FIG. 4.

Ultrastructural images of the nucleolus of Arabidopsis root meristematic cells grown for 4 days under simulated microgravity in a magnetic levitation instrument (upper part) and under real microgravity in the ISS (lower part). Nucleolar parameters represent an accurate estimation of the rate of ribosome biogenesis and, consequently, of the level of protein synthesis and cell growth. The nucleolus is smaller in both real and simulated microgravity compared to the corresponding 1g controls. Furthermore, the levels of the nucleolar protein nucleolin, an essential factor for pre-rRNA synthesis and processing, estimated by ultrastructural immunogold procedure, were lower under both real and simulated microgravity than in the respective 1g ground controls. Bars indicate 1 μm in each experiment. Data taken from Matía et al. (2010) and Manzano et al. (personal communication). DFC, dense fibrillar component; GC, granular component; arrows indicate fibrillar centers. N, nucleus; Nu, nucleolus.