Tissue distribution of avenanthramides and gene expression of hydroxycinnamoyl-CoA:hydroxyanthranilate N-hydroxycinnamoyltransferase (HHT) in benzothiadiazole treated oat (Avena sativa)

Authors: Wise, Mitchell

Submitted to: Canadian Journal of Plant Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/7/2017
Publication Date: 9/23/2017
Citation: Wise, M.L. 2017. Tissue distribution of avenanthramides and gene expression of hydroxycinnamoyl-CoA:hydroxyanthranilate N-hydroxycinnamoyltransferase (HHT) in benzothiadiazole treated oat (Avena sativa). Canadian Journal of Plant Science. 98(2):444-456. https://doi.org/10.1139/cjps-2017-0108.
DOI: https://doi.org/10.1139/cjps-2017-0108

Interpretive Summary: Oats produce a group of compounds termed avenanthramides that are unique, among food crops, to oat. Here we employ a commercially available plant defense activator (BTH) to elicit high levels of avenanthramide biosynthesis and dissect the responding plants to determine the levels of avenanthramide produced in the various organs of the plants. We also measured the activity levels of the final enzyme in the biosynthetic pathway (HHT) as well as the level of gene expression for HHT. Also clearly demonstrated is the presence of avenanthramides in the phloem sap, indicating source to sink transport of these metabolites. A surprising result was that very high levels of HHT were found in both treated and untreated plant roots, but only the BTH treated plants showed enhanced avenanthramide production, indicating the likelihood of transport being their principle source. Stem and panicle stem tissue, while demonstrating the presence of low level HHT activity, showed much higher avenanthramide content than what might be expected from de novo biosynthesis and poor correlation between avenanthramide concentrations and HHT activity. Leaves, glumes and lemma/palea, on the other hand, showed strong correlation between avenanthramide levels and HHT activity. The impact of this report is to illustrate that plant defense activators can be employed to investigate the biosynthesis of certain phytonutrients in planta as well as to determine the tissue distribution of avenanthramide biosynthesis in whole oat plants. These experiments also demonstrate that extremely high concentrations of naturally produced avenanthramides can be obtained in the vegetative tissue of oat seedlings in response to BTH treatment.

Technical Abstract: Oats produce a group of natural products termed avenanthramides. These compounds are produced in both the vegetative tissue and the grain. In leaf tissue they are produced in response to crown rust infection or by treatment with chemical plant defense activators and likely other environmental stresses. Grain avenanthramide production tends to be constitutive, but concentrations are highly variable and strongly influenced by environmental conditions. Here we report the effect of a plant defense activator (Benzothiadiazole (BTH)) on the temporal expression and tissue distribution of avenanthramides in the leaf, stem, root, panicle stem, glumes, lemma/palea and filling grain in the oat plant. HHT activity, the final enzyme in the biosynthetic pathway to the avenanthramides, is also determined in these tissues as well as the relative expression ratios of HHT mRNA resulting from benzothiadiazole treatment. Evidence for phloem transport of the avenanthramides is also presented. In summary, following BTH treatment, leaf tissue is the predominate location for avenanthramide biosynthesis. However, significant amounts are also found in the upper and lower stems, roots, panicle stems and glumes. The lemma/palea and filling/mature grain contained demonstrable, but substantially lower amounts of the avenanthramides. Avenanthramides were also detected in the phloem sap, indicating source to sink transport of these metabolites following BTH treatment. Interestingly, only in the leaf tissue, developing seeds and glumes is the expression of HHT mRNA significantly up-regulated relative to the untreated controls. In the roots both treated and untreated plants demonstrated substantial levels of HHT activity however, only the treated roots contained high (> 400 mg/kg) concentrations of avenanthramides, suggesting source to sink transport in this tissue.