These substances of the seed coat are considered to play important roles in protecting the embryo from influences of UV radiation or toxic chemicals.
The epidermal cells (oi2) of Arabidopsis thaliana integuments are characterized by a deposition of pectic polysaccharide, called mucilage, around a columella, which is a volcano-shaped structure of secondary cell wall ( Beeckman et al., 2000 Windsor et al., 2000 Western et al., 2001 Debeaujon et al., 2003), while the endothelial cells (ii1) are characterized by the production of flavonoids such as proanthocyanidin ( Beeckman et al., 2000 Windsor et al., 2000 Western et al., 2001 Debeaujon et al., 2003).
The seed coat consists of two integuments (the outer and inner integuments) of maternal tissues, and multiple cell layers of these integuments develop after fertilization, resulting in specialized structures of the seed coat. In angiosperm seeds, the embryo and endosperm are surrounded by the seed coat, which protects the embryo from mechanical stress and pathogen infections. Our findings suggest that there is an intertissue communication between the embryo and the maternal integument. Taken together, these results indicate that NARS1 and NARS2 regulate embryogenesis by regulating the development and degeneration of ovule integuments. Wild-type pistils pollinated with nars1 nars2 pollen generated normal seeds, while the reverse crossing generated abnormal seeds. Unexpectedly, however, neither NARS1 nor NARS2 was expressed in the embryo at this stage, although they were found to be expressed in the outer integument. Additionally, nars1 nars2 showed a defect in embryogenesis: some nars1 nars2 embryos were developmentally arrested at the torpedo-shaped embryo stage.
Degeneration of the integuments in nars1 nars2 was markedly delayed, while that of the wild type occurred around the torpedo-shaped embryo stage. Double knockout mutant nars1 nars2 exhibited abnormally shaped seeds moreover, neither nars1 nor nars2 produced abnormal seeds, indicating that NARS1 and NARS2 redundantly regulate seed morphogenesis. We show that expression of chimeric repressors derived from NAC-REGULATED SEED MORPHOLOGY1 and -2 (NARS1 and NARS2, also known as NAC2 and NAM, respectively) caused aberrant seed shapes in Arabidopsis thaliana. Seed morphogenesis consists of embryogenesis and the development of maternal tissues such as the inner and outer integuments, both of which give rise to seed coats.
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