A transcriptional hub integrating gibberellin-brassinosteroid signals to promote seed germination in Arabidopsis

Seed germination is regulated by multiple phytohormones, including gibberellins (GAs) and brassinosteroids (BRs), however, the molecular mechanism underlying GA and BR co-induced seed germination is not well elucidated. We demonstrated that BRs induce seed germination through promoting testa and endosperm rupture in Arabidopsis. BRs promote cell elongation, rather than cell division, at the hypocotyl-radicle transition region of the embryonic axis during endosperm rupture. Two key basic helix-loop-helix transcription factors in the BR signaling pathway, HBI1 and BEE2, are involved in the regulation of endosperm rupture. Expression of HBI1 and BEE2 was induced in response to BR and GA treatment. In addition, HBI1- or BEE2-overexpressing Arabidopsis plants are less sensitive to the BR biosynthesis inhibitor, brassinazole, and the GA biosynthesis inhibitor, paclobutrazol. HBI1 and BEE2 promote endosperm rupture and seed germination by directly regulating the GA-Stimulated Arabidopsis 6 (GASA6) gene. Expression of GASA6 was altered in Arabidopsis overexpressing HBI1, BEE2, or SRDX-repressor forms of the two transcription factors. In addition, HBI1 interacts with BEE2 to synergistically activate GASA6 expression. Our findings define a new role for GASA6 in GA and BR signaling and reveal a regulatory module that controls GA and BR co-induced seed germination in Arabidopsis.

Automated summary

This study revealed that brassinosteroids induce seed germination in Arabidopsis by promoting the rupture of seed coat and endosperm, while two key basic helix-loop-helix transcription factors, HBI1 and BEE2, are involved in the regulation of this process. Furthermore, HBI1 and BEE2 promote the expression of the GA-Stimulated Arabidopsis 6 (GASA6) gene, which plays a role in GA and BR signaling, thus outlining a regulatory module controlling seed germination in Arabidopsis.