The evolution of the expansin gene superfamily in basal land plant lineages: Studies of expansins in Physcomitrella patens and Selaginella moellendorffii

Expansins are an important superfamily of cell wall loosening proteins consisting of four families named EXPA, EXPB, EXLA, and EXLB. These proteins loosen the linkage between cellulose microfibrils in a non-enzymatic fashion, allowing these microfibrils to slide past one another. This loosening action has been implicated as being important in a wide variety of growth and developmental processes in plants. Because this protein superfamily appears to be critical for the proper growth and development of land plants, its origin and evolutionary history has become a question of great interest. Two recent papers from Sampedro et al. used gene colinearity data integrated with traditional phylogenetic analyses to define 17 orthologous groups of angiosperm expansins between Arabidopsis, Populus and rice. The work presented in this dissertation provides the first look at the makeup of the expansin superfamily in basal land plant lineages. The sequencing of the genomes of the bryophyte Physcomitrella patens and the lycophyte Selaginella moellendorffii by the Joint Genome Initiative of the Department of Energy allowed for the assembly of the expansin superfamily in these organisms. What is revealed are gene superfamilies containing EXPA and EXPB families but lacking EXLA or EXLB families. This indicates that the EXLA and EXLB families arose sometime after the divergence of the lycophytes, or that these divergent expansin families have been lost from early land plant lineages. The Physcomitrella EXPA sequences show a high degree of conservation at putatively critical amino acid residues, suggesting a conservation of biochemical function with angiosperm expansins. Phylogenetic analyses seem to indicate, however, that most of these genes are bryophyte-specific lineages paraphyletic to all angiosperm EXPA genes. The Physcomitrella EXPB genes appear to form a monophyletic group sister to all angiosperm EXPB genes, and they also show several changes at normally conserved amino acid residues. Both the EXPA and EXPB genes of Physcomitrella show a relatively variable intron pattern, but these patterns do not yet warrant a change to the ancestral expansin intron patterns hypothesized in the literature. Although Physcomitrella expansins appear to be evolving independently from their angiosperm counterparts, EXPA and EXPB gene families of similar sizes have evolved in parallel, indicating some critical importance of gene number for proper expansin function. In contrast to the apparently divergent history of the expansin superfamily in bryophytes, Selaginella possesses a superfamily, which, although still difficult to definitively relate to angiosperm clades, seems to be much more closely related to angiosperm expansins than to the Physcomitrella superfamily. It is even possible to firmly place two Selaginella EXPA genes as a sister group to an angiosperm clade. Preliminary expression pattern analysis of these two Selaginella EXPA genes indicates that they do not share the root-hair-specific expression pattern observed for the Arabidopsis members of this angiosperm EXPA clade. Selaginella EXPA and EXPB intron patterns are highly conserved and reflect the ancestral states proposed in earlier works. Preliminary expression pattern data for two Physcomitrella EXPA genes indicates that, like their angiosperm counterparts, bryophyte expansins are expressed in a tissue-specific manner consistent with a role in plant growth and development. This work also reveals that, like angiosperm expansins, certain Physcomitrella expansins seem to have their expression regulated by plant hormones. Although functionality studies (both in vivo and in vitro ) of Physcomitrella expansins presented here were not able to show a role for bryophyte expansins in cell growth or wall loosening, future work may well illuminate such a role using the lessons learned here as a starting point.