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Summary: Unlike bibenzyls derived from the vascular plants, lunularic acid (LA), a key precursor for macrocyclic bisbibenzyl synthesis in nonvascular liverworts, exhibits the absence of one hydroxy group within the A ring. It was hypothesized that both polyketide reductase (PKR) and stilbenecarboxylate synthase 1 (STCS1) were involved in the LA biosynthesis, but the underlined mechanisms have not been clarified.This study used bioinformatics analysis with molecular, biochemical and physiological approaches to characterize STCS1s and PKRs involved in the biosynthesis of LA.The results indicated that MpSTCS1s from Marchantia polymorpha catalyzed both C2→C7 aldol‐type and C6→C1 Claisen‐type cyclization using dihydro‐p‐coumaroyl‐coenzyme A (CoA) and malonyl‐CoA as substrates to yield a C6‐C2‐C6 skeleton of dihydro‐resveratrol following decarboxylation and the C6‐C3‐C6 type of phloretin in vitro. The protein–protein interaction of PKRs with STCS1 (PPI‐PS) was revealed and proved essential for LA accumulation when transiently co‐expressed in Nicotiana benthamiana. Moreover, replacement of the active domain of STCS1 with an 18‐amino‐acid fragment from the chalcone synthase led to the PPI‐PS greatly decreasing and diminishing the formation of LA. The replacement also increased the chalcone formation in STCS1s.Our results highlight a previously unrecognized PPI in planta that is indispensable for the formation of LA. See also the Commentary on this article by Davies & Andre, 237: 371–373. [ABSTRACT FROM AUTHOR]

The distribution of type I and II chalcone isomerases (CHIs) in plants is highly family specific. We have previously reported that ancient land plants, such as the liverworts and Selaginella moellendorffii , harbor type II CHIs. To better understand the function and evolution of CHI-fold proteins, transcriptomic data obtained from 52 pteridophyte species were subjected to sequence alignment and phylogenetic analysis. The residues determining type I/II CHI identity in the pteridophyte CHIs were identical to those of type I CHIs. The enzymatic characterization of a sample of 24 CHIs, representing all the key pteridophyte lineages, demonstrated that 19 of them were type I enzymes and that five exhibited some type II activity due to an amino acid mutation. Two pteridophyte chalcone synthases (CHSs) were also characterized, and a type IV CHI (CHIL) was demonstrated to interact physically with CHSs and CHI, and to increase CHS activity by decreasing derailment products, thus enhancing flavonoid production. These findings suggest that the emergence of type I CHIs may have coincided with the divergence of the pteridophytes. This study deepens our understanding of the molecular mechanism of CHIL as an enhancer in the flavonoid biosynthesis pathway. [ABSTRACT FROM AUTHOR]