Portrait of the Expansin Superfamily in Physcomitrella patens: Comparisons with Angiosperm Expansins

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Abstract
Expansins are plant cell wall loosening proteins important in a variety of physiological processes. They comprise a large superfamily of genes consisting of four families (EXPA, EXPB, EXLA and EXLB) whose evolutionary relationships have been well characterized in angiosperms, but not in basal land plants. This work attempts to connect the expansin superfamily in bryophytes with the evolutionary history of this superfamily in angiosperms. The expansin superfamily in Physcomitrella patens has been assembled from the Physcomitrella sequencing project data generated by the Joint Genome Institute and compared with angiosperm expansin superfamilies. Phylogenetic, motif, intron and distance analyses have been used for this purpose. A gene superfamily is revealed that contains similar numbers of genes as found in arabidopsis, but lacking EXLA or EXLB genes. This similarity in gene numbers exists even though expansin evolution in Physcomitrella diverged from the angiosperm line approx. 400 million years ago. Phylogenetic analyses suggest that there were a minimum of two EXPA genes and one EXPB gene in the last common ancestor of angiosperms and Physcomitrella. Motif analysis seems to suggest that EXPA protein function is similar in bryophytes and angiosperms, but that EXPB function may be altered. The EXPA genes of Physcomitrella are likely to have maintained the same biochemical function as angiosperm expansins despite their independent evolutionary history. Changes seen at normally conserved residues in the Physcomitrella EXPB family suggest a possible change in function as one mode of evolution in this family.