Salicylic Acid Is Involved in the Growth Inhibition Caused by Excessive Ammonium in Oilseed Rape (Brassica napus L.)

Rapeseed (Brassica napus L.) is extremely sensitive to excessive NH₄⁺ toxicity. There remains incomplete knowledge of the causal factors behind the growth suppression in NH₄⁺-nourished plants, with limited studies conducted specifically on field crop plants. In this study, we found that NH₄⁺ toxicity significantly increased salicylic acid (SA) accumulation by accelerating the conversion of SA precursors. Moreover, exogenous SA application significantly aggravated NH₄⁺ toxicity symptoms in the rapeseed shoots. Genome-wide differential transcriptomic analysis showed that NH₄⁺ toxicity increased the expression of genes involved in the biosynthesis, transport, signaling transduction, and conversion of SA. SA treatment significantly increased shoot NH₄⁺ concentrations by reducing the activities of glutamine synthase and glutamate synthase in NH₄⁺-treated rapeseed plants. The application of an SA biosynthesis inhibitor, ABT, alleviated NH₄⁺ toxicity symptoms. Furthermore, SA induced putrescine (Put) accumulation, resulting in an elevated ratio of Put to [spermidine (Spd) + spermine (Spm)] in the NH₄⁺-treated plants, while the opposite was true for ABT. The application of exogenous Put and its biosynthesis inhibitor DFMA induced opposite effects on NH₄⁺ toxicity in rapeseed shoots. These results indicated that the increased endogenous SA contributed noticeably to the toxicity caused by the sole NH₄⁺-N supply in rapeseed shoots. This study provided fresh perspectives on the mechanism underlying excessive NH₄⁺-induced toxicity and the corresponding alleviating strategies in plants.