Pinoresinol-Lariciresinol Reductases with Opposite Enantiospecificity Determine the Enantiomeric Composition of Lignans in the Different Organs ofLinum usitatissimumL.

Abstract

Lignans in higher plants represent an ideal class of natural products to be investigated for the origin of stereochemical diversity since chiral lignans occur in pure enantiomeric form as well as in enantiomeric mixtures. Seeds of Linum usitatissimum contain 8S, 8'S-(+)- and 8R, 8'R-(-)-secoisolariciresinol [SS-(+)- and RR-(-)-secoisolariciresinol, respectively] as diglucosides (SS- and RR-secoisolariciresinol diglucosides) whereas aerial parts of flowering L. usitatissimum accumulate only lignans derived from RR-(-)-secoisolariciresinol. Pinoresinol-lariciresinol reductase (PLR) catalyzes two early steps in lignan biosynthesis. Up to now, only a cDNA encoding a PLR ( PLR-LU1) which is enantiospecific for the conversion of 8S, 8'S-(-)-pinoresinol (SS-pinoresinol) via 8S, 8'S-(-)-lariciresinol (SS-lariciresinol) to SS-(+)-secoisolariciresinol was cloned. Here we present the cloning of a cDNA encoding a RR-pinoresinol-RR-lariciresinol reductase ( PLR-LU2) from the leaves of L. usitatissimum which converts only RR-pinoresinol to RR-secoisolariciresinol. In leaves and stems of L. usitatissimum accumulating the 8R, 8'R-enantiomers of lignans, only PLR-LU2 was transcriptionally active. Both PLR-LU1 and PLR-LU2 transcripts were observed in seeds and contribute to the synthesis of SS- and RR-secoisolariciresinol, respectively. Thus, the enantiomeric composition of lignans in the organs of L. usitatissimum appears to be determined by the relative action of two PLRs with opposite enantiospecificities rather than by a single enzyme of low enantiospecificity.

Natural Hypolignification Is Associated with Extensive Oligolignol Accumulation in Flax Stems

“…Although promoter-GUS analyses and semiquantitative PCR indicated that the LuPLR1 gene cloned was not expressed in flax stem tissues (Hano et al, 2006b), the recent cloning of a second PLR gene (LuPLR2; Hemmati et al, 2010) from leaves and stems of flax plants, together with our results, suggest that PLR genes in flax form a small multigene family with organ-/tissuespecific expression patterns. Two other highly abundant neolignan unigene transcripts in flax inner stem tissues correspond to the enzyme PCBER, catalyzing the reduction of DDC into IDDDC and identified previously as the most abundant protein in poplar xylem (Gang et al, 1999;Vander Mijnsbrugge et al, 2000).…”

Plant cell wall lignification and monolignol metabolism

“…(+)-Secoisolariciresinol is then glycosylated to secoisolariciresinol diglucoside (SDG) that accumulates to high levels in flax seeds (linseed; Ford et al, 2001). The complexity of lignan stereochemistry was underlined in a recent study (Hemmati et al, 2010) that isolated two PLRs (PLR-Lu1, PLR-Lu2) showing opposite enantiospecificity. PLR-Lu1 utilizes 8S,8′S-pinoresinol, whereas PLR-Lu2 uses 8R,8′R-pinoresinol.…”

“…PLR-Lu1 utilizes 8S,8′S-pinoresinol, whereas PLR-Lu2 uses 8R,8′R-pinoresinol. Interestingly, gene expression analyses have shown that the Lu PLR1 gene is expressed in different flax seed tissues, whereas the Lu PLR2 gene is expressed in flax stem and leaf tissues (Hano et al, 2006; Hemmati et al, 2010; Venglat et al, 2011). In agreement with this organ-specific gene expression, 8S,8′S-SDG accumulates in flax seeds whereas aerial green parts accumulate 8R,8′R-SDG.…”

Laser Microdissection and Spatiotemporal Pinoresinol-Lariciresinol Reductase Gene Expression Assign the Cell Layer-Specific Accumulation of Secoisolariciresinol Diglucoside in Flaxseed Coats

“…LuPLR1 is responsible for the synthesis of the major (+)-secoisolariciresinol enantiomer (87%) while a minor quantity of the (–)-secoisolariciresinol enantiomer (13%) is synthesized by LuPLR2 (Hemmati et al, 2010). The very weak β-glucuronidase ( GUS ) activity detected in embryos has to be attributed to non-specific GUS that has already been reported in seeds of a number of plants (Hu et al, 1990; Hänsch et al, 1995).…”

“…RNAi technology applied to down regulate LuPLR1 gene expression in flax ( Linum usitatissimum L.) seeds has evidenced the direct in vivo implication of the LuPLR1 gene in the biosynthesis of (+)-SDG (Renouard et al, 2014). LuPLR2 , another PLR gene, encoding a protein with an enantiospecificity opposite to LuPLR1 exists in flaxseeds (Hemmati et al, 2010). LuPLR2 is responsible for the synthesis of the minor (–)-secoisolariciresinol enantiomer.…”