Abstract
Malus hupehensis, as an apple rootstock, is an economically important tree species popular due to its excellent fruit yield and stress resistance. Nitrogen is one of the critical limiting factors of plant growth and fruit yield, so it is crucial to explore new methods to improve nitrogen use efficiency. Melatonin and dopamine, as multifunctional metabolites, play numerous physiological roles in plants. We analyzed the effects of exogenous melatonin and dopamine treatments on the growth, root system architecture, nitrogen absorption, and metabolism of M. hupehensis when seedlings were exposed to nitrate-deficient conditions. Under low nitrate stress, plant growth slowed, and chlorophyll contents and 15NO3– accumulation decreased significantly. However, the application of 0.1 μmol/L melatonin or 100 μmol/L exogenous dopamine significantly reduced the inhibition attributable to low nitrate levels during the ensuing period of stress treatment, and the effect of dopamine was more obvious. In addition to modifying the root system architecture of nitrate-deficient plants, exogenous melatonin and dopamine also changed the uptake, transport, and distribution of 15NO3–. Furthermore, both exogenous melatonin and dopamine enhanced tolerance to low nitrate stress by maintaining the activity of enzymes (NR, NiR, GS, Fd-GOGAT, and NADH-GOGAT) and the transcription levels of related genes involved in leaf and root nitrogen metabolism. We also found that exogenous melatonin and dopamine promoted the expression of nitrate transporter genes (NRT1.1, NRT2.4, NRT2.5, and NRT2.7) in nitrate-deficient plant leaves and roots. Our results suggest that both exogenous melatonin and dopamine can mitigate low nitrate stress by changing the root system architecture, promoting the absorption of nitrate, and regulating the expression of genes related to nitrogen transport and metabolism. However, according to a comprehensive analysis of the results, exogenous dopamine plays a more significant role than melatonin in improving plant nitrogen use efficiency.
Highlights
Nitrogen (N) is required in large amounts by plants, and plant growth and productivity can be restricted when it is deficient (Maathuis, 2009; Liu et al, 2021)
Under low-NO3− stress, plant growth was severely inhibited in comparison with the normal control, leading to significant decreases in plant length (PL), trunk diameter (TD), LN, total dry weight (TDW), and relative growth rate (RGR) of 33.4, 24.0, 33.2, 47.8, and 52.5%, respectively
Our results demonstrate that melatonin and dopamine improve the uptake of NO3− in M. hupehensis plants by enhancing the activities of key enzymes involved in N assimilation, so that plants can absorb and use
Summary
Nitrogen (N) is required in large amounts by plants, and plant growth and productivity can be restricted when it is deficient (Maathuis, 2009; Liu et al, 2021). The application of N fertilizer has contributed greatly to crop yields, largely alleviating the pressure. Melatonin and Dopamine Affect Nitrate Stress caused by the global population surge. There is evidence that crops use only 30–50% of the N applied, and the loss of reactive N from agricultural soils causes serious environment pollution (Guo et al, 2010; Su et al, 2020). It is necessary to enhance crop nitrogen use efficiency (NUE) to improve yield quality with low N investment (Liu et al, 2021).
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