A
lignan,
lariciresinol, was isolated from Arabidopsis thaliana, the most widely used model plant in plant bioscience sectors, for the first time. In the A. thaliana genome database, there are two genes (At1g32100 and At4g13660) that are annotated as
pinoresinol/
lariciresinol reductase (PLR). The recombinant AtPLRs showed strict substrate preference toward
pinoresinol but only weak or no activity toward
lariciresinol, which is in sharp contrast to conventional PLRs of other plants that can reduce both
pinoresinol and
lariciresinol efficiently to
lariciresinol and
secoisolariciresinol, respectively. Therefore, we renamed AtPLRs as A. thaliana
pinoresinol reductases (AtPrRs). The recombinant AtPrR2 encoded by At4g13660 reduced only (-)-
pinoresinol to (-)-
lariciresinol and not (+)-
pinoresinol in the presence of
NADPH. This enantiomeric selectivity accords with that of other PLRs of other plants so far reported, which can reduce one of the enantiomers selectively, whatever the preferential enantiomer. In sharp contrast, AtPrR1 encoded by At1g32100 reduced both (+)- and (-)-pinoresinols to (+)- and (-)-lariciresinols efficiently with comparative k(cat)/K(m) values. Analysis of
lignans and spatiotemporal expression of AtPrR1 and AtPrR2 in their functionally deficient A. thaliana mutants and wild type indicated that both genes are involved in
lariciresinol biosynthesis. In addition, the analysis of the enantiomeric compositions of
lariciresinol isolated from the mutants and wild type showed that
PrRs together with a dirigent
protein(s) are involved in the enantiomeric control in
lignan biosynthesis. Furthermore, it was demonstrated conclusively for the first time that differential expression of PrR
isoforms that have distinct selectivities of substrate enantiomers can determine enantiomeric compositions of the product,
lariciresinol.