1 | INTRODUCTION
Root exudates are a complex mixture of biochemical compounds that are secreted actively or passively by plant roots. These compounds comprise diverse small molecules, such as amino acids and organic acids, and macromolecules, including polysaccharides, proteins, and biologically active enzymes (Chai and Schachtman, 2022). Biological and abiotic stress conditions alter the composition and quantity of plant root exudates, thereby altering the rhizosphere ecosystem and assisting plants in adapting to different stress environments (Chai and Schachtman, 2022, Calvo et al., 2017). During the process of plant growth, development and reproduction, about 21% of the net photosynthetic products of plants enter the soil as root exudates, and when plants adapt to different stress environments (Wang et al., 2021, Panchal et al., 2022, Pramanik and Phukan, 2020). Moreover, root exudates provide nutrients and energy to the rhizosphere microbiome, regulate the structure of microbial communities in soil, affect enzyme activities produced by microorganisms, and ultimately influence the decomposition, mineralization, and availability of organic compounds and soil nutrients (Lu et al., 2021). It can be seen that root exudates affect the action strength and direction of soil enzymes, and play a key role in the soil material cycle, energy conversion and plant carbon storage (Panchal et al., 2022). Especially, it plays an important role in the soil nitrogen and phosphorus cycle (Malek et al., 2021).
Soil enzymes are a complex mixture of both plant and microbial sources. Root secretion is not only an important source of soil enzymes but also the main factor driving microorganisms to secrete soil enzymes. Many enzymes play key roles in soil nitrogen cycling, such as Solid-Nitrate Reductase (S-NR), Solid-Nitrite reductase (S-NiR) and Solid-Urease (S-UE). Specifically, S-NR catalyzes the reduction of nitrate to nitrite, S-NiR catalyzes the reduction of nitrite to nitric oxide (Jian-guo and Wei-guo, 2018), and S-UE hydrolyzes urea to produce ammonia and carbonic acid (Fisher et al., 2017). The solid-phosphotransferase (S-PT) plays a crucial role in phosphorus uptake and utilization by catalyzing the transfer of phosphate groups from donors to acceptors (Wohlgemuth et al., 2017). Solid-Catalase (S-CAT) prevents the accumulation of toxic substances by promoting the degradation of hydrogen peroxide (Cusack et al., 2011). Additionally, root-secreted polysaccharides can enhance the growth of polysaccharide-utilizing microbial communities and stimulate the production of extracellular enzymes, thereby facilitating the decomposition of soil organic matter and nutrient cycling (Morcillo and Manzanera, 2021). Furthermore, these polysaccharides also serve as a significant source of organic matter in the soil (Pansu and Gautheyrou, 2006). Abiotic stresses such as drought (Staszel et al., 2022) and nitrogen (Jia et al., 2020) can change soil enzyme activities, thereby affecting soil-plant interactions. Nevertheless, root secretion plays an important role in plant response to environmental stress, especially soil enzymes secreted by roots and soil enzymes secreted by microorganisms driven by root exudates. However, reports on soil enzymes and polysaccharides in root exudates under sterile conditions less.
Global climate change is one of the important factors limiting crop yields (Yuan et al., 2009), mainly causing the loss of nutrients such as nitrogen and phosphorus in the soil, resulting in reduced crop yields (Bojko and Kabala, 2016, Widdig et al., 2020, Twining et al., 2022). High-temperature, drought, nitrogen deficiency, and phosphorus deficiency are common factors that limit crop yield. In nature, plants are often under combined stress of multiple environmental factors, but how they affect plant growth and development is poorly understood (Zandalinas et al., 2021). There is also a lack of direct evidence that roots secrete soil enzymes and polysaccharides into the soil when plants respond to these adverse environments. Based on this, under sterile conditions, 15 stress models were designed with high-temperature (T), drought (D), nitrogen deficiency (N), phosphorus deficiency (P) and their combinations. After the stress culture of S. miltiorrhizatest-tube seedlings, the secretion of S-UE, S-NIR, S-NR, S-PT, and S-CAT activities and total polysaccharide content changes were measured. To understand the effects of S. miltiorrhiza on soil nitrogen, phosphorus cycling and soil polysaccharides in response to abiotic stress, explore the ecological strategy and role of plant root secretion response under abiotic stress.