An array of Zymoseptoria tritici effectors suppress plant immune responses

Plant-pathogenic fungi can secrete proteins (effectors) with a variety of host- and host-environment-manipulating activities. Studying these effectors can enhance our understanding about how these destructive organisms are able to colonize and induce disease symptoms, without the plant immune system first inhibiting the pathogen’s development and spread. One such fungal pathogen is Zymoseptoria tritici, which is responsible for a devastating disease in wheat, known as septoria tritici blotch (STB). Researchers have identified multiple effectors that are expressed by Z. tritici during infection that can actively suppress the plant immune system.

Like animals, plants have immune systems that monitor for invading pathogens. The plant immune system comprises of extracellular and intracellular receptors that can recognize specific molecules associated with pathogens (such as components of a microorganism’s cell-wall). Upon sensing the presence of a pathogen, the receptors signal for an immune response to be initiated. This response can include the secretion of antimicrobial compounds and/or additional signalling molecules to further enhance the activation of immunity. Accordingly, pathogens need to evade these host defences. Evasion can be achieved by secretion of small proteins (effectors). These effectors can directly target plant proteins that are involved in signalling immune responses. Dr Thynne and colleagues at the University of Kiel, Germany, and the University of Birmingham, UK, undertook a collaborative project to identify immune-suppressing effectors from the wheat pathogen, Zymoseptoria tritici. 

Z. tritici is the causal agent of an economically significant wheat disease, septoria tritici blotch (STB). STB is identifiable by characteristic necrotic lesions on infected wheat leaves. However, the story of Z. tritici’s infection starts approximately two weeks before STB symptoms are observable. During this time, the pathogen spreadsasymptomatically throughout the host, without triggering a significant immune response. It is assumed that effectors are used during this growth phase to suppress immune responses, but few Z. tritici effectors have previously been characterized.

To address this lack of knowledge, the teams in Kiel and Birmingham used the bacterium, Agrobacterium tumefaciens, to inject effector genes into a model plant species, Nicotiana benthamiana, to allow these plants to heterologously produce these effector proteins. These effector-expressing leaves were tested for their ability to respond to immune-triggering molecules, such as bacterial flagellin protein or fungal cell-wall chitin. Via this method, the authors identified multiple effectors that were able to suppress plant immune responses. This work is an important first step towards understanding the molecular interactions between Z. tritici and wheat.

TITLE IMAGE: We screened two independent libraries of Zymoseptoria tritici effectors, via Agrobacterium-mediated, transient expression, in Nicotiana benthamiana and identified effectors with immune-suppressing activity.