Armadillo repeat-containing kinesin represents the versatile plus-end-directed transporter in Physcomitrella

Kinesin-1, also known as conventional kinesin, is widely utilised for microtubule plus-end-directed ("anterograde") transport of various cargos in animal cells. However, a motor functionally equivalent to the conventional kinesin has not been identified in plants, which lack the kinesin-1 genes. Here, we show that plant-specific armadillo repeat-containing kinesin (ARK) is the long sought-after versatile anterograde transporter in plants. In ARK mutants of the moss Physcomitrium patens, the anterograde motility of nuclei, chloroplasts, mitochondria, and secretory vesicles was suppressed. Ectopic expression of non-motile or tail-deleted ARK did not restore organelle distribution. Another prominent macroscopic phenotype of ARK mutants was the suppression of cell tip growth. We showed that this defect was attributed to the mislocalisation of actin regulators, including RopGEFs; expression and forced apical localisation of RopGEF3 suppressed the growth phenotype of the ARK mutant. The mutant phenotypes were partially rescued by ARK homologues in Arabidopsis thaliana, suggesting the conservation of ARK functions in plants.