The evolutionary path to wide angles: Domain V is required for LAZY1 function, but its loss alone does not confer TAC1-like activity in poplar.

When ectopically expressed in hybrid poplar, the gymnosperm protein PdeLAZY1 confers LAZY1-like activity, whereas deletion of Domain V abolishes this activity without conferring TAC1-like function. The IGT gene family regulates plant architecture by controlling lateral organ angles. TAC1 is proposed to have evolved from an ancestral LAZY-like gene through loss of the conserved C-terminal Domain V, but this has not been tested functionally. We expressed PdeLAZY1, a LAZY1 homolog from the gymnosperm Pinus densiflora, and its Domain V-deleted variant (PdeLAZY1ΔV) under the 35S promoter in wild-type hybrid poplar (clone BH) and in a CRISPR-generated tac1 mutant. In wild-type poplar, PdeLAZY1 significantly reduced petiole angle, showing that a gymnosperm LAZY1 is competent to promote upright growth in an angiosperm context, whereas PdeLAZY1ΔV significantly increased it, indicating that Domain V is required for this activity. In the tac1 background, however, neither construct significantly increased petiole angle relative to the untransformed control, so PdeLAZY1ΔV did not complement loss of TAC1. Domain V is therefore required for the LAZY1-type activity of PdeLAZY1, but its loss alone is not sufficient to confer TAC1-like function in poplar. Because these are ectopic overexpression experiments, they test competence rather than endogenous function; within this limit, the data are consistent with a stepwise model in which changes beyond Domain V loss were required for TAC1 evolution.