Division Plane Orientation Defects Revealed by a Synthetic Double Mutant Phenotype.

TANGLED1 (TAN1) and AUXIN-INDUCED-IN-ROOTS9 (AIR9) are microtubule-binding proteins that localize to the division site in plants. Their function in Arabidopsis () remained unclear because neither nor single mutants have a strong phenotype. We show that double mutants have a synthetic phenotype consisting of short, twisted roots with disordered cortical microtubule arrays that are hypersensitive to a microtubule-depolymerizing drug. The double mutants have significant defects in division plane orientation due to failures in placing the new cell wall at the correct division site. Full-length TAN1 fused to yellow fluorescent protein, , and several deletion constructs were transformed into the double mutant to assess which regions of TAN1 are required for its function in root growth, root twisting, and division plane orientation. expressed in significantly rescued the double mutant phenotype in all three respects. Interestingly, TAN1 missing the first 126 amino acids, TAN1-ΔI-YFP, failed to rescue the double mutant phenotype, while TAN1 missing a conserved middle region, TAN1-ΔII-YFP, significantly rescued the mutant phenotype in terms of root growth and division plane orientation but not root twisting. We use the double mutant to discover new functions for TAN1 and AIR9 during phragmoplast guidance and root morphogenesis.