The transcription factor PagLBD3 contributes to the regulation of secondary growth in Populus

Lateral organ boundaries domain (LBD) genes encode plant-specific
transcription factors that participate in regulating various developmental
processes. In this study, we genetically characterized PagLBD3 as an
important regulator of secondary growth in Populus. Overexpression of
PagLBD3 increased stem secondary growth in Populus with significantly
higher rate of cambial cells differentiated into phloem, while dominant
repression of PagLBD3 significantly decreased the rate of cambial cells
differentiated into phloem. Furthermore, we identified 1756 PagLBD3
genome-wide putative direct target genes (DTGs) through RNA sequencing
(RNA-seq) coupled DNA affinity purification followed by sequencing
(DAP-seq) assays. Gene Ontology analysis revealed that genes regulated by
PagLBD3 were enriched in biological pathways regulating meristem
development, xylem development, and auxin transport. Several central
regulator genes for vascular development, including phloem intercalated
with xylem (PXY), wuschel related homeobox4 (WOX4), Secondary
Wall-Associated NAC Domain 1s (SND1-B2) and Vascular-Related NAC-Domain 6s
(VND6-B1), were identified as PagLBD3 DTGs. Together, our results
suggested that PagLBD3 and its DTGs form a complex transcriptional network
to modulate cambium activity and phloem/xylem differentiation.