Defective control of growth rate and cell diameter in tip-growing root hairs of the rhd4 mutant of Arabidopsis thaliana

A recessive mutation in the RHD4 gene of Arabidopsis thaliana L. affects the control of tip growth in seedling root hairs. Fully grown rhd4 root hairs are half the length of wild-type (WT) hairs. The hairs are wider, and they vary in diameter during tip growth. Light microscopy and motion analysis revealed that rhd4 hairs grow more slowly and that hair growth rate varies more than in WT hairs. Hair diameter increases at the rhd4 hair tips when tip growth slows. Ultrastructural analysis revealed cell wall thickenings in some mutant hairs. WT hairs were grown in a hyperosmotic medium in an attempt to mimic the rhd4 hairs and investigate the control of root hair morphology. Osmotic stress increased WT hair diameter and induced hair bulging and also increased the diameters of rhd4 hairs. Osmotic stress could disrupt tip growth through reduced turgor pressure and (or) reduced concentrations of cytosolic calcium. Together these results indicate that RHD4 is required to maintain a uniform rate of tip growth in r...