Expression of cotton GhMYB109 complements non-hair cell fate in the root epidermis of the Arabidopsis thaliana werewolf mutant.

Root cell fate and patterning in plants are orchestrated by the expression of cell-type-specific genes, including WEREWOLF (WER). Phylogenetic analysis of WER, functional WER homologs in Type III species (Rhodiola rosea and Boehmeria nivea), and related R2R3 MYB proteins identified in the cotton genome revealed that GhMYB109-a known regulator of fiber development in cotton ovules-clustered in a clade with Arabidopsis thaliana WER. To determine whether GhMYB109 is a functional homolog of WER, we expressed GhMYB109 under the control of the CaMV 35S promoter in the Arabidopsis thaliana wer-1 mutant and analyzed root epidermal cell patterning by counting root hairs. GhMYB109 expression significantly decreased the percentage of root hairs at both H and N positions. We found that most epidermal cells in the cotton root develop into root hairs (Type I pattern) although the cotton genome contains a functional WER homolog, GhMYB109. Additionally, GhMYB109 has been reported not to be expressed in cotton roots. These support the idea that GhMYB109 is a functional homolog of WER and that Arabidopsis thaliana and cotton diverged in root epidermal morphology through modifications in cis-regulatory elements rather than a functional divergence of their WER-like R2R3 MYB transcription factors.