Deciphering the mechanisms of non-host resistance to Zymoseptoria tritici

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The air- and water-borne spores of plant pathogenic fungi are ubiquitous in nature and will often encounter many diverse plant species. However, plants remain immune to the majority of would be invaders. This highly effective and broad-spectrum immunity, termed non-host resistance (NHR), is thought to be under polygenic control and is therefore highly durable. Despite the paramount importance of NHR to interactions between plants and microbes, in most cases the precise mechanisms involved are poorly understood. The fungus Zymoseptoria tritici is only able to cause disease on leaves of wheat, and is the causal agent of Septoria tritici blotch (STB) disease. This disease is of global importance and currently one of the foremost threats, and economic constraints, to wheat production in the UK and Western Europe. Z. tritici does not physically penetrate plant cells during infection. Instead it colonises the intercellular space between plant cells (apoplast) and, thus, likely relies on the functions of apoplastic small secreted proteins (effectors) to manipulate plant defences and enable infection. This infection strategy is shared by many other species of plant pathogenic fungi causing globally important plant diseases. What prevents Z. tritici and fungi with similar lifestyles from causing disease on the non-host plant species they encounter remains virtually unknown. We recently discovered a dozen of Z. tritici effectors that trigger strong, defence reactions in the form of cell death, when infiltrated into Nicotiana benthamiana (tobacco), which is a non-host for Z. tritici. Most of these effectors required secretion into the apoplast to elicit cell death responses, and induction of cell death depended on the plant regulatory receptor-like kinases BAK1 and SOBIR1. Because the latter are known to be required for defence signalling following activation of cell surface pattern recognition immune receptors we hypothesised that tobacco possesses numerous receptors that can recognise different Z. tritici apoplastic effectors and initiate defence signalling. We also hypothesised that these multiple, likely distinct receptors play an important role in NHR against Z. tritici, and might be deployed for engineering disease resistance in wheat (New Phytologist 213: 7-9 [2017]; New Phytologist 213: 338-350 [2017]). The principal aim of the proposed PhD studentship project is to identify the putative immune receptor(s) that facilitate recognition of Z. tritici effectors in tobacco, functionally characterise these proteins, and assess their contribution to the NHR phenomenon. This will be done using a combination of functional genomics and biochemical approaches including Virus-induced gene silencing of cell surface receptor encoding genes (Nature Communications 9:594 [2018]) and proteomic screening to identify tobacco receptors interacting /binding to Z. tritici effectors (Proceedings National Academy Sciences USA 113:3389-94 [2016]).