Abstract
Fusarium graminearum is one of the primary causal agents of Fusarium head blight (FHB) on wheat and barley. FHB reduces grain yield and contaminates grain with various mycotoxins, including deoxynivalenol (DON). DON acts as a virulence factor to promote the fungus passing the rachis node and spreading throughout the head of wheat but not barley. Reactive oxygen species (ROS) are one of the earliest defense responses during plant and pathogen interactions. However, the complex roles of ROS during FHB development remain unclear. We investigated immune responses in wheat triggered by chitin, a major component of fungal cell walls. Although no ROS burst was detected in chitin-treated wheat leaves from eight tested varieties, a robust ROS peak was triggered by chitin in tested barley leaves. Interestingly, ROS were induced by chitin in wheat rachises and rachis nodes, which are critical barriers for FHB spread in wheat. We demonstrated that ROS were induced in wheat rachis nodes from both FHB susceptible and resistant wheat varieties. Further, we showed different defense gene expression patterns in rachis nodes and wheat heads treated with chitin, and wheat heads inoculated with F. graminearum. Our study showed the tissue-specific immune responses induced by chitin in wheat, which may play an important role during F. graminearum infection.
Highlights
Plants have evolved multi-layered immune responses toward microbial pathogen attacks
No reactive oxygen species (ROS) burst was observed in wheat leaves from four Fusarium head blight (FHB) moderately resistant or four susceptible varieties with chitin treatment (Figure 1A)
TaPR1 was moderately induced in chitin-treated wheat heads (6-fold) and F. graminearum infected heads (12-fold), but not in chitin-treated rachis nodes. These results showed different defense gene expression patterns in rachis nodes treated with chitin, wheat heads treated with chitin, and wheat heads during F. graminearum infection
Summary
Plants have evolved multi-layered immune responses toward microbial pathogen attacks. The perception of pathogen (microbe or damage)-associated molecular patterns (PAMPs, MAMPs, or DAMPs) by membrane-bound pattern recognition receptors (PRRs) leads to PAMPs−, MAMPs−, or DAMPs-triggered immunity (PTI, MTI, or DTI), including calcium ion (Ca2+) influx, reactive oxygen species (ROS) burst, and defense gene activation (Bigeard et al, 2015). Chitin is a long-chain polymer of N-acetylglucosamine, an amide derivative of glucose. It is a primary component of cell walls in fungi, the exoskeletons of arthropods, and the shells of crab and shrimp. In addition to the direct toxic effects on pathogens, ROS function as cellular signaling molecules to trigger plant defense responses, such as cell wall strengthening, hormone synthesis, and programmed cell death. Due to the critical role of ROS during plant and pathogen interactions, effectors secreted from many pathogens target multiple steps in the ROS signaling pathway to promote infection and disease (Jwa and Hwang, 2017)
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