FUSCA3 from barley unveils a common transcriptional regulation of seed-specific genes between cereals and Arabidopsis

Accumulation of storage compounds in the embryo and endosperm of developing seeds is a highly regulated process that allows seedling growth upon germination until photosynthetic capacity is acquired. A critical regulatory element in the promoters of seed storage protein (SSP) genes from dicotyledonous species is the RY box, a target of B3-type transcription factors. However, the functionality of this motif in the transcriptional regulation of SSP genes from cereals has not been fully established. We report here the identification and molecular characterization of barley FUSCA3, a B3-type transcription factor as yet uncharacterized in monocotyledonous plants. Our results show that both the barley and Arabidopsis FUS3 genes maintain a conserved functionality for the regulation of SSP genes and anthocyanin biosynthesis in these two distantly related phylogenetic groups. Complementation of the loss-of-function mutant fus3 in Arabidopsis by the barley HvFus3 gene resulted in restored transcription from the At2S3 gene promoter and normal accumulation of anthocyanins in the seed. In barley, HvFUS3 participates in transcriptional activation of the endosperm-specific genes Hor2 and Itr1. HvFUS3, which specifically binds to RY boxes in EMSA experiments, trans-activates Hor2 and Itr1 promoters containing intact RY boxes in transient expression assays in developing endosperms. Mutations in the RY boxes abolished the HvFUS3-mediated trans-activation. HvFus3 transcripts accumulate in the endosperm and in the embryo of developing seeds, peaking at mid maturation phase. Remarkably, HvFUS3 interacts with the Opaque2-like bZIP factor BLZ2 in yeast, and this interaction is essential for full trans-activation of the seed-specific genes in planta.