Figure 11.

A proposed model for O2 transactivation at the initiation of endosperm filling. The expression levels of ZmbZIP29 and ZmABI19 are high at the early seed development and low at the endosperm-filling stage. The encoded proteins act as two hub coordinators connecting early seed development and grain filling. During seed development, ABA contents are substantially increasing at 8 DAP, at which time endosperm filling is initiated. ZmABI19 and ZmbZIP29 co-transactivate O2 expression at the endosperm initiation stage, which is promoted by ABA. When O2 is activated at the endosperm-filling stage, O2 can auto-transactivate its own promoter, which is also mediated by ABA. The ABA-induced action of SnRK2.2 further phosphorylates ZmABI19, ZmbZIP29, and O2, which confers enhanced transactivation capacity to these TFs. Numbers from 0 to 36 indicate serial DAP during seed development. “An” indicates the O2 mRNA. “P” indicates phosphorylation.