DECREASE IN DNA METHYLATION 1-mediated epigenetic regulation maintains gene expression balance required for heterosis in Arabidopsis thaliana

Heterosis, or hybrid vigor, is characterized by superior phenotypic performance in F₁ hybrids compared to their parental lines, and its expression is influenced by both genetic and epigenetic factors. In this study, we investigated the role of epigenetic regulation, particularly DNA methylation mediated by DECREASE IN DNA METHYLATION 1 (DDM1), in early seedling biomass heterosis using hybrids between Arabidopsis thaliana accessions Columbia-0 and C24. Loss of DDM1 function in F₁ hybrids resulted in a significant reduction of rosette diameter, confirming that DDM1 is essential for heterosis. Transcriptomic and epigenomic analyses revealed extensive genotype-specific changes in gene expression and DNA methylation patterns in ddm1 mutants. Notably, ddm1-F₁ hybrids exhibited upregulation of genes categorized into 'xyloglucan:xyloglucosyl transferase activity' and downregulation of genes categorized into 'circadian rhythm', which may contribute to reduced growth vigor. Whole-genome bisulfite sequencing showed widespread hypomethylation in ddm1 mutants, yet the overlap between differentially methylated and expressed genes was limited, suggesting indirect or context-dependent regulatory effects. Additionally, we examined the role of salicylic acid (SA) in heterosis using SA-deficient mutants and found that variations in endogenous SA levels did not correlate with biomass heterosis under normal conditions. Our findings suggest that heterosis in A. thaliana is dependent on the maintenance of parental epigenetic divergence, particularly in DNA methylation patterns mediated by DDM1. Disruption of DDM1 compromises this epigenetic complementarity, leading to transcriptomic imbalances that reduce heterosis.