Global cellulose biomass, horizontal gene transfers and domain fusions drive microbial expansin evolution

Abstract

Summary Plants must rearrange the network of complex carbohydrates in their cell walls during normal growth and development. To accomplish this, all plants depend on proteins called expansins that nonenzymatically loosen noncovalent bonding between cellulose microfibrils. Surprisingly, expansin genes have more recently been found in some bacteria and microbial eukaryotes, where their biological functions are largely unknown. Here, we reconstruct a comprehensive phylogeny of microbial expansin genes. We find these genes in all eukaryotic microorganisms that have structural cell wall cellulose, suggesting expansins evolved in ancient marine microorganisms long before the evolution of land plants. We also find expansins in an unexpectedly high diversity of bacteria and fungi that do not have cellulosic cell walls. These bacteria and fungi inhabit varied ecological contexts, mirroring the diversity of terrestrial and aquatic niches where plant and/or algal cellulosic cell walls are present. The microbial expansin phylogeny shows evidence of multiple horizontal gene transfer events within and between bacterial and eukaryotic microbial lineages, which may in part underlie their unusually broad phylogenetic distribution. Overall, expansins are unexpectedly widespread in bacteria and eukaryotes, and the contribution of these genes to microbial ecological interactions with plants and algae has probbaly been underappreciated.

A horizontally acquired expansin gene increases virulence of the emerging plant pathogen Erwinia tracheiphila

“…2 A). Because mobile DNA are common agents of horizontal gene transfer 35 , 51 , 52 , we reconstructed the phylogenies of both the exlx and gh5 genes and compared the phylogenies of these two genes to the bacterial species phylogeny 7 , 35 , 51 – 57 . We find that the phylogenies of both Et – exlx and Et – gh5 genes are in conflict with the species phylogeny, which is consistent with horizontal acquisition of these genes by E. tracheiphila (Fig.…”

“…This has driven the evolution of diverse molecular mechanisms for plant colonization throughout commensal, beneficial and pathogenic microbes 3 . Proteins called ‘expansins’ are particularly intriguing, and genes coding for expansins are being identified in the genomes of an increasing number of plant-associated bacterial and fungal species 4 – 7 . Expansins are non-enzymatic, two-domain proteins of ~ 250 amino acids.…”

“…Expansin-encoding genes have also been identified in hundreds of taxonomically diverse bacterial and fungal species which do not have cellulosic cell walls, but interact with live or dead plant or algal matter 4 , 6 , 7 , 16 . In these microbial species, expansin proteins are hypothesized to promote colonization of plants through interactions with structural cellulose and/or hemicellulose in plant cell walls 4 , 7 , 17 . However, the functions and importance of microbial expansins for plant colonization have been empirically investigated in very few microbial species 4 .…”

“…For Clavibacter michiganensis , studies have described contradictory expansin roles for virulence and ability to colonize xylem 21 , 23 – 26 . Outside of these few examples, fundamental questions surrounding how microbial expansins mediate plant colonization, and the molecular mechanism(s) by which these proteins interact with plant structural carbohydrates remain unknown for almost all species 4 , 6 , 7 , 16 , 17 , 21 , 24 , 27 , 28 .…”

“…The bacterial plant pathogen Erwinia tracheiphila Smith (Enterobacteriaceae) , the causative agent of bacterial wilt of cucurbits, contains an operon with an expansin gene ( exlx ) and a glycoside hydrolase family 5 gene ( gh5 ) 7 , 29 – 31 . E. tracheiphila colonizes host plant xylem and causes a fatal bacterial wilt infection in only two genera of cucurbit host plants: cultivars of Cucurbita spp.…”