The quiescent/colorless alleles of viviparous1 show that the conserved B3 domain of VP1 is not essential for ABA-regulated gene expression in the seed

A series of vp1 alleles distinguish at least two classes of maturation-related genes that are regulated by the VP1 factor and abscisic acid (ABA). The intermediate vp1-c821708 and vp1-McW alleles have quiescent (non-viviparous) anthocyanin-deficient phenotypes while maintaining significant levels of maturation-specific gene expression in the developing embryo. However, expression of the C1 regulatory gene of the anthocyanin pathway is not detected in these mutants, Reduced steady-state levels of structurally altered VP1 proteins are detected in quiescent mutant embryos, The VP1-McW protein sequence lacks the highly conserved region encoded by exons 3-5 of the Vp1 gene. A sensitive RT-PCR assay was used to rule out significant amounts of intact transcripts in the vp1-McW mutant that could account for the quiescent phenotype. In transient expression assays, the VP1-McW protein and other mutants with a truncated B3 domain of VP1 retained a strong capacity to synergistically enhance ABA-regulation of the Em-GUS reporter gene; whereas transactivation of both Em-GUS and C1-sh-GUS genes in the absence of hormone was strongly inhibited. These results indicate that the largest conserved region in VP1 homologs (B3) is critical for gene activation at low or insignificant ABA dosages; whereas the N-terminal domain provides a key interface with ABA signaling pathways in the developing seed.