Mutually opposing activity of PIN7 splicing isoforms is required for auxin‐mediated tropic responses in Arabidopsis thaliana

Advanced transcriptome sequencing has uncovered that the majority of eukaryotic genes undergo alternative splicing (AS). Nonetheless, little effort has been dedicated to investigating the functional relevance of particular splicing events, even those in the key developmental and hormonal regulators. Combining approaches of genetics, biochemistry and advanced confocal microscopy, we describe the impact of alternative splicing on the PIN7 gene in the plant model Arabidopsis thaliana. PIN7 encodes a polarly localized transporter for the phytohormone auxin and produces two evolutionary-conserved transcripts PIN7a and PIN7b. PIN7a and PIN7b, differing in a 4-amino acid motif, exhibit almost identical expression pattern and subcellular localization. We reveal that they closely associate and mutually influence their mobility within the plasma membrane. Phenotypic complementation tests indicate that the functional contribution of PIN7b per se is minor, but it markedly reduces the prominent PIN7a activity, which is required for correct seedling apical hook formation and auxin-mediated tropic responses. Our results establish alternative splicing of the PIN family as a conserved, functionally relevant mechanism, unveiling an additional regulatory level of auxin-mediated plant development