Epidermal expression of a sterol biosynthesis gene regulates root growth by a non-cell autonomous mechanism in Arabidopsis
The epidermis has been hypothesized to play a signalling role during plant development. One class of mutants showing defects in signal transduction and radial patterning are those in sterol biosynthesis. The expectation is that sterol biosynthesis is a constitutive cell-autonomous process for the maintenance of basic cellular functions. The HYDRA1 (HYD1) gene of Arabidopsis encodes an essential sterol {Delta}8-{Delta}7 isomerase, and although hyd1 mutant seedlings are defective in radial patterning of several tissues, we show that the HYD1 gene is expressed primarily in the root epidermis. Cell type-specific transgenic activation of HYD1 transcription reveals that HYD1 expression in the epidermis of hyd1 null mutants is sufficient to rescue root patterning and growth. Unexpectedly, expression of HYD1 in the vascular tissues and root meristem, though not endodermis or pericycle, also leads to phenotypic rescue. Phenotypic rescue is associated with rescued patterning of the PIN1 and PIN2 auxin efflux carriers. The importance of the epidermis is in part due to its role as a site for tissue-specific sterol biosynthesis, and auxin is a candidate for a non-cell autonomous signal.