Abstract
Abscisic acid (ABA) is a crucial phytohormone for the regulation of seed germination. The ABA content of seeds is regulated by synthesis and catabolic pathways. Coumarin, an important plant allelochemical, can inhibit seed germination effectively, although whether it is involved in the regulation of ABA content during seed germination has not been elucidated. For the study reported herein, we show that coumarin effectively inhibits rice seed germination and vivipary. We found that the ABA content gradually decreased in water-imbibed rice seeds and that the content and activity of the Oryza sativa 9-cis epoxycarotenoid dioxygenases (OsNCEDs), which are ABA synthases, decreased during seed germination. At the transcription level, the expression of OsNCED1–3 appeared to decrease, whereas the expression of the ABA 8′-hydroxylase 2 and 3 genes (OsABA8’ox2/3) first appeared to increase and then decrease. Samples of rice seeds were also imbibed in water containing coumarin, which increased their ABA content but did not significantly increase the activity or content of their OsNCEDs or OsNCED1–3 transcription. Interestingly, coumarin imbibition remarkably reduced OsABA8’ox2/3 expression in rice embryos, which partially explained how coumarin increased the ABA content of germinating rice embryos. Coumarin also inhibited the accumulation of reactive oxygen species (ROS) in rice embryos and increased the activity of superoxide dismutase and catalase, which are indispensable for seed germination. These results indicate that coumarin delays seed germination by inhibiting ABA catabolism, particularly by decreasing the expression of OsABA8’ox2/3 rather than by increasing ABA synthesis. Moreover, coumarin increases the ABA content while decreasing the ROS content in rice embryos. Our results enhance our understanding of the regulation of ABA and ROS during seed germination and provide theoretical support for application of coumarin to prevent sprouting before crop harvesting.
Highlights
Seed germination is a critical step in the life cycle of higher plants
For the study reported we comprehensively examined the effects of coumarin on germination, abscisic acid (ABA) synthesis and its degradation, reactive oxygen species (ROS) production, and expression of related enzyme genes in rice seeds
The ABA content in seeds is regulated by the relative rates of its synthesis and degradation
Summary
Seed germination is a critical step in the life cycle of higher plants. Seed germination is regulated by various factors, including phytohormones, e.g., gibberellin, ethylene, and brassinosteroids, which promote germination, and abscisic acid (ABA), which promotes dormancy (Koornneef et al, 1998; Millar et al, 2006). For rice and Arabidopsis, a decrease in the ABA content of their seeds is necessary for germination (Ali-Rachedi et al, 2004; Zhu et al, 2009). Many studies have shown that certain seed germination inhibitors regulate germination by altering the content of ABA in seeds. Glucose and copper inhibit rice seed germination by increasing the amount of ABA (Zhu et al, 2009; Ye et al, 2014). It is apparent that the ABA content of seeds determines whether germination will occur
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