Degradation of phytochrome A and the high irradiance response in Arabidopsis: a kinetic analysis

The ability to respond to far-red-rich light is essential for seedlings ger minating below dense canopies. Physiological and genetic studies have demon strated that phytochrome A is the only photoreceptor mediating responses to far-red light. However, all phytochromes including phytochrome A are belie ved to be activated by red light and to be inactivated by far-red light. To address the fundamental question of why phytochrome A has its highest phys iological activity at presumably inactivating wavelengths, we analysed ligh t-induced degradation of phytochrome A in Arabidopsis. Rate constants were obtained for all reaction events in a two-step model of degradation. Based on biochemical data, the model includes a tagging mechanism preceding degra dation. The parameterized model describes Pr accumulation, wavelength depen dencies of degradation kinetics and steady-state levels as well as Pfr-indu ced Pr degradation. Subsequently, experimentally derived fluence rate respo nse curves, action spectrum and response curves to dichromatic irradiation were compared to simulations based on the model of degradation. Two kinetic ally defined phytochrome subspecies, untagged Pfr and tagged Pr, have stead y-state levels closely matching the physiological response curves. Therefor e, sensing of far-red light by phytochrome A can be quantitatively explaine d based exclusively on regulated protein degradation.