The Endosperm-Derived Embryo Sheath Is an Anti-adhesive Structure that Facilitates Cotyledon Emergence during Germination in Arabidopsis

Abstract

Germination sensu stricto in Arabidopsis involves seed coat and endosperm rupture by the emerging seedling root. Subsequently the cotyledons emerge rapidly from the extraembryonic tissues of the seed, allowing autotrophic seedling establishment. Seedling survival depends upon the presence of an intact seedling cuticle that prevents dehydration, and which has hitherto been assumed to form the interface between the newly germinated seedling and its environment. Here we show that in Arabidopsis this is not the case. The primary interface between the emerging seedling and its environment is formed by an extra-cuticular endosperm-derived glycoprotein-rich structure called the sheath, which is maintained as a continuous layer at seedling surfaces during germination, and becomes fragmented as cotyledons expand. Mutants lacking an endosperm specific cysteine-rich peptide (KERBEROS (KRS)), show a complete loss of sheath production. Although krs mutants have no defects in germination sensu stricto they show a delay in cotyledon emergence, a defect not observed in seedlings with defects in cuticle biosynthesis. Biophysical analyses reveal that the surfaces of wild-type cotyledons show minimal adhesion to silica beads in an aqueous environment at cotyledon emergence, but that adhesion increases as cotyledons expand. In contrast krs mutant cotyledons show enhanced adhesion at germination. Mutants with defects in cuticle biosynthesis, but no sheath defects, show a similar adhesion profile to wild-type seedlings at germination. We propose that the sheath reduces the adhesiveness of the cotyledon surface under the humid conditions necessary for seed germination, and thus promotes seed coat shedding and rapid seedling establishment.

An Updated Overview on the Regulation of Seed Germination

“…Finally, the ZOU/RGE1 TF is also responsible for the production of the KRS endosperm-specific peptide, which triggers deposition of the embryo sheath, a layer of endosperm-derived material rich in extensin-like molecules [389]. This sheath is deposited outside the embryonic cuticle reducing the adhesion to the endosperm during germination and thus facilitating seed-coat shedding and rapid seedling establishment [390].…”

Rapid identification of mutations caused by fast neutron bombardment in Medicago truncatula

“…MtrunA17_Chr7g0223481 is the homolog of At1g50650 in Arabidopsis thaliana , which encodes the STIG1 family peptide KERBEROS (KRS). KRS regulates the development of embryo sheath [ 25 ], and krs mutant showed the absence of sheath production by endosperm [ 25 ]. Another deletion was located in the second exon of MtrunA17_Chr7g0253961 , which encodes a putative peptidase M50 protein (Fig.…”

Embryo–Endosperm Interactions

“…After germination, seed coat shedding from the cotyledons (Figure 1) is also impaired (41). The observed adhesion between the embryo and endosperm, both before and during germination suggest that the embryo sheath acts as an anti-adhesive structure.…”

“…The embryo sheath has an organized and homogeneous structure with a regular thickness, suggesting that its formation involves active assembly on the embryo surface (Figure 1). The embryo sheath is strongly and specifically recognized by anti-EXTENSIN antibodies and has thus been proposed to include a 3D EXTENSIN-based scaffold (41,121). EXTENSINS can selfassemble in vitro through isodityrosine crosslinking (22,103).…”