NAC family proteins NARS1/NAC2 and NARS2/NAM in the outer integument regulate embryogenesis in Arabidopsis

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. 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. 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. Degeneration of the integuments in nars1 nars2 was markedly delayed, while that of the wild type occurred around the torpedo-shaped embryo stage. Additionally, nars1 nars2 showed a defect in embryogenesis: some nars1 nars2 embryos were developmentally arrested at the torpedo-shaped embryo stage. 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. Wild-type pistils pollinated with nars1 nars2 pollen generated normal seeds, while the reverse crossing generated abnormal seeds. Taken together, these results indicate that NARS1 and NARS2 regulate embryogenesis by regulating the development and degeneration of ovule integuments. Our findings suggest that there is an intertissue communication between the embryo and the maternal integument.