GARP transcription factors repress Arabidopsis nitrogen starvation response via ROS-dependent and -independent pathways.

Plants need to cope with strong variations of nitrogen availability in the soil. Although many molecular actors are being discovered concerning how plants perceive NO3  - provision, it is less clear how plants recognize a lack of Nitrogen. Following N removal, plants activate their Nitrogen Starvation Response (NSR) being characterized in particular by the activation of very high-affinity nitrate transport systems (NRT2.4, NRT2.5) and other sentinel genes involved in N-remobilization such as GDH3. Here, we show using a combination of functional genomics via Transcription Factor (TF) perturbation and molecular physiology studies, that the TFs belonging to the HHO sub-family are important regulators of NSR through two potential mechanisms. First, HHOs directly repress the high-affinity nitrate transporters, NRT2.4 and NRT2.5. hho mutants display increased high-affinity nitrate transport activity opening promising perspectives for biotechnological applications. Second, we show that Reactive Oxygen Species (ROS) are important to control NSR in wild type plants and that HRS1 and HHO1 over-expressors and mutants are affected in their ROS content, defining a potential feed-forward branch of the signaling pathway. Taken together our results define the relationships of two types of molecular actors controlling the NSR including: ROS and the HHO TFs. This work (i) opens perspectives on a poorly understood nutrient-related signaling pathway, and (ii) defines targets for molecular breeding of plants with enhanced NO3  - uptake.