Antibiotic resistance genes (ARGs) in rice: Source attribution and putative mobility patterns

Rice grains can harbor antibiotic resistance genes (ARGs), yet the relative roles of seed-associated and environmental reservoirs remain unclear. We used shotgun metagenomics on rice tissues (grain, seed, leaf, stem, root) and surrounding matrices (bulk/rhizosphere soil, irrigation water, rainwater, PM₁₀). In total, 1019 ARG subtypes were detected; grains contained 395, the largely overlapping with seeds (290) and environmental samples (322). FEAST source tracking revealed contrasting attribution patterns: seed sources explained nearly half of the grain microbiome (average contribution 49.49%) versus 8.45% from environmental sources, whereas environmental sources contributed more strongly to the grain resistome (20.68%). 747 metagenome-assembled genomes (MAGs) were reconstructed, including 275 ARG-carrying MAGs. Phylogenetic screening identified 39 near-identical (≥99%) ARG linkages across samples, operationally classified by host consistency (same vs different predicted hosts) into 11 putative VGT-like and 28 putative HGT-like patterns. For example, bla_GOB-50 in grains and seeds shared near-identical sequences within Elizabethkingia anopheles (VGT-like), while APH(9)-Ic in grains (Burkholderia) matched PM₁₀ (Comamonas), consistent with an HGT-like linkage. In selected cases, ARG-MGE co-localization (e.g., umuC, cca) further supported mobility interpretations. Together, these results indicate seedborne signatures in the grain microbiome but comparatively stronger environmental association for the grain resistome, informing efforts to trace ARG reservoirs in rice systems.