Click chemistry-based tracking reveals putative cell wall-located auxin binding sites in expanding cells

Auxin is a key plant regulatory molecule, which acts upon a plethora of cellular processes, including those related to cell differentiation and elongation. Despite the stunning progress in all disciplines of auxin research, the mechanisms of auxin-mediated rapid promotion of cell expansion and underlying rearrangement of cell wall components are poorly understood. This is partly due to the limitations of current methodologies for probing auxin. Here we describe a click chemistry-based approach, using an azido derivative of indole-3-propionic acid. This compound is as an active auxin analogue, which can be tagged in situ. Using this new tool, we demonstrate the existence of putative auxin binding sites in the cell walls of expanding/elongating cells. These binding sites are of protein nature but are distinct from those provided by the extensively studied AUXIN BINDING PROTEIN 1 (ABP1). Using immunohistochemistry, we have shown the apoplastic presence of endogenous auxin epitopes recognised by an anti-IAA antibody. Our results are intriguingly in line with previous observations suggesting some transcription-independent (non-genomic) activity of auxin in cell elongation.