Mechanical stress mediated by both endosperm softening and embryo growth underlies endosperm elimination in Arabidopsis seeds

Abstract

Seed development in angiosperms demands the tightly co-ordinated development of three genetically distinct structures. The embryo is surrounded by the endosperm, which is in turn enclosed within the maternally derived seed coat. In Arabidopsis final seed size is determined by early expansion of the coenocytic endosperm, which then cellularizes and subsequently undergoes developmental Programmed Cell Death, breaking down as the embryo grows. Endosperm breakdown requires the endosperm-specific basic Helix Loop Helix transcription factor ZHOUPI. However, to date the mechanism underlying the Arabidopsis endosperm breakdown process has not been elucidated. Here we provide evidence that ZHOUPI does not induce the developmental Programmed Cell Death of the endosperm directly. Instead ZHOUPI indirectly triggers cell death by regulating the expression of cell wall modifying enzymes, thus altering the physical properties of the endosperm to condition a mechanical environment permitting the compression of the cellularized endosperm by the developing embryo.