Cryo-SEM and large volume FIB-SEM of Arabidopsis cotyledons: Degradation of lipid bodies, biogenesis of glyoxysomes and reorganisation of organelles during germination.

Until recently, the lack of three-dimensional visualisation of whole cells at the electron microscopic (EM) level has led to a significant gap in our understanding of the interaction of cellular organelles and their interconnection. This is particularly true with regard to the role of the endoplasmic reticulum (ER). In this study, we perform three-dimensional reconstructions of serial FIB/SEM stacks and anaglyphs derived from volume rendering, cryo-scanning electron microscopy (cryo-SEM) and state-of-the-art electron microscopy immobilisation and imaging techniques. The results show that glyoxysomes are formed de novo in large numbers and in characteristic clusters on the ER upon germination in mesophyll cells of Arabidopsis cotyledons. The degradation of lipid bodies during germination occurs not only via the ER, which enlarges by taking up polar lipids resulting from enzymatic degradation by lipases, but also via glyoxysomes, which engulf lipid bodies. Dictyosomal (Golgi-derived) vesicles, which fuse with glyoxysomes or their precursors, also appear to be involved in the differentiation of glyoxysomes from segments of the ER. The formation of the central vacuole is the result of the fusion of protein storage vacuoles (protein bodies), which become complex three-dimensional structures during germination. Our observations also suggest that the vacuole plays a role in the degradation of glyoxysomes. The evidence provided in three dimensions shows that the endoplasmic reticulum plays a central role in the biogenesis and degradation of lipid bodies, the ontogeny of glyoxysomes and the development of plastids in the mesophyll cells of Arabidopsis cotyledons.