Interaction of PKR with STCS1: an indispensable step in the biosynthesis of lunularic acid in Marchantia polymorpha

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.