Biochar-immobilized Burkholderia pyrrocinia P10 alleviates cadmium stress in pepper throughout the growth cycle: physiological responses and transcriptional regulatory features.

Cadmium (Cd) is a primary contaminant deteriorating agricultural soil quality in China. Pepper (Capsicum annuum L.) is a vegetable crop capable of accumulating high levels of Cd. This study was conducted to investigate the effects of a biochar-based microbial agent (Burkholderia pyrrocinia P10) on pepper growth and development under Cd stress conditions. The results showed that P10 had strong Cd resistance-related and plant growth-promoting properties. Morever, it effectively colonized the porous biochar structure. Under Cd stress (10-70 mg kg⁻¹), the biochar-immoblized P10 (PC) treatment consistently enhanced pepper growth and Cd tolerance throughout the growth period, as reflected by increased plant biomass, decreased malondialdehyde content. PC also significantly increased the number of fruits per plant and improved fruit nutritional quality, while markedly reducing soil available Cd and the acuumulation of Cd plants and fruits. These beneficial effects were superior to those of bacterial inoculation or biochar application alone. According to root transcriptomic profiling at the full-fruiting stage, compared with the effects of 50 mg kg- 1 Cd2+ stress conditions, the PC treatment upregulated the expression of genes related to cell wall and cell membrane synthesis (e.g., CYP98A3, ALDH2C4, PER4, and FAD2); phytohormone biosynthesis and signaling (e.g., GH3.1, CisZOGT, amiE, GA3ox, KS, and KO); base excision repair, alleviation of spliceosome dysfunction, and protein homeostasis (e.g., NTH, AlkA, prp16, and Ubx); and glutathione and ascorbate metabolism (e.g., APX1, GME, and multiple GST genes). These findings elucidate the molecular basis of the superior efficacy of biochar-immobilized bacterial agent in enhancing Cd tolerance and reducing Cd accumulation in pepper.