Maize transcription factor ZmBES1/BZR1-5 positively regulates kernel size.

The BES1/BZR1 transcription factors regulate the expression of brassinosteroid responsive genes and play pivotal roles in plant development. However, the function of BES1/BZR1 regulating kernel development remains unclear. In this study, maize ZmBES1/BZR1-5 is found to positively regulate kernel size. Candidate-gene association analysis showed that four and three SNPs related to ZmBES1/BZR1-5 were significantly associated with kernel width and 100-kernel weight in 513 diverse maize inbred lines, respectively. Overexpression of ZmBES1/BZR1-5 gene in Arabidopsis and rice both significantly increased seed size and weight, as well as smaller kernel produced in maize Mu transposon insertion and EMS mutants. The ZmBES1/BZR1-5 protein contains bHLH and BAM domains, shows no transcriptional activity as monomer but forms homodimer through BAM domain, and locates in nucleus. Chromatin immunoprecipitation sequencing (ChIP-seq), yeast one-hybrid (Y1H) and dual-luciferase assay demonstrate that ZmBES1/BZR1-5 protein binds to the promoter of AP2/EREBP genes (Zm00001d010676 and Zm00001d032077) and inhibits their transcription. cDNA library screening shows that ZmBES1/BZR1-5 interacts with casein kinase II subunit β4 (ZmCKIIβ4) and ferredoxin 2 (ZmFdx2) in vitro and vivo, respectively. Taken together, the study suggests that ZmBES1/BZR1-5 positively regulates kernel size and provides new insights into understanding the mechanism of kernel development in maize.