The N6-methyladenosine reader ECT1 regulates seed germination via gibberellic acid- and phytochrome B-mediated signaling

Abstract Seed germination, a pivotal stage in plant growth, is governed by phytohormones such as gibberellic acid (GA) and influenced by phytochromes, which are key photoreceptors in plants. The N6-methyladenosine (m6A) RNA modification is fundamental to plant growth and development. However, the molecular mechanisms underlying the regulation of PHYTOCHROME B (phyB) and the function of m6A signaling in GA-mediated seed germination remain elusive. Here, we discovered EVOLUTIONARILY CONSERVED C-TERMINAL REGION 1 (ECT1) as an m6A reader protein that directly binds to m6A and forms homodimers to enhance its stability in Arabidopsis (Arabidopsis thaliana). We observed that the ect1-1 mutant exhibits attenuated GA3 responsiveness in seed germination. Restoration of ECT1 function in ect1-1 confirmed the role of ECT1 in promoting seed germination. Our findings indicate that ECT1 promotes seed germination by destabilizing m6A-modified REPRESSOR OF GA1-3 1 (RGA1), a key inhibitor of GA-mediated seed germination. Moreover, ECT1 establishes a regulatory circuit with DOF AFFECTING GERMINATION 2 (DAG2), another regulator of GA-mediated seed germination. DAG2 directly binds to the ECT1 promoter and controls its transcription, and ECT1 modulates DAG2 mRNA stability through m6A binding. Furthermore, we identified PHYB as a common downstream target of DAG2 and ECT1. ECT1 binds directly to m6A-modified PHYB and influences its stability, and DAG2 binds to the PHYB promoter to regulate its transcription. Our findings demonstrate that ECT1 fine-tunes m6A-regulated seed germination via complex and multifaceted molecular mechanisms, particularly through interactions with GA and phyB, broadening our understanding of m6A-regulated processes in Arabidopsis.