Defective Control of Growth Rate and Cell Diameter in Tip-Growing Root Hairs of the RHD4 Mutant of Arabidopsis Thaliana

The study investigated the effects of a recessive mutation in the RHD4 gene of Arabidopsis thaliana on root hair growth. It was found that rhd4 mutant root hairs were half the length of wild-type (WT) hairs, wider, and exhibited variable diameters during tip growth. The growth rate of rhd4 hairs was slower and more variable compared to WT hairs, with increased diameter at the tips when growth slowed. Ultrastructural analysis showed cell wall thickenings in some mutant hairs. When WT hairs were grown in a hyperosmotic medium to mimic rhd4 conditions, they exhibited increased diameter and bulging, similar to rhd4 hairs. The study suggested that osmotic stress could disrupt tip growth by reducing turgor pressure and cytosolic calcium concentrations. Overall, the results indicated that the RHD4 gene was essential for maintaining a uniform rate of tip growth in root hairs.