Resolving the Compartmentation and Function of C₄ Photosynthesis in the Single-Cell C₄ Species Bienertia sinuspersici

• Published in: Plant physiology (Bethesda) Vol. 155; no. 4; pp. 1612 - 1628
• Main Authors: Offermann, Sascha, Okita, Thomas W., Edwards, Gerald E.
• Format: Journal Article
• Language: English
• Published: United States American Society of Plant Biologists 01.04.2011
• Subjects: Amaranthaceae - enzymology; Amaranthaceae - physiology; C4 photosynthesis; Cell separation; Centrifugation; Chlorophyll - analysis; Chlorophylls; Chloroplasts; Chloroplasts - enzymology; Chloroplasts - physiology; Enzymes; FOCUS ISSUE ON PLASTID BIOLOGY; Oxygen; Oxygen - metabolism; Photosynthesis; Plant cells; Plant Proteins - analysis; Plants; Protoplasts
• DOI: 10.1104/pp.110.170381
• ISSN: 0032-0889; 1532-2548

Bienertia sinuspersici is a land plant known to perform C₄ photosynthesis through the location of dimorphic chloroplasts in separate cytoplasmic domains within a single photosynthetic cell. A protocol was developed with isolated protoplasts to obtain peripheral chloroplasts (P-CP), a central compartment (CC), and chloroplasts from the CC (C-CP) to study the subcellular localization of photosynthetic functions. Analyses of these preparations established intracellular compartmentation of processes to support a NAD-malic enzyme (ME)-type C₄ cycle. Western-blot analyses indicated that the CC has Rubisco from the C₃ cycle, the C₄ decarboxylase NAD-ME, a mitochondrial isoform of aspartate aminotransferase, and photo-respiratory markers, while the C-CP and P-CP have high levels of Rubisco and pyruvate, Pidikinase, respectively. Other enzymes for supporting a NAD-ME cycle via an aspartate-alanine shuttle, carbonic anhydrase, phosophoenolpyruvate carboxylase, alanine, and an isoform of aspartate aminotransferase are localized in the cytosol. Functional characterization by photosynthetic oxygen evolution revealed that only the C-CP have a fully operational C₃ cycle, while both chloroplast types have the capacity to photoreduce 3-phosphoglycerate. The P-CP were enriched in a putative pyruvate transporter and showed light-dependent conversion of pyruvate to phosphoenolpyruvate. There is a larger investment in chloroplasts in the central domain than in the peripheral domain (6-fold more chloroplasts and 4-fold more chlorophyll). The implications of this uneven distribution for the energetics of the C₄ and C₃ cycles are discussed. The results indicate that peripheral and central compartment chloroplasts in the single-cell C₄ species B. sinuspersici function analogous to mesophyll and bundle sheath chloroplasts of Kranz-type C₄ species.