Łucja Kowalewska - 3D Visualization of Thylakoid Membrane Development

Abstract: Chloroplast biogenesis is a complex process that is integrated with plant development, leading to fully differentiated and functionally mature plastids. At the structural level of chloroplast biogenesis, the regular network of paracrystalline prolamellar bodies (PLBs) and the flattened porous membranes of prothylakoids develop into the chloroplast thylakoids. Three-dimensional reconstruction is required to provide us with a more complete understanding of this transformation. We used electron tomography and confocal microscopy to reconstruct the process of structural membrane transformation during the etioplast-to-chloroplast transition in runner bean (Phaseolus coccineus). We also point out the importance of particular chlorophyll-protein complex components in the membrane appression during the subsequent stages of biogenesis process. We provide 3D models of the bean chloroplast biogenesis that allow spatial reconstruction of the internal membranes of the developing chloroplast and visualize the transformation from the tubular arrangement to the linear system of parallel lamellae. The results show that the transformation of PLBs consists of the untwining of tubules from the PLB structure in a continuous process, without dispersion to vesicles. The tubular structure of the PLB transforms directly into flat slats that eventually form grana. We demonstrate that grana membranes, from the beginning of their formation, associate with stroma thylakoids in a helical way. The main structural stages of chloroplast internal membrane biogenesis are presented in a movie that shows the time development of the chloroplast biogenesis as a theoretical dynamic model of this process.