Effect of Nickel Ions on the Physiological and Transcriptional Responses to Carbon and Nitrogen Metabolism in Tomato Roots under Low Nitrogen Levels

Nickel (Ni) is an essential trace element for plant growth and a component of the plant body that has many different functions in plants. Although it has been confirmed that nickel ions (Ni(2+)) havea certain regulatory effect on nitrogen (N) metabolism, there are not enough data to prove whether exogenous Ni(2+) can increase the carbon (C) and N metabolism in the roots of tomato seedlingsunder low-nitrogen (LN) conditions. Therefore, through the present experiment, we revealed the key mechanism of Ni(2+)-mediated tomato root tolerance to LN levels. Tomato plants were cultured at two different N levels (7.66 and 0.383 mmol L(−1)) and two different Ni(2+) levels (0 and 0.1 mg L(−1) NiSO(4) 6H(2)O) under hydroponic conditions. After nine days, we collected roots for physiological, biochemical, and transcriptome sequencing analyses and found that the activities of N assimilation-related enzymes decreased at LN levels. In contrast, Ni(2+) significantly increased the activities of N assimilation-related enzymes and increased the contents of nitrate (NO(3)(−)), ammonium (NH(4)(+)), and total amino acids. Through root transcriptomic analysis, 3738 differentially expressed genes (DEGs) were identified. DEGs related to C and N metabolism were downregulated after LN application. However, after Ni(2+) treatment, PK, PDHB, GAPDH, NR, NiR, GS, GOGAT, and other DEGs related to C and N metabolism were significantly upregulated. In conclusion, our results suggest that Ni(2+) can regulate the C and N metabolism pathways in tomato roots to alleviate the impact of LN levels.