RED LIGHT-INDUCED MEMBRANE-POTENTIAL TRANSIENTS IN THE MOSS PHYSCOMITRELLA PATENS - ION-CHANNEL INTERACTION IN PHYTOCHROME SIGNALING

In caulonemal filaments of the moss Physcomitrella patens red light (f luence rate 85 mu mol m(-2) s(-1)) triggers within 2-15 s a transient membrane depolarization, and 3d later the development of side branch i nitial cells. Both the rapid electrical events at the plasma membrane and the morphological response are Ca2+-dependent, phytochrome-mediate d and effectively inhibited by the cation channel blocker TEA (10 mM) and the anion channel blocker niflumic acid (1 mu M). This suggests th at both responses are connected. Current voltage analyses of the red l ight-induced current combined with ion flux measurements revealed that Ca2+, K+ and anion-permeable channels are open at the peak of the dep olarization. While Ca2+ influx and anion efflux coincide with the depo larizing phase, K+ entry occurs during the first 30 s only and is foll owed by a dramatic transient K+ efflux leading to repolarization, Patc h clamp studies have revealed the presence of a class of depolarizatio n-activated outward rectifying cation channel with a likely role in th e rapid repolarization, Furthermore, the channel has a small permeabil ity for Ca2+ and could provide an additional mode of Ca2+ entry. The s equence of events which underlie the red light-induced membrane potent ial transient is discussed with respect to ion channel interaction dur ing phytochrome signalling.