ZmZHOUPI, an endosperm-specific basic helix-loop-helix transcription factor involved in maize seed development

Summary In angiosperm seeds the embryo is embedded within the endosperm, which is in turn enveloped by the seed coat, making inter‐compartmental communication essential for coordinated seed growth. In this context the basic helix–loop–helix domain transcription factor At ZHOUPI (At ZOU ) fulfils a key role in both the lysis of the transient endosperm and in embryo cuticle formation in Arabidopsis thaliana . In maize ( Zea mays ), a cereal with a persistent endosperm, a single gene, Zm ZOU , falls into the same phylogenetic clade as At ZOU . Its expression is limited to the endosperm where it peaks during the filling stage. In Zm ZOU ‐ RNA interference knock‐down lines embryo size is slightly reduced and the embryonic suspensor and the adjacent embryo surrounding region show retarded breakdown. Ectopic expression of Zm ZOU reduces stomatal number, possibly due to inappropriate protein interactions. Zm ZOU forms functional heterodimers with At ICE /At SCREAM and the closely related maize proteins Zm ICE b and Zm ICE c, but its interaction is more efficient with the Zm ICE a protein, which shows sequence divergence and only has close homologues in other monocotyledonous species. Consistent with the observation that these complexes can trans‐ activate target gene promoters from Arabidopsis, Zm ZOU partially complements the Atzou‐4 mutant. However, structural, trans ‐activation and gene expression data support the hypothesis that Zm ZOU and Zm ICE a may have coevolved to form a functional complex unique to monocot seeds. This divergence may explain the reduced functionality of Zm ZOU in Arabidopsis, and reflect functional specificities which are unique to the monocotyledon lineage.