A GTPase-Dependent Fine ER Is Required for Localized Secretion in Polarized Growth of Root Hairs

The endoplasmic reticulum (ER) is a cellular network comprising membrane tubules and sheets stretching throughout the cytoplasm. Atlastin GTPases, including Atlastin-1 in mammals and RHD3 in plants, play a role in the generation of the interconnected tubular ER network by promoting the fusion of ER tubules. Root hairs in rhd3 are short and wavy, a defect reminiscent of axon growth in cells with depleted Atlastin-1. However, how a loss in the ER complexity could lead to a defective polarized cell growth of root hairs or neurons remains elusive. Using live-cell imaging techniques, we reveal that, a fine ER distribution, which is found in the subapical zone of growing root hairs of wild-type plants, is altered to thick bundles in rhd3. The localized secretion to the apical dome as well as the apical localization of root hair growth regulator ROP2 is oscillated in rhd3. Interestingly, the shift of ROP2 precedes the shift of localized secretion as well as the fine ER distribution in rhd3. Our live imaging and pharmacologic modification of root hair growth defects in rhd3 suggest that there is interplay between the ER and microtubules in the polarized cell growth of root hairs. We hypothesize that, under the guidance of ROP2, RHD3, together with the action of microtubules, is required for the formation of a fine ER structure in the subapical zone of growing root hairs. This fine ER structure is essential for the localized secretion to the apical dome in polarized cell growth.