MYB Superfamily in Brassica napus: Evidence for Hormone-Mediated Expression Profiles, Large Expansion, and Functions in Root Hair Development

Abstract

MYB proteins are involved in diverse important biological processes in plants. Herein, we obtained the <i>MYB</i> superfamily from the allotetraploid <i>Brassica napus</i>, which contains 227 <i>MYB</i>-related (<i>BnMYBR</i>/<i>Bn1R-MYB</i>), 429 <i>R2R3-MYB</i> (<i>Bn2R-MYB</i>), 22 <i>R1R2R3-MYB</i> (<i>Bn3R-MYB</i>), and two <i>R1R2R2R1/2-MYB</i> (<i>Bn4R-MYB</i>) genes. Phylogenetic analysis classified the Bn2R-MYBs into 43 subfamilies, and the BnMYBRs into five subfamilies. Sequence characteristics and exon/intron structures within each subfamily of the <i>Bn2R-MYB</i>s and <i>BnMYBR</i>s were highly conserved. The whole superfamily was unevenly distributed on 19 chromosomes and underwent unbalanced expansion in <i>B. napus</i>. Allopolyploidy between <i>B. oleracea</i> and <i>B. rapa</i> mainly contributed to the expansion in their descendent <i>B. napus</i>, in which <i>B. rapa</i>-derived genes were more retained. Comparative phylogenetic analysis of 2R-MYB proteins from nine Brassicaceae and seven non-Brassicaceae species identified five Brassicaceae-specific subfamilies and five subfamilies that are lacking from the examined Brassicaceae species, which provided an example for the adaptive evolution of the 2R-MYB gene family alongside angiosperm diversification. Ectopic expression of four <i>Bn2R-MYB</i>s under the control of the viral <i>CaMV35S</i> and/or native promoters could rescue the lesser root hair phenotype of the <i>Arabidopsis thaliana wer</i> mutant plants, proving the conserved negative roles of the <i>2R-MYB</i>s of the S15 subfamily in root hair development. RNA-sequencing data revealed that the <i>Bn2R-MYB</i>s and <i>BnMYBR</i>s had diverse transcript profiles in roots in response to the treatments with various hormones. Our findings provide valuable information for further functional characterizations of <i>B. napus</i><i>MYB</i> genes.