Transcriptome Analysis Of Diversity In Osmotic Tolerance Between Two Medicago Truncatula Genotypes

Drought and salt stress are important factors in crop yield reduction,quality reduction,and seed dormancy,and are natural factors that limit the crop area,yield,and quality in China.Alfalfa(Medicago sativa)is an important legume forage and is widely cultivated in the North China Plain,Songnen Plain,and arid and semi-arid areas in the northwest China.Previous studies of plant responses to osmotic stress have limited guidance for breeding a drought or salt tolerant species of alfalfa.However,Medicago truncatula,the model plant with the closest relationship to alfalfa,could be used in plant stress research for a better guidance in alfalfa breeding.In this study,two M.truncatula genotypes Jemalong A17 and R108 were treated with osmotic stress,and their stomatal switches,ABA level and transcriptomic characteristics were investigated.The main findings were summarized as follows:1.After 24 h of osmotic treatment,the stomata of A17 and R108 leaves remained open.The stomata of A17 leaves completely closed,while the stomata in R108 remained open after 72 h of osmotic treatment.The observation time was shortened based on the results of leaf stomata clousure.2.The endogenous ABA content of leaves and roots of A17 and R108 was measured by ultra-efficient liquid chromatography-mass spectrometry during 24 h of osmotic stress.The endogenous ABA content of A17 roots increased significantly after 1 h of stress treatment,and continued to increase to 24 h,while the A17 leaves showed a significant increase in ABA content for the first time at 24 h after osmotic treatment.R108 leaves or roots did not show a significant increase in endogenous ABA content during the entire treatment period.Based on the changes in the stomata closure and the endogenous ABA content under osmotic stress,we named the period from 0 to 24 h under osmotic stress the transduction stage,named the period after 1 h the tolerance stage,and set the time points for the Medicago truncatula transcriptome under osmotic stress.3.Among all the transcriptome levels except the control,genes with higher expression levels in A17 than R108 were annotated in the GO molecular function annotation,and a larger part was annotated with nucleoid binding,catalytic activity and transporter activity.Further analysis found that the main genes encoding functional proteins for signal transduction,post-transcriptional regulation and modification,and molecular chaperone.4.Based on tissue-and stage-specific osmotic stress response in M.truncatula genotypes Jemalong A17 and R108,we focused on a total of 48 genes that may play an important role in the signal transduction of A17 showes better than R108 in osmotic stress,all of which are specifically up-regulated by A17 over R108.In transduction stage,two genes were picked encoding potential sensors in leaves,and three genes were picked encoding potential molecular function regulators in roots.In tolerance stage,there are 8 common up-regulated genes in leaves and roots were screened out,which 4 encode possible signal transduction related proteins,4 encode unknown protein.In tolerance stage,the only one picked gene encodes a potential receptor-like kinase with predicted transmembrane structure which located in the plasma membrane.5.Since R108 exhibits a phenotype similar to the delay of ABA signal in A17 in this study,we regard R108 after 24 h' treatment as the same stage of A17 after 2 h' treatment.By comparing gene expression co-upregulated in A17 and R108 during the delay stage,potential genes responsible for signal transduction were screened out.11 genes were screened from the leaves of the delay stage,5 of which were predictively encoding transcription factors or molecular chaperones and 6 encoding protein kinases.23 genes were screened from the leaves of the delay stage,8 of which were predicted to encode transcription factors or molecular chaperones,11 were predicted to be protein kinases,and the other 4 were annotated into the ABA signaling pathway.In conclusion,this study has refined the dynamic processes of ABA and stomata closure of Medicago truncatula A17 and R108 under osmotic stress,and the results of transcriptome data can provide basis and ideas for the studies of legumes,especially medicagos in response to osmotic stress.