Defects in division plane positioning in the root meristematic zone affect cell organization in the differentiation zone.

Cell-division-plane orientation is critical for plant and animal development and growth. TANGLED1 (TAN1) and AUXIN-INDUCED IN ROOT CULTURES 9 (AIR9) are division-site-localized microtubule-binding proteins required for division-plane positioning. The single mutants tan1 and air9 of Arabidopsis thaliana have minor or no noticeable phenotypes, but the tan1 air9 double mutant has synthetic phenotypes including stunted growth, misoriented divisions and aberrant cell-file rotation in the root differentiation zone. These data suggest that TAN1 plays a role in non-dividing cells. To determine whether TAN1 is required in elongating and differentiating cells in the tan1 air9 double mutant, we limited its expression to actively dividing cells using the G2/M-specific promoter of the syntaxin KNOLLE (pKN:TAN1-YFP). Unexpectedly, in addition to rescuing division-plane defects, expression of pKN:TAN1-YFP rescued root growth and cell file rotation defects in the root-differentiation zone in tan1 air9 double mutants. This suggests that defects that occur in the meristematic zone later affect the organization of elongating and differentiating cells.