Endosperm and Nucellus Develop Antagonistically in Arabidopsis Seeds

Abstract

In angiosperms, seed architecture is shaped by the coordinated development of three genetically different components: embryo, endosperm, and maternal tissues. The relative contribution of these tissues to seed mass and nutrient storage varies considerably among species. The development of embryo, endosperm, or nucellus maternal tissue as primary storage compartments defines three main typologies of seed architecture. It is still debated whether the ancestral angiosperm seed accumulated nutrients in the endosperm or the nucellus. During evolution, plants shifted repeatedly between these two storage strategies through molecular mechanisms that are largely unknown. Here, we characterize the regulatory pathway underlying nucellus and endosperm tissue partitioning in Arabidopsis thaliana. We show that Polycomb-group proteins repress nucellus degeneration before fertilization. A signal initiated in the endosperm by the AGAMOUS-LIKE62 MADS box transcription factor relieves this Polycomb-mediated repression and therefore allows nucellus degeneration. Further downstream in the pathway, the TRANSPARENT TESTA16 (TT16) and GORDITA MADS box transcription factors promote nucellus degeneration. Moreover, we demonstrate that TT16 mediates the crosstalk between nucellus and seed coat maternal tissues. Finally, we characterize the nucellus cell death program and its feedback role in timing endosperm development. Altogether, our data reveal the antagonistic development of nucellus and endosperm, in coordination with seed coat differentiation.

Auxin production in the endosperm drives seed coat development in Arabidopsis

“…This suggests that AGL62, like auxin, is involved in mediating both endosperm and seed coat developmental pathways. It was recently proposed that AGL62 in the endosperm also mediates non-autonomous cell signalling driving nucellus degeneration (Xu et al, 2016). This transcription factor thus seems to be a central player in modulating diverse developmental pathways during seed development.…”

Endosperm and Imprinting, Inextricably Linked

“…Seed formation requires extensive intergenerational and intertissue communication and coordination (Garcia et al, 2005;Ingouff et al, 2006;Ingram, 2010;Li et al, 2013;Xu et al, 2016). Angiosperm seeds consist of three genetically distinct entities: the diploid embryo, with one paternal and one maternal genome; the triploid endosperm, with two maternal and one paternal genome; and the protective integuments/seed coat, consisting of diploid maternal tissue from the previous generation (Fig.…”

Seed tissue and nutrient partitioning, a case for the nucellus

“…A few layers of proximal nucellus cells persist and expand with the rest of the ovules to form a gate between chalaza and endosperm till embryo maturity (Xu et al 2016). …”

“…2) (Roszak and Kohler 2011; Xu et al 2016). Single fertilization of the central cell is necessary and sufficient to initiate nucellus degeneration.…”