Root hair initiation-dependent and -independent nuclear movements during root hair development in Arabidopsis thaliana

Nuclear migration toward the tip is an essential intracellular structural reorganization that drives tip growth in root hairs. Previous studies have demonstrated that the nucleus is delivered to the root hair tip via three sequential steps. However, it remains poorly understood what triggers each step. Here, we show that the 1st step-the shift from the cell center toward the inner lateral membrane of root hair precursor cells-normally occurs in root hair precursor cells lacking ROOT HAIR DEFECTIVE 6 (RHD6)-induced root hair initiation, which is characterized by hair bulge formation. In contrast, the 2nd step-migration from the inner lateral membrane toward the root hair initiation site-is abolished in RHD6-deficient cells, indicating that the 1st and 2nd steps represent root hair initiation-independent and -dependent nuclear movements, respectively. Unlike the subsequent steps, the 1st step occurs normally even in the absence of the major actin genes, namely ACTIN2 and ACTIN7, raising the possibility that an actin-independent force steers the 1st step of nuclear migration in root hair cells. Furthermore, undergoing the inward movement of the first step is unexpectedly not mandatory to proceed to the next stage. Altogether, our findings highlight the existence of elaborate yet flexible mechanisms that ensure precise nuclear delivery to its destination in root hairs.