PagIPT5 Mediates Cambial Development in Poplar via Cytokinin–Auxin Crosstalk

Background/Objectives: Cytokinin and auxin are essential for vascular development in plants. This study aims to explore whether these two hormones exhibit crosstalk in the cambium, analogous to that observed in the apical meristem. Methods: Using the hybrid poplar (Populus alba × Purshia glandulosa clone ‘84K’), we integrated gravitropic induction with transcriptomic analysis and identified the cytokinin biosynthesis gene PagIPT5 as differentially expressed in a tension wood induction system. PagIPT5 overexpression lines were generated and assessed for growth-related phenotypes. The interaction between cytokinin and auxin was investigated via anatomical observation, cell proliferation assays, in situ PCR, and immunofluorescence detection of auxin and cytokinin. Results: Compared with the wild type, PagIPT5 overexpression lines showed growth inhibition and an auxin-deficient phenotype. High PagIPT5 expression in the vascular cambium elevated cytokinin levels while reducing auxin levels, leading to decreased cambial cell proliferation and suppressed xylem development. However, in the tension wood induction system, both auxin and cytokinin levels increased in the vascular cambium of tension wood relative to opposite wood. Treatment with a superoxide anion activator promotes the accumulation of both auxin and cytokinin in 84K plants. Conclusions: These results revealed an antagonistic interaction between the two hormones in the cambium zone. However, this antagonism is attenuated in tension wood, which may be induced by the accumulation of superoxide anion in tension wood.