Could the transcription factor HHO5 hold the key to breaking fertilizer dependency?

Efficient nitrogen uptake is essential for plant growth. Hinckley et al. have characterized HRS1 HOMOLOG 5 (HHO5), a transcription factor in Arabidopsis that had been previously overlooked compared to the NIGT1 family. Through a single T-DNA insertion mutation, the authors show HHO5 acts as a non-redundant and physiologically distinct regulatory player in the nitrogen uptake and response. hho5 mutants exhibit reduced nitrogen-dose-dependent growth, lower seed nitrogen content and blunted root responses to glutamate. The authors applied six nitrogen concentrations across a 10,000-fold range to hydroponically grown seedlings and sequenced whole-root transcriptomes two hours later. They found that HHO5 expression is repressed by inorganic nitrogen but induced by organic nitrogen. When organic nitrogen accumulates, HHO5 levels rise and directly repress crucial nitrate transporter genes, including NRT1.1 and NRT2.1. At the same time, HHO5 activates organic nitrogen-response genes indirectly via a WRKY partner. The authors deployed DoubleTARGET, a cell-based co-perturbation assay that tests transcription factor pairs simultaneously to untangle dense regulatory networks. They then revealed that HHO5 controls around 828 nitrogen-responsive genes and behaves according to enzyme kinetics by sensing dose instead of just presence or absence. The authors conclude by suggesting HHO5 as an agronomically significant gene with obvious implications for engineering crops that yield more from less fertilizer. (Summary by Sonal Sachdev, sci3ntyst , sci3ntyst.bsky.social) Plant Cell 10.1093/plcell/koag201