Existing infection with Rhynchosporium secalis compromises the ability of barley to express induced resistance

It has been suggested that if plants in the field are already induced, their ability to further enhance induced resistance might be compromised. This was examined in barley by inoculating the first two leaves with Rhynchosporium secalis prior to treatment of leaves three and four with an elicitor combination, followed by inoculation with R. secalis. The elicitor combination used consisted of acibenzolar-S-methyl, beta-aminobutyric acid, and cis-jasmone, which was shown previously to provide higher levels of disease control in barley than any of the components used individually. The elicitor combination reduced infection by R. secalis, and led to an up-regulation of PR1-b, a marker gene for systemic acquired resistance, and increased activities of the defence-related enzymes cinnamyl alcohol dehydrogenase (CAD), peroxidase (POX), and beta-1,3-glucanase. It also led to down-regulation of LOX2, a gene involved in biosynthesis of jasmonic acid. In plants where the first two leaves were inoculated with R. secalis prior to treatment of leaves three and four with elicitor, these increased defence responses did not occur, and control of R. secalis infection on leaves three and four was also reduced. These results suggest that, at least in young barley plants, prior infection with R. secalis compromises their ability to respond effectively to elicitors. The results might help to explain the relatively poor performance of induced resistance in the field, particularly in cereals, compared to plants grown under controlled conditions.