Proposed model for HAM4a- and HAM4b-mediated secondary xylem formation in poplar. HAM4a directly activates SCL21 expression to promote the differentiation of cambium into secondary xylem. However, HAM4b directly activates TCP20 expression, and then TCP20 activates WND6 transcription and also interacts with WOX4, which is activated by ARF7 and the HD-ZIP III transcription factor HB4, to promote cambial activity and differentiation of cambium into secondary xylem. In the model, the roles of transcription factors with names in white are supported by this study, and those with names in black have been confirmed by other groups (Zhu et al., 2018; Hou et al., 2020; Hu et al., 2022).