4 Impermeabilizing biopolymers
The evolution of hydrophobic extracellular biopolymers was a critical innovation for plant terrestrialization, contributing to permeability and water transport control. Some of these compounds include complex phenolic-derived polymers such as lignin, cutin and suberin (Niklas, Cobb, & Matas, 2017). Lignin generally works in the reinforcement of secondary cell walls, cutin is commonly found as part of the impermeabilizing cuticle of aerial parts and was recently reported as being also part of the root cap cuticle, and suberin is found in many tissues including specialized root cells, tubers, fruit skin, and seed coat (Berhin et al., 2019; Niklas et al., 2017; Philippe et al., 2020; Renault et al., 2017). In general these compounds function in mechanical support, defense against pathogens and herbivores, and in the control of the movement of water, nutrient and gases (Niklas et al., 2017; Philippe et al., 2020; Renault et al., 2017; Pei Wang et al., 2020).
Convergent evolution has also played an important role on the appearance and shaping of the biosynthetic pathways of some of these biopolymers across diverging plant taxa. For example the ancestral green algae and red-algae present the ability to produce “lignin-like” compounds, leading to the hypothesis that ancient biosynthesis pathways have been rewired in the vascular plant lineage (Delwiche et al., 1989; Labeeuw et al., 2015; Martone et al., 2009). Furthermore, it has been identified that lycophytes and spermatophytes have independently developed the ability to produce monomers necessary for lignin biosynthesis and assembly (Renault et al., 2017; Weng et al., 2010; Weng et al., 2008).
It is possible that convergent evolution also played a role on the appearance of cutin and suberin across distantly related plant species. Both cutin and suberin share similar initial biosynthetic steps and lipid precursors, however different enzymes belonging to the same superfamily and with different mechanisms of action work on their modification and assembly (Philippe et al., 2020; Pollard et al., 2008). A recent large scale comparative genomic study has revealed that the ability to synthesize precursor molecules of cutin and suberin emerged prior the evolution of land plants, however the subsequent steps of the biosynthetic pathway may have evolved independently across land plants (Cannell et al., 2020). Still, little is known about how the other parts of the pathway, such as polymerization and distribution in plant cells, evolved in land plants, what opens up a myriad of possibilities for future research (Niklas et al., 2017; Philippe et al., 2020; Pollard et al., 2008).