Repeated microgravity incidents induce repeated bursts of superoxide anion radicals in roots

Plant cells are exquisitely mechano-sensitive. Besides light, gravity is the most important physical factor shaping the plant body. Moreover, penetrating plant pathogens are exerting mechanical pressure on plant cells and ability to perceive sensitively mechanical signals may be important for rapid mounting of defence responses. The most well understood is induction of the oxidative burst with pathogen-derived elicitors. Whereas early studies focused almost exclusively on production of reactive oxygen species (ROS) from the plant defense perspective of attacking pathogens, more recent studies revealed another perspective. Both in animals and plants, but not yeast, ROS emerge as an important signalling molecules acting as second messenger. In plants, several recent studies documented signaling roles of ROS in cell death, stress adaptation, stomata movements, root hair tip growth, as well as auxin and abscisic acid signaling. Here we have investigated effects of hypergravity and microgravity repeatedly imposed on roots of maize during parabolic flight experiment. In the first campaign, we have monitored production of superoxide anion radical (O2‾ ) along surfaces of intact plant root apices. In the second campaign, we have monitored O2‾ in the whole chamber in which five isolated roots were placed. Both these different approaches resulted in the same surprising results. Not the hypergravity, but the microgravity induced all the time immediate oxidative bursts. As this activation can be repeated many times and the hypergravity which is imposed in-between does not inteferes with the microgravity-induced burst of superoxide anion radicals, our data strongly suggest that the sensing mechanism is specific only for the microgravity and that the receptor NADPH oxidase(s) can be rapidly resetted back to the sensing modus. Monitoring of the root apices with the non-invasive oxygen sensitive electrode in the first campaign showed that the microgravity sensing via NADPH oxidases activity is specific for cells located in the root apex, including the root cap, meristem and transition zone and that cells of the root elongation region do not respond to the microgravity. Our future studies will aim in the identification of those cells which accomplish the microgravity-induced burst of superoxide anion radicals and of the identity of NADPH oxidases which underly this one of the fastest response to microgravity ever recorded in plants.