CbuBIM1 Antagonizes CbuMYB27 to Regulate Tension Wood Formation in <i>Catalpa bungei</i>

Catalpa bungei is a valuable timber species famous for its high-quality wood properties, which are greatly compromised by tension wood (TW) formation. However, the molecular mechanisms underlying TW remain unclear. In this study, we identified that CbuMYB27 was specifically upregulated during TW formation in C. bungei. CbuMYB27 exhibits the highest expression level in the stem and its encoded protein possesses transcriptional activation activity. Overexpression of CbuMYB27 in hybrid poplar (84K) resulted in increased cambium layers and xylem width. Further analysis revealed that CbuMYB27 negatively regulates lignin biosynthesis. Molecular analysis indicated that CbuMYB27 positively regulates cambial activity and negatively regulates lignin biosynthesis by directly activating cambium-related genes and suppressing lignin-biosynthetic genes, respectively. Protein interaction assays indicated that CbuMYB27 physically interacts with a bHLH transcription factor, CbuBIM1. Although CbuBIM1 cannot directly regulate the target genes of CbuMYB27, it antagonizes CbuMYB27-mediated transcriptional regulation of these downstream genes. Moreover, CbuBIM1 attenuates the promoting effect of CbuMYB27 in TW formation caused by bending stress. In summary, our findings identify a functionally antagonistic module of CbuMYB27- CbuBIM1, which is involved in TW formation by modulating cambium activity and lignin biosynthesis in C. bungei and thus provides a potential target for the improvement of wood quality in C. bungei.