Drought resistance and DNA methylation of interspecific hybrids between Fraxinus mandshurica and Fraxinus americana

Key Message The advantages of F1 hybrid progenies of Fraxinus mandshurica and Fraxinus americana were analyzed under drought conditions in the field and greenhouse. For cross-pollination trees, the optimal breeding method is hybridization. Tree heterosis is commonly present and is the main research focus in tree crossbreeding. In this study, Fraxinus mandshurica was used as female and Fraxinus americana was used as male parents to generate F1 hybrid generation. The adaptability and overwintering survival of F1 hybrids were evaluated under natural conditions. The advantages of F1 generation were analyzed under drought conditions in the field and greenhouse. The results showed that hybrid F1 generation (IH-10 and IH-15) had significantly higher growth and survival rate than the intraspecific progenies from either F. mandshurica or F. americana (MA-13 and FA-43, respectively) in the field. After 12 days of drought stress, growth, chlorophyll a and b, superoxide dismutase and peroxidase of IH-10 and IH-15 were higher than those of MA-13 and FA-43, suggesting an advantage of drought resistance for the hybrid progenies. Relationship of drought resistance and DNA methylation of interspecific hybrids was analyzed. Without drought treatment, methylation degree in the hybrid progenies (IH-10 and IH-15) was lower than that in the paternal or maternal progenies (MA-13 and FA-43). Drought stress enhanced the methylation rate in both hybrid and parental progenies, with higher increases in MA-13 and FA-43 than those in IH-10 and IH-15. Methylation in MA-13 and FA-43 mainly occurred in the full methylation of the internal cytosine, while in IH-10, methylation occurred in both full methylation of the internal cytosine and hemi-methylation of the external cytosine. These results showed that DNA methylation pattern was reconstructed in the hybrid progenies. Further analysis indicated that methylated DNA fragments were associated with growth and cell metabolism. Southern blot and transcriptional analysis showed that DNA methylation in FmaHSP70, FmaCO, FmaWRKY2 and FmaEF1A altered their gene expression. We observed that alteration of DNA methylation is closely correlated with the adaptation to the drought stress and provided epigenetic mechanisms of drought resistance in the interspecific hybridization of trees.