Abstract
Adventitious rooting is an essential biological process in the vegetative propagation of economically important horticultural and forest tree species. It enables utilization of the elite genotypes in breeding programmes and production. Promotion of adventitious root (AR) formation has been associated with starvation of inorganic phosphate and some factors involved in low phosphorus (LP) signalling. However, the regulatory mechanism underlying LP-mediated AR formation remains largely elusive. We established an efficient experimental system that guaranteed AR formation through short-term LP treatment in Populus ussuriensis. We then generated a time-course RNA-seq data set to recognize key regulatory genes and regulatory cascades positively regulating AR formation through data analysis and gene network construction, which were followed by experimental validation and characterization. We constructed a multilayered hierarchical gene regulatory network, from which PuMYB40, a typical R2R3-type MYB transcription factor (TF), and its interactive partner, PuWRKY75, as well as their direct targets, PuLRP1 and PuERF003, were identified to function upstream of the known adventitious rooting genes. These regulatory genes were functionally characterized and proved their roles in promoting AR formation in P. ussuriensis. In conclusion, our study unveiled a new hierarchical regulatory network that promoted AR formation in P. ussuriensis, which was activated by short-term LP stimulus and primarily governed by PuMYB40 and PuWRKY75.