PDX1.1-dependent biosynthesis of vitamin B6 protects roots from ammonium-induced oxidative stress

Despite serving as a major inorganic nitrogen source for plants, ammonium causes toxicity at elevated concentrations, inhibiting root elongation early on. While previous studies have shown that ammonium-inhibited root development relates to ammonium uptake and formation of reactive oxygen species (ROS) in roots, it remains unclear about the mechanisms underlying the repression of root growth and how plants cope with this inhibitory effect of ammonium. In this study, we demonstrate that ammonium-induced apoplastic acidification co-localizes with Fe precipitation and hydrogen peroxide (H2O2) accumulation along the stele of the elongation and differentiation zone in root tips, indicating Fe-dependent ROS formation. By screening ammonium sensitivity in T-DNA insertion lines of ammonium-responsive genes, we identified PDX1.1, which is upregulated by ammonium in the root stele and whose product catalyzes de novo biosynthesis of vitamin B-6. Root growth of pdx1.1 mutants is hypersensitive to ammonium, while chemical complementation or overexpression of PDX1.1 restores root elongation. This salvage strategy requires non-phosphorylated forms of vitamin B-6 that are able to quench ROS and rescue root growth from ammonium inhibition. Collectively, these results suggest that PDX1.1-mediated synthesis of non-phosphorylated B-6 vitamers acts as a primary strategy to protect roots from ammonium-dependent ROS formation.