Functional analysis of CAPRICE S1 motif residues via alanine scanning in Arabidopsis

The CAPRICE (CPC) transcription factor regulates root hair initiation and inhibits trichome formation in Arabidopsis thaliana. CPC transits between root epidermal cells via its N-terminal S1 motif; however, the contribution of individual amino acid residues of this motif remains unclear. We generated seven CPC variants with alanine substitutions at positions 2-4 of the S1 motif and analyzed their functions in transgenic Arabidopsis. All variants promoted root hair initiation and inhibited trichome formation with reduced GL2 expression. Confocal imaging of green fluorescent protein-tagged proteins revealed partial reduction in their cell-to-cell movement compared with that of CPC wild-type. Protein structural analysis suggested that the biochemical properties of Phe-2, Arg-3, and Ser-4 within the disordered N-terminal region may contribute to CPC transport. These findings indicate that the S1 motif fine-tunes the intercellular mobility of CPC without compromising epidermal cell differentiation in the root epidermis. (143 words).