Cbuhdz34, a Homeodomain Leucine Zipper Transcription Factor, Positively Regulates Tension Wood Formation and Xylem Fibre Cell Elongation in Catalpa bungei

Catalpa bungei is a highly valued timber species renowned for its superior wood properties. However, the development of tension wood (TW) induced by wind and other mechanical stresses during the growing season significantly reduces its economic value. Although Homeodomain Leucine Zipper (HD-Zip), a plant-specific transcription factor family, has been reported to play various roles in plant growth, development, and stress resistance, a systematic characterisation of the HD-Zip gene family in C. bungei, particularly regarding the regulatory mechanisms involved in TW formation, is still lacking. Here, we identified a total of 48 HD-Zip genes (Cbuhdzs) in C. bungei and analysed their phylogeny, structure, and expression profiles. In particular, Cbuhdz34, a member of the HD-Zip I subfamily, was specifically upregulated during TW formation. To further explore its function, we overexpressed Cbuhdz34 (OE-Cbuhdz34) in poplar '84 K', which led to noticeable changes in plant growth and fibre cell length. Moreover, compared with wild-type plants, the OE-Cbuhdz34 plants presented increased TW formation under bending stress, as indicated by increased TW width, gelatinous layer width, and eccentric growth rate, suggesting a positive regulatory role in TW formation. Additionally, hierarchical genetic regulatory network analysis revealed the direct targets of Cbuhdz34, including CbuMYB63 and three genes involved in cell wall synthesis (CbuGATL1, CbuFLA17, and CbuLRR14). Further, yeast one-hybrid and dual-luciferase reporter assays confirmed the activation of these targets by Cbuhdz34. In conclusion, our results provide insights into the molecular mechanisms by which Cbuhdz34 regulates TW formation and lay a genetic foundation for the potential improvement of wood quality in C. bungei.