Figure 2.

Hypothetical model of auxin concentration-dependent control over apoplastic pH. At low concentrations, auxin activates plasma membrane (PM) H⁺ ATPases, thereby lowering apoplastic pH and increasing apoplastic Ca²⁺ concentrations via arabinogalactan glycoproteins (AGBs). At high auxin concentrations, auxin induces a Ca²⁺ signal that inactivates H⁺ ATPases. The auxin-induced Ca²⁺ signal can be inhibited by La³⁺.