Abolishing ANAC017-Mediated Mitochondria Retrograde Signalling Alleviates Ammonium Toxicity in Arabidopsis thaliana

ABSTRACTAmmonium (NH4+), an important nitrogen source, often fails to stimulate plant growth as a sole nitrogen source, a phenomenon known as ammonium toxicity syndrome. NH4+ is believed to disrupt cellular redox status by increasing chloroplast reducing capacity and exporting excess reducing equivalents, which trigger retrograde signalling. The precise role of mitochondria in this process remains unclear. Here, we demonstrate that the loss of ARABIDOPSIS NAC DOMAIN TRANSCRIPTION FACTOR17 (ANAC017, rao2‐1), a master regulator of mitochondrial retrograde signalling, significantly increased shoot biomass under both nitrate (NO3−) and NH4+, thus utilising NH4+ more effectively than Col‐0 wildtype. In contrast, loss of function of ALTERNATIVE OXIDASE1A (aox1a) improved recovery of nitrogen‐starved seedlings with NO3− but had no effect with NH4+. Metabolomic analysis revealed that the rao2‐1 mutant assimilated NH4+ more efficiently than the wild type, incorporating it into nitrogen‐rich metabolites. Transcriptomic analyses showed that with NO3− resupply, ANAC017 acted to stimulate photosynthesis, carbon fixation, and NO3− reduction. Under NH4+ resupply, however, ANAC017 suppressed plastid biogenesis and metabolism through classical retrograde signalling pathways. In rao2‐1, a variety of chloroplast retrograde pathways were de‐repressed. Unlike NO3−, NH4+ fails to generate the signals necessary to suppress ANAC017‐dependent retrograde stress responses, thereby impairing nitrogen assimilation and growth.