Effect of different gravity environments on DNA fragmentation and cell death in Kalanchoe leaves

Different gravity environments have been shown to significantly affect leaf-plantlet formation and asexual reproduction in Kalanchoë daigremontiana Ham. and Perr. In the present work, we investigated the effect of gravity at tissue and cell levels. Leaves and leaf-plantlets were cultured for different periods of time (min to 15 d) in different levels of gravity stimulation: simulated hypogravity (1 rpm clinostats; 2 x 10(-4) g), 1 g (control) and hypergravity (centrifugation; 20 and 150 g). Both simulated hypogravity and hypergravity affected cell death (apoptosis) in this species, and variations in the number of cells showing DNA fragmentation directly correlated with nitric oxide (NO) formation. Apoptosis in leaves was more common as gravity increased. Apoptotic cells were localized in the epidermis, mainly guard cells, in leaf parenchyma, and in tracheary elements undergoing terminal differentiation. Exposures to acute hypergravity (up to 60 min) showed that chloroplast DNA fragmentation occurred prior to nuclear DNA fragmentation, marginalization of chromatin, nuclear condensation, and nuclear blebbing. Addition of sodium nitroprusside (NO donor) mimicked centrifugation. NO and DNA fragmentation decreased with N(G)-monomethyl-L-arginine (NO-synthase inhibitor). The variations in NO levels, nucleoid DNA fragmentation, and cell death show how chloroplasts, cells and leaves may respond (and adapt) to gravity changes.