A seed coat bedding assay shows that RGL2-dependent release of abscisic acid by the endosperm controls embryo growth in Arabidopsis dormant seeds

摘要

Seed dormancy is an ecologically important adaptive trait in plants whereby germination is repressed even under favorable germina tion conditions such as imbibition with water. In Arabidopsis and most plant species, dormancy absolutely requires aη unidentified seed coat germination-repressive activity and constitutively higher abscisic acid (ABA) levels upon seed imbibition. The mechanisms un derlying these processes and their possible relationship are incom pletely understood. We developed a "seed coat bedding" assay monitoring the growth of dissected embryos cultured on a layer of seed coats, allowing combinatorial experiments using dormant, nondormant, and various genetically modified seed coat and embry onic materials. This assay, combined with direct ABA measurements, revealed that, upon imbibition, dormant coats, unlike nondormant coats, actively produce and release ABA to repress embryo germina tion, whatever the embryo origin, i.e., from dormant, nondormant, or never dormant aba seeds, unable to synthesize ABA. The persis tent high ABA levels in imbibed dormant seeds requires the perma nent expression of the DELLA gene RGL2, where it remains insensitive to gibberellins (GA) unlike in nondormant seeds. These findings present the seed coat as an organ actively controlling ger mination upon seed imbibition and provide a framework to investi gate how environmental factors break seed dormancy.