Abstract
It is commonly known that exogenously applied melatonin can alleviate the impact of drought stress, but the mechanism used by melatonin to regulate stomatal behavior and carbon (C) and nitrogen (N) metabolism to increase drought resistance remains elusive. Herein, our aim was to investigate the influence of exogenous melatonin on the regulation of C and N metabolism in maize plants under water deficit. In this study, we analyzed stomatal behavior, the key components of C and N metabolism, and the gene expression and activity of enzymes involved in the C and N metabolism in maize plants. The results showed that the application of melatonin (100 μM) significantly increased maize growth and sustained the opening of stomata, and secondarily increased the photosynthetic capacity in maize. Under drought stress, foliar application of melatonin induced the gene transcription and activities of sucrose phosphate synthetase, ADP-glucose pyrophosphorylase, phosphoenolpyruvate carboxylase, and citrate synthase, resulting in the enhancement of sucrose and starch synthesis and the tricarboxylic acid (TCA) cycle. This enhancement in sugar biosynthesis and the TCA cycle might lead to stronger N assimilation. As anticipated, NO3– reduction and NH4+ assimilation were also strengthened after melatonin treatment under drought stress. An increase was observed in some key enzymatic activities and transcription involved in nitrogen metabolism, such as that of nitrate reductase, nitrite reductase, glutamate synthase, and glutamine synthetase, in melatonin-treated, drought-stressed maize. Moreover, melatonin attenuated the drought-induced damage by reducing protein degradation and increasing the level of proline. Conclusively, our results indicate that exogenous melatonin enhances drought tolerance in maize via promoting stomatal opening and regulating C and N metabolism and related gene expression.
Highlights
Maize (Zea mays L.) is one of the most important grain crops cultivated worldwide but is extremely sensitive to drought stress (Li Z. et al, 2021)
We evaluated the effects of melatonin plus ample water (MT), drought stress, and their combination on the growth of maize to understand the role of melatonin in drought tolerance in maize plants
Drought stress resulted in a sharp decrease in chlorophyll and the relative water content of leaves (RWC), while the application of exogenous melatonin reversed these trends to some extent (Figures 1C,D)
Summary
Maize (Zea mays L.) is one of the most important grain crops cultivated worldwide but is extremely sensitive to drought stress (Li Z. et al, 2021). Drought is complex abiotic stress, and a series of morphological, physiological, and biochemical changes take place during the response of plants to drought stress (Shah et al, 2020), which include plant growth (Todaka et al, 2017), leaf stomatal behavior (Indira et al, 2021), photosynthetic activity (Zhou et al, 2019), cellular redox homeostasis (Zhang et al, 2019), and metabolism homeostasis (Pinheiro and Chaves, 2011). Following stomatal closure and the decrease in Ci, the activity of the carboxylating enzyme Rubisco has been shown to decrease, which leads to electron accumulation and reactive oxygen species (ROS) overproduction, eventually resulting in oxidative damage and a series of subsequent side effects, such as leaf peroxidation, and degradation of chlorophyll, proteins, and nucleic acids (Campos et al, 2019; Sharma and Zheng, 2019; Sharma et al, 2020)
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