Defence mechanisms associated with mycorrhiza-induced resistance in wheat against powdery mildew

To develop a more sustainable agriculture using alternative control strategies, mechanisms involved in the biocontrol ability of the arbuscular mycorrhizal fungus Funneliformis mosseae to protect wheat against the foliar biotrophic pathogen Blumeria graminis f. sp. tritici were investigated under controlled conditions. B. graminis infection on wheat leaves was reduced by 78% in mycorrhizal plants compared with non-mycorrhizal ones (control). Wheat roots inoculated with F. mosseae revealed a systemic resistance in leaves to B. graminis, after a 6-week co-culture. Accordingly, this resistance was associated with a significant reduction of B. graminis haustorium formation in epidermal leaf cells of mycorrhizal wheat and an accumulation of phenolic compounds and H2O2 at B. graminis penetration sites. Moreover, gene expression analysis demonstrated upregulation of genes encoding for several defence markers, such as peroxidase, phenylalanine ammonia lyase, chitinase 1 and nonexpressor of pathogenesis-related proteins 1 in mycorrhizal wheat only in the absence of the pathogen. This study showed that protection of wheat obtained against B. graminis in response to mycorrhizal inoculation by F. mosseae could be interpreted as a mycorrhiza-induced resistance (MIR). Our findings also suggest that MIR-associated mechanisms impaired the B. graminis development process and corresponded to a systemic elicitation of plant defences rather than a primed state in wheat leaves.