IN BARLEY LEAF-CELLS, JASMONATES DO NOT ACT AS A SIGNAL DURING COMPATIBLE OR INCOMPATIBLE INTERACTIONS WITH THE POWDERY MILDEW FUNGUS (ERYSIPHE-GRAMINIS F-SP HORDEI)

We have studied a possible function of jasmonates as mediators in the host-pathogen interaction of barley (Hordeum vulgare L.) with the powd ery mildew fungus Egh (Erysiphe graminis f. sp. hordei). Previous find ings from whole-leaf extracts demonstrated that (i) extracts from infe cted barley leaves did not contain enhanced levels of jasmonates, (ii) transcripts of jasmonate-inducible genes were not expressed upon infe ction, and (iii) exogenous application of jasmonates did not induce re sistance to Egh (Kogel et al., 1995). Nevertheless, the question arise s whether or not jasmonates are involved in the interaction of barley with the powdery mildew fungus at the local site of infection. Using a n immunocytological approach the analysis of leaf cross-sections from a susceptible barley cultivar and its near-isogenic mlo(5)-resistant l ine revealed no accumulation of JIP-23, the most abundant jasmonate in ducible protein, neither in epidermal cells attacked by the pathogen n or in adjacent mesophyll cells. As a positive control, cross-sections from methyl jasmonate-treated leaf segments showed a strong signal for JIP-23 accumulation. Because the presence of the jasmonate-inducible protein is highly indicative for an already low threshold level of end ogenous jasmonate (Lehmann et al., 1995), the lack of JIP-23 accumulat ion at the sites of attempted fungal infection clearly demonstrates th e absence of enhanced levels of jasmonates. This excludes even a local rise of jasmonate confined to those single cells penetrated (Mlo geno type) or attacked (mlo(5) genotype) by the fungus.