Metabolism of avenanthramide phytoalexins in oats

Abstract

SummaryOat leaves produce phytoalexins, avenanthramides, in response to infection by pathogens or treatment with elicitors. The metabolism of avenanthramides was investigated using low molecular weight, partially deacetylated chitin as an elicitor. When oat leaf segments are floated on the elicitor solution, avenanthramides accumulate in the solution. The transfer of elicited oat leaves to solutions containing stable-isotope-labeled avenanthramides resulted in a rapid decrease in the labeled avenanthramides, suggesting the metabolism of avenanthramides. The rate of decrease was enhanced by elicitor treatment, and was dependent on the species of avenanthramides, with avenanthramide B decreasing most rapidly. The rates of biosynthesis and metabolism of avenanthramides A and B were measured using a model of isotope-labeling dynamics. Avenanthramide B was found to be more actively biosynthesized and metabolized than avenanthramide A. Radiolabeled avenanthramide B was incorporated into the cell wall fraction and 99% of incorporated activity was released by alkaline treatment. Gel filtration indicated that high-molecular-weight compounds derived from avenanthramide B were released by alkaline treatment. The decrease in stable-isotopelabeled avenanthramides was suppressed by catalase, salicylhydroxamic acid, and sodium ascorbate, suggesting the involvement of peroxidase in the metabolism. Consistent with this, peroxidase activity that accepts avenanthramide B as a substrate was induced in apoplastic fractions by elicitor treatment. The appearance of multiple basic isoperoxidases was observed by activity staining with 3-amino-9-ethylcarbazole coupled with isoelectric focusing of proteins from elicitor-treated leaves. These findings suggest that accumulated avenanthramides are further metabolized in apoplasts in oat leaves by inducible isoperoxidases.

Accumulation of hydroxycinnamic acid amides induced by pathogen infection and identification of agmatine coumaroyltransferase in Arabidopsis thaliana

“…HCAAs of tyramine have been demonstrated to be a component of suberin that accumulates in mechanically wounded potato tuber tissues (Bernards et al 1995). In addition, the feeding of radiolabeled avenanthramide B indicated the incorporation of the compound into the cell wall fraction (Okazaki et al 2004). Although, in either case, these Wndings strongly suggest that HCAAs function in the defense response of plants, the establishment of mutant lines that lack HCAA production is required to unequivocally prove their physiological signiWcance.…”

Phenolic amides (avenanthramides) in oats - a review

“…AVAs are generally determined by various chromatographic methods. For the determination of the AVAs amounts, highperformance liquid chromatography (HPLC) (Bryngelsson et al 2002;Peterson et al 2002;Chen et al 2004;Mattila et al 2005;Jastrebova et al 2006;Skoglund et al 2008;Wise et al 2011;Ortiz-Robledo et al 2013;Ishihara et al 2014;Koening et al 2014), liquid chromatography-mass spectrometry (LC-MS) (Okazaki et al 2004), liquid chromatography-mass/ mass spectrometry (LC-MS/MS) (Ishihara et al 2014), and ion-exchange chromatography (Collins 1989) have been used. HPLC currently represents the most popular technique for the analysis of AVAs.…”

“…Among cereals, only oats contain AVAs (Kováčová & Malinová 2007;Moglia et al 2010;Shi et al 2014). These compounds are antipathogens (phytoalexins), which are produced by the plant in response to the exposure to pathogens such as fungus (Collins & Mullin 1988;Collins 1989;Okazaki et al 2004). The three most abundant AVAs in oats are AVA-A, AVA-B, and AVA-C, which contain hydroxyanthranilic acid and a kind of hydroxycinnamic acids like p-coumaric, caffeic, or ferulic acids, respectively (Peterson et al 2002;Okazaki et al 2004;Gani et al 2012).…”

Bioactivity of oats as it relates to cardiovascular disease

“…Chemically, avenanthramides are amides of different cinammic acids with different anthranilic acids 128 . They are distinguished based on their particular anthranilic acid component which may include anthranilic, 5-hydroxy-anthranilic, 5-hydroxy-4-methoxy-anthranilic or 4-hydroxy-anthranilic acids 129 .…”