Origins of xyloglucan-degrading enzymes in fungi.

The origin story of land plants - the pivotal evolutionary event that paved the way for terrestrial ecosystems of today to flourish - lies within their closest living relatives: the streptophyte algae. Streptophyte cell wall composition has evolved such that profiles of cell wall polysaccharides can be used as taxonomic markers. Since xyloglucan is restricted to the streptophyte lineage, we hypothesized that fungal enzymes evolved in response to xyloglucan availability in streptophyte algal or land plant cell walls. The record of the origins of these enzymes is embedded in fungal genomes, and comparing genomes of fungi that share an ancient common ancestor can provide insights into fungal interactions with early plants. This Viewpoint contributes a review of evidence underlying current assumptions about the distribution of xyloglucan in plant and algal cell walls. We evaluate evolutionary scenarios that may have given rise to the observed distribution of putative xyloglucanases in fungi and discuss possible biological contexts in which these enzymes could have evolved. Our findings suggest that fungal xyloglucanase evolution was more likely driven by land plant diversification and biomass accumulation than by the first origins of xyloglucan in streptophyte algal cell walls.