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Abstract
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 (Arabidopsis thaliana): ODR1 (for reversal of rdo5), an ortholog of the rice (Oryza sativa) Seed dormancy4 (Sdr4), and the basic helix-loop-helix transcription factor bHLH57. ODR1, 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 9-CIS-EPOXYCAROTENOID DIOXYGENASE6 (NCED6) and NCED9, which encode ABA biosynthetic enzymes, and thus leads to higher ABA levels. ODR1 interacts with bHLH57 and inhibits bHLH57-modulated NCED6 and NCED9 expression in the nucleus. bhlh57 loss-of-function alleles can partially counteract the enhanced NCED6 and NCED9 expression seen in odr1 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.
Highlights
Dormancy prevents germination when seeds are exposed to short but temporary favorable periods before the return of adverse conditions, and delays seedling establishment until the start of the growing season
We evaluated ODR1 expression levels in different Arabidopsis tissues by RT-qPCR: ODR1 transcript was undetectable in roots, stems, leaves, or flowers, but it gradually increased during seed maturation and reached its highest level in seeds 20 d after pollination, while it sharply declined in hydrated seed (Figure 1D)
Since abscisic acid (ABA) content was higher in odr1-2 seeds compared to Col-0, we evaluated the expression levels of pivotal ABA metabolism genes (Supplemental Figure 4), including NCEDs (NCED2, NCED3, NCED5, NCED6, NCED9, involved in ABA biosynthesis) and CYP707As (CYP707A1-4, linked to ABA degradation) in Col-0 and odr1-2 freshly harvested and in seeds that had been hydrated for 6 hours
Summary
Dormancy prevents germination when seeds are exposed to short but temporary favorable periods before the return of adverse conditions, and delays seedling establishment until the start of the growing season. Previous studies revealed that phytohormones, including abscisic acid (ABA), gibberellins (GA), ethylene, strigolactones, and brassinosteroids all play important roles in the control of seed dormancy (Seo et al, 2006; Shu et al, 2016a). Among these hormones, ABA and GA have central and antagonistic roles: ABA enhances dormancy while GA stimulates germination. The roles of ABA and GA biosynthesis and signal transduction in the control of seed dormancy and germination have been intensively studied in the past decades (Gubler et al, 2005; Lefebvre et al, 2006; Née et al, 2017b)
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