TWISTED DWARF1 mediates myosin XI-associated vesicle trafficking required for auxin transport

Defects in plant development caused by loss-of the FKBP42, TWISTED DWARF1 (TWD1), have so far been accounted to a dual function of TWD1 acting as an ABCB chaperone that positively regulates ABCB biogenesis and transport activity. On the other hand, TWD1 was characterized as a modulator of actin cytoskeleton bundling and dynamics by interaction with ACTIN7, however, currently it is unclear if both events are connected. Here, we show that TWD1 positively regulates pollen tube germination and growth by controlling actin organization. We identify and verify myosin XI-K as TWD1 interacting protein, which is most likely linking the action of TWD1 on the actin cytoskeleton. We provide evidence that myosin XI-K is required for proper auxin exporter trafficking and auxin export. Further, we show that ER-localized TWD1 reshapes the ER network to overlay actin cables similar to mutations of myosin-XI and thus controls cytoplasmic streaming. In summary, our data support a model in that TWD1 functions as an ER-actin adapter proteins involved in myosin-dependent ER motility and cargo trafficking. Our findings provide a molecular explanation for the defects in early ABCB biogenesis in twd1 that are caused by defects in the three-way interaction between the ER, cytosolic myosin-XI and F-actin.