A molecular module controlling silicon efflux from glandular trichomes is required for fruit bloom formation in cucumber

Silicon plays a vital role in plant physiology. Although the silicon transport mechanisms in monocots are well characterized, the molecular basis of silicon deposition in dicots remains elusive. Fruit bloom, an off-white substance covering the fruit surface and affecting its appearance, is crucial for the market-driven breeding and production of cucumbers (Cucumis sativus). However, the mechanisms regulating fruit bloom formation are not well understood. In this study, we aimed to elucidate the molecular mechanisms underlying silicon deposition in glandular trichomes (GTs) and GT's role in fruit bloom formation. Using map-based cloning, we identified a single-nucleotide polymorphism in CsaV3_3G017280, encoding a homolog of the rice (Oryza sativa) silicon efflux transporter Low Silicon Rice 2 (Lsi2), causing a premature translation termination mutation linked to the nonfruit-bloom phenotype. Knocking out CsLsi2 prevented silicon deposition on the fruit surface, leading to a nonfruit-bloom phenotype. The MYB transcription factor CsRAX3 directly activated CsLsi2, and the GT development-related factor TINY BRANCHED HAIR (TBH) regulated both CsRAX3 and CsLsi2, linking silicon deposition with GT development. Collectively, our observations establish a direct connection between Si deposition and GT development and provide a perspective on the mechanisms regulating fruit bloom formation.