IONIC MECHANISM AND ROLE OF PHYTOCHROME-MEDIATED MEMBRANE DEPOLARIZATION IN CAULONEMAL SIDE BRANCH INITIAL FORMATION IN THE MOSS PHYSCOMITRELLA PATENS

In caulonemal filaments of the moss Physcomitrella patens (Hedw.), red light triggers a phytochrome-mediated transient depolarisation of the plasma membrane and the formation of side branch initials. Three-elec trode voltage clamp and ion flux measurements were employed to elucida te the ionic mechanism and physiological relevance of the red-light-in duced changes in ion transport. Current-voltage analyses indicated tha t ion channels permeable to K+ and Ca2+ are activated at the peak of t he depolarisation. Calcium influx evoked by red light coincided with t he depolarisation in various conditions, suggesting the involvement of voltage-gated Ca2+ channels. Respective K+ fluxes showed a small init ial influx followed by a dramatic transient efflux. A role of anion ch annels in the depolarising current is suggested by the finding that Cl - efflux was also increased after red light irradiation. In the presen ce of tetraethylammonium (10 mM) or niflumic acid (1 mu M), which bloc k the red-light-induced membrane depolarisation and ion fluxes, the re d-light-promoted formation of side branch initials was also abolished. Lanthanum (100 mu M), which inhibits K+ fluxes and part of the initia l Ca2+ influx activated by red light, reduced the development of side branch initials in red light by 50%. The results suggest a causal link between the red-light-induced ion fluxes and the physiological respon se. The sequence of events underlying the red-light-triggered membrane potential transient and the role of ion transport in stimulus-respons e coupling are discussed in terms of a new model for ion-channel inter action at the plasma membrane during signalling.