Auxin Represses Soybean Seed Germination And Regulates Plant Shade Avoidance Response Antagonistically With Karrikins

Phytohormones are the main internal factors that regulate plant growth and development.Although different phytohormones possess distinct and specific biological functions,they can regulate the various stages of plant growth and development by interacting with each other.Both seed germination and post-germination seedling establishment are key processes during plant life cycle.Abscisic acid（ABA）and gibberellin（GA）are the key regulators of seed dormancy and germination.Recent study has revealed that auxin has also been involved in regulat ing seed dormancy and germination,but in this process,the relationship among auxin and ABA,GA pathways are largely unknown.Shade was a special light environmental cue,which leads to a significant decrease in photosynthetically active radiation and the ratio of red light to far red light,and the seedling hypocotyl was significantly elongated due to the increased auxin under shade stress.Karrikins are a small molecular compounds found in the smoke of wild fire,which have the biological functions of regulating seed germination and promoting seedling establishment.Remarkably,Karrikins can inhib it the hypocotyl elongation under red light conditions.However,there is no research on molecular function of Karrikins under shade stress,and the interactions with auxin in this process are needed further exploration.In the present thesis,using oil crop soybean and model plant Arabidopsis thaliana as experimental materials and through genetics,plant physiology,molecular biology and biochemistry approaches,we studied the regulatory effect of auxin on soybean seed germination,and revealed the crosstalk between auxin and ABA,GA in those processes.Finally,we dissected the molecular function of Karrikins and auxin interactions to regulate the hypocotyl elongation under shade conditions.The main results of this study are presented as follows:（1）The present study showed that both natural and artificial synthetic auxin had significant inhibitory effect on soybean seed germination,and the inhibitory effect exhibited in a concentration-dependent manner.The microscopic observations of the imbibed seeds showed that both the rupture of seed coat and the extension of the radicle were delayed by auxin treatment.Analysis of q RT-PCR showed that in the processes of soybean seed germination,auxin treatment promoted the transcription of several ABA biosynthesis genes,including Gm AAO and Gm ABA2,and suppressed the transcript io n of ABA catabolism gene Gm CYP707A1,and positively regulated the transcription of several ABA signaling pathway genes,including Gm ABI4,Gm ABI5 and Gm RD29-A.While,the transcription of several GA biosynthesis genes,including Gm GA3,Gm GA3ox1,Gm GA3ox2 and Gm KAO,were inhibited by auxin treatment,and auxin up-regulated the transcription of negative regulatory gene Gm RGL of GA signa ling pathway.Accordingly,phytohormone quantification shows that auxin significa nt ly increased ABA content,whereas the active GA₁ and GA₄ levels were decreased,resulting significant decrease in the ratio GA/ABA.Further,ABA synthesis inhib itor fluridone promoted soybean seed germination,when fluridone and IAA treatment jointly,fluridone reversed the delayed-germination phenotype of IAA-treatment.These results showed that exogenous auxin treatment repressed soybean seed germination by enhancing ABA biosynthesis,while impairing GA biogenesis,and finally decreasing GA/ABA ratio.（2）The present study showed that Karrikin 2（KAR₂）treatment significa nt ly inhibited the hypocotyl elongation under shade conditions,but had no obvious effect under normal light conditions.Analysis of q RT-PCR showed that KAR₂ treatment reduces the transcription of several shade response genes,including HFR1,ATHB2,IAA29 and PIL1,and weakens the response of plants to the shade at the transcriptio na l level.The exogenous auxin can reverse the elongated hypocotyl of KAR₂ treatment.The transcription analysis of auxin biosynthesis genes,including TAA1,YUCCA2,YUCCA5,YUCCA8 and YUCCA9,showed that KAR₂ treatment inhibited the biosynthesis of auxin under shade conditions.Accordingly,phytohormone quantification shows that KAR₂ significa nt ly decreased IAA content.Subsequently,genetic analysis and the experimental results of auxin biosynthesis inhibitor yucasin showed that KAR₂inhibited the hypocotyl elongation by weakening the Auxin biosynthesis.Further analysis of the mutants related to light signaling pathways indicated that KAR₂ inhibition of hypocotyl elongation may depend on transcription factor PIF4.These results showed that KAR₂ may through phosphorylated PIF4 and reduced its activity,thereby inhibited the downstream auxin biosynthesis expression,thus reduced the content of endogenous auxin,finally inhib ited hypocotyl elongation under shade.