Septoria leaf blotch is a highly damaging disease of wheat and scientists are looking for ways to manage it more effectively. Most studies have looked directly at the interaction between wheat and Septoria but, in a novel approach, scientists at Rothamsted Research, who are strategically funded by the BBSRC, have instead looked at how plant species that do not get infected by Septoria achieve resistance. Most plants are resistant to the majority of microbes, a phenomenon known as non-host resistance, or NHR. Using non-host tobacco plants and advanced molecular techniques, the scientists characterised several genes linked to protection against infection in a study just published in the journal New Phytologist.

Fungi and their potential host plants are locked in a battle the outcome of which is strongly influenced by the expression and function of their genes as they recognise and respond to each other during attempted infection. Understanding this interplay unlocks the mysteries of host and non-host plant resistance and opens them up to manipulation. Zymoseptoria tritici, the causal agent of Septoria leaf blotch, only infects wheat and no other plant species. Unlike many fungi that directly invade plant cells, this fungus is strictly extracellular (i.e. non-plant cell penetrating or apoplastic) entering the plant through stomata and remaining in between the plant cells throughout its life.

Dr Jason Rudd, a senior scientist at Rothamsted Research and co-author of this paper commented that “Using a combination of fluorescence microscopy, quantitative PCR, transient gene-expression and gene-silencing techniques we were able to show that gene expression and function in both the fungus and the non-host plant during attempted infection were surprisingly similar to those we had previously observed during successful infection of wheat by Septoria.”

The scientists found that genes for many of the secreted proteins, called “effectors”, used by Septoria to manipulate defences of its host plant wheat, were also expressed when this fungus was inoculated onto the non-host plant tobacco. Moreover, when these fungal effectors were expressed in tobacco leaves, they were recognised by the plant stimulating defence reactions.

Gene-silencing methods allowed the researchers to demonstrate that two tobacco genes in particular, encoding NbBAK1 and NbSOBIR1 proteins known to associate with a range of cell surface immune receptors, were involved in induction of plant responses following recognition of effectors secreted by the pathogen. “This interesting finding suggests that non-host plants may possess specific cell surface immune receptors recognising Septoria effectors, and this opens up avenues for the development of new methods to protect susceptible wheat plants from Septoria” said Dr Kostya Kanyuka, a senior scientist at Rothamsted Research and co-author of this paper.  

“It is exciting to find this receptor mediated widespread recognition of proteins from a wheat-infecting fungal pathogen in a taxonomically distant non-host plant species and suggests that recognition of multiple non-adapted pathogen proteins may contribute to non-host resistance”, Jason added.

Septoria tritici blotch disease is one of the most severe foliar pathogens of wheat accounting for 70 % of fungicide applications; with fungicide insensitivity on the increase, this makes effective control very difficult. Studies like this one that increase our understanding of how plants defend themselves against infection are incredibly important. This study is particularly informative because it is one of the first to consider non-host resistance to a plant disease with an extracellular necrotrophic lifestyle.