Effects of inhibiting phenolic biosynthesis on penetration resistance of barley isolines containing seven powdery mildew resistance genes or alleles

Barley leaf epidermal cells actively resist penetration by germinated conidia of the powdery mildew fungus Blumeria graminis f. sp. hordei (Bgh). Penetration resistance to the appropriate forma specialis (f. sp.) of Bg is sensitive to inhibition of phenylpropanoid biosynthesis but its genetic basis relative to known race-specific powdery mildew resistance genes (R-genes) and alleles is confusing. In this study, the effects of phenylpropanoid inhibition on penetration resistance in near-isogenic barley lines with genetically defined R-genes or alleles were investigated. Seven members of the Pallas (P) near-isogenic barley set were used. Phenylpropanoid biosynthesis inhibition was accomplished using α-Aminooxy-β-phenylpropionic acid (AOPP), an inhibitor of phenylalanine ammonia lyase (PAL), and ([2-hydroxyphenyl amino] sulfinyl) acetic acid, 1,1-dimethyl ester (OH-PAS) an inhibitor of cinnamyl-alcohol dehydrogenase. Phenylpropanoid inhibition decreased localized autofluorescence at Bgh germ tube contact sites and significantly (P < 0.05) increased fungal penetration. This was true of the parent Pallas and the near-isogenic lines containing Mla1 (P01), Mla12 (P10), Mla13 (P11), Mlg (P21), Mlk (P17) and Mlp (P19) R-genes. These R-genes had no apparent effect on phenylpropanoid sensitivity of penetration resistance. However, in P22, with the race non-specific recessive allele mlo5, penetration resistance was unaffected by either AOPP or OH-PAS treatments, even though both inhibitors reduced the frequency and intensity of localized autofluorescence at fungal germ tube contact sites. The sensitivity of penetration resistance to the phenylpropanoid inhibitors was shared by all isolines, except P22, and appeared to be a property of the Pallas genetic background rather than being associated with race-specific R-genes or their alleles. Thus mlo5 penetration resistance shares a physiological phenomena i.e. insensitivity to phenylpropanoid inhibition, with "non-host" penetration resistance exhibited by barley, wheat and oat when they are attacked by inappropriate formae speciales (IT. spp.) of Bg.