Abscisic acid regulates pinoresinol-lariciresinol reductase gene expression and secoisolariciresinol accumulation in developing flax (Linum usitatissimum L.) seeds

Secoisolariciresinol diglucoside (SDG), the main phytoestrogenic lignan of Linum usitatissimum, is accumulated in the seed coat of flax during its development and pinoresinol–lariciresinol reductase (PLR) is a key enzyme in flax for its synthesis. The promoter of LuPLR1, a flax gene encoding a pinoresinol lariciresinol reductase, contains putative regulatory boxes related to transcription activation by abscisic acid (ABA). Gel mobility shift experiments evidenced an interaction of nuclear proteins extracted from immature flax seed coat with a putative cis-acting element involved in ABA response. As ABA regulates a number of physiological events during seed development and maturation we have investigated its involvement in the regulation of this lignan synthesis by different means. ABA and SDG accumulation time courses in the seed as well as LuPLR1 expression were first determined in natural conditions. These results showed that ABA timing and localization of accumulation in the flax seed coat could be correlated with the LuPLR1 gene expression and SDG biosynthesis. Experimental modulations of ABA levels were performed by exogenous application of ABA or fluridone, an inhibitor of ABA synthesis. When submitted to exogenous ABA, immature seeds synthesized 3-times more SDG, whereas synthesis of SDG was reduced in immature seeds treated with fluridone. Similarly, the expression of LuPLR1 gene in the seed coat was up-regulated by exogenous ABA and down-regulated when fluridone was applied. These results demonstrate that SDG biosynthesis in the flax seed coat is positively controlled by ABA through the transcriptional regulation of LuPLR1 gene.