Expansins are small proteins that loosen plant cell walls and cellulosic materials without lytic activity. First discovered in plants, expansin genes are found in the genomes of numerous bacteria and fungi that interact with plants in pathogenic and mutualistic patterns, as well as in microbes that feed on plant debris. Horizontal gene transfer from plants to microbes and between microbes accounts for expansins’ irregular taxonomic distribution. Expansins facilitate plant colonization by Bacillus, Clavibacter, and Trichoderma species, a list likely to grow as knowledge of microbial expansin function deepens. Studies have documented a synergistic action of expansins for cellulose digestion by cellulases, but only rarely to an extent that is commercially relevant. Expansins’ biophysical actions remain enigmatic because of limited understanding of cell wall structure. Deeper understanding of microbial expansins may lead to novel approaches for biomass deconstruction and biocontrol of plant diseases.

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