Abstract
BackgroundExpansins are plant cell wall loosening proteins that are involved in cell enlargement and a variety of other developmental processes. The expansin superfamily contains four subfamilies; namely, α-expansin (EXPA), β-expansin (EXPB), expansin-like A (EXLA), and expansin-like B (EXLB). Although the genome sequencing of soybeans is complete, our knowledge about the pattern of expansion and evolutionary history of soybean expansin genes remains limited.ResultsA total of 75 expansin genes were identified in the soybean genome, and grouped into four subfamilies based on their phylogenetic relationships. Structural analysis revealed that the expansin genes are conserved in each subfamily, but are divergent among subfamilies. Furthermore, in soybean and Arabidopsis, the expansin gene family has been mainly expanded through tandem and segmental duplications; however, in rice, segmental duplication appears to be the dominant process that generates this superfamily. The transcriptome atlas revealed notable differential expression in either transcript abundance or expression patterns under normal growth conditions. This finding was consistent with the differential distribution of the cis-elements in the promoter region, and indicated wide functional divergence in this superfamily. Moreover, some critical amino acids that contribute to functional divergence and positive selection were detected. Finally, site model and branch-site model analysis of positive selection indicated that the soybean expansin gene superfamily is under strong positive selection, and that divergent selection constraints might have influenced the evolution of the four subfamilies.ConclusionThis study demonstrated that the soybean expansin gene superfamily has expanded through tandem and segmental duplication. Differential expression indicated wide functional divergence in this superfamily. Furthermore, positive selection analysis revealed that divergent selection constraints might have influenced the evolution of the four subfamilies. In conclusion, the results of this study contribute novel detailed information about the molecular evolution of the expansin gene superfamily in soybean.
Highlights
Expansins are plant cell wall loosening proteins that are involved in cell enlargement and a variety of other developmental processes
The results showed that, with the exception of chromosomes 8 and 16, expansin genes could be mapped on all chromosomes from 1 to 20
The soybean expansin genes were classified into the four known subfamilies: α-expansin (EXPA), β-expansin (EXPB), expansin-like A (EXLA), and expansin-like B (EXLB)
Summary
Expansins are plant cell wall loosening proteins that are involved in cell enlargement and a variety of other developmental processes. Expansins are encoded by a multi-gene family, and are composed of a superfamily of plant cell wall loosening proteins that induce pH-dependent wall extension and stress relaxation in a characteristic and unique manner [1]. The plant cell wall is composed of cellulose microfibrils, which bind to various glycans, including xyloglucan and xylan. The extension of the cell wall involves the movement and separation of cellulose microfibrils by the process of molecular creeping. Α-Expansinis hypothesized to promote such movement, by inducing the local dissociation and slippage of xyloglucans, whereas β-expansin is theorized to work in a similar manner on a different glycan, perhaps xylan [7]. No assays have demonstrated that expansins have hydrolytic activity or any other enzymatic activities [15,16,17]
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