Accumulation of a putative guanidine compound in relation to other early defence reactions in epidermal cells of barley and wheat exhibiting resistance to Erysiphe graminis f.sp. hordei

The infection process was compared in interactions between Erysiphe graminis f.sp. hordei isolate A6 and two near-isogenic barley lines (the susceptible line P-01 and the resistant line P-02), and the non-host wheat. A low frequency of successful infections, measured as the percentage of germinated conidia forming primary haustoria, was detected in both susceptible (7·5%), and resistant (73%) barley isolines and wheat plants (223%). In all three types of plants, papilla formation around the sites of attempted penetration seems to be a general resistance factor and thus not the determining step for distinguishing between race-specific and non-host resistance. In wheat plants, further resistance was manifested early after penetration and characterized by hypersensitively reacting (HR) single cells, no or limited growth of elongating secondary hyphae and no visible sign of infection. The race-specific resistance of barley isoline P-02 occurred later after penetration, and was characterized by plant tissue necrosis and no or weak fungal sporulation. In the susceptible barley isoline P-01, successful infections resulted in heavily sporulating powdery mildew colonies. Putative guanidine compounds, indicated by the Sakaguchi test, occurred in papillae 4–5 h after inoculation and subsequently in HR cells too, and are apparently associated with the very early resistance responses in barley and wheat. The highly basic guanidine compounds, not identified previously in plant defence responses, were found to be bound to the cell wall, and referred to as “basic staining material”. Accumulation of polyphenolic substances and callose were demonstrated in papillae and HR cells of both barley and wheat. Lignification occurred in wheat, but did not occur or did so only minimally in barley.