The localization of PHRAGMOPLAST ORIENTING KINESIN1 at the division site depends on two microtubule binding proteins TANGLED1 and AUXIN-INDUCED-IN-ROOT-CULTURES9 in Arabidopsis.

Proper plant growth and development requires spatial coordination of cell divisions. Two unrelated microtubule-binding proteins, TANGLED1 (TAN1) and AUXIN-INDUCED-IN-ROOT-CULTURES9 (AIR9), are together required for normal growth and division-plane orientation in Arabidopsis. tan1 air9 double mutants have synthetic growth and division-plane orientation defects while single mutants lack obvious defects. Here we show that the division-site localized protein, PHRAGMOPLAST-ORIENTING-KINESIN1 (POK1), was aberrantly lost from the division site during metaphase and telophase in tan1 air9 mutants. Since TAN1 and POK1 interact via the first 132 amino acids of TAN1 (TAN11-132), we assessed its localization and function in the tan1 air9 double mutant. TAN11-132 rescued tan1 air9 mutant phenotypes and localized to the division site in telophase. However, replacing six amino-acid residues within TAN11-132 that disrupts POK1-TAN1 interaction in the yeast-two-hybrid system caused loss of both rescue and division-site localization of TAN11-132 in tan1 air9 mutants. Full-length TAN1 with the same alanine substitutions had defects in phragmoplast guidance and reduced TAN1 and POK1 localization at the division site but rescued most tan1 air9 mutant phenotypes. Together, these data suggest that TAN1 and AIR9 are required for POK1 localization, and yet unknown proteins may stabilize TAN1-POK1 interactions. One sentence summarySpecific amino acids within TAN1 are required for its correct localization and function partially through interaction with POK1; both TAN1 and AIR9 mediate POK1 division site localization.