Elucidating the Genetic Basis of Columnar Upright Architecture in <i>Populus</i> Through <scp>CRISPR</scp> Disruption of <i>TILLER ANGLE CONTROL1</i>

Narrow or upright branch angles in shoots and leaves lead to columnar, upright-growing tree architectures, as observed in various tree species such as Lombardy poplar (Populus nigra var. italica). However, the genetic mechanism underlying this unique growth habit in Lombardy poplar has not yet been elucidated. In this study, we identified a nonsense mutation in the PnTAC1-1 gene of Lombardy poplar, an ortholog of the rice TILLER ANGLE CONTROL 1 (TAC1) gene known to regulate branch angles. To confirm the functional role of TAC1 in regulating tree architecture, we generated transgenic hybrid poplar (Populus alba × Populus glandulosa, clone BH) with targeted mutations in TAC1 homologues using CRISPR/Cas9 gene editing. The resulting TAC1-CRISPR hybrid poplars exhibited a stable upright branching phenotype closely resembling that of Lombardy poplar, as confirmed by two consecutive years of living modified organism (LMO) field trials. Anatomical analysis revealed increased cell elongation specifically in the lower petiole region and significantly enhanced gravitropic responses in TAC1-CRISPR hybrid poplars compared to wild-type BH clones. RNA sequencing analysis further demonstrated that TAC1 disruption triggered extensive transcriptomic reprogramming of axillary meristem, notably altering hormonal and photomorphogenic signalling pathways, which redirected auxin accumulation toward the abaxial region and increased gibberellin biosynthesis, ultimately promoting upright growth. This research uncovers the genetic and molecular mechanisms behind columnar growth in poplar and provides a promising approach for engineering tree architecture to enhance planting density, harvest efficiency and woody biomass productivity.