The Arabidopsis root exhibits a highly stereotypical radial pattern, with layers of pericycle, endodermis, cortex and epidermis surrounding a central file of vascular cells. Although numerous factors that influence radial patterning have been identified, the precise mechanism that coordinates patterning across the root remains unclear and is proposed to involve both cell-autonomous and non-cell-autonomous mechanisms. Here, Keith Lindsey and colleagues investigate this issue and reveal that sterol synthesis, specifically within the epidermis, plays a key role in Arabidopsis root patterning. They first report that hyd1 mutants, which exhibit defective sterol biosynthesis owing to a mutation in the sterol isomerase encoding gene HYD1, show abnormal root patterning and growth. They further demonstrate that HYD1 is expressed predominantly in the epidermis. Following on from this, the authors show that expression of HYD1 specifically in the epidermis is able to rescue the root defects seen in hyd1 mutants. Finally, they reveal that polar localisation of the auxin transporters PIN1 and PIN2 is perturbed in the roots of hyd1 mutants but restored following epidermal-specific expression of HYD1. Overall, these findings indicate that sterol biosynthesis within the epidermis regulates root patterning, potentially acting non-cell autonomously via its effects on PIN localisation and hence auxin transport across the root.
