Effects of PAL-specific inhibition on suppression of activated defence and quantitative susceptibility of oats to Erysiphe graminis

Abstract

Conidia of Erysiphe graminis DC f.sp. avenae ex. Merat were applied at various densities (low = approximately 30 mm −2, medium = approximately 100 mm −2, high = approximately 300–400 mm −2) to first leaves of spring oat ( Avena sativa) cvs Selma and Maldwyn. As previously published, high inoculum density results in activated (enhanced) host cell defence to fungal penetration, and this correlates with elevation in the frequency and intensity of host cell autofluorescent responses. In the current study, leaves were infused with either water (control treatment) or the phenylalanine ammonia lyase (PAL) inhibitors α-aminooxy-β-phenylpropionic acid (AOPP) or α-aminooxy acetic acid (AOA) at concentrations known to be inhibitory to PAL from oats. Results confirm and extend previous observations showing that these PAL inhibitors suppress localized autofluorescent host cell responses to fungal germ tube contact and increase susceptibility to penetration (haustorium formation) from appressoria. The PAL inhibitors, particularly AOPP, suppressed activated defence due to inoculum density and decreased the frequency and intensity of host cell autofluorescence responses. The data support the hypotheses that the activated defence is related to autofluorogenic host responses, and that the autofluorogens are phenolic compounds. Involvement of phenolic compound metabolism in activated defence was shown by assays of the specific activity of PAL from control cv. Selma leaves inoculated with low and high densities of conidia. Compared to uninoculated controls, the specific activity of PAL was enhanced at 6 h after inoculation (when responses to the primary germ tubes of E. graminis were evident), but there was little difference between low and high density inoculations at this time. At 18 h after inoculation (when responses to fungal appressoria were evident), the specific activity of PAL was substantially greater from high than from low inoculum density inoculations. In a coincidental observation, brown colouration (suggesting phenol oxidation) visible in many epidermal cell papillae associated with appressorial attack in control leaves, was not present in most papillae from AOPP-treated leaves.