Abstract
Phytoalexins are metabolites biosynthesized in plants in response to pathogen, environmental, and chemical stresses that often have potent bioactivities, rendering them promising for use as therapeutics or scaffolds for pharmaceutical development. Glyceollin I is an isoflavonoid phytoalexin from soybean that exhibits potent anticancer activities and is not economical to synthesize. Here, we tested a range of source tissues from soybean, in addition to chemical and biotic elicitors, to understand how to enhance the bioproduction of glyceollin I. Combining the inorganic chemical silver nitrate (AgNO3) with the wall glucan elicitor (WGE) from the soybean pathogen Phytophthora sojae had an additive effect on the elicitation of soybean seeds, resulting in a yield of up to 745.1 µg gt−1 glyceollin I. The additive elicitation suggested that the biotic and chemical elicitors acted largely by separate mechanisms. WGE caused a major accumulation of phytoalexin gene transcripts, whereas AgNO3 inhibited and enhanced the degradation of glyceollin I and 6″-O-malonyldaidzin, respectively.
Highlights
Plants, like other organisms, have metabolic pathways that remain silent until activated by stresses.Phytoalexins are defense metabolites biosynthesized in response to pathogens, but which for unknown reasons accumulate in response to specific environmental stresses and inorganic chemicals, such as heavy metals [1,2,3]
We have identified that combining the biotic elicitor wall glucan elicitor (WGE) from P. sojae and the chemical elicitor AgNO3 stimulated the accumulation of the anticancer phytoalexin glyceollin I in an additive fashion, because they functioned by largely distinct elicitation mechanisms
WGE elicited a massive accumulation of biosynthesis gene mRNAs, and AgNO3 stimulated the hydrolysis of the isoflavone conjugate 600 -O-malonyldaidzin
Summary
Like other organisms, have metabolic pathways that remain silent until activated by stresses.Phytoalexins are defense metabolites biosynthesized in response to pathogens, but which for unknown reasons accumulate in response to specific environmental stresses and inorganic chemicals, such as heavy metals [1,2,3]. Much of what is known about phytoalexin elicitation mechanisms comes from studies of the glyceollins in soybean, camalexins in Arabidopsis, diterpenoids and flavonoids in rice, stilbenes in grapevine, alkaloids in California poppy, and the 3-deoxyanthocyanidins, terpenoids, and phytodienoic acids in maize. Very few studies have attempted to distinguish the elicitation mechanisms of biotic and chemical elicitors. Mitogen-activated protein kinase (MAPK) or phospholipase signaling results in the expression of transcription factors (TFs) that directly activate the transcription of phytoalexin biosynthesis genes. MYB-, bHLH-, or WRKY-type TFs directly activate some or all of the phytoalexin biosynthesis genes in cotton, sorghum, rice, Arabidopsis, and grapevine [4,5,6]. No phytoalexin TF has been identified, but transcription of glyceollin biosynthesis genes was coordinately induced in response to the pathogen Phytophthora sojae [7,8]
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