Actin and microtubule cross talk mediates persistent polarized growth.

Coordination between actin and microtubules is important for numerous cellular processes in diverse eukaryotes. In plants, tip-growing cells require actin for cell expansion and microtubules for orientation of cell expansion, but how the two cytoskeletons are linked is an open question. In tip-growing cells of the moss Physcomitrella patens, we show that an actin cluster near the cell apex dictates the direction of rapid cell expansion. Formation of this structure depends on the convergence of microtubules near the cell tip. We discovered that microtubule convergence requires class VIII myosin function, and actin is necessary for myosin VIII-mediated focusing of microtubules. The loss of myosin VIII function affects both networks, indicating functional connections among the three cytoskeletal components. Our data suggest that microtubules direct localization of formins, actin nucleation factors, that generate actin filaments further focusing microtubules, thereby establishing a positive feedback loop ensuring that actin polymerization and cell expansion occur at a defined site resulting in persistent polarized growth.