Towards a systems understanding of the gravitropic growth response in plants: lessons from ground and space experiments

Understanding the mechanisms through which cells perceive and respond to gravity is of fundamental relevance to space biology. In plants altered gravity induces asymmetric cell growth leading to root bending. This process depends on the differential distribution of the phytohormone auxin which is regulated by auxin efflux transporters. The objective of our studies is directed towards defining the molecular networks that underlie the perception and transduction of gravitropic growth in response to microgravity and to develop methods to directly visualize molecular components acting in these networks. We have developed advanced imaging techniques and innovative sensors for high-content imaging of cellular processes using the toolbox provided by synthetic biology. We also developed an intrinsic root coordinate system (iRoCS) which enables rapid, standardized parameterization of image data within a single framework and provides insight into dynamic changes in root growth, the distribution of cell divisions in different cell layers and the role of components acting in signalling networks. We have performed ground- and space-bound experiments during the Chinese-German Space Mission (Shenzhou-8) and participated in the German Aerospace Center´s (DLR) and European Space Agency´s (ESA) Microgravity Programmes (International Space Station ISS, Texus, Parabolic Flights). Based on data obtained in different missions we elucidated gene networks underlying the plants’ response to microgravity and their organization. Using wild type and mutants of the model plant Arabidopsis thaliana we explored its phenotypic plasticity to gravitropic reponse. First RNAseq analysis identified a µg-mediated decrease in energy metabolism following an immediate stress response and a major deregulation of central hormone signalling pathways putatively leading to altered root growth