Characterization of auxin transporter PIN 6 plasma membrane targeting reveals a function for PIN 6 in plant bolting

Summary Auxin gradients are sustained by series of influx and efflux carriers whose subcellular localization is sensitive to both exogenous and endogenous factors. Recently the localization of the Arabidopsis thaliana auxin efflux carrier PIN ‐ FORMED ( PIN ) 6 was reported to be tissue‐specific and regulated through unknown mechanisms. Here, we used genetic, molecular and pharmacological approaches to characterize the molecular mechanism(s) controlling the subcellular localization of PIN 6. PIN 6 localizes to endomembrane domains in tissues with low PIN 6 expression levels such as roots, but localizes at the plasma membrane ( PM ) in tissues with increased PIN 6 expression such as the inflorescence stem and nectary glands. We provide evidence that this dual localization is controlled by PIN 6 phosphorylation and demonstrate that PIN 6 is phosphorylated by mitogen‐activated protein kinases ( MAPK s) MPK 4 and MPK 6. The analysis of transgenic plants expressing PIN 6 at PM or in endomembrane domains reveals that PIN 6 subcellular localization is critical for Arabidopsis inflorescence stem elongation post‐flowering (bolting). In line with a role for PIN 6 in plant bolting, inflorescence stems elongate faster in pin6 mutant plants than in wild‐type plants. We propose that PIN 6 subcellular localization is under the control of developmental signals acting on tissue‐specific determinants controlling PIN 6‐expression levels and PIN 6 phosphorylation.