Nitrogen sources modulate auxin transport to fine-tune root system architecture

KEY MESSAGE: Increased auxin transport toward and within young organs may promote greater root branching when plants are grown with ammonium as a nitrogen source. Accessibility to different forms of nitrogen is a critical condition for shaping plant root morphology. Nitrate promotes and ammonium impedes primary root growth, but they show a converse effect on root branching. In this study, we demonstrated how using ammonium as the sole nitrogen source modifies auxin distribution in Arabidopsis thaliana to adjust root system development to this nutritional stress. Local changes in PIN-FORMED protein abundance and auxin levels were detected using GFP and DR5 reporter lines. Higher PIN1 abundance correlated with auxin enrichment in the primary root apex, whereas increased abundance of PIN1, PIN2, and PIN3 correlated with auxin accumulation in root primordia under ammonium nutrition. These findings suggest that the ammonium-induced pattern of the auxin transporters may facilitate auxin buildup in these tissues. Consequently, lateral root density increased in higher-order lateral roots of ammonium-grown plants, resulting in a branched root phenotype in mature Arabidopsis thaliana.