Diverse Expression Landscapes and Functional Characterization of the GRF Family in Paulownia Fortunei

GENERAL REGULATORY FACTOR (GRF) proteins constitute an evolutionarily conserved family of phosphoserine/phosphothreonine-binding adaptors that orchestrate multiple signaling pathways governing plant development and adaptive responses to environmental cues. Here, we performed a genome-wide characterization of the GRF family in Paulownia fortunei, a commercially important fast-growing timber tree, and identified 15 PfGRF genes that are non-uniformly distributed across nine chromosomes. Phylogenetic reconstruction assigned these genes into two distinct clades, ε and non-ε, both characterized by conserved motif compositions and exon–intron structures. Promoter analysis uncovered a wide spectrum of cis-regulatory elements associated with hormone signaling and stress responses, and synteny examination revealed that segmental duplication, rather than tandem duplication, drove the expansion of this family, with the strongest collinearity detected against Populus trichocarpa. Expression profiling across eight organs demonstrated marked tissue-preferential patterns, with the majority of PfGRF transcripts most abundant in mature leaves and male buds. In addition, PfGRF1 was experimentally verified to localize to the nucleus. Functional assays further showed that PfGRF4–7 possess transcriptional self-activation capacity, whereas PfGRF15 does not, and yeast two-hybrid experiments confirmed a specific protein interaction between PfGRF4 and PfGRF5.