Integrating metagenomics into legume breeding: A breeder-centered roadmap from core microbiomes to precision inoculation.

Metagenomics, culture-independent profiling of genetic material recovered from environmental samples, provides a powerful route to characterize microbial communities associated with legumes and to translate their functional potential into breeding targets that enhance resilience and productivity. Across analyses of rhizosphere, endosphere, and seed microbiomes, repeated studies consistently identify a conserved set of microbial functions linked to nutrient cycling, responses to abiotic and biotic stress, and biological control of pathogens, thereby offering mechanistic support that community-level functional capacities can shape host outcomes, including seedling vigor, nutrient-use efficiency, and stress tolerance. To move from descriptive discovery to actionable breeding, three complementary translational strategies have emerged: (i) synthetic microbial communities (SynComs) engineered to deliver targeted metabolic functions while enabling rigorous assessment of community stability and functional consistency; (ii) predictive model systems that integrate metagenomic features with phenotypic measurements to prioritize candidate taxa or functions for subsequent validation; and (iii) precision inoculation approaches that deploy validated microbes or consortia in agronomic settings to test whether metagenome-inferred functions confer robust performance under field-relevant conditions. A critical appraisal of metagenomic, multi-omics, and translational studies indicates that functional-phenotypic mappings are promising, yet substantial barriers continue to constrain reproducibility and scalability, including heterogeneity in sampling and experimental design, biases introduced by DNA extraction and sequencing, variability across bioinformatics workflows and reference databases, and overarching biosafety and regulatory constraints that can obscure true biological signals and weaken the reliability of functional inferences intended to guide selection decisions. To mainstream metagenomics in conventional legume breeding, we propose a breeders' roadmap centered on coordinated standardization and decision-ready analytics, encompassing standardized metagenomics-compatible sampling and sequencing platforms, harmonized computational frameworks and metabolic inference tools to ensure comparable functional calls, high-throughput phenotyping protocols aligned to microbiome-sensitive host traits, and selection frameworks that explicitly incorporate microbiome-oriented decision rules rather than treating microbial signals as ancillary. Finally, integrating machine learning with multi-omics datasets alongside precision delivery systems offers a practical route to generate actionable holobiont-level selection indices, and, when coupled with clearly defined translational pipelines and methodological standardization, metagenomics can broaden breeding gains beyond those achievable using host genomics alone, enabling more reliable, function-driven microbiome-assisted improvement of legume performance.