Annals of Botany

Model moss provides family portrait

Professor David Cove first introduced me to the moss Physcomitrella patens, enthusing about its potential as a “model”. His enthusiasm was not misplaced, as is evident from the paper by Carey and Cosgrove (Pennsylvania State University, USA, pp. 1131–1141). Cosgrove’ group is well-known for work on expansins, non-enzyme proteins involved in rapid cell wall expansion in angiosperms. Expansin genes exist as a super-family comprised of four families, EXPA, EXPB, EXLA and EXLB, of which EXPA and EXPB encode the “classic” expansins that weaken linkages between cellulose microfibrils. In order to understand further the evolution of the expansin super-family the authors have focussed on Physcomitrella as a less-complex land plant. The data show clearly that Physcomitrella possesses EXPA and EXPB families of expansin genes but not the EXLA or EXLB families. There are 27 genes in the EXPA family (one more than Arabidopsis but one less than Populus) and seven in the EXPB family (one more than Arabidopsis and four more than Populus). Therefore, the individual genes, many of which show cell-specific expression patterns in angiosperms, have arisen at least 400 million years ago, before the increase in the range of cell types associated with the evolutionary route from simple to complex land plants. Further analysis of gene structure places the EXPA family into six groups (the latter two, E and F, each consisting of one gene). Groups D, E and F are more similar to angiosperm expansins than groups A, B and C. This analysis, taken with other data, suggests that the common ancestor of Physcomitrella and angiosperms had at least two EXPA genes that gave rise to the A, B, C and to the D, E, F lineages. EXPB genes by contrast fall into one group (there are two in Arabidopsis), which appears to be a sister group (i.e. derived from a common ancestor) to those in angiosperms.