Seed germination compromises expansion pressure, cell wall alterations, and the cuticular layer: New insights

Abstract

The seed is a complex structure composed of different functional tissues that interact to ensure successful germination. This organization supports embryo protrusion and its transition from heterotrophic to photoautotrophic growth. During germination, the seed reactivates its genome, and embryo cells undergo changes through distinct transcriptional states. Once germination is triggered, endosperm expansion occurs, driven by the growing embryonic axis, though the exact mechanism remains unknown. The GA/DELLA-NAC25/NAC1L-AtEXP2 module is essential for regulating endosperm expansion under high-gibberellin (GA) conditions, thereby supporting germination. The mechanical anisotropy of the cell wall (CW) governs the direction of expansion, a process that involves the alignment of microtubules. The expansion process, along with the induction of more deformable CWs through CW remodeling enzymes (CWRE), creates an interplay of dormacy-related mechanical forces that facilitate in seed-coat rupture and "sensu stricto" germination. Abscisic acid (ABA) levels and signaling sharply decrease at the onset of germination; however, the regulatory mechanisms underlying the loss of ABA sensitivity remain unclear. Recently, MBF1 family genes have been shown to regulate ABA and GA levels at the onset of seed germination. In endosperm seeds, endosperm and seed-coat ruptures must be overcome for successful germination. In contrast, during monocot germination, the coleorhiza first penetrates the surrounding structures, followed by the emergence of the radicle. The presence of a cuticle (CU) associated with the endosperm plays a key role throughout the seed's life, particularly during the onset of germination, by controlling endosperm permeability through tannic CWs attached to it. This recently discovered layer relies on two receptor-like kinases, GSO1 and GSO2, as well as the peptides CIF2 and PSY1 from the endosperm. However, it remains unclear whether the CU tissue softens or alters its structure to facilitate radicle protrusion. In summary, this review highlights recent advances in the understanding of seed germination, with a focus on its molecular regulation, biomechanical properties, and inter-tissue communication. To conclude, these insights underscore the CU as a dynamic and multifunctional barrier that adapts to developmental cues, ensuring its dual role in seed protection during dormancy and facilitating a controlled transition to growth.

Keywords: CW remodeling enzymes; Endosperm cuticle; Expansion of endosperm and embryonic axis; Gibberellins and abscisic acid; Microtubules; Seed-coat; Tannins.