Piriformospora indica colonization alleviates selenium stress in Broussonetia papyrifera: Insights from transcriptomic approaches and the key role of BpSBP1

Broussonetia papyrifera is an emerging forage tree species with significant potential as a selenium-enriched feed resource. However, a high concentration of selenium (Se) can severely inhibit its growth, thereby limiting its practical application. This study aimed to elucidate the effects of Piriformospora indica on the growth and developmental fate of B. papyrifera under high-dose selenium stress. We employed a 2 × 2 factorial design involving Piriformospora indica inoculation and sodium selenate treatment to investigate their combined effects. Se alone reduced biomass, elevated Se accumulation, induced reactive oxygen species (ROS), disrupted osmolyte balance, and altered antioxidant enzyme activities. Co-inoculation with P. indica (P+Se) restored growth, normalized selenomethionine, and reversed these stress markers. Comparative transcriptomics showed P+Se modulates the MAPK signaling pathway, amino acid metabolism, and key transcription factors while suppressing Se-metabolism genes ( APR1, SAT3, HMT3, SBP1 ). Weighted gene co-expression network analysis identified WRKY, MYB, and bHLH transcription factors as central regulators. BpSBP1 , highly correlated with selenium content ( r = 0.76), was chosen as a candidate gene for functional validation. Heterologous overexpression of BpSBP1 in Nicotiana tabacum confirmed its role in enhancing root growth and reducing SeMet accumulation under Se stress. These findings outline an integrated strategy of endophyte-mediated host signaling modulation and BpSBP1-driven selenium assimilation control, laying the pavement for microbial remediation and developing Se-tolerant forage trees.