Abstract: It is of great importance to better understand how trees regulate nitrogen (N) uptake under N deficiency conditions which severely challenge afforestation practices, yet the underlying molecular mechanisms have not been well elucidated. Here, we functionally characterized PuHox52, a Populus ussuriensis HD-ZIP transcription factor, whose overexpression greatly enhanced nutrient uptake and plant growth under N deficiency. We first conducted an RNA sequencing experiment to obtain root transcriptome using PuHox52-overexpression lines of P. ussuriensis under low N treatment. We then performed multiple genetic and phenotypic analyses to identify key target genes of PuHox52 and validated how they acted against N deficiency under PuHox52 regulation. PuHox52 was specifically induced in roots by N deficiency, and overexpression of PuHox52 promoted N uptake, plant growth, and root development. We demonstrated that several nitrate-responsive genes (PuNRT1.1, PuNRT2.4, PuCLC-b, PuNIA2, PuNIR1, and PuNLP1), phosphate-responsive genes (PuPHL1A and PuPHL1B), and an iron transporter gene (PuIRT1) were substantiated to be direct targets of PuHox52. Among them, PuNRT1.1, PuPHL1A/B, and PuIRT1 were upregulated to relatively higher levels during PuHox52-mediated responses against N deficiency in PuHox52-overexpression lines compared to WT. Our study revealed a novel regulatory mechanism underlying root adaption to N deficiency where PuHox52 modulated a coordinated uptake of nitrate, phosphate, and iron through ‘PuHox52-PuNRT1.1’, ‘PuHox52-PuPHL1A/PuPHL1B’, and ‘PuHox52-PuIRT1’ regulatory relationships in poplar roots.

Key words: HD-Zip transcription factor, iron transport, nitrate transport, nitrogen deprivation, phosphate absorption, Populus ussuriensis, root architecture, target gene