Molecular and cellular basis of infection-related dimorphism in Zymoseptoria tritici

Lead Research Organisation: University of Exeter
Department Name: Biosciences

Abstract

Crop-infecting fungi are a major threat to the global food security. In fact, losses of wheat, rice, and maize to fungal pathogens, per year, is the same as the annual spend by US Department of Homeland Security - some 60 billion US dollars. Fungi are kept under control by treatment with anti-fungal chemicals. However, as fast growing microbes, fungi adapt to become resistant to fungicide treatments, and so we need to develop new fungicides all the time. This requires an in-depth knowledge of the invasion strategies of fungal pathogens.
In this project, we focus on the number-one wheat pathogen in Europe, Zymoseptoria tritici, the causative agent of Septoria tritici blotch in wheat. The fungus lives on debris in the field, where it grows as a small "yeast-like" structure. However, rain-splash can transport this spore onto the leaf surface. Here, the fungus undergoes a change in growth and forms an elongated string of cells, the hypha, which expands at the tip and invades the plant leaf. Published data and our preliminary results clearly demonstrate that this "dimorphic switch" is an essential of infection of the plant by the fungus. Thus, understanding the process of the yeast-to-hypha transition is important to develop novel antifungal chemistries.
Despite its importance for wheat infection, our knowledge of the requirements for dimorphic switching in Z. tritici is fragmentary. This project aims to understand the processes underlying host invasion by the fungus and to identify pathways and requirement within the fungus that control the dimorphic switching. We will undertake this work by genetic screening, identifying protein-protein interactions, live cell imaging in the plant and observing the fungal cell itself during its transition from a yeast to a hypha. Moreover, we collaborate with a company that provides a novel antifungal product and investigate the way by which fungicide inhibits infection of wheat by Z. tritici. It is important to reiterate that the dimorphic switch occurs very early in the fungal infection process, at a time when the fungi are most accessible to fungicide treatment. Disabling the process could therefore result in a new generation of "preventive" fungicides that are able to act on STB before the fungus has damaged its host plant , wheat.

Technical Summary

Many pathogenic fungi initiate pathogenic development by a dimorphic switch from a "yeast-like" stage to a tip-growing hypha to explore and enter the host. The wheat blotch fungus Zymoseptoria tritici undergoes such a dimorphic switch. In this fungus, only two factors are known to be required for morphogenic transition from "yeast-like" form to a hypha. However, it is likely that a numerous pathways cross-talking control dimorphic switching. Identifying the key component of this network promises a valuable insight into this infection-associated process.
We have undertaken a forward genetic screen and obtained 39 mutants defective in pathogenicity in wheat. Whole genome sequencing of 10 mutants identified 12 affected genes, amongst which a transcription factor was found twice. This makes this protein a very likely candidate for dimorphism-associated transcriptional regulation. Moreover, this also demonstrates that extending the screen to saturation, thereby will provide a holistic picture of the genetic requirements for dimorphism in Z. tritici. Under Objective 1, we will extend the established genetic screen to reach saturation. This will be complemented by an extensive yeast-two-hybrid approach (Objective 2), using our recently published Z. tritici hyphal-specific libraries, and a ChIP-seq analysis of the target genes of the identified transcription factor (Objective 4). Our screen, together with other preliminary data, has also provides evidence that microtubule-associated transport is essential for dimorphism in Z. tritici. We will investigate this further (Objective 3), using the wealth of cytological tools, recently published in numerous papers (special issue, FGB, Vol. 79, 2015; ed. by GS). Finally, the project will investigate the use of a novel anti-fungal compound on dimorphism and virulence of Z. tritici (Objective 5). This part is done in collaboration with a UK company, and allows direct translation of the results obtained under Objective 1, 2 and 4.

Planned Impact

This project addresses the importance of the dimorphic switch in the pathogenesis of the plant pathogenic fungus causing the most important disease of EU wheat, Septoria tritici wheat blotch. The application stems directly from our expertise in studying host pathogen interactions and from the toolkit of cellular markers we have gathered in the fungus, thanks to a recent BBSRC initiative (published in a special edition of Fungal Genetic & Biology, 2015, edited by the PI). This project extends this work to utilise these tools in combining a molecular and cellular interrogation of the processes underpinning the critical events which lead from the yeast-like state to the formation of infective hyphae. It also brings into play a new antifungal which interferes with this process.

Academia:
The research activity around fungal pathogens in humans and plants is increasing, as the awareness of the threat posed by fungi increases. In particular, research on life-cycle strategies, infection-related processes and fungal effectors represent dynamic and rapidly moving fields. Indeed, over the past decade our work has influenced and led much of this work. The importance of the dimorphic switch and interference / prevention of this change in morphology will inform scientists in both arenas of plant-fungal and animal-fungal biology.

The PI and Co-Is are well-known and are influential in the field of fungal biology. They edit various journals (Molecular Biology of the Cell, Fungal Genetics and Biology: New Phytologist: Plant Cell and Food Security); draw attention to fungal research in learned societies (BMS, BSPP and The Royal Society) and advise RCUK (BBSRC Council and committee work) and Government (policy documents and talks) and as member of the Research Advisory Board of the British Mycological Society. They are invited routinely to speak at international conferences. The PI will disseminate the outcome of this project in publications and talks, to maximise the awareness of the scientific community of the various findings.

Society: The UK government has recently invested significant amounts of funding in a new AgriTechnology strategy, aiming to develop sustainable agricultural practises. Fungi are recognised as a major threat to food security. Understanding of essential and fundamental processes in establishing fungal-induced plant disease will ultimately lead to efficient and environmentally benign strategies of pathogen management. Therefore, the outcome of this project promises to be of high societal impact.

Democratic policy is rooted in public understanding of the challenges facing society. Therefore, the PI and co-Is will use opportunities to disseminate scientific knowledge about fungal pathogenesis in a series of public presentations and MOOC as outlined in the pathways to impact document. This will be freely available in the internet.

Industry:
This project covers an area of fundamental research, pivotal to the discovery of new anti-fungal target sites. Agricultural industry is very interested in identifying new pathways to develop alternative fungal pathogen control strategies. The findings of this work will influence strategic decisions and applied research in industry.

Publications

10 25 50
publication icon
Kilaru S (2017) Fluorescent markers of various organelles in the wheat pathogen Zymoseptoria tritici. in Fungal genetics and biology : FG & B

publication icon
Steinberg G (2017) Woronin body-based sealing of septal pores. in Fungal genetics and biology : FG & B