Root Hair Development Is Suppressed by Long-Term Mild Heat Through Down-Regulation of RHD6 and RHD6-like Genes.

Roots located in the upper soil layers are prone to experiencing high temperatures. Despite their importance for water and nutrient absorption, little is known about the effect of high temperature on root hairs. Here, we found that exposure of Arabidopsis thaliana seedlings to long-term mild heat suppressed root hair initiation. Epidermal patterning of hair and non-hair cells was maintained, as observed with GL2- and CPC-based marker genes, and the suppression was independent of the activity of GL2 and its upstream regulators. Instead, we found that expression of downstream RHD6 and RHD6-like bHLH transcription factor genes RSL2 and RSL4 was reduced and that overexpression of RHD6 via an inducible transgene or ethylene treatment maintained the transcriptional expression of RSL2 and RSL4 and fully rescued the root hair phenotype. We conclude that GL2-independent downregulation of RHD6 and its homologues mediates the inhibition in root hair initiation under long-term mild heat stress. This finding may contribute to the development of strategies for improving plant performance under high temperature.