A Ca(2+)- and voltage-modulated flagellar ion channel is a component of the mechanoshock response in the unicellular green alga Spermatozopsis similis

In flagellate green algae, behavioral responses to photo- and mechanoshock are induced by different external stimuli within 10-15 ms. In the accompanying changes in flagella beat, Ca(2+) has important regulatory roles. Although the axonemal Ca(2+) responsive elements are well characterized, analyses of flagellar channels involved in Ca(2+) signalling as well as other ion channels at the single-channel level were not yet conducted in green algae. To gain a further understanding of these important signaling elements in movement responses, intact flagella of Spermatozopsis similis were isolated and characterized and the solubilized flagellar membrane proteins were reconstituted into liposomes. We observed three types of channel activity, two of which were weakly anion and cation-selective and in the high-conductance regime typical for porin-like solute channels. The dominating channel activity was a voltage dependent, rectifying, low conductance (Lambda=80 pS in 50 mM KCl) cation-selective channel modulated by, and highly permeable to, Ca(2+) ions (SFC1: Spermatozopsis flagellar cation channel 1). Depolarizations necessary to activate SFC1 probably only occur in vivo during avoidance reactions of this alga. Ca(2+)-activation of SFC1 points to a direct link to Ca(2+)-mediated signaling pathway(s) in the flagella. Both the response to mechanoshock and SFC1 activity were inhibited by Gd(3+) and Ba(2+), thus supporting our assumption that SFC1 represents a major flagellar ion channel involved in this green algal avoidance reaction.

Receptor-mediated activation of a plant Ca2+-permeable ion channel involved in pathogen defense

Pathogen recognition at the plant cell surface typically results in the initiation of a multicomponent defense response. Transient influx of Ca2+ across the plasma membrane is postulated to be part of the signaling chain leading to pathogen resistance. Patch-clamp analysis of parsley protoplasts revealed a novel Ca2+-permeable, La3+-sensitive plasma membrane ion channel of large conductance (309 pS in 240 mM CaCl2). At an extracellular Ca2+ concentration of 1 mM, which is representative of the plant cell apoplast, unitary channel conductance was determined to be 80 pS. This ion channel (LEAC, for large conductance elicitor-activated ion channel) is reversibly activated upon treatment of parsley protoplasts with an oligopeptide elicitor derived from a cell wall protein of Phytophthora sojae. Structural features of the elicitor found previously to be essential for receptor binding, induction of defense-related gene expression, and phytoalexin formation are identical to those required for activation of LEAC. Thus, receptor-mediated stimulation of this channel appears to be causally involved in the signaling cascade triggering pathogen defense in parsley.