Two-step, long-distance nuclear migration guided by distinct actin networks prior to plant root hair formation.

One of the most fundamental tasks for a cell to fulfill its function is to move the nucleus to its proper place through a process known as "nuclear migration." In plant cells, the nucleus relocates spontaneously in response to developmental and environmental cues. Root hairs are tip-growing projections of root epidermal cells that are required for the acquisition of water and nutrients from the soil. Before its development, the nucleus travels ∼50 µm across the cell body from the cell center toward the root hair initiation site to support continuous tip growth; however, the dynamics and molecular basis underlying this nuclear migration remain unclear. In this study, we show that long-distance nuclear migration to the root hair initiation site consists of two steps, each driven by distinct F-actin networks. Our cytological and genetic data indicate that the nucleus initially moves downward with the aid of F-actin, which consists of two classes of actin isoforms, ACTIN 7 (ACT7) and ACT2/ACT8, followed by the lateral movement to the root hair initiation site, which is guided by F-actin primarily, incorporating ACT2/ACT8. Moreover, RHO-OF-PLANTS small GTPases play an important role in the formation of F-actin, which connects the nucleus to the root hair initiation site, thus facilitating lateral nuclear movement. This study provides an example of distinct F-actin networks steering the nucleus in a stepwise manner to enable long-distance and precise nuclear migration within plant cells.