Importance of the B2 domain of the Arabidopsis ABI3 protein for Em and 2S albumin gene regulation

Genetic and molecular studies have shown that the Arabidopsis ABSCISIC ACID-INSENSITIVE3 (ABI3) protein plays a prominent role in the control of seed maturation. The ABI3 protein and its orthologues from various other plant species share four domains of high sequence identity, including three basic domains designated as B1, B2 and B3. The leaky abi3-1 mutation is a single amino acid substitution within the B3 domain. A new abi3 allele, abi3-7, was generated by mutagenizing abi3-1 seeds. The abi3-7 line contains, in addition to the abi3-1 mutation, a point mutation that converts residue Ala-458 into Thr within the B2 domain of the ABI3 protein. This Ala residue is absolutely conserved in all known ABI3 orthologues. Abi3-7 seeds display reductions in dormancy and in sensitivity to abscisic acid which are intermediate between those of the leaky abi3-1 and of the severe abi3-4 and abi3-5 mutants. Accumulation and distribution of At2S1 and At2S2 albumin mRNA as well as of AtEm1 and AtEm6 late embryogenesis-abundant proteins and mRNA have been analyzed. Both At2S1 and At2S2 mRNA are reduced in abi3-7, but distribution of At2S2 is spatially restricted. Accumulation of AtEm6 protein is more sensitive to abi3-7 mutation than AtEm1. However both mRNAs are considerably reduced in this mutant. Their distribution is also differentially affected. These results provide genetic evidence for the importance of the conserved B2 domain for ABI3 function in vivo.