Host genotype modulates melatonin-associated drought resilience in citrus accompanied by rhizosphere microbiome...

Exogenous melatonin (MT) has emerged as a potent biostimulant for mitigating plant drought stress; however, whether its efficacy depends on the host genetic background to orchestrate rhizosphere microbiome assembly remains unclear. Here, we integrated plant physiological traits, soil biochemical properties, and amplicon sequencing to decipher the genotype-dependent responses of drought-tolerant (DR) and drought-sensitive (DS) citrus cultivars to MT application under water deficit. While MT alleviated oxidative damage and growth inhibition in both genotypes, the DR cultivar exhibited a superior, system-level resilience characterized by a profound enhancement of rhizosphere fertility and microbial biomass. Co-occurrence network analysis showed that MT treatment was associated with marked topological reorganization in the DR rhizosphere, including increased bacterial modularity and fungal connectivity, whereas such changes were comparatively limited in the DS cultivar. Furthermore, Random Forest models and Mantel tests identified divergent patterns of candidate bacterial taxa associated with plant and soil traits. The DR genotype showed a relative enrichment of Paenibacillus and Rugosimonospora, which were strongly associated with soil nutrient-related indicators and microbial biomass. In contrast, the DS genotype showed a relative enrichment of Sphingobacterium, which was more closely associated with host antioxidant-related traits than with soil nutrient-related indicators. Collectively, our findings provide correlative evidence that melatonin-associated drought resilience co-occurs with coordinated changes in host physiological traits and rhizosphere microbiome properties, and that these associations vary with host genotype. This study provides preliminary insights into genotype-dependent plant-microbiome associations under biostimulant application and suggests that host genetic background should be considered when evaluating rhizosphere responses to exogenous agents.