Abstract：
In natural and agricultural situations, ammonium ( NH 4 + ) is a preferred nitrogen (N) source for plants, but excessive amounts can be hazardous to them, known as NH 4 + toxicity. Nitrate ( NO 3 - ) has long been recognized to reduce NH 4 + toxicity. However, little is known about Brassica napus, a major oil crop that is sensitive to high NH 4 + . Here, we found that NO 3 - can mitigate NH 4 + toxicity by balancing rhizosphere and intracellular pH and accelerating ammonium assimilation in B. napus. NO 3 - increased the uptake of NO 3 - and NH 4 + under high NH 4 + circumstances by triggering the expression of NO 3 - and NH 4 + transporters, while NO 3 - and H+ efflux from the cytoplasm to the apoplast was enhanced by promoting the expression of NO 3 - efflux transporters and genes encoding plasma membrane H+ -ATPase. In addition, NO 3 - increased pH in the cytosol, vacuole, and rhizosphere, and down-regulated genes induced by acid stress. Root glutamine synthetase (GS) activity was elevated by NO 3 - under high NH 4 + conditions to enhance the assimilation of NH 4 + into amino acids, thereby reducing NH 4 + accumulation and translocation to shoot in rapeseed. In addition, root GS activity was highly dependent on the environmental pH. NO 3 - might induce metabolites involved in amino acid biosynthesis and malate metabolism in the tricarboxylic acid cycle, and inhibit phenylpropanoid metabolism to mitigate NH 4 + toxicity. Collectively, our results indicate that NO 3 - balances both rhizosphere and intracellular pH via effective NO 3 - transmembrane cycling, accelerates NH 4 + assimilation, and up-regulates malate metabolism to mitigate NH 4 + toxicity in oilseed rape.
摘要：
在自然和农业情况下，铵(NH4+$${\\mathrm{NH}}_4^{+}$$)是植物的首选氮源,但是过量会对他们有害，称为NH4+$${\\mathrm{NH}}_4^{+}$$毒性。硝酸盐(NO3-$${\\mathrm{NO}}_3^{-}$$$)长期以来被认为可降低NH4+${\\mathrm{NH}}_4^{+}$$$毒性。然而,人们对甘蓝型油菜知之甚少，对高NH4+$${\\mathrm{NH}}_4^{+}$敏感的主要油料作物。这里,我们发现NO3-$${\\mathrm{NO}}_3^{-}$$${\\mathrm{NH}}_4^{}$$可以通过平衡根际和细胞内pH和加速氨氮吸收来减轻NH4的毒性。NO3-$${\\mathrm{NO}}_3^{-}$$${\\mathrm{NO}}_3^{-}$和NH4+$${\\mathrm{NH}}_4^{+}$$$$$${^_4^}在高NH4+$$$${NO\^3+NH}情况下而NO3-$${\\mathrm{NO}}_3^{-}$$和H+从细胞质到质外体的外排通过促进NO3-$${\\mathrm{NO}}_3^{-}$$$$外排转运蛋白和编码质膜H-ATPase的基因的表达而增强。此外,NO3-$${\\mathrm{NO}}_3^{-}$$增加细胞溶质的pH值,液泡,和根际，以及酸胁迫诱导的下调基因。在高NH4+$${\\mathrm{NH}}_4^{+}_4^{+}$$$条件下,根谷氨酰胺合成酶(GS)活性提高了NO3-$${\\mathrm{NO}}_3^{-}$$$$,从而减少NH4+$${\\mathrm{NH}}_4^{+}$$在油菜籽中的累积和移位。此外,根GS活性高度依赖于环境pH。NO3-$${\\mathrm{NO}}_3^{-}$$可能诱导参与三羧酸循环中氨基酸生物合成和苹果酸代谢的代谢产物,抑制苯丙素代谢，减轻NH4+${\\mathrm{NH}}_4^{+}$$$毒性。总的来说，我们的结果表明，NO3-$${\\mathrm{NO}}_3^{-}$$$$通过有效的NO3-${\\mathrm{NO}}_3^{-}$$$跨膜循环平衡根际和细胞内pH,加速NH4+$${\\mathrm{NH}}_4^{+}$$同化,并上调苹果酸代谢，以减轻油菜中NH4+${\\\mathrm{NH}}_4^{+}$$毒性。
