Elicitor-induced chloride efflux and anion channels in tobacco cell suspensions

Membrane depolarization associated with changes in transmembrane cation and anion effluxes constitute early events in the signalling cascades which mediate elicitor-induced defense reactions in plant cells. The aim of the present study is to investigate whether membrane depolarization can be a key step in the signalling cascade by triggering the opening of voltage-dependent anion channels which would mediate anion effluxes. To address this question, tobacco (Nicotiana tabacum cv. Xanthi) cells were subjected to three elicitors (pectolyase, oligogalacturonides and cryptogein) and four other depolarizing agents (vanadate, azide, cyanide and CCCP) and their plasma membrane potential was monitored. In parallel, chloride effluxes induced by these treatments were quantified. Membrane depolarizations induced by the three elicitors were of similar extent, but the intensity of elicitor-induced chloride effluxes was markedly different, pectolyase being the most potent inducer and oligogalacturonides the less active ones. The tested depolarizing agents were all as active as elicitors in terms of membrane depolarization, but again displayed very different efficiencies in the induction of chloride effluxes. The sensitivity of chloride effluxes induced by pectolyase and vanadate to several anion channel blockers was studied. The following sequence of efficiency was found: NPPB (nitrophenyl-propylamino-benzoic-acid) > 9-AC (anthracene-9-carboxylic acid) > DIDS (4,4'-diisothiocyanato-stilbene-2,2'-disulfonic acid) > ETH (ethacrinic acid). The sole anion channel described up to now in tobacco cells is the plasma membrane channel TSAC (tobacco suspension anion channel). Reinvestigations of its properties in the light of the results described above suggest that this depolarization-activated channel may be a candidate to play a role in elicitation and signalling processes accompanied by membrane depolarizations.