Maize early endosperm development is initiated in coordination with elimination of maternal nucellar tissues. This paper characterizes a major quantitative trait locus for maize kernel size and weight that encodes an EXPANSIN gene, *ZmEXPB15*. The β-expansin protein is expressed specifically in nucellus and positively controls kernel size and weight by promoting nucellus elimination. Two nucellus-enriched transcription factors (*ZmNAC11* and *ZmNAC29*) activate *ZmEXPB15* expression, also promoting kernel size and weight. Hybrids from a *ZmEXPB15* overexpression line show increased kernel weight, highlighting its breeding potential. The study reveals a pathway modulating nucellus elimination and early endosperm development.
