The role of the barley testa layer and its flavonoid content in resistance to Fusarium infections

Developing testa layers of the barley proanthocyanidin-free mutants ant13-152, ant17-148, an118-159, ant19-109, ant22-1508, ant25-264, ant26-485, ant27-489, ant28-484 and ant29-2110 and their mother varieties, were analysed for accumulation of proanthocyanidins and their flavonoid precursors. In vitro infection of developing barley caryopses of wild type and mutants with Fusarium poae, F. culmorum and F. graminearum revealed all mutants except ant 18-159 to be more sensitive to Fusarium attack than wildtype. Mutant ant18-159 showed extreme resistance. Histological investigations of the infection process revealed that the hyphae were unable to penetrate the testa of this mutant. The testa layer of ant18-159 accumulates small amounts of dihydroquercetin as a result of nonsense mutations in the structural gene for dihydroflavonol reductase. Authentic dihydroquercetin and an autographic assay proves this flavonoid to be a strong inhibitor of Fusarium growth and macrospore formation. Mutant ant17-148, which accumulates the flavone chrysoeriol as a consequence of a mutation in the step catalysed by flavanone 3-hydroxylase, demonstrates that this flavone is not an inhibitor of Fusarium growth in vivo. Comparison of flavonoid standards and monomers and polymers from other plants reveal monomeric flavonoids generally to be potent inhibitors of Fusarium.