. The thylakoid is assembled during chloroplast biogenesis through collaboration between the plastid and nuclear genomes. Recently, the membrane dynamics involved in the assembly process has been investigated with 3D electron microscopy, and molecular factors required for thylakoid construction have been characterized. The two classes of chloroplasts in C4 plants arise from common precursors, but little is known about how a single type of chloroplasts grow, divide, and differentiate to mature into distinct chloroplasts. Here, we outline the thylakoid structure and its assembly processes in C3 plants to discuss ultrastructural analyses of dimorphic chloroplast biogenesis in C4 plant species. Future directions for electron microscopy research of C4 photosynthetic systems are also proposed.
Highlights
Since the early days of microscopy, chloroplasts have attracted attention from microscopists
We summarize the macromolecular organizations of thylakoids in C3 plants and discuss their assembly process during chloroplast biogenesis with an emphasis on what has been learned from electron microscopy/tomography analyses
Light-induced chloroplast biogenesis was examined in chemically fixed bean leaf samples with electron tomography (ET) by Kowalewska et al (2016) They showed that the paracrystalline prolamellar bodies lose lattice-like regularity upon illumination to become planar precursors of thylakoids and that grana stacks arise from the flattened membranes
Summary
Since the early days of microscopy, chloroplasts have attracted attention from microscopists. We summarize the macromolecular organizations of thylakoids in C3 plants and discuss their assembly process during chloroplast biogenesis with an emphasis on what has been learned from electron microscopy/tomography analyses. We will review the link between structures and functions of dimorphic chloroplasts in mature leaves of maize (a C4 plant with Kranz anatomy) and Bienertia sinuspersici (a SCC4 plant).
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