Regulation of Gene Expression Programs during Arabidopsis Seed Development: Roles of the AB13 Locus and of Endogenous Abscisic Acid

The accumulation kinetics of 18 mRNAs were characterized during Arabidopsis silique development. These marker mRNAs could be grouped in distinct classes according to their coordinate temporal expression in the wild type and provided a basis for further characteriration of the corresponding regulatory pathways. The abscisic acid (ABA)-insensitive abi3-4 mutation modified the expression pattern of several but not all members of each of these wild-type temporal mRNA classes. This indicates that the A613 protein directly participates in the regulation of several developmental programs and that multiple regulatory pathways can lead to the simultaneous expression of distinct mRNA markers. The A613 gene is specifically expressed in seed, but ectopic expression of A613 conferred the ability to accumulate several seed-specific mRNA markers in response to ABA in transgenic plantlets. This suggested that expression of these marker mRNAs might be contmlled by an AB13dependent and AEA-dependent pathway(s) in seed. However, characterization of the ABA-biosynthetic aba mutant revealed that the accumulation of these mRNAs is not correlated to the ABA content of seed. A possible means of regulating gene expression by developmental variations in ABA sensitivity is apparently not attributable to variations in AB13 cellular abundance. The total content of A613 protein per seed markedly increased at certain developmental stages, but this augmentation appears to result primarily from the simultaneous multiplication of embryonic cells. Our current findings are discussed in relation to their general implications for the mechanisms controlling gene expression programs in seed.