Myosin XI-K is involved in Root Organogenesis, Polar Auxin Transport and Cell Division

The interplay between myosin- and auxin-mediated processes was investigated by following root development in the triple myosin knockout mutant xi-k xi-1 xi-2 (3KO). Surprisingly, it was found that the 3KO plants generated significantly more lateral and adventitious roots than wild type plants or rescued line expressing functional myosin XI-K:YFP (3KOR). Using the auxin-dependent reporter DR5:venus, a significant change in the auxin gradient toward the root tip was found in 3KO plants, which correlated with the loss of polar localization of the auxin transporter PIN1 in the stele and with the increased number of the stele cells with oblique cell walls. Interestingly, the myosin XI-K:YFP in the 3KOR plants was localized to the cell division apparatus. In anaphase and early telophase, XI-K:YFP was concentrated in the midzone and the forming cell plate. In late telophase, XI-K:YFP formed a ring that overlapped with the growing phragmoplast. The principal myosin receptors MyoB1 and MyoB2 that are highly expressed throughout the plant, were undetectable in dividing cells suggesting that the myosin function in cell division relies on distinct adaptor proteins. Therefore, loss of myosins XI activity might affect root formation via effects on auxin gradient, cell division, or both.