HHO5: A key orchestrator of dose-dependent nitrogen signaling pathways in Arabidopsis

A major goal in agriculture is to engineer crops that can maintain yield with less nitrogen (N) fertilizer input. Major orchestrators of plant responses to N include members of the HRS1 HOMOLOG (HHO) family of transcription factors (TFs). However, HHO TFs have been difficult targets for functional studies in planta due to their redundancy. Here, we highlight a unique role for a phylogenetically diverged HHO TF, HHO5, whose expression is regulated in an N-dose dependent fashion and is specifically expressed in phloem. We found that an HHO5 single mutant displays significant misregulation of N-dose dependent genes and plant growth rates. HHO5 is also unique as it displays a dual activator/repressor activity on N-dose dependent gene regulation. HHO5 specifically acts as a direct gene repressor when binding DNA targets. In contrast, genes activated by HHO5 include indirect targets regulated by TFs downstream of HHO5 (TF2s). To validate the influence of HHO5 via its direct TF2s, we used validated TF2 data to build a gene regulatory network that links HHO5-TF2 targets to ~70% of the N-dose genes regulated by HHO5 in planta. By these means, we define HHO5 as a novel dual activator/repressor of plant N-dose signaling.