Intracellular MIZU-KUSSEI1 movement and hydrotropism in <i>Arabidopsis</i> require F-actin organization

Tropism is directed growth toward or away from stimuli such as light, gravity, and moisture gradient, by which plants can adapt to their surroundings. Hydrotropism is a response to a moisture gradient across the soil, which enables plants to grow their roots toward wet soil and thus avoid drought conditions. MIZU-KUSSEI 1 (MIZ1) is a pivotal gene for root hydrotropism, and its function inside cortical cells at the transition zone is indispensable for hydrotropic bending. However, how MIZ1 is regulated in the cortical cells remains unclear. Here, we found that treatment with the actin depolymerizing drug latrunculin B (Lat B) reduces hydrotropic bending of the Arabidopsis (Arabidopsis thaliana) wild-type and a MIZ1 overexpression line. Moreover, lines with knocked down actin depolymerizing factors showed enhanced hydrotropic root bending, partly due to an increase in MIZ1 expression. We further explored intracellular MIZ1 dynamics using a GFP-fused MIZ1 (MIZ1-GFP) expressing line and found that MIZ1-GFP moves in the cytosol. Lat B treatment diminished MIZ1-GFP movement, indicating the movement of MIZ1-GFP is actin-dependent. These results indicate that actin filaments are required for proper hydrotropic root bending, probably by controlling MIZ1 expression and localization.