Figure 9. Pyrenoid tubules connect thylakoids with the pyrenoid.
(A) A slice from a tomographic volume and (B) corresponding 3D segmentation showing thylakoid membranes (dark green), thylakoid lumina (light green), minitubules (orange), the chloroplast envelope (blue), and the pyrenoid starch sheath (tan). Cylindrical pyrenoid tubules extend from thylakoid stacks into the center of the pyrenoid through fenestrations in the starch sheath. Once inside the pyrenoid, the tubules twist (top tubule in A), can abruptly change direction by ∼90° (bottom tubule in B), lose their cylindrical geometry, and narrow to form an interconnected membrane network at the pyrenoid center. (C–D) Slices from two additional tomograms detailing how pyrenoid tubules extend from thylakoid stacks. Thylakoid lumina are continuous with pyrenoid tubule lumina. (E and G) Slices from higher magnification tomograms of the pyrenoid matrix and (F and H) corresponding 3D segmentations showing cross-sections of pyrenoid tubules (green) surrounded by RuBisCO complexes (grey). Multiple smaller minitubules (orange) are bundled within each pyrenoid tubule. The tomograms were 2× binned. Unbinned pixel size: 9.6 Å in A–D, 5.7 Å in E–H. Segmented tomogram thickness: 229 nm in B, 165 nm in F, 108 nm in H. Scale bars: 500 nm in A, 100 nm in C, D, E, and G. Figure accompanied by Videos 4 and 5.
Figure 9—figure supplement 1. Longitudinal views of pyrenoid minitubules.
