2.4.3. Metabolic engineering
Another efficient, stable, and sustainable approach for lignans
production in metabolic engineering of enzymes involved in the
biosynthesis of lignans in Linum , Forsythia , andPodophyllum species (Ionkova,
2011; Malik et al., 2014). Silencing ofForsythia cell cultures by pinoresinol-lariciresinol
reductase-RNA interference (PLR-RNAi) triggered an approximately 20-fold
increase in total pinoresinol (pinoresinol aglycone and glucoside),
compared with levels observed in the control wild-type cells
(Kim et al., 2009). Stable transfection
of PLR-RNAi and the Sesamum CYP81Q1 gene in Forsythia cell
cultures triggered an approximately 20-fold increase in pinoresinol
(pinoresinol aglycone and glucoside) (Kim
et al., 2009) along with sesamin (0.01 mg/g DW of the cell)
(Satake et al., 2015). The reaction
catalyzed by PLR is crucial in plant defense mechanisms against
pathogens. In the presence of a fungal elicitor, the lignan deficient
roots usually divert the phenylpropanoid synthesis pathway to the
production of defense metabolites to enhance the expression of genes
involved in lignan synthesis to cope pathogen attack. Hence, enhanced
synthesis of lignan has been noticed in silenced PLR plants compared to
controls, and no other pathway seems able to bypass downregulated PLR
(Tashackori et al., 2019).
Similarly, transcriptomic analyses of P. hexandrum Royle (having
anticancer lignan PPT) were used to investigate the expression of key
genes involved in the production of this compound. Quantitative
expression analysis of pathway genes exhibited ~23-fold
increase in transcript abundance of nine genes namely
secoisolariciresinol dehydrogenase (SD), prephenate dehydrogenase (PD),
p-coumarate 3-hydroxylase (PCH), chorismate mutase (CM), caffeic acid
3-O-methyltransferase (CMT), cinnamyl alcohol dehydrogenase (CAD),
cinnamoyl-CoA reductase (CCR), cinnamate 4-hydroxylase (C4H), and
arogenate dehydratase (ADH). This higher expression of genes also
correlated with a higher yield of PPT in the root and rhizomes ofP. hexandrum (Kumar et al., 2015)
(Table1 ).