Pinoresinol–lariciresinol reductases, key to the lignan synthesis in plants

Main conclusion This paper provides an overview on activity, stereospecificity, expression and regulation of pinoresinol–lariciresinol reductases in plants. These enzymes are shared by the pathways to all 8–8′ lignans derived from pinoresinol. Pinoresinol–lariciresinol reductases (PLR) are enzymes involved in the lignan biosynthesis after the initial dimerization of two monolignols. They catalyze two successive reduction steps leading to the production of lariciresinol or secoisolariciresinol from pinoresinol. Two secoisolariciresinol enantiomers can be synthetized with different fates. Depending on the plant species, these enantiomers are either final products (e.g., in the flaxseed where it is stored after glycosylation) or are the starting point for the synthesis of a wide range of lignans, among which the aryltetralin type lignans are used to semisynthesize anticancer drugs such as Etoposide ® . Thus, the regulation of the gene expression of PLRs as well as the possible specificities of these reductases for one reduction step or one enantiomer are key factors to fine-tune the lignan synthesis. Results published in the last decade have shed light on the presence of more than one PLR in each plant and revealed various modes of action. Nevertheless, there are not many results published on the PLRs and most of them were obtained in a limited range of species. Indeed, a number of them deal with wild and cultivated flax belonging to the genus Linum . Despite the occurrence of lignans in bryophytes, pteridophytes and monocots, data on PLRs in these taxa are still missing and indeed the whole diversity of PLRs is still unknown. This review summarizes the data, published mainly in the last decade, on the PLR gene expression, enzymatic activity and biological function.