The B-type response regulator ZmRR5 controls maize kernel size by regulating BETL development

Maize ( Zea mays L.) is one of the world’s most important staple crops, and is used for manufacturing food, feed, and industrial products. A key factor in maize yield is the grain weight, which directly influences productivity. In this study, we revealed the role of smk23 in maize kernel development. The ethyl methanesulfonate mutant smk23 is characterized by substantially reduced kernel weight. Through map-based cloning, smk23 was found to be located on Chr5 and encode a putative B-type response regulator, ZmRR5. A change from G to A occurs in the coding sequence of ZmRR5 , resulting in the early termination of smk23 . In Arabidopsis , B-type response regulators are involved in cytokinin signaling. Histological analysis and in situ hybridization of the mutant revealed abnormal endosperm development, particularly in the basal endosperm transfer layer (BETL), a specialized tissue critical for nutrient transport from the maternal tissues to the developing kernel. ZmRR5 positively regulates key genes involved in BETL development and function, including MRP1 and TCRR1 . Furthermore, RNA sequencing revealed that several genes closely linked to BETL development, including BETL2 , MEG1 , and MN1 , were significantly downregulated in smk23 . These genes are essential for nutrient transport, tissue development and signal transduction. In addition, haploid analysis of ZmRR5 revealed natural variations (Hap 2) that may contribute to the increased kernel yield. Disruption of ZmRR5 function in smk23 leads to defects in BETL development, impairing its ability to transport nutrients, and ultimately resulting in a smaller kernel size. This study provides new insights into the molecular mechanisms through which ZmRR5 regulates maize kernel development and offers potential strategies for improving grain yield.