BLUE LIGHT-REVERSED AND GENETICALLY-REVERSED GRAVITROPIC RESPONSE IN PROTONEMATA OF THE MOSS CERATODON-PURPUREUS

In darkness, protonemal filaments of Ceratodon purpureus (Brid.) grow negatively gravitropically (upwards). Red light induces a positive pho totropic response mediated by the photoreceptor phytochrome. A red lig ht treatment also has an inhibitory effect on the gravitropic response , an effect also mediated by phytochrome. In this study the effects of blue light on phototropism and on gravitropism were analysed. Unilate ral blue light resulted in only a weak phototropic response, but marke dly randomised growth direction. Blue light given together with a grav itropic stimulus reversed the gravitropism, changing it from negative to positive (filaments grow downward). The effect of blue light was al so analysed with the mutant ptr116, which is defective in the biosynth esis of the phytochrome chromophore, and in a newly isolated mutant ww r2, which is positively gravitropic in darkness. Blue light induced th e same reversal of gravitropism in ptr116 as in the wild type, indicat ing that phytochrome is not involved in this process. In wwr2 the dire ction of gravitropism was unaltered by the blue light treatment. Light also affects chlorophyll content and the size of plastids, potential statoliths for gravitropism. Red light induced an increase in plastid size and chlorophyll content in the wild type but not in ptr116. Blue light induced a similar change in wild type plastids. It seems as thou gh light-induced alterations of gravitropism are not simply mediated b y alterations in plastid properties, and that red light and blue light evoke fundamentally different responses.