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2080 SONG Hui et al. Journal of Integrative Agriculture 2018, 17(9): 2074–2081 with the time-cost hypothesis, in which rapidly rather than highly expressed genes, tend to be expressed well in time- cost efficient manner. 5. Conclusion Our results support that nature selection played a pivotal role in forming codon usage pattern, and no relatedness between gene expression and either CDS length or GC content in M . truncatula . In addition, we have identified a total of 30 optimal codons for the model legume. These results could provide valuable information on genetic engineering of the model legume and forage crop. Acknowledgements We thank Dr. Wen Jiangqi (Noble Research Institute, USA) and Dr. Wang Hongliang (United States Department of Agriculture-Agricultural Research Service, Tifton) for critical reviews and comments. This study was supported by the National Basic Research Programof China (2014CB138702) and the National Natural Science Foundation of China (31502001). 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