ELEMENTARY AUXIN RESPONSE CHAINS AT THE PLASMA-MEMBRANE INVOLVE EXTERNAL ABP1 AND MULTIPLE ELECTROGENIC ION-TRANSPORT PROTEINS

Studies of membrane electrical responses of isolated protoplasts to au xin have demonstrated the existence of elementary response chains to a uxin at the plasma membrane, presently defined only by their uttermost ends. At one side, as demonstrated by several lines of evidence, the auxin perception unit involves proteins homologous to ZmER-abpl (abpl) , the most abundant auxin-binding protein from maize coleoptiles. At t he other side, multiple ion transport proteins appear as targets of th e auxin signal; the proton pump ATPase, an anion channel and potassium channels. We investigated early electrical responses to auxin at the plasma membrane of tobacco protoplasts. The work presented here will i nitially focus on abpl and its functional role at the membrane. The C- terminus abpl peptide (Pz151-163) was recently reported to modulate K currents at the plasma membrane of intact guard cells from broad bean [23] and induce plasma membrane hyperpolarisation of tobacco mesophyl l protoplasts. These results further demonstrate that proteins involve d in plasma membrane responses to auxin are related to maize abpl, and provide clues as to the region of the protein possibly involved in th e interaction of abpl with the plasma membrane. Secondly, this report concentrates on one of the targets of auxin, a voltage-dependent and A TP-regulated anion channel that we characterised on protoplasts from t obacco cell suspensions. This anion channel was specifically modulated by auxin, as already observed for the anion channel of guard cells [1 4]. Further work will be needed to assess if this auxin modulation inv olves a direct interaction between the hormone and the anion channel p rotein(s), or follows from the activation of a perception chain includ ing abpl homologues.