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.
The plant allelochemical coumarin effectively inhibits the germination of Brassica parachinensis (B. parachinensis) seeds. Quantification of endogenous phytohormones showed that contents of abscisic acid (ABA), ABA glucose ester, gibberellin A20 (GA20), GA3, GA15, GA24, GA9 and GA4 were higher in germinating seeds than in seedlings. Moreover, the presence of coumarin significantly reduced the content of bioactive GA4 which is thought to positively regulate seed germination. Histochemical staining and spectrophotometry of reactive oxygen species (ROS) revealed that exogenous GA3 and GA4+7 could effectively promote the production of endogenous ROS during germination and that the GA synthesis inhibitor paclobutrazol could effectively inhibit production of ROS. Coumarin significantly inhibited the accumulation of ROS, especially superoxide anion radical ( ${\rm O}_2^{{\cdot}{-}} $ ). This inhibitory effect could be restored by the addition of exogenous GA3 and GA4+7. Coumarin also inhibited the activity of the ROS-degrading enzymes such as superoxide dismutase, catalase and peroxidase as well as β-amylase in seeds and seedlings. Taken together, we propose a model for the regulation of seed germination in B. parachinensis by coumarin, Gas and ROS, in which coumarin may delay seed germination by reducing endogenous GA4, thus decreasing the accumulation of ROS.
The purpose of this study was to explore the relationship between β-galactosidase and rice seed germination. Exogenous β-galactosidase (1 U) facilitated germination while abscisic acid (ABA) delayed germination. Histochemical staining and spectrophotometry revealed that the activity of endogenous β-galactosidase increased gradually during germination. 1 µM ABA and GA4+7 could, respectively, inhibit and promote the activity of β-galactosidase in the rice embryo. This indicated that the regulation of rice seed germination by these two phytohormones may be through mediation of β-galactosidase activity. During seed germination, the transcriptional expressions of OsBAGL1, OsBAGL4, OsBAGL8 and OsBAGL11 were higher than those of other homologous genes, suggesting that these four genes may be involved in the germination process. GA4+7 significantly increased the expression of the above four genes. By contrast, the transcriptional expression of OsBAGL8 and OsBAGL11 were significantly reduced by ABA, but not that of OsBAGL1 and OsBAGL4. Our study has important theoretical and practical significance for illustrating the relationship between β-galactosidase and seed germination.
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