Members of the LATERAL ORGAN BOUNDARIES DOMAIN transcription factor family are involved in the regulation of secondary growth in Populus.

Regulation of secondary (woody) growth is of substantial economic and environmental interest but is poorly understood. We identified and subsequently characterized an activation-tagged poplar (Populus tremula × Populus alba) mutant with enhanced woody growth and changes in bark texture caused primarily by increased secondary phloem production. Molecular characterization of the mutation through positioning of the tag and retransformation experiments shows that the phenotype is conditioned by activation of an uncharacterized gene that encodes a novel member of the LATERAL ORGAN BOUNDARIES DOMAIN (LBD) family of transcription factors. Homology analysis showed highest similarity to an uncharacterized LBD1 gene from Arabidopsis thaliana, and we consequently named it Populus tremula × Populus alba (Pta) LBD1. Dominant-negative suppression of Pta LBD1 via translational fusion with the repressor SRDX domain caused decreased diameter growth and suppressed and highly irregular phloem development. In wild-type plants, LBD1 was most highly expressed in the phloem and cambial zone. Two key Class I KNOTTED1-like homeobox genes that promote meristem identity in the cambium were downregulated, while an Altered Phloem Development gene that is known to promote phloem differentiation was upregulated in the mutant. A set of four LBD genes, including the LBD1 gene, was predominantly expressed in wood-forming tissues, suggesting a broader regulatory role of these transcription factors during secondary woody growth in poplar.