Abstract
Indole is a volatile compound and emitted from plants challenged by insect infestation or mechanic wounding. It has been shown to prime defense against herbivory. Here we identified that indole induced defense either directly or as a priming agent against necrotrophic pathogens Fusarium graminearum and F. moniliforme in maize and Magnaporthe oryzae in rice. With indole pretreatment, smaller lesions were developed in infected leaves, as well as less fungal growth. Indole induced H2O2 burst in the priming stage like other priming substances did. Such priming relied on mitogen-activated protein kinase (MAPK) cascade, which potentially activated downstream defense signaling. In addition, indole priming resulted in earlier and stronger defensive gene expression upon pathogen infection, including genes of jasmonate and phytoalexin biosynthesis, pathogenesis-related proteins (PRs) and anti-oxidant enzymes, which enhanced plant resistance. Meanwhile, H2O2 was also identified as the priming agent to induce plant defense. Taken together, indole exhibited priming function not only against herbivory but also necrotrophic pathogens. The common emission of indole in plants suggests that it plays important roles as the universal and endogenous priming substance in plant defense.
Highlights
Plant activates several layers of defense systems in response to pathogen infection, including defensive phytohormones, reactive oxygen species (ROS), specialized metabolites like phytoalexins, physical barriers like callose and lignin, and defense related gene expression
In systemic acquired resistance (SAR), ROS are accumulated at infecting sites rapidly upon biotrophic pathogen infection to kill local plant cells and prevent further expansion of pathogens, which is called hypersensitive reaction (HR)
Indole was reported to prime herbivory resistance through inducing jasmonic acid (JA), abscisic acid (ABA) biosynthesis and volatile terpene emission [22]
Summary
Plant activates several layers of defense systems in response to pathogen infection, including defensive phytohormones, reactive oxygen species (ROS), specialized metabolites like phytoalexins, physical barriers like callose and lignin, and defense related gene expression. All these defense responses fall into two categories, systemic acquired resistance (SAR) and induced systemic resistance (ISR) [1]. Necrotrophic fungi stimulate plant ROS accumulation to promote plant cell death, which is beneficial to fungal infection and growth In such case, ROS scavenging system is activated to eliminate ROS at infection sites to prevent necrotrophic pathogen infection
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