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 ).