The melatonin delivery system based on nano-SiO2 optimizes photosynthetic carbon metabolism and yield formation of soybeans in saline-alkali land
Nanomaterials show a great potential in regulating plant growth and improving stress resistance due to their unique physical and chemical characteristics. In this study, nano-silicon dioxide (nano-SiO2) was prepared by the sol-gel method and used to construct a highly efficient melatonin delivery system. Soybean cultivars Hefeng 50 and Henong 95 were field-grown in saline-alkali soil and foliar-sprayed with 200 mg/L nano-SiO2, 300 µM melatonin (MT), or their combination at the V3 stage. In the MT-nano-SiO2system, the encapsulation efficiency and drug-loading capacity of melatonin by nano-SiO2were 35% and 53%, respectively. The system can significantly enhanced melatonin photostability. Scanning electron microscopy results showed that the MT-nano-SiO2 composite can be directly adsorbed on the leaf surface. The MT-nano-SiO2 treatment significantly reduced salt-alkali stress induced reactive oxygen species (ROS) accumulation by activated the enzymatic and non-enzymatic antioxidant system expressed as enhancing the activities of SOD, CAT, and APX and regulating glutathione (GSH/GSSG) levels. The results of physiology, RNA-seq and qRT-PCR indicated that MT-nano-SiO2 treatment can repair photosynthetic carbon metabolism damage induced by salt-alkali stress. Specifically, it increased leaf area, photosynthetic pigment content, photosystem II (PS?) efficiency (Fv/Fm, Qy), net photosynthetic rate (Pn), stomatal conductance (Gs), and intercellular CO2concentration (Ci). It also promoted the activities of sucrose phosphate synthase (SPS), sucrose synthase (SuSy), but inhibited the activities of a-amylase, ß-amylase to enhance soluble sugar, sucrose, and starch accumulation. Ultimately, MT-nano-SiO2 treatment significantly improved pod number per plant, seed number per plant, 100-seed weight, and yield of soybean plants. Overall design: RNA-seq analysis was performed on the functional leaves of soybean cultivar HN95 at 7 days after MT,  nano-SiO2 and nano-SiO2+MT foliar application