The poplar root system efficiently accumulates cadmium (Cd), which is a heavy metal pollutant in soil. The regulatory effects of various forms of nitrogen on the absorption and accumulation of Cd in roots and the underlying mechanisms remain unclear. This study investigated the effects of diverse nitrogen forms on Cd accumulation in young poplar seedling roots. The 3 mM nitrate treatment resulted in the highest root Cd levels and root weights. Examinations of physiological parameters and hormone contents as well as an untargeted metabolomic analysis indicated that the nitrogen application influenced antioxidant systems, osmoregulation, nitrogen assimilation, primary metabolic processes, and hormone metabolism. The transcriptome sequencing analysis identified key genes encoding regulators of different processes. The Spearman correlation analysis and random forest modeling showed that the nitrate-induced Cd accumulation in young poplar seedling roots was associated with abscisic acid and zeatin riboside. According to the weighted gene co-expression network analysis, abscisic acid regulated diverse pathways, including those associated with glutathione metabolism, the tricarboxylic acid cycle, and amino acid synthesis, whereas zeatin riboside controlled pathways related to starch and sucrose metabolism, glycolysis, secondary metabolite synthesis, ascorbic acid and aldehyde metabolism, and arginine and proline metabolism. Hence, nitrate nitrogen significantly enhances the accumulation of Cd in poplar roots, with abscisic acid and zeatin riboside playing crucial roles in this process. The study findings may be useful for breeding Cd-hyperaccumulating varieties and for remediating Cd-polluted agricultural fields.

中文翻译：

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脱落酸和玉米素核苷对硝酸盐诱导的杨根镉超积累的重要性

杨树根系有效积累土壤中的重金属污染物镉（Cd）。各种形式的氮对根系吸收和积累镉的调节作用及其潜在机制尚不清楚。本研究调查了不同氮形态对杨幼苗根部镉积累的影响。 3 mM 硝酸盐处理导致根部镉水平和根部重量最高。生理参数和激素含量的检查以及非目标代谢组学分析表明，施氮影响抗氧化系统、渗透调节、氮同化、初级代谢过程和激素代谢。转录组测序分析确定了编码不同过程调节因子的关键基因。 Spearman相关分析和随机森林模型表明，硝酸盐诱导的杨幼苗根部Cd积累与脱落酸和玉米素核苷有关。根据加权基因共表达网络分析，脱落酸调节多种途径，包括与谷胱甘肽代谢、三羧酸循环和氨基酸合成相关的途径，而玉米素核苷控制与淀粉和蔗糖代谢、糖酵解、次级代谢产物相关的途径。合成、抗坏血酸和醛代谢、以及精氨酸和脯氨酸代谢。因此，硝态氮显着增强了杨树根部镉的积累，其中脱落酸和玉米素核苷在此过程中发挥着至关重要的作用。该研究结果可能有助于培育镉超积累品种和修复镉污染的农田。