REGULATION OF GENE-EXPRESSION PROGRAMS DURING ARABIDOPSIS SEED DEVELOPMENT - ROLES OF THE ABI3 LOCUS AND OF ENDOGENOUS ABSCISIC-ACID

The accumulation kinetics of 18 mRNAs were characterized during Arabid opsis silique development. These marker mRNAs could be grouped in dist inct classes according to their coordinate temporal expression in the wild type and provided a basis for further characterization of the cor responding regulatory pathways. The abscisic acid (ABA)-insensitive ab i3-4 mutation modified the expression pattern of several but not all m embers of each of these wild-type temporal mRNA classes. This indicate s that the ABI3 protein directly participates in the regulation of sev eral developmental programs and that multiple regulatory pathways can lead to the simultaneous expression of distinct mRNA markers. The ABI3 gene is specifically expressed in seed, but ectopic expression of ABI 3 conferred the ability to accumulate several seed-specific mRNA marke rs in response to ABA in transgenic plantlets. This suggested that exp ression of these marker mRNAs might be controlled by an ABI3-dependent and ABA-dependent pathway(s) in seed. However, characterization of th e ABA-biosynthetic aba mutant revealed that the accumulation of these mRNAs is not correlated to the ABA content of seed. A possible means o f regulating gene expression by developmental variations in ABA sensit ivity is apparently not attributable to variations in ABI3 cellular ab undance. The total content of ABI3 protein per seed markedly increased at certain developmental stages, but this augmentation appears to res ult primarily from the simultaneous multiplication of embryonic cells. Our current findings are discussed in relation to their general impli cations for the mechanisms controlling gene expression programs in see d.