Clock-regulatory networks contribute to growth vigor in maize hybrids

Heterosis, or hybrid vigor, has been widely used in agriculture for more than a century. Despite extensive investigation and various models proposed, the molecular basis for heterosis remains largely elusive. In Arabidopsis interspecific and intraspecific hybrids, increased photosynthetic and metabolic activities are linked to altered expression of central circadian clock regulators, including CIRCADIAN CLOCK ASSOCIATED 1 (CCA1). It is unknown whether a similar mechanism mediates maize heterosis. In this dissertation, I investigated whether and how the circadian clock regulation contributes to growth heterosis in maize. I reported that higher levels of carbon fixation and starch accumulation in maize hybrids are associated with altered temporal gene expression. Two maize CCA1 homologs, ZmCCA1a and ZmCCA1b, are diurnally up-regulated in the hybrids. In Arabidopsis ZmCCA1 complements the cca1 mutant phenotype, and overexpressing ZmCCA1b disrupts circadian rhythms and heterosis. Furthermore, overexpressing ZmCCA1b in maize reduced chlorophyll content and plant height. Reduced height stems from reduced node elongation but not total node number in both greenhouse and field conditions. Chromatin immunoprecipitation followed by deep sequencing (ChIP-seq) analysis revealed a temporal shift of ZmCCA1-binding targets to the early morning in the hybrids, suggesting that activation of morning-phased genes in the hybrids promotes photosynthesis and growth vigor. This temporal shift of ZmCCA1-binding targets correlated with nonadditive and additive gene expression in early and late stages of seedling development. These results could guide breeding better hybrid crops to meet the growing demand in food and bioenergy.