Abstract: Silicon (Si) plays a crucial role in plant growth, development, and stress tolerance. However, in some consumable plant products, such as fruits, Si deposition leads to the formation of a white powdery layer known as bloom, which diminishes glossiness and consumer appeal. Despite its significance, the genetic basis of bloom formation remains largely unexplored. Here, we identified a unique cucumber backbone parent line exhibiting bloomless fruit, which was designated bloomless cucumber 1 (bec1). Map-based cloning of the bec1 locus revealed that BEC1, harboring a natural C-to-T variation at the 754th base of its coding region, is a strong candidate gene for the bloomless trait. Functional validation through gene-editing mutants and BEC1::BEC1-GFP transgenic lines confirmed that BEC1, encoding a Si efflux transporter, is responsible for bloom formation. Mutation of BEC1 impaired Si uptake, thereby preventing the deposition of Si on the surface of glandular trichomes and resulting in bloomless fruits. Additionally, Si deficiency in BEC1 mutants compromised resistance to Corynespora cassiicola and chilling stress. Interestingly, grafting bec1 scions onto bloom rootstocks restored the Si accumulation and stress resistance, while maintaining bloomless phenotype. Overall, our findings elucidate the role of BEC1 in bloom formation and provide a valuable genetic target for breeding bloomless cucumber with enhanced stress resilience.

Key words: bloomless cucumber, graft, Si transporter, silicon, stress