Function of the HYDROXYCINNAMOYL-CoA:SHIKIMATE HYDROXYCINNAMOYL TRANSFERASE is evolutionarily conserved in embryophytes

Abstract

The plant phenylpropanoid pathway generates a major class of specialized metabolites and precursors of essential extracellular polymers that initially appeared upon plant terrestrialization. Despite its evolutionary significance, little is known about the complexity and function of this major metabolic pathway in extant bryophytes, which represent the non-vascular stage of embryophyte evolution. Here, we report that the HYDROXYCINNAMOYL-CoA:SHIKIMATE HYDROXYCINNAMOYL TRANSFERASE (HCT) gene, which plays a critical function in the phenylpropanoid pathway during seed plant development, is functionally conserved in Physcomitrium patens (Physcomitrella), in the moss lineage of bryophytes. Phylogenetic analysis indicates that bona fide HCT function emerged in the progenitor of embryophytes. In vitro enzyme assays, moss phenolic pathway reconstitution in yeast and in planta gene inactivation coupled to targeted metabolic profiling, collectively indicate that P. patens HCT (PpHCT), similar to tracheophyte HCT orthologs, uses shikimate as a native acyl acceptor to produce a p-coumaroyl-5-O-shikimate intermediate. Phenotypic and metabolic analyses of loss-of-function mutants show that PpHCT is necessary for the production of caffeate derivatives, including previously reported caffeoyl-threonate esters, and for the formation of an intact cuticle. Deep conservation of HCT function in embryophytes is further suggested by the ability of HCT genes from P. patens and the liverwort Marchantia polymorpha to complement an Arabidopsis thaliana CRISPR/Cas9 hct mutant, and by the presence of phenolic esters of shikimate in representative species of the three bryophyte lineages.

The evolution of the phenylpropanoid pathway entailed pronounced radiations and divergences of enzyme families

“…Overall, the topology of the tree corroborates the findings of Kriegshauser et al. (2021) that a clear clade of HCT proteins first emerged in land plants—streptophyte algal sequences were few, divergent from HCT, and scattered over the tree without clear affinity to characterized acyltransferases. Without functional analyses, there is no solid foundation for predicting their function—making them exciting candidates for future studies.…”

“…Recently, Kriegshauser et al. (2021) reported on the functional conservation of the HCT homologs found in bryophytes with those of seed plants.…”

Plant cytochrome P450 plasticity and evolution

“…Thus, already in early land plants, the canonical CYP98 has been required for plant aerial growth and for the formation of essential biopolymers. Early on, it used the p-coumaroyl shikimate ester as a main substrate for phenolic ring meta-hydroxylation forming a regulatory shikimate-sensing loop for the biochemical regulation of the lignin pathway (Kriegshauser et al, 2021).…”

Phylogenetic Occurrence of the Phenylpropanoid Pathway and Lignin Biosynthesis in Plants

“…The orthologs of HCTs are present among all the land plants, which suggests that this enzyme evolved before the occurrence of lignin. A recent study showed that P. patens HCT and Marchantia polymorpha HCT can complement the deficiency of Arabidopsis hct mutant in terms of morphology and metabolite levels, suggesting that the function of HCT is likely conserved in all embryophytes (Kriegshauser et al, 2021). It appears that gene duplication of HCT occurred in dicots that produced the HQT gene.…”

“…Lignins appeared as specialized metabolites with the evolution of tracheophytes. The identification of progenitors of lignin biosynthetic genes in bryophytes provides new insights into the origin of lignin biosynthesis (Kriegshauser et al, 2021). The recent progress on genome sequencing of Charophyte algae, bryophytes, lycophytes, and ferns have also provided unprecedented opportunities to study the origin of phenylpropanoid biosynthetic pathway (Szövényi et al, 2021).…”