Grienenberger, E. | Search bioRxiv

bioRxiv2026

Discovery of Scrophularia nodosa harpagoside synthase, a novel BAHD cinnamoyltransferase, bridges a key gap in the iridoid biosynthetic pathway

Harpagoside, a high-value anti-inflammatory iridoid compound, is traditionally extracted from the roots of Harpagophytum procumbens (Pedaliaceae, Lamiales), a Southern African desert plant widely used in traditional medicine but currently threatened by overexploitation. Scrophularia nodosa (Scrophulariaceae, Lamiales) is a perennial annual plant widely distributed in Western Europe and accumulates several iridoid compounds with known biological activities, such as catalpol, aucubin, harpagide and particularly harpagoside. We gathered extensive genomic and transcriptomics resources for this species and aimed at deciphering the biosynthetic pathway leading to the most abundant iridoid in S. nodosa, harpagoside. We found that the early iridoid pathway is well conserved with other iridoid-producing plants and validated the enzyme activities by transient co-expression in N. benthamiana. Investigation into the large BAHD family showed subclade 6i expanding in Scrophulariaceae, with an atypical VYPWG motif instead of the canonical DFGWG. In this branch, we discovered and characterized harpagoside synthase, a BAHD-type cinnamoyl transferase enzyme showing unique high specificity to the uncommon cinnamoyl-CoA acyl donor and catalyzing the final step of harpagoside biosynthesis. These results establish S. nodosa as a new model to investigate unexplored branches of the iridoids metabolism, and are a first step towards sustainable harpagoside and high-value cinnamoyl-containing conjugates production.

plant biology↗

bioRxiv2020

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

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 that 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 bonafide 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.

plant biology↗