Pairs polyphylla var. yunnanensis is a perennial herb, and its rhizomes are used as medicine to invigorate blood circulation, dissipate blood stasis, relieve swelling and pain, clear heat and detoxify the body. However, due to its slow growth, most of the saponin content is low, while the content of saponins is higher in Paris fargesii Franch of the same genus. The isolation and screening of strains from P. fargesii Franch that promote the accumulation of saponin content in P. polyphylla var. yunnanensis is important to improve the growth rate of artificially cultivated P. polyphylla var. yunnanensis and increase its medicinal activity. Plant symbiotic strains are biologically active microorganisms that can promote the growth and development of host plants through direct probiotic effects and indirect disease resistance mechanisms. Here, we isolated four strains of symbiotes, including two bacteria, Bacillus cereus (LgD2) and Bacillus thuringiensis (LgD10), and two fungi, Aspergillus tamari (TPD11) and Fusarium oxysporum (TPB). All of them promoted the biomass, chlorophyll content and photosynthetic efficiency of P. polyphylla var. yunnanensis seedlings to different degrees and effectively stimulated the accumulation of saponin content in P. polyphylla var. yunnanensis seedlings to improve the expression of saponin synthesis pathway genes. The symbiotes also promoted the enzymatic activities of the enzymes superoxide dismutase (SOD), peroxidase (POD), polyphenol oxidase (PPO) and phenylalanine aminolase (PAL), improved the physiological indicators of tobacco plants (plant height, crown width, leaf length and leaf width), reduced the incidence of tobacco black shank, and improved the growth and resistance-related genes of tobacco. In conclusion, the four symbiotic strains TPD11, TPB, LgD10 and LgD2 play an important role in regulating the growth, promoting saponin accumulation, and disease resistance of P. polyphylla var. yunnanensis.

Arbuscular mycorrhizal fungi (AMF) are important members of the plant microbiome and affect the uptake and transfer of mineral elements by forming a symbiotic relationship with plant roots. Nitrogen (N), as an important mineral element, can directly affect plant growth and development at different N levels. It has been confirmed that inoculation with AMF can improve the efficiency of N utilization by plants. However, there are still fewer reports on the dynamic relationship between arbuscular mycorrhizal and plant secondary metabolites at different nitrogen levels. In this experiment, the physiological indexes and genes related to saponin synthesis were determined by applying different concentration gradients of nitrogen to the medicinal plant P. polyphylla var. yunnanensis infested by AMF as the test material. It was found that nitrogen addition increased the biomass, chlorophyll content, and nutrient content of above- and below-ground plant parts and increased the content of saponin content of P. polyphylla var. yunnanensis to some extent, but AMF inoculation increased the saponin content of P. polyphylla var. yunnanensis more significantly. AMF inoculation also promoted the expression of genes related to the saponin synthesis pathway, including 3-hydroxy-3-methylglutaryl coenzyme A synthase (HMGS), squalene epoxidase 1 (SE1), and cycloartenol synthase (CAS), which is in according with the accumulation of saponin in plants. It also may increase the saponin content of AMF plants by altering the expression of P450s and UGTs related to saponin synthesis.

Drought stress (DS) is one of the important abiotic stresses facing cash crops today. Drought can reduce plant growth and development, inhibit photosynthesis, and thus reduce plant yield. In this experiment, we investigated the protective mechanism of AMF on plant photosynthetic system by inoculating Paris polyphylla var. yunnanensis with a clumping mycorrhizal fungus (AMF) under drought conditions. The drought environment was maintained by weighing AM plants and non-AM plants. The relative water content (RWC) of plant leaves was measured to determine its drought effect. DS decreased the RWC of plants, but AMF was able to increase the RWC of plants. chlorophyll a fluorescence curve measurements revealed that DS increased the OKJIP curve of plants, but AMF was able to reduce this trend, indicating that AMF increased the light absorption capacity of plants. dS also caused a decrease in plant Y(I) and Y(II). ETRI and ETRII, and increased Y(NO) and Y(NA) in plants, indicating that DS caused photosystem damage in plants. For the same host, different AMFs did not help to the same extent , but all AMFs were able to help plants reduce this damage and contribute to the increase of plant photosynthesis under normal water conditions.This work was supported by the Key R & D program of Yunnan Province, China (grant no. 202103AC100003；202101AS070228)；Major special projects of the Ministry of science and technology (2021YFD1000202)；the National Nature Science Foundation of China (Grant No. 31860075)