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 auxin have demonstrated the existence of elementary response chains to auxin 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-abp1 (abp1), the most abundant auxin-binding protein from maize coleoptiles. At the other side, multiple ion transport proteins appear as targets of the 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 initially focus on abp1 and its functional role at the membrane. The C-terminus abp1 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 mesophyll protoplasts. These results further demonstrate that proteins involved in plasma membrane responses to auxin are related to maize abp1, and provide clues as to the region of the protein possibly involved in the interaction of abp1 with the plasma membrane. Secondly, this report concentrates on one of the targets of auxin, a voltage-dependent and ATP-regulated anion channel that we characterised on protoplasts from tobacco cell suspensions. This anion channel was specifically modulated by auxin, as already observed for the anion channel of guard cells 14]. Further work will be needed to assess if this auxin modulation involves a direct interaction between the hormone and the anion channel protein(s), or follows from the activation of a perception chain including abp1 homologues.