Electron Tomography of Prolamellar Bodies and Their Transformation into Grana Thylakoids in Cryofixed Arabidopsis Cotyledons

The para-crystalline structures of prolamellar bodies (PLBs) and light-induced etioplast-to-chloroplast transformation have been investigated via electron microscopy. However, such studies suffer from chemical fixation artifacts and limited volumes of 3D reconstruction. Here, we examined Arabidopsis thaliana cotyledon cells by electron tomography (ET) to visualize etioplasts and their conversion into chloroplasts. We employed scanning transmission ET to image large volumes and high-pressure freezing to improve sample preservation. PLB tubules were arranged in a zinc blende-type lattice-like carbon atoms in diamonds. Within 2 h after illumination, the lattice collapsed from the PLB exterior and the disorganized tubules merged to form thylakoid sheets (pre-granal thylakoids), which folded and overlapped with each other to create grana stacks. Since the nascent pre-granal thylakoids contained curved membranes in their tips, we examined the expression and localization of CURT1 (CURVATURE THYLAKOID1) proteins. CURT1A transcripts were most abundant in de-etiolating cotyledon samples, and CURT1A was concentrated at the PLB periphery. In curt1a etioplasts, PLB-associated thylakoids were swollen and failed to form grana stacks. In contrast, PLBs had cracks in their lattices in curt1c etioplasts. Our data provide evidence that CURT1A is required for pre-granal thylakoid assembly from PLB tubules during de-etiolation, while CURT1C contributes to cubic crystal growth in the dark.