The relationship between growth, nucleus migration and cytoskeleton in root hairs of radish

Abstract

The aim of this study was to investigate whether and how the cytoskeleton is involved in the mechanism keeping the nucleus in the sub-apical region (ca. 50–150 pm, from the tip) of root hairs of radish. In actively-growing root hairs of 2 days-old plants grown at 23iaC the nucleus was always localized in the sub-apical region, whereas the nucleus was frequently located toward the base in non-growing root hairs of 3 days-old plants. The root-hair tip grew at a constant rate while the nucleus sometimes changed its migration speed within the sub-apical region. Colchicine liberated the nucleus from the sub-apical region. This phenomenon also occured in plants exposed to 0–1°C for a long time. When the cold-treated plants were transferred to a 23°C incubator, some root hairs began to grow again after a lag phase (1–2 hrs). During this lag phase, the nucleus always returned to the sub-apical region. The root hairs whose nucleus did not recover its position in the sub-apical region never grew again. According to the immunofluorescence pattern of microtubules (MTs), actively-growing root hairs were discriminated into three parts: (1) an amorphous zone located at the tip and which emits uniform strong fluorescence, (2) an Mt-containing (MT+) zone immediately behind the amorphous zone and is occupied by MTs running parallel to the root-hair axis, and (3) a zone where MTs were absent which succeeds the MT+ zone. The nucleus was always localized at the basal end of the MT+ zone. The ends of MTs were anchored at the surface of the nucleus. In elongated root hairs, the MT+ zone shortened and the nucleus was frequently free. Non-growing root hairs differentiated neither the amorphous nor the MT+ zones. In these hairs the nucleus was most frequently located toward the base. Fluorescence microscopy using rhodamine-phalloidin clearly revealed well-developed bundles of actin filaments, especially in actively-growing root hairs, which ran the entire axis of the hairs. In conclusion, the present study reconfirmed that the nucleus is always kept in the sub-apical region while root hairs are actively growing, but it begins to be liberated from this region when the hairs weaken or cease their tip-growth. MTs are responsible for the maintenance of the nucleus in the sub-apical region.