Study On Agronomic Traits And Transcriptomics Of Exogenous Gibberellin-induced Internode Elongation In Cenchrus Purpureus

Elephant grass（Cenchrus purpureus（Schumach.）Morrone）is a high quality forage with soft texture,abundant leaves,high nutritional value,strong adaptability,and good regenerability.Increasing the grass yield is one of the important target traits of forage improvement,while the yield of elephant grass is related to internode elongation.Gibberellin is an important hormone that regulates plant growth,and was reported to promote internode elongation in rice（Oryza sativa）,rape（Brassica campestris）,sugarcane（Saccharum officinarum）and other plants.In this study,we observed the agronomic traits of six elephant grass cultivars,and then picked out Cp GMY[（C.americanus×C.purpureus）×（C.purpureus cv.Mott）]and Cp Mott（C.purpureus cv.Mott）with the biggest difference between internode length and biomass.Then,we analyzed the effects of exogenous GA₃ treatment on agronomic traits and endogenous hormone content of Cp GMY and Cp Mott.Furthermore,the molecular mechanism of internode elongation was analyzed by comparative transcriptome.This study will provide the foundation for revealing the mechanism of internode elongation in elephant grass and molecular improvement of grass yield.The main results are as follows:1.Determination and analysis of agronomic traits including plant height,stem diameter,internode length,internode number,fresh weight,dry weight,and stem-to-leaf ratio of the six elephant grass cultivars.They were Hybrid Guimu[（C.purpureus cv.Mott）×（C.americanus×C.purpureus）]cv.Guimu No.1,Cp GMY,Cp Red（C.purpureus cv.Red）,Cp Mott,Pg Jujun（Pennisetum giganteum z.x.lin）and King grass（C.purpureus×C.americanus cv.Reyan No.4）.The results showed that the plant height,internode length,fresh weight,dry weight,and stem-to-leaf ratio of Cp GMY were the highest among the six cultivars,while those of Cp Mott were the lowest.The internode number of Cp Mott was more than the other four cultivars,but less than Cp GMY.Therefore,we selected Cp GMY and Cp Mott as the cultivars for exogenous GA₃ treatment.2.We analyzed the response of agronomic traits and endogenous hormone content of Cp GMY and Cp Mott after exogenous GA₃ treatment.The results showed that exogenous GA₃ treatment significantly increased the plant height and stem diameter（p<0.05）,advanced the appearance time of the first internode in Cp GMY;significantly increased the endogenous GA_3,IAA and ZR content and reduced the endogenous ABA content of Cp GMY and Cp Mott（p<0.05）.Generally speaking,the effect of exogenous GA₃ treatment on the promotion or inhibition of target traits in Cp GMY was stronger than those of Cp Mott.3.The shoot apical meristem of 0,1 and 48 h after GA₃ treatment were collected from Cp GMY and Cp Mott,transcriptome sequencing analysis was performed to identify 8859 differentially expressed genes（DEGs）.GO（Gene Ontology）,KEGG（Kyoto Encyclopedia of Genes and Genomes）enrichment analysis and transcription factor prediction showed that genes were mainly enriched in three metabolic pathways of translation,amino acid metabolism and carbon metabolism.Some DEGs were enriched to response to hormone and plant hormone signal transduction pathway,and the number of genes in Cp GMY was more than in Cp Mott.Meanwhile,we found that the bHLH,MYB,ERF and NAC transcription factor families related to plant responsing to gibberellin signal contained more DEGs.The weighted gene co-expression analysis（WGCNA）indicated that some key genes in the biosynthesis and signal transduction pathways of IAA,CTK and BR may be involved in the regulation of plant height and internode elongation of Cp GMY and Cp Mott,such as AUX/IAA、SAUR、B-ARR and CYP90D2.Real-time fluorescence quantitative PCR further verified that the expression of these genes was basically consistent with the transcriptome sequencing.