Abstract
Abscisic acid (ABA) inhibits seed germination and the regulation of ABA biosynthesis has a role in maintenance of seed dormancy. The key rate-limiting step in ABA biosynthesis is catalyzed by 9-cis-epoxycarotenoid dioxygenase (NCED). Two hydroxamic acid inhibitors of carotenoid cleavage dioxygenase (CCD), D4 and D7, previously found to inhibit CCD and NCED in vitro, are shown to have the novel property of decreasing mean germination time of tomato (Solanum lycopersicum L.) seeds constitutively overexpressing LeNCED1. Post-germination, D4 exhibited no negative effects on tomato seedling growth in terms of height, dry weight, and fresh weight. Tobacco (Nicotiana tabacum L.) seeds containing a tetracycline-inducible LeNCED1 transgene were used to show that germination could be negatively and positively controlled through the chemical induction of gene expression and the chemical inhibition of the NCED protein: application of tetracycline increased mean germination time and delayed hypocotyl emergence in a similar manner to that observed when exogenous ABA was applied and this was reversed by D4 when NCED expression was induced at intermediate levels. D4 also improved germination in lettuce (Lactuca sativa L.) seeds under thermoinhibitory temperatures and in tomato seeds imbibed in high osmolarity solutions of polyethylene glycol. D4 reduced ABA and dihydrophaseic acid accumulation in tomato seeds overexpressing LeNCED1 and reduced ABA accumulation in wild type tomato seeds imbibed on polyethylene glycol. The evidence supports a mode of action of D4 through NCED inhibition, and this molecule provides a lead compound for the design of NCED inhibitors with greater specificity and potency.
Highlights
Seed dormancy is “an innate seed property that defines the environmental conditions in which the seed is able to germinate” (Finch-Savage and Leubner-Metzger, 2006)
D8 showed the greatest specificity toward NCED because inhibition of LeCCD1a in vitro was 61% compared to >95% for the other hydroxamic acids, and inhibition of LeNCED1 was marginally the highest at 40% (Sergeant et al, 2009), D8 inhibited rather than promoted germination of sp12 seed
Abamine, which inhibited LeCCD1a (35%) and LeNCED1 (20%) in vitro (Sergeant et al, 2009), inhibited germination of sp12 seed when applied at 1 mM; this was not surprising because a positive effect was seen on Arabidopsis germination at 10–30 μM (Li et al, 2012), it was inhibitory to cress growth at 100 μM (Han et al, 2004)
Summary
Seed dormancy is “an innate seed property that defines the environmental conditions in which the seed is able to germinate” (Finch-Savage and Leubner-Metzger, 2006). The mechanisms vary between plant species, but physiological seed dormancy is the most prevalent dormancy class (Baskin and Baskin, 2004) and is the focus of this study. Primary dormancy can be released in dry seeds through after-ripening, or in imbibed seeds by particular environmental stimuli such as short cold or warm periods and this reduction in dormancy depth enables seeds to germinate over a wider range of environmental conditions. Dormancy depth can increase in response to prolonged absence of conditions which allow germination, and this is known as secondary dormancy (Finch-Savage and Leubner-Metzger, 2006). The balance of the levels of, and sensitivity to, the antagonistic phytohormones abscisic acid (ABA), and gibberellic acid (GA) is a predominant regulator of seed dormancy and germination (Kucera et al, 2005; Finkelstein et al, 2008)
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