Phytochrome-mediated rapid changes of cyclic AMP in mustard seedlings (Sinapis alba L.).

A 5-min red light pulse which operates exclusively through phytochrome leads to a strong but transient decrease of the level of cAMP in the cotyledons of the mustard seedling. The strongest response is observed approximately 30 min after the red light pulse. In the axis system (hypocotyl plus taproot) the red light pulse causes a rapid but modest increase of the cAMP contents. The question of whether the observed changes of the tissue contents of cAMP can be regarded as intermediate steps in the causal sequence between Pfr (far-red absorbing form of phytochrom) and some photoresponses is discussed.