The Transcription Factor PtrWOX4 Promotes Secondary Growth by Regulating Cambium Activity in Citrus

In citrus cultivation, successful grafting depends on the vigorous growth of rootstock, particularly robust stem thickening, which is crucial for a strong and healthy graft union. However, the factors and molecular mechanisms that accelerate stem thickening remain largely unknown. Here, we functionally characterize PtrWOX4, a key regulator of secondary growth in trifoliate orange (Poncirus trifoliata L. Raf.). This study shows that PtrWOX4 encodes a nuclear protein with a conserved WUS-box motif activated by auxin and strigolactone but repressed by abscisic acid and gibberellin. Overexpressing PtrWOX4 in tobacco markedly increased stem diameter, reduced internode length, and upregulated the cambium-associated gene KNAT6 and the phloem-associated gene NEN4. These changes were concomitant with elevated auxin and jasmonic acid levels in tobacco. Furthermore, overexpressing PtrWOX4 in trifoliate orange accelerated stem thickening and activation of the cambium and vascular tissue development. Collectively, this study established PtrWOX4 as a transcriptional coordinator that promotes stem secondary growth in trifoliate orange by orchestrating cambium activity and vascular differentiation.