Temperature during grain filling regulates subsequent germination in maize ( <i>Zea mays</i> L.) through seed DNA methylation

Environmental conditions after anthesis not only affect final crop plant yields, but also profoundly affect subsequent seed germination. The grain yield of maize (Zea mays L.), a cereal important to the global economy and food security, is known to be reduced by high temperature during grain filling, but the effects of this stress on subsequent seed germination have not been elucidated. In this study, maize plants were divided into two temperature regimes during grain filling, control group (20°C) and heat-stress group (30°C), after which harvested seeds from both treatments were germinated at 25°C. We found that heat stress during grain filling significantly delayed subsequent seed germination in maize. During imbibition, expression of an abscisic acid (ABA) biosynthesis–related gene (ZmNCED1) was significantly increased in heat-stressed seeds whereas expression of an ABA catabolism gene (ZmABA8ox1b) and of gibberellic acid (GA) biosynthesis genes (ZmGA20ox2 and ZmGA3ox2) was significantly suppressed, suggesting the roles of these phytohormones in delaying maize seed germination. The expression of an α-amylase gene (ZmAMY), encoding a starch degradation enzyme, was also significantly suppressed in heat-stressed seeds. Analysis of DNA methylation in the promoter regions of these target genes showed significant hypomethylation of ZmNCED1 and hypermethylation of ZmABA8ox1b, ZmGA3ox2 and ZmAMY in heat-stressed seeds, consistent with their transcriptional changes during imbibition. In all, these results suggest that high temperature during grain filling delays subsequent germination through the epigenetic regulation of seed DNA methylation in maize.