Evolution of the Expansin Superfamily in Bryophytes and Green Algae

Expansins are a superfamily of non-enzymatic proteins that mediate plant cell wall loosening and play essential roles in growth and development. The expansin superfamily consists of four families: EXPA, EXPB, EXLA, and EXLB. Although the expansin gene families have been well characterized in angiosperms, their evolutionary history in early-diverging land plants and green algae remains incompletely resolved. This study examines expansin superfamilies in two liverworts ( Marchantia polymorpha and Conocephalum conicum ), a hornwort ( Anthoceros agrestis ), a moss ( Ceratodon purpureus ), and two green algal species ( Chara braunii and Spirogloea muscicola ). Expansin genes were identified from these organisms using BLAST searches. These newly assembled gene families were then analyzed to determine the relationships between them and gain insight into early expansin evolution. No EXLA or EXLB genes were found in bryophytes or green algae. Bryophytes contain EXPA genes; however, EXPBs were not detected in liverworts and are present only in mosses and hornworts. Although green algal expansins share key characteristics of both EXPA and EXPB genes, evidence suggests they are not members of either family. The data presented here raises interesting questions about the timing of EXPA and EXPB evolution in land plants.