Identification of a candidate gene underlying qHKW3, a QTL for hundred-kernel weight in maize.

KEY MESSAGE: qHKW3, a quantitative trait locus for hundred-kernel weight, harbors the proposed causal gene Zm00001d044081, encoding a homeobox-leucine zipper protein (ATHB-4) that might affect kernel size and weight. Kernel size and weight are key traits that contribute greatly to grain yield per year in maize (Zea mays). Here, we developed the chromosome segment substitution line (CSSL), H15-6-2, with smaller kernel size and lower kernel weight across environments compared to the background line Ye478. Histological analysis suggested that a slower kernel filling rate of H15-6-2 contributes to its small-kernel size and reduced hundred-kernel weight. We identified a quantitative trait locus (QTL) explaining 23% of the phenotypic variation in hundred-kernel weight. This QTL, qHKW3, was fine mapped to an interval of approximately 40.66-kb harboring the gene Zm00001d044081. The upstream sequence and its expression level of Zm00001d044081 in kernels at 6 days after pollination (DAP) showed obvious differences between the near-isogenic lines HKW3Ye478 and HKW3H15-6-2. We further confirmed the effects of the Zm00001d044081 promoter on maize kernel size and weight in an independent association mapping panel with 513 lines by candidate regional association analysis. We propose that Zm00001d044081, which encodes the homeobox-leucine zipper protein ATHB-4, is the causal gene of qHKW3, representing an attractive target for the genetic improvement of maize yield.

A NAC-EXPANSIN module enhances maize kernel size by controlling nucellus elimination.

Maize early endosperm development is initiated in coordination with elimination of maternal nucellar tissues. However, the underlying mechanisms are largely unknown. Here, we characterize a major quantitative trait locus for maize kernel size and weight that encodes an EXPANSIN gene, ZmEXPB15. The encoded ß-expansin protein is expressed specifically in nucellus, and positively controls kernel size and weight by promoting nucellus elimination. We further show that two nucellus-enriched transcription factors (TFs), ZmNAC11 and ZmNAC29, activate ZmEXPB15 expression. Accordingly, these two TFs also promote kernel size and weight through nucellus elimination regulation, and genetic analyses support their interaction with ZmEXPB15. Importantly, hybrids derived from a ZmEXPB15 overexpression line have increased kernel weight, demonstrates its potential value in breeding. Together, we reveal a pathway modulating the cellular processes of maternal nucellus elimination and early endosperm development, and an approach to improve kernel weight.