BIN2-mediated phosphorylation of KAN1 integrates brassinosteroid and auxin signaling during poplar secondary growth

The spatial arrangement of phloem, cambium and xylem is crucial for secondary growth in tree species. During secondary growth, cambium cells produce secondary xylem inwards and secondary phloem outwards. While phytohormone regulators and differentiation mediators coordinate vascular development, their signaling crosstalk remains poorly understood. Here, we reveal that in poplar (Populus tomentosa), the GSK3 kinase BRASSINOSTEROID INSENSITIVE 2.1 (PtoBIN2.1), integrates brassinosteroid (BR) and auxin signaling during secondary growth by phosphorylating the transcription factor KANADI1 (KAN1), which functions as abaxial determinant. In the phloem/abaxial side, BIN2-mediated phosphorylation stabilizes KAN1, enhancing its suppression of auxin biosynthesis/signaling and HD-ZIP III adaxial determinants, thereby promoting phloem development while inhibiting cambial activity and xylem differentiation. Conversely, BR and auxin synergistically promote xylem formation, with auxin signaling being required for BR-mediated secondary growth. BRs or lower BIN2.1 levels decrease KAN1 stability through diminished phosphorylation, attenuating KAN1-driven inhibition of auxin signaling and consequently enhancing cambial proliferation and xylem development. Our findings establish a BIN2–KAN1 regulatory module that orchestrates phloem–xylem patterning and demonstrate how poplar integrates BR and auxin signaling to control secondary growth.