Seed-borne bacteria-mediated seedling microbiome assembly: genetic and metabolic mechanisms of salt tolerance

Seed-borne bacteria (SbRB) are the initial colonizers of the seedling microbiome, yet their priority effects and community-shaping functions under salt stress have been underexplored. This review synthesizes recent advances in three interconnected areas: vertical transmission and genetic conservation of SbRB; ecological assembly mechanisms driven by priority effects, including niche preemption, environmental modification, and niche differentiation between rhizosphere and phyllosphere; and molecular pathways that confer salt tolerance, with a focus on the potential "pre‑adaptation" and "immune window" mechanisms of SbRB over common rhizosphere plant growth‑promoting rhizobacteria. We further discuss how SbRB recruit and modulate downstream microbiota through metabolic complementation, siderophore‑mediated competition, and host immune reprogramming. Key challenges (low culturability, agricultural disruption, inconsistent field performance) and future directions (flower‑inoculation, synthetic communities, multi‑omics integration) are highlighted. Harnessing seed‑borne bacteria offers a promising strategy for salt‑tolerant microbiome breeding and green agriculture in saline soils.