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.