Objectives We studied the effect of Se fertilizer on the endophytic bacterial communities and its subsequent impact on the growth and quality of tea leaves.

Methods A field experiment was conducted in Taoyuan, Hunan Province, using the tea cultivar 'Taoyuandaye'. The treatments included spraying water (CK), 90 mg/L Se fertilizer (Se1), and 180 mg/L Se fertilizer (Se2) before budding. When new twigs with one bud and two leaves covered 30% of the canopy, we assessed twig number and length, bud weight, and leaf chlorophyll content. Additionally, Illumina sequencing was used to analyze the endophytic bacterial community, and Se, total polyphenols, catechins, and total free amino acids in the twigs were measured.

Results Compared to CK, Se1 and Se2 significantly increased leaf Se content to 3.06 mg/kg and 4.78 mg/kg, respectively, but did not significantly affect one-hundred-bud weight, leaf length, bud length, or chlorophyll content. However, Se1 and Se2 increased free amino acid content by 7.04% and 16.15%, respectively, and reduced the TP/AA ratio, indicating improved tea flavor quality. The result of Illumina sequence analysis showed that Se1 did not significantly influence the α diversity of the endophytic bacterial community, whereas the Se2 significantly decreased it. Compared with CK, the Se1 reduced the relative abundance of endophytic bacteria affiliated with Herbaspirillum, Sphingomonas, and unclassified Rhodanobacteraceae. However, it became the dominant endophytic bacterial group in leaves and increased the proportion of some minority taxa, such as bacterial communities belonging to Corynebacterium, unclassified Acetobacteraceae, Cellvibrio, Reyranella, and Mesorhizobium. Moreover, Se2 restructured the endophytic bacterial communities more profoundly than Se1 in tea leaves. PICRUSt analysis showed that selenium fertilizer significantly increased endophytic bacteria with metabolic functions, such as amino acids, carbohydrates, coenzymes, vitamins, terpenoids, and ketones, and also enhanced the relative abundance of endophytic bacteria potentially involved in pathways related to environmental adaptation, replication and repair, energy metabolism, and the biodegradation and metabolism of xenobiotics. Correlation analysis showed that the relative abundance of Pseudomonas positively correlated with the content of total free amino acids and theanine. Several species, such as Corynebacterium, unclassified Acetobacteraceae, and Methylobacillus, negatively correlated with ester-catechins (EGCG, GCG, ECG).

Conclusions Applied prior to budding, selenium fertilization significantly elevated the selenium content in tea leaves, thereby promoting the abundance of endophytic bacterial groups with metabolic functions associated with amino acids, carbohydrates, coenzymes, vitamins, terpenoids, and ketones. Furthermore, it enhanced the abundance of endophytic bacteria involved in processes related to environmental adaptation, DNA repair, energy metabolism, and the biodegradation and metabolism of xenobiotics.