Phyllospheric application of Bacillus mucilaginosus mediates the recovery of tea plants exposed to low-temperature stress by alteration of leaf endophytic community and plant physiology.

Background: In winter, tea plants are highly susceptible to low-temperature freezing damage. The rapid recovery of tea plant vigor in spring is crucial for tea yield and quality. Some studies have reported that Bacillus mucilaginosus could improve the stress resistance of plants. However, there were no reports on the effect of B. mucilaginosus on the recovery of tea plant vigor after low-temperature stress. This study firstly used different concentrations of B. mucilaginosus to spray tea leaves and used 16S rRNA high-throughput sequencing technology to study the impact of different treatments on tea leaf endophytic populations. Meanwhile, physiological indexes such as Soil and plant analyzer development values (SPAD), maximum photochemical quantum yield of PS II (Fv/Fm), and superoxide dismutase (SOD) were measured and analyzed in tea plant leaves of different treatments, and the correlation between them and the bacterial community was studied.

Results: Microbial results showed that the diversity of leaf endophytic populations treated with different concentrations of Bacillus mucilaginosus (T1, T2, T3) was higher than that in control group (CK) leaves, and T2 treatment had the highest diversity. The dominant bacterial phyla of all samples were Proteobacteria, Actinobacteriota, Firmicutes, and Bacteroidota. At the phylum level, the relative abundance of Actinobacteriota, Firmicutes, and Bacteroidota in leaves treated with B. mucilaginosus was significantly higher than that in the control. At the genus level, the relative abundance of Paenibacillus, Nocardioides, and Marmoricola in leaves treated with B. mucilaginosus was significantly higher than that in the control. Different concentrations of B. mucilaginosus affected the distribution of leaf endophytic populations. At the level of bacterial function, abundant metabolic functional features were observed, including amino acid transport and metabolism, as well as energy production and conversion, indicating that bacterial metabolism in tea plant leaf samples tends to be vigorous. The treatment with B. mucilaginosus significantly increased the activity of antioxidant enzymes and osmolyte content, promoted the recovery of Fv/Fm in tea plants after low-temperature stress, and improved the resistance of tea leaves to low-temperature stress, thereby promoting recovery.

Conclusions: This study showed that B. mucilaginosus could significantly change the community structure of leaf endophytic populations, and increase antioxidant enzyme activity and osmolyte content in tea plants after low-temperature stress, promoting the rapid recovery of photosynthesis, and thereby benefiting the recovery of tea plant leaves. This study provided a theoretical basis for the application of B. mucilaginosus in practical production and also provided new ideas for the recovery of tea plants exposed to low-temperature stress.