The Mechanism Underlying Karrikins Delaying Seed Germination And Post-germination Growth In Soybean

Karrikins (KARs) are a class of signal compounds which are discovered in wildfire smoke. Because of the significant promotion effect on seed germination and seedling development in Arabidopsis, KARs are becoming a new and attractive topics in plant molecular biology field. Current studies on the effect of KARs on plant growth and development are mainly focusing on the model plant Arabidopsis. The effect of KARs on crop seed germination and seedling development and relative molecular mechanisms still needs more research.As the main oil crop, the germination rate of soybean seeds shares a close relationship with the development of soybean industry. Firstly, low germination rate of soybean seeds will affect the seedling emergency rate and effective stalks, and then lower the soybean production. The seedling emergency rate and effective stalks will be further reduced if the soybean seed coat is thick and impervious. In recent years, the increasing promoted maize-soybean intercropping system has contributed a lot to the increasing of crop yield and the income of farmers, because of high nitrogen use efficiency and high land equivalent ratio (LER) compared to mono cropping system. But the canopy made by maize plants in the maize-soybean intercropping system can cause shade stress which would result in shade avoidance response to soybean plants. Shade avoidance would lead to weak soybean plants and lodging. All of these would result in reduction of soybean yield. What’s more, this is the main reason which have definitely hindered the increasing of yield and quality of soybean in the maize-soybean intercropping system. In addition, food industry also has a high requirement for soybean seed germination. Therefore, research the effect of KARs on soybean seed germination and post seedling development has an important significance to maize-soybean intercropping system, even to the development of soybean industry.Therefore, in this research the real-time PCR was used to study the effect of KAR2 on soybean germination, post seedling development in order to illustrate the effect of KAR2 on soybean seed germination and post seedling development and relative molecular mechanisms. Results are as follows:KAR2 treatment could delay soybean seed germination under shade condition, but had no effect under dark or white light (2340 Lx) conditions; consistent with that, KAR2 treatment could significantly inhibit the fresh weight of hypocotyl and root, the length of root was also decreased; KAR2 delayed soybean seed germination and post seedling development in a concentration dependent manner obviously; consistently, the fresh weight of hypocotyl and root and the length of root would be decreased more significantly.In order to prove the effectiveness of KAR2 treatment, we measured the relative expression of KARs response genes, GmKUF1 and GmSTH7. As the results showed, KAR2 treatment did up-regulated the relative expression of the response genes. KAR2 could enhance seed germination by enhancing the sensitivity of Arabidopsis seeds, and regulating the biosynthesis of endogenous abscisic acid and gibberellin at the same time. We need to ask if light can affect seed germination and seedling development of soybean. As the results showed, compared with dark treatment, light (2430 Lx) could delay soybean seed germination significantly; fresh weight of hypocotyl and root, and root length are decreased significantly. Abscisic acid could inhibit soybean seed germination, and gibberellin induced that of soybean; consistent with that, abscisic acid could decrease the fresh weight of hypocotyl and root, and the length of root, gibberellin increased those of soybean.The results of gene relative expression showed that, KAR2 treatment could up-regulate the expression of abscisic acid biosynthesis gene GmAAO and abscisic acid signal transduction genes GmABI5、GmABI4 and GmRD29A; gibberellin biosynthesis genes GmGA3ox1、GmGA3ox、GmKAO and GmGA3 were down-regulated. Further endogenous phytohormone concentration detection showed that KAR2 could decrease the content of endogenous GA4; the content of endogenous GA1 did not change; the content of endogenous ABA increased; the ratio of GA4/ABA decreased. The inhibitor of abscisic acid biosynthesis, Fluridone (FL) could induce soybean seed germination and enhanced the fresh weight of hypocotyl and root, and root length; the inhibitor of gibberellin biosynthesis, Paclobutrazol (PAC) could inhibit soybean seed germination and decrease the fresh weight of hypocotyl and root, and the root length; interestingly, FL could recover the delay effect of KAR2 on soybean seed germination, and there is no significant changes of the germination rate, fresh weight of hypocotyl and root, and the root length in FL+KAR2 treatment, compared with the control.In conclusion, KAR2 could delay soybean seed germination and post seedling development by enhancing sensitivity to light, promoting biosynthesis and signal transduction of ABA and decreasing biosynthesis of GA under shade condition.