Physiological and DNA methylation analysis provides epigenetic insights into kenaf cadmium tolerance heterosis

Soil heavy metal pollution is one of the most challenging problems. Kenaf is an important natural fiber crop with strong heterosis and a higher tolerance to heavy metals. However, little is known about the molecular mechanisms of kenaf heavy metal tolerance, especially the mechanism of genomic DNA methylation regulating heterosis. In this study, kenaf cultivars CP085, CP089, and their hybrid F1 seedlings were subjected to 300 µM cadmium stress and found obvious heterosis of cadmium resistance in morphology and antioxidant enzyme activity of F1 hybrid seedlings. Through methylation-sensitive amplification polymorphism (MSAP) analysis, we highlighted that the total DNA methylation level under cadmium decreased by 16.9 % in F1 and increased by 14.0 % and 3.0 % in parents CP085 and CP089, respectively. The hypomethylation rate was highest (21.84 %), but hypermethylation was lowest (17.24 %) in F1 compared to parent cultivars. In particular, principal coordinates analysis (PCoA) indicates a significant epigenetic differentiation between F1 and its parents under cadmium. Furthermore, 21 differentially methylated DNA fragments (DMFs) were analyzed. Especially, the expression of NPF2.7, NADP-ME, NAC71, TPP-D, LRR-RLKs, and DHX51 genes were changed due to cadmium stress and related to cytosine methylation regulation. Finally, the knocked-down of the differentially methylated gene NPF2.7 by virus-induced gene silencing (VIGS) resulted in increased sensitivity of kenaf seedlings under cadmium stress. It is speculated that low DNA methylation levels can regulate gene expression that led to the heterosis of cadmium tolerance in kenaf.