Red Light-Induced Membrane Potential Transients in the Moss Physcomitrella Patension Channel Interaction in Phytochrome Signalling

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In caulonemal filaments of the moss Physcomitrella patens red light (fluence rate 85 μmol m-2 s-1) triggers within 2-15 s a transient membrane depolarization, and 3 d later the development of side branch initial cells. Both the rapid electrical events at the plasma membrane and the morphological response are Ca2+dependent, phytochrome-mediated and effectively inhibited by the cation channel blocker TEA (10 μM) and the anion channel blocker niflumic acid (1 μM). This suggests that both responses are connected. Current voltage analyses of the red light-induced current combined with ion flux measurements revealed that Ca2+, K+ and anion-permeable channels are open at the peak of the alepolarization. While Ca2+ influx and anion efflux coincide with the depolarizing phase, K+ entry occurs during the first 30 s only and is followed by a dramatic transient K+ efflux leading to repolarization. Patch clamp studies have revealed the presence of a class of depolarization-activated outward rectifying cation channel with a likely role in the rapid repolarization. Furthermore, the channel has a small permeability for Ca2+ and could provide an additional mode of Ca2+ entry. The sequence of events which underlie the red light-induced membrane potential transient is discussed with respect to ion channel interaction during phytochrome signalling.

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