Distribution of avenacins A-1, A-2, B-1 and B-2 in oat roots: Their fungicidal activity towards ‘take-all’ fungus

Abstract

Oat roots contain a group of four major in situ inhibitors of the ‘take-all’ fungus Gaeumannomyces graminis, avenacins A-1, A-2, B-1 and B-2: these are trisaccharide-bearing triterpenes esterified (A-1, B-1) with N-methylanthranilic acid or (A-2, B-2) benzoic acid. Tests using the more virulent var. avenae, which can attack oats as well as the more susceptible wheat, show the N-methylanthranilate esters to be considerably more fungicidal. Avenacin contents (0.22–1.0 mg/g dry weight) and composition (47–60 % A-1, 5–7 % B-1; 30–43 % A-2, 3–6 % B-2) for eleven species and varieties of oat roots (at 77 days) are recorded. Young roots of oats, Avena sativa (var. Peniarth) have a high content of A-1 (73 %) relative to A-2 (14 %) which gradually shifts to a more even distribution of A-1 (55 %) to A-2 (44 %) as the root ages. Total avenacins content of young (3 day) root tips is very high (12.8 mg/g dry wt) and can be estimated at ∼8 μg/root tip: the remainder of the young root has 5 mg/g dry wt of avenacins. The nutritional status of the var. avenae fungus is important in determining its vulnerability to avenacins. The latter provide oat roots with little defence against Fusarium attack.