Silicon induces resistance to the brown spot fungus Cochliobolus miyabeanus by preventing the pathogen from hijacking the rice ethylene pathway

Although numerous studies have shown the ability of silicon (Si) to mitigate a wide variety of abiotic and biotic stresses, relatively little is known about the underlying mechanism(s). Here, we have investigated the role of hormone defense pathways in Si-induced resistance to the rice brown spot fungus Cochliobolus miyabeanus. To delineate the involvement of multiple hormone pathways, a multidisciplinary approach was pursued, combining exogenous hormone applications, pharmacological inhibitor experiments, time-resolved hormone measurements, and bioassays with hormone-deficient and/or -insensitive mutant lines. Contrary to other types of induced resistance, we found Si-induced brown spot resistance to function independently of the classic immune hormones salicylic acid and jasmonic acid. Our data also rule out a major role of the abscisic acid (ABA) and cytokinin pathways, but suggest that Si mounts resistance to C.miyabeanus by preventing the fungus from hijacking the rice ethylene (ET) machinery. Interestingly, rather than suppressing rice ET signaling per se, Si probably interferes with the production and/or action of fungal ET. Together our findings favor a scenario whereby Si induces brown spot resistance by disarming fungal ET and argue that impairment of pathogen virulence factors is a core resistance mechanism underpinning Si-induced plant immunity.