Signal Transduction in Arabidopsis Root Gravitropism

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Gravity is a fundamental signal that regulates plant growth and form. Despite its importance to plant success, the cellular and molecular events whereby higher plants sense and respond to the gravity signal are essentially unknown. It is thought that in the root gravity is perceived in cap cells and that this signal is converted to asymmetrical growth through a mechanism involving asymmetrical auxin and Ca2+ distribution. Much of the debate can be traced to our relative ignorance of the interacting signaling systems that tranduce, coordinate and integrate these tropic responses. The goal of this research is therefore to define these critical gravetropic signaling systems and how they interact, to the cellular level, using the roots of Arabidopsis thaliana as a model system. Experiments are proposed using laser ablation to define the graviperceptive cells of the root cap with single cell resolution. Fluorescence tagging of single cell walls will then be used to map the temporal and spatial components of individual cellular growth response in the elongating region of the root. Confocal and ratio fluorescent imaging will be applied to the intact root to visualize Ca2+- and pH-dependent signaling responses as the root responds to gravity. Signal related changes in these second messengers will then be mimicked or blocked by microinjecting the defined perceptive and responsive cells with Ca2+ and pH-related signaling molecules and their antagonists. Finally, experiments will be performed to generate defined gradients of auxin, Ca2+ and pH in root tissue in intact plants using localized release of caged forms of these putative regulators. These experiments will directly test the hypothesis that asymmetries in these regulators are involved in coordinating the tropic growth along the intact root. Results from the research will extend our understanding of gravity responses in roots specifically, and help define the interactions that integrate the growth responses of plant organs to environmental signals in general.