The seed embryo sends signals to the encapsulating endosperm to promote cell expansion events critical for germination

A key component of seed germination is the interplay of mechanical forces governing embryo growth and the surrounding restraining endosperm tissue. Endosperm cell separation is therefore thought to play a critical role in the control of this developmental transition. In our article published in the journal “Molecular Plant”, we demonstrate that in Arabidopsis thaliana seeds, endosperm cell expansion is a key component of germination. By measuring changes in endosperm cell surface area using confocal imaging and 3D geometry reconstruction, we observed that all endosperm cells expand during imbibition but at different rates to accommodate embryo growth and to facilitate germination. Gibberellins (GAs) are plant hormones that positively regulate endosperm function in the germination process, and DELLA proteins repress responses to these hormones. We have found that two NAC transcription factors (NAC25 and NAC1L), when released from repression by the RGL2 DELLA protein, perceive appropriate signals from embryo and activate the expression of a cohort of cell-wall remodeling enzymes (CWREs) required for expansion of the endosperm. Our results suggest a regulatory model where RGL2 blocks GA-signaling in the endosperm by sequestering NACs. Upon imbibition, a signal from embryo and/or GA biosynthesis destabilizes RGL2 and releases NACs to activate CWRE gene expression, having an impact on endosperm cell elongation to accommodate embryo growth and facilitate root protrusion (germination “sensu stricto”).