Phytochrome Activation of K+ Channels and Chloroplast Rotation in Mougeotia: the Escape Times

Abstract

Red light mediates chloroplast movement and increased activity of calcium-activ ated potassium channels on the plasma membrane of the alga Mougeotia sp. (UTEX LB 734). When activation is mediated by phytochrome, a far-red light irradiation given some time after the red light irradiation will reverse the effect of the red light, due to phytochrome photoreversibility. We characterized the escape times (time required for loss of photoreversib ility) for these two processes to compare the transduction pathways involved in chloroplast rotation and channel activation. The escape time for chloroplast rotation was 2.5 min after red light irradiation (red and far-red light irradiations were 30 s). For channel activation, shorter red and far-red light irradiations (10 s) had to be used to obtain an escape time of 20 s. The difference in the escape times suggests that there is relatively rapid divergence in the transduction pathways leading from phytochrome activation (only one molecular species of phytochrome is found in Mougeotia) to each of the two responses in the same cellular system. Because channel activation occurs 2-4 min after irradiation while the escape time is 20 s, it is unlikely that phytochrome acts directly on the channel.