REVERSAL OF RDO5 1, a Homolog of Rice Seed Dormancy4, Interacts with bHLH57 and Controls ABA Biosynthesis and Seed Dormancy in Arabidopsis

The control of seed dormancy by abscisic acid (ABA) has been extensively studied, but the underlying mechanism is not fully understood. Here, we report the characterization of two ABA-related seed dormancy regulators in Arabidopsis (<i>Arabidopsis thaliana</i>): ODR1 (for reversal of <i>rdo5</i>), an ortholog of the rice (<i>Oryza sativa</i>) Seed dormancy4 (Sdr4), and the basic helix-loop-helix transcription factor bHLH57. <i>ODR1</i>, whose transcript levels are directly suppressed by the transcription factor ABA INSENSITIVE3 (ABI3), negatively regulates seed dormancy by affecting ABA biosynthesis and ABA signaling. By contrast, bHLH57 positively regulates seed dormancy by inducing the expression of the genes <i>9-CIS-EPOXYCAROTENOID DIOXYGENASE6</i> (<i>NCED6</i>) and <i>NCED9</i>, which encode ABA biosynthetic enzymes, and thus leads to higher ABA levels. ODR1 interacts with bHLH57 and inhibits bHLH57-modulated <i>NCED6</i> and <i>NCED9</i> expression in the nucleus. <i>bhlh57</i> loss-of-function alleles can partially counteract the enhanced <i>NCED6</i> and <i>NCED9</i> expression seen in <i>odr1</i> mutants and can therefore rescue their associated hyper-dormancy phenotype. Thus, we identified a novel ABI3-ODR1-bHLH57-NCED6/9 network that provides insights into the regulation of seed dormancy by ABA biosynthesis and signaling.