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 con-centrations,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 apo-plastic acidification co-localizes with Fe precipitation and hydrogen peroxide(H_(2)O_(2))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 biosyn-thesis of vitamin B6.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 B6 that are able to quench ROS and rescue root growth from ammo-nium inhibition.Collectively,these results suggest that PDX1.1-mediated synthesis of non-phosphorylated B6 vitamers acts as a primary strategy to protect roots from ammonium-dependent ROS formation.

Nitric oxide regulation of plant metabolism

Nitric oxide(NO)has emerged as an important signal molecule in plants,having myriad roles in plant devel-opment.In addition,NO also orchestrates both biotic and abiotic stress responses,during which intensive cellular metabolic reprogramming occurs.Integral to these responses is the location of NO biosynthetic and scavenging pathways in diverse cellular compartments,enabling plants to effectively organize signal transduction pathways.NO regulates plant metabolism and,in turn,metabolic pathways reciprocally regu-late NO accumulation and function.Thus,these diverse cellular processes are inextricably linked.This re-view addresses the numerous redox pathways,located in the various subcellular compartments that pro-duce NO,in addition to the mechanisms underpinning NO scavenging.We focus on how this molecular dance is integrated into the metabolic state of the cell.Within this context,a reciprocal relationship be-tween NO accumulation and metabolite production is often apparent.We also showcase cellular pathways,including those associated with nitrate reduction,that provide evidence for this integration of NO function and metabolism.Finally,we discuss the potential importance of the biochemical reactions governing NO levels in determining plant responses to a changing environment.