The seed is a key evolutionary adaptation of land plants that facilitates dispersal and allows for germination when the environmental conditions are adequate. Mature seeds are dormant and desiccated, with accumulated storage products that are to be used by the seedling after germination. These properties are imposed on the developing embryo by a maturation program, which operates during the later part of embryogenesis. A number of "master regulators" (the "LEC genes") required for the induction of the maturation program have been described, but it is not known what prevents this program from being expressed during early embryogenesis. Here, we report that Arabidopsis (Arabidopsis thaliana) embryos mutant for strong alleles of DICER-LIKE1, the enzyme responsible for the biosynthesis of microRNAs (miRNAs), mature earlier than their wildtype counterparts. This heterochronic phenotype indicates that miRNAs are key regulators of the timing of the maturation program. We demonstrate that miRNAs operate in part by repressing the master regulators LEAFY COTYLEDON2 and FUSCA3 and identify the trihelix transcription factors ARABIDOPSIS 6B-INTERACTING PROTEIN1-LIKE1 (ASIL1) and ASIL2 and the histone deacetylase HDA6/SIL1 as components that act downstream of miRNAs to repress the maturation program early in embryogenesis. Both ASIL1 and HDA6/SIL1 are known to act to prevent the expression of embryonic maturation genes after germination, but to our knowledge, this is the first time they have been shown to have a role during embryogenesis. Our data point to a common negative regulatory module of maturation during early embryogenesis and seedling development.

Pattern formation and morphogenesis require coordination of cell division rates and orientations with developmental signals that specify cell fate. A viable mutation in the TILTED1 locus, which encodes the catalytic subunit of DNA polymerase e of Arabidopsis thaliana, causes a lengthening of the cell cycle by ;35% throughout embryo development and alters cell type patterning of the hypophyseal lineage in the root, leading to a displacement of the root pole from its normal position on top of the suspensor. Treatment of preglobular and early globular stages, but not later stage, embryos with the DNA polymerase inhibitor aphidicolin leads to a similar phenotype. The results uncover an interaction between the cell cycle and the processes that determine cell fate during plant embryogenesis.

Early embryonic development in the flowering plant Arabidopsis thaliana follows a predictable sequence of cell divisions. Anatomical hallmarks and the expression of marker genes in dynamic patterns indicate that new cell fates are established with virtually every round of mitosis. Although some of the factors regulating these early patterning events have been identified, the overall process remains relatively poorly understood. Starting at the globular stage, when the embryo has approximately 100 cells, the organization of development appears to be taken over by programs that regulate postembryonic patterning throughout the life cycle.