Poplar Sex-Determining Gene Directly Affects Tree Growth Through Regulating Cytokinin Signalling Pathway

Sex separation in dioecious plants is triggered by the sex-determining gene, which primarily drives sexual dimorphism in reproductive organs. However, whether the plant sex-determining gene directly affects plant growth remains controversial. Here, we investigate the effect of the FERR gene, the sex-determining gene in Populus deltoides, on tree growth. Our results showed that the knockout of FERR in the female poplar 'Nanlin 895' significantly enhanced plant height, stem diameter and the number of cambial cell layers, while overexpressing FERR in the male poplar '84 K' reduced tree growth, diminished cambial cell proliferation and disrupted apical dominance with more branching. To illustrate the underlying molecular mechanism, we detected a type-B response regulator (BRR13/21) that activated the transcription of FERR. BRR13/21 is a key gene in the cytokinin signalling pathway, which binds to the FERR promoter at the conserved binding site of 5'-GATT(T/C)-3'. FERR got involved in cytokinin signalling through a negative feedback to BRR13/21 expression. Structural analysis showed that FERR contains the conserved aspartate-aspartate-lysine residues, among which aspartate acts as the phosphorylation receptor. Knocking out this receptor in FERR resulted in the elimination of phosphorylation and consequently blocked the cytokinin signalling transduction. Findings in this study illustrated that FERR negatively affected tree growth through the cytokinin signalling pathway, which provided direct evidence that the poplar sex-determining gene triggered a direct effect on tree growth. This study also provided a feasible strategy for modifying the female poplars into trichomeless and fast-growing trees for poplar plantations.