Phytochrome-controlled phototropism of protonemata of the moss Ceratodon purpureus: physiology of the wild type and class 2 ptr-mutants

Phototropism and polarotropism in protonemata of the moss Ceratodon purpure us are controlled by the photoreceptor phytcochrome. One class of phototrop ism mutants is characterised by growing randomly when kept for a prolonged time (5 d or longer) in unilateral red light. II was found that a subclass of these mutants grows faster than the wild type, the rate of cell division and the length of the cells being increased. This difference is found for light-grown and dark-grown filaments. It is therefore suggested that the mu tant phenotype neither results from a defect in phytochrome photoconversion nor from a defect in phytochrome-gradient formation. Instead, it is possib le that a factor which is involved in both signal transduction of phototrop ism and regulation of cell size and cell division is deregulated, If dark-g rown mutant filaments are phototropically stimulated for 24 h, they show a weak phototropic response. Phototropism and polarotropism fluence-rate effe ct curves for mutants were flattened and shifted to higher fluence rates co mpared with those for the wild type. With wild-type filaments, a previously unreported response was observed. At a low fluence rate, half of the filam ents grew positively phototropically, while the other half grew negatively phototropically. It seems that under these conditions, a phytochrome gradie nt with two maxima for the far-red-absorbing form of phytochrome (Pfr) with in the cross-section of the cell is displayed by the response of the filame nts. At higher fluence rates, all filaments of the wild type grew towards t he light. These data and results from microbeam irradiation experiments and from phototropism studies with filaments growing within agar, indicate tha t light refraction plays an important role in the formation of the Pfr grad ient in phototropism of Ceratodon.