4.1 | Root secretion and soil nitrate cycling and utilization
Plants require nitrogen to form new cells (Ke et al., 2020), and nitrogen deficiency is a limiting factor in crop productivity (Zhao et al., 2005). Soil nitrogen exists in organic and inorganic forms, with nitrate and ammonium being the plant-available types (Xu et al., 2012). Root secretion of soil nitrogen cycle enzymes influences nitrogen use (Dong et al., 2021). This experiment revealed that S-NR activity secreted by S. miltiorrhiza roots was significantly down-regulated under 15 different abiotic stress conditions. This suggests that under stress, S. miltiorrhiza can maintain the supply of nitrate-nitrogen in the root environment by down-regulating the activity of root-secreted S-NR and reducing its nitrate-nitrogen reduction ability (Coskun et al., 2017). This is consistent with nitrogen stress down-regulating NR secretion from maize roots (Qiang et al., 2021) and phosphorus stress down-regulating NR activity from cowpea roots (Qi bing-lin et al., 2010). The suggestion is that reducing nitrogen loss is a strategy for plants to cope with abiotic stress. However, the results of this study differ from the drought stress induced upregulation of NR activity in wheat roots (Hosseini et al., 2022) and the improvement of S-NR enzyme activity in soil by nutrient stress treatment (Meng et al., 2021). The involvement of microorganisms in root exudation activities, and the increased activity of denitrifying bacteria, may contribute to the differences observed in nitrate reductase activity in the soil.
This experiment shows that S. miltiorrhiza often up-regulates the activity of root-secreted S-NiR under most stress conditions, which is consistent with the fact that drought stress significantly increases the activity of S-NiR in the rhizosphere of maize (Lin et al., 2021) but different from the fact that low nitrogen reduces the activity of NiR in the rhizosphere of tobacco (Xihuan et al., 2020). Notably, S. miltiorrhiza significantly increased the activity of S-NiR in both DN and TN stresses, indicating improved denitrification, which may be related to the high-temperature causing soil oxygen overflow (Guan hui-lin et al., 2010). It shows that after stress, plants can reduce the damage of nitrite salt in the rhizosphere environment by up-regulating the activity of S-NiR secreted from the root system.
S-UE plays a critical role in converting organic nitrogen in soil into ammonium nitrogen available for plants. This study shows that S. miltiorrhiza can significantly up-regulate the activity of S-UE secreted by roots under N, D, T, DT, DN, DP, and TNP stresses, consistent with the results that increasing temperature increases the activity of S-UE in soil (Bai et al., 2017). Moreover, nitrogen deficiency and its synergistic stress with drought can significantly up-regulate the activity of root-secreting S-UE, but the synergistic stress of high-temperature or synergistic stresses of drought and high-temperature can significantly down-regulate the activity of root-secreting S-UE (Figure 2-a) (e.g., DN, TN, and DTN stresses). In addition, nitrogen deficiency, phosphorus deficiency and their synergistic stress with drought or high-temperature or drought and high-temperature can significantly up-regulate the activity of root-secreting S-UE (Figure 2-c) (e.g., DNP, TNP, and DTNP stresses). These findings suggest that plants demonstrate unique responses to different synergistic stress. Under abiotic environmental stress, plants can reduce the reduction of nitrate nitrogen and accelerate the transformation of organic nitrogen by altering the activities of enzymes involved in root secretion and soil nitrogen cycling to maintain nitrogen available to plants and improve the reduction of nitrite to reduce its harmfulness, and then improve nitrogen conversion and use efficiency.