Abstract
SummarySeed dormancy is a widespread and key adaptive trait that is essential for the establishment of soil seed banks and prevention of pre‐harvest sprouting. Herein we demonstrate that the endosperm‐expressed transcription factors ZHOUPI (ZOU) and INDUCER OF CBF EXPRESSION1 (ICE1) play a role in determining the depth of primary dormancy in Arabidopsis. We show that ice1 or zou increases seed dormancy and the double mutant has an additive phenotype. This increased dormancy is associated with increased ABA levels, and can be separated genetically from any role in endosperm maturation because loss of ABA biosynthesis or DELAY OF GERMINATION 1 reverses the dormancy phenotype without affecting the aberrant seed morphology. Consistent with these results, ice1 endosperms had an increased capacity for preventing embryo greening, a phenotype previously associated with an increase in endospermic ABA levels. Although ice1 changes the expression of many genes, including some in ABA biosynthesis, catabolism and/or signalling, only ABA INSENSITIVE 3 is significantly misregulated in ice1 mutants. We also demonstrate that ICE1 binds to and inhibits expression of ABA INSENSITIVE 3. Our data demonstrate that Arabidopsis ICE1 and ZOU determine the depth of primary dormancy during maturation independently of their effect on endosperm development.
Highlights
We demonstrate that loss of function of INDUCER OF C-REPEAT BINDING FACTORS (CBFs) EXPRESSION1 (ICE1) and/or ZOU result in seeds with increased primary dormancy and elevated accumulation of Abscisic Acid (ABA) (Figures 1, 2 & 4)
The characterisation of the dormancy effects of ICE1 and ZOU is complicated by the co-occurrence of the effects on seed development caused by the failure of endosperm consumption
The aberrant endosperm consumption alone is insufficient to explain the dormancy phenotype, because in the aba2 and dog1 mutant backgrounds normal germination is restored without an effect on seed morphology (Figure 3)
Summary
After fertilisation seeds enter a rigid developmental program which proceeds through embryogenesis to seed maturation, where the basic body plan of the plant is established, desiccation tolerance is gained, and primary dormancy is imposed (Baud et al, 2002; Fourquin et al, 2016).The plant hormone abscisic acid (ABA) and a small network of B3family transcription factors including ABA INSENSITIVE 3 (ABI3), FUSCA3 and LEAFY COTYLEDON 2, otherwise known as the AFL subfamily of B3 transcription factors, induce the seed maturation programme in the embryo and endosperm, as well as seed dormancy (Karssen et al, 1983; Koornneef et al, 1984; Giraudat et al, 1992; Parcy et al, 1994; Nambara et al, 1995; Lopez-Molina et al, 2002)ABA and ABI3 continue to be important upon seed imbibition where they are required to block the germination of dormant seeds (reviewed in (Koornneef et al, 2002; Carbonero et al, 2017; Leprince et al, 2017). After shedding primary dormancy can be broken by environmental signals such as seasonal changes in temperature or soil nitrate levels, or signals of canopy disturbance such as compounds in smoke from forest fires (Cadman et al 2006). In the laboratory, these environmental responses are exploited to create simple dormancy-breaking treatments such as cold stratification or dry after-ripening which are often used as methods for comparing seed dormancy depth between genotypes. Depending on the plant species, primary dormancy can be conferred either by the embryo or imposed by the surrounding tissues (Finch-Savage and Leubner-Metzger, 2006) The latter is known as coat-imposed dormancy, and is prevalent in the Brassicaceae including Arabidopsis. Coat-imposed dormancy requires properties of both the seed coat and endosperm in Arabidopsis (Debeaujon et al, 2000; Bethke et al, 2007; Doherty and Kay, 2010; Lee et al, 2010; Lee et al, 2012b; Piskurewicz and Lopez-Molina, 2016; Fedi et al, 2017)
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