Wheat Defence Against Septoria: Two Genes in the Front Line

Wheat is the most important UK crop with an annual value of about £1.2 Bn.
One of the most economically important diseases of wheat is Septoria leaf blotch, also known as Septoria tritici blotch (STB). The disease is caused by the fungus Mycosphaerella graminicola (Mg) and it is a major threat to crop yields in the UK and worldwide.

Rothamsted researchers have previously identified a fungal gene that is critical in evading wheat immune responses early during disease establishment.

Now, using modern biotechnology methods two wheat genes have been identified whose functions are to activate the wheat defence response. This finding can pave the road for developing molecular approaches to combat the disease in the future. The study has been published in the journal Molecular Plant-Microbe Interactions.

When pathogens attack wheat plant leaves they release signals that the plants have evolved to recognise and subsequently initiate a response within the leaf cells to protect themselves against the pathogen. However, pathogens are successful in evading the immune response of the host plant because they have also evolved other signals that are able to suppress the first layer of plant defence, often making themselves "invisible". The most commonly known and studied fungal signal that both plants and animals can recognise is chitin, which is a major component of fungal cell walls. In some plants, like Arabidopsis, just one gene that codes for the protein Chitin Elicitor Receptor Kinase 1 (CERK1) is sufficient for recognition of fungal chitin and initiation of defence responses. In other plants, e.g. rice, not only CERK1 but also another gene encoding a different protein, Chitin Elicitor Binding Protein (CEBiP), are required. Despite the fact that wheat is a major crop in the UK and STB a highly prevalent disease, very little was known about the mechanism that wheat may have evolved to recognise the invading fungus. This study demonstrates that wheat is more like rice, having a two gene system for recognition of fungal chitin and elicitation of the immune response. Moreover, these genes are capable of conferring resistance against STB in the absence of the interfering fungal gene.

Dr Kostya Kanyuka, lead researcher at Rothamsted said: "We are very excited about the findings of this study. To identify the exact role of the two candidate wheat genes we had to temporarily inhibit their function (i.e. silence) and investigate whether the pathogens can be successful or not in causing disease in the silenced plants. Virus-induced gene silencing (VIGS) is a powerful method used in plant science for inhibiting plant gene function for a short period of time. In this study we demonstrate that gene silencing using this method can also have long lasting effects, thus allowing the study of plant-pathogen interactions that have a long symptomless infection phase, like Mycosphaerella graminicola in wheat".

Professor Kim Hammond-Kosack of Rothamsted Research said: "There is a long symptomless infection phase of between 7 to 14 days, which is followed by rapid deterioration of the leaf tissue. This life cycle of the disease makes it difficult to identify and apply curative control methods before it is too late for the crop. Having identified the molecules that are involved in this interaction in wheat we can now think of different ways that we can develop to detect the presence of the pathogen and to stop symptoms arising before the effect of the disease on crop performance, final grain yield and final grain quality is too costly for the farmers".

Dr Jason Rudd of Rothamsted said: "This work has identified two genes that are already present in wheat which are perfectly able to provide resistance against STB. The remaining problem, and the reason why they currently don't do this in the field, resides in the fact that the fungus contains a single gene that prevents the two wheat genes from functioning. On this basis it is extraordinary that only three genes in total (two from wheat and one from the fungus) can decide the outcome of the interaction. Their identification opens the way to future biotechnological approaches that could be used to either enhance (for wheat genes) or inhibit (for the pathogen gene) their functions to favour the disease-free plant."

In addition, Professor Kim Hammond-Kosack of Rothamsted Research said: "A closely related fungus Mycosphaerella fijiensis causes the globally important Black Sigatoka disease that can devastate banana plantations in just a few months. The similarity between the wheat and the banana pathogen's mode of leaf infection suggests that this new knowledge on wheat defence could have potential application in the protection of banana crops".