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).