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Background  GRAS proteins belong to a plant transcription factor family that is involved with multifarious roles in plants. Although previous studies of this protein family have been reported for  Arabidopsis , rice, Chinese cabbage and other species, investigation of expansion patterns and evolutionary rate on the basis of comparative genomics in different species remains inadequate.    Results  A total of 289 GRAS genes were identified in  Arabidopsis ,  B. distachyon , rice, soybean,  S. moellendorffii , and  P. paten s and were grouped into seven subfamilies, supported by the similarity of their exon–intron patterns and structural motifs. All of tandem duplicated genes were found in group II except one cluster of rice, indicating that tandem duplication greatly promoted the expansion of group II. Furthermore, segment duplications were mainly found in the soybean genome, whereas no single expansion pattern dominated in other plant species indicating that GRAS genes from these five species might be subject to a more complex evolutionary mechanism. Interestingly, branch-site model analyses of positive selection showed that a number of sites were positively selected under foreground branches I and V. These results strongly indicated that these groups were experiencing higher positive selection pressure. Meanwhile, the site-specific model revealed that the GRAS genes were under strong positive selection in  P. paten s. DIVERGE v2.0 was used to detect critical amino acid sites, and the results showed that the shifted evolutionary rate was mainly attributed to the functional divergence between the GRAS genes in the two groups. In addition, the results also demonstrated the expression divergence of the GRAS duplicated genes in the evolution. In short, the results above provide a solid foundation for further functional dissection of the GRAS gene superfamily.    Conclusions  In this work, differential expression, evolutionary rate, and expansion patterns of the GRAS gene family in the six species were predicted. Especially, tandem duplication events played an important role in expansion of group II. Together, these results contribute to further functional analysis and the molecular evolution of the GRAS gene superfamily.    Electronic supplementary material  The online version of this article (doi:10.1186/s12870-014-0373-5) contains supplementary material, which is available to authorized users.

Unusual tandem expansion and positive selection in subgroups of the plant GRAS transcription factor superfamily