Regulation of maize kernel development via divergent activation of α-Zein genes by transcription factors O11, O2, and PBF1.

α-Zeins, the major maize endosperm storage proteins, are transcriptionally regulated by Opaque2 (O2) and PROLAMIN-BOX BINDING FACTOR1 (PBF1), with Opaque11 (O11) functioning upstream of them. However, whether O11 directly binds to α-zein genes and its regulatory interactions with O2 and PBF1 remain unclear. Using the small-kernel mutant sw1, which exhibits decreased 19-kDa and increased 22-kDa α-zein, we positionally cloned O11 and found it directly binds to G-box/E-box motifs. O11 activates 19-kDa α-zein transcription, stronger than PBF1 but weaker than O2. Notably, PBF1 competitively binds to an overlapping E-box/P-box motif, and represses O11-mediated transactivation. Although O11 does not physically interact with O2, it participates in the O2-centered hierarchical network to enhance α-zein expression. sw1 o2 and sw1 pbf1 double mutants exhibit smaller, more opaque kernels with further reduced 19-kDa and 22-kDa α-zeins compared to the single mutants, suggesting distinct regulatory effects of these transcription factors on 19-kDa and 22-kDa α-zein genes. Promoter motif analysis suggests that O11, PBF1, and O2 directly regulate 19-kDa α-zein genes, while O11 indirectly controls 22-kDa α-zein genes via O2 and PBF1 modulation. These findings identify the unique and coordinated roles of O11, O2, and PBF1 in regulating α-zein genes and kernel development.