Phototropin Phosphorylation

Abstract

While the phenomenon of phototropism, bending of plants towards or away from a blue light source, has been investigated for more than a century, studies on the photoreceptor phototropin started less than two decades ago with the investiga- tion of the phosphorylation of a plasma membrane-associated protein (reviewed by [Short and Briggs 1994]).The phosphorylation in vitro was enhanced by irra- diating the membranes before addition of [γ-³² P]ATP and strongly reduced when the plant tissue was illuminated before membrane isolation. The latter effect, which was in fact the starting observation in this field ([Gallagher et al 1988]), was later explained by occupation of the potential phosphorylation sites with endoge- nous phosphate. Several photobiological correlations between the light-inducible phosphorylation and phototropism were established, for example similar tissue distribution, similar fluence-response curves and action spectra, and similar kinetics of recovery in the dark period after a light pulse ([Short and Briggs 1994]). However, the postulate of a molecular difference between irradiated and shaded cells on unilateral irradiation of phototropic sensitive plant tissue was not sub- stantiated for phototropin phosphorylation in these early investigations.