Abstract
The seed, the reproductive unit of angiosperms, is generally protected by the seed coat. The seed coat is made of one or two integuments, each comprising two epidermal cells layers and, in some cases, extra sub-epidermal cell layers. The thickness of the seed-coat affects several aspects of seed biology such as dormancy, germination and mortality. In Arabidopsis, the inner integument displays one or two sub-epidermal cell layers that originate from periclinal cell divisions of the innermost epidermal cell layer. By contrast, the outer integument was considered to be two-cell layered. Here, we show that sub-epidermal chalazal cells grow in between the epidermal outer integument cell layers to create an incomplete three-cell layered outer integument. We found that the MADS box transcription factor TRANSPARENT TESTA 16 represses growth of the chalaza and formation of sub-epidermal outer integument cells. Finally, we demonstrate that sub-epidermal cells of the outer and inner integument respond differently to the repressive mechanism mediated by FERTILIZATION INDEPENDENT SEED Polycomb group proteins and to fertilization signals. Our data suggest that integument cell origin rather than sub-epidermal cell position underlies different responses to fertilization.
Highlights
Angiosperm seeds are generally protected by the seed coat, which develops from the ovule integuments after fertilization [1]
Arabidopsis thaliana plants, ecotype Columbia (Col) or Wassilewskija (Ws), were used as wildtype controls when appropriate. kpl-1, and tt16-3 lines are in the Ws accession [14, 17]. goa-1, Development of sub-epidermal cells in the Arabidopsis outer integument fie-12/+ and msi1-1/+, lines are in the Col accession [9, 18, 19]
Whereas the ii primordium is compact and comprises three cells, the oi primordium extends to cover the entire proximal region of the chalaza and its convexity is partially due to periclinal cell divisions of sub-epidermal chalazal cells (Fig 1A)
Summary
Angiosperm seeds are generally protected by the seed coat, which develops from the ovule integuments after fertilization [1]. Angiosperm ovules display three main integument architectures: bitegmic ovules are covered by two integuments, unitegmic ovules carry a single integument, and ategmic ovules lack any integuments. Ovule integuments are either two- or multicell layered. Both external integument cell layers originate from the epidermal tissue of the chalaza. Sub-epidermal cell layers develop from periclinal cell divisions of epidermal integument cells or from sub-epidermal chalazal tissue [2]. Seed coat thickness is considered a stable character [2] and affects seed germination, dormancy and mortality [3,4,5]
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