R2R3-MYB gene pairs in Populus: evolution and contribution to secondary wall formation and flowering time.

In plants, the R2R3-MYB gene family contains many pairs of paralogous genes, which play the diverse roles in developmental processes and environmental responses. The paper reports the characterization of 81 pairs of Populus R2R3-MYB genes. Chromosome placement, phylogenetic, and motif structure analyses showed that these gene pairs resulted from multiple types of gene duplications and had five different gene fates. Tissue expression patterns revealed that most duplicated genes were specifically expressed in the tissues examined. qRT-PCR confirmed that nine pairs were highly expressed in xylem, of which three pairs (PdMYB10/128, PdMYB90/167, and PdMYB92/125) were further functionally characterized. The six PdMYBs were localized to the nucleus and had transcriptional activities in yeast. The heterologous expression of PdMYB10 and 128 in Arabidopsis increased stem fibre cell-wall thickness and delayed flowering. In contrast, overexpression of PdMYB90, 167, 92, and 125 in Arabidopsis decreased stem fibre and vessel cell-wall thickness and promoted flowering. Cellulose, xylose, and lignin contents were changed in overexpression plants. The expression levels of several genes involved in secondary wall formation and flowering were affected by the overexpression of the six PdMYBs in Arabidopsis. This study addresses the diversity of gene duplications in Populus R2R3-MYBs and the roles of these six genes in secondary wall formation and flowering control.