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.

Arogenate dehydratase (ADT) catalyzes the final step in phenylalanine synthesis and is crucial for plant development and metabolism. Previously, we demonstrated that the ADT/prephenate dehydratase ZmADT2 is essential for maize resistance to Ustilago maydis and for overall plant development. In this study, we explored the role of ZmADT2 in maize kernel development. The mmsu mutant, a dysfunctional ZmADT2 variant, exhibits delayed embryo and endosperm development, along with deficiencies in carbohydrate and protein storage. Transcriptome analysis revealed differential expression of many kernel compartment-specific genes between mmsu and wild-type (WT) kernels, with impaired nutrient accumulation and auxin signaling pathway in the mmsu endosperm. Compared to WT, ZmADT2 mutation led to reduced auxin levels and smaller endosperm cell size. Exogenous auxin rescued the small kernel phenotype of mmsu. Additionally, auxin distribution was reduced in the basal endosperm transfer layer (BETL), causing defects in its development and function, including reduced transfer cell elongation, cell wall ingrowth and nutrient uptake. These findings suggest that ZmADT2 mediated mediates an auxin signaling pathway that is essential for maize kernel development.