Importance of abscisic acid and zeatin nucleosides for the nitrate-induced cadmium hyperaccumulation in Populus roots

Abstract

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.