We have previously described a homeotic leafy cotyledon (lec) mutant of Arabidopsis that exhibits striking defects in embryonic maturation and produces viviparous embryos with cotyledons that are partially transformed into leaves. In this study, we present further details on the developmental anatomy of mutant embryos, characterize their response to abscisic acid (ABA) in culture, describe other mutants with related phenotypes, and summarize studies with double mutants. Our results indicate that immature embryos precoclously enter a germination pathway after the torpedo stage of development and then acquire characteristics normally restricted to vegetative parts of the plant. In contrast to other viviparous mutants of maize (vp7) and Arabidopsis (abi3) that produce ABA-insensitive embryos, immature lec embryos are sensitive to ABA in culture. ABA is therefore necessary but not sufficient for embryonic maturation in Arabidopsis. Three other mutants that produce trichomes on cotyledons following precocious germination in culture are described. One mutant is allelic to lecl, another is a fusca mutant (fusg, and the third defines a new locus (lec2). Mutant embryos differ in morphology, desiccation tolerance, pattern of anthocyanin accumulation, presence of storage materials, size and frequency of trichomes on cotyledons, and timing of precocious germination in culture. The leafy cotyledon phenotype has therefore allowed the identification of an important network of regulatory genes with overlapping functions during embryonic maturation in Arabidopsis.

In an attempt to elucidate the physiological role of ABA in seed dormancy and the adaptive response to dehy-dration, we isolated an ABA-deficient mutant of Arabi-dopsis thaliana (L.) Heynh. which germinated in the presence of a gibberellin biosynthetic inhibitor. Genetic analysis showed this mutation is a new allele of a recently reported locus aba2, and therefore has been designated aba2-2. The levels of endogenous ABA in fresh and dehy-drated tissues of the aba2-2 mutant were highly reduced compared to those of wild-type plants. As a consequence, aba2-2 plants wilt and produce seeds with reduced dorman-cy. Dark germinated seedlings of the aba2-2 mutant show-ed true leaves, which were not observed in those of the wild type, indicating that abal-2 embryos grew precociously dur-ing seed maturation. In the dehydrated tissues of the wild-type plants, the levels of free proline, isoleucine and leucine were elevated to a content approximately 100-fold higher than those in fresh tissues. In contrast to the wild-type plants, dehydration-induced accumulation of proline was highly suppressed in the aba2-2 mutant plants while that of leucine and isoleucine accumulated. Furthermore, exoge-nous application of ABA to wild-type plants promoted ac-cumulation of free proline, but not leucine nor isoleucine. These results suggest that dehydration-induced accumula-tion of free leucine and isoleucine is achieved independent of ABA.

[LEC1], and LEC2) control most aspects of seed maturation, such as accumulation of storage compounds, cotyledon identity, acquisition of desiccation tolerance, and dormancy. The molecular basis for complex genetic interactions among these regulators is poorly understood. By analyzing ABI3 and FUS3 expression in various single, double, and triple maturation mutants, we have identified multiple regulatory links among all four genes. We found that one of the major roles of LEC2 was to upregulate FUS3 and ABI3. The lec2 mutation is responsible for a dramatic decrease in ABI3 and FUS3 expression, and most lec2 phenotypes can be rescued by ABI3 or FUS3 constitutive expression. In addition, ABI3 and FUS3 positively regulate themselves and each other, thereby forming feedback loops essential for their sustained and uniform expression in the embryo. Finally, LEC1 also positively regulates ABI3 and FUS3 in the cotyledons. Most of the genetic controls discovered were found to be local and redundant, explaining why they had previously been overlooked. This works establishes a genetic framework for seed maturation, organizing the key regulators of this process into a hierarchical

licence dowing the