The fungus Zymoseptoria tritici causes Septoria tritici blotch (STB), a devastating foliar disease in wheat that causes dramatic losses in yield.
While the fight against STB often employs synthetic fungicides, significant energy is also being directed towards more sustainable approaches, namely the development of resistant wheat varieties1. The problem is that pathogen populations can adapt rather rapidly and thus evade resistance mechanisms, limiting the efficacy of such varieties.
Consequently, to make more strategic use of wheat resistance, it is necessary to identify and functionally characterize the genes involved.
Twenty-one major genes involved in resistance against Z. tritici—the Stb genes—have been found in wheat, although none of them had been cloned or characterized until now. The scientific community has been particularly interested in Stb6, a gene commonly found in both ancestral and modern wheat varieties. Although its resistance mechanism can be evaded, the gene's presence in wheat crops can still confer protection against some members of the fungal population.
INRA researchers, working with colleagues at the Rothamsted Research Institute (UK), recently isolated and cloned Stb6, the first time this has been accomplished. They managed the feat by drawing on recent discoveries in wheat genetics and genomics and by employing highly effective, novel methods. Found on the short arm of wheat's 3A chromosome,Stb6 encodes a 647-amino-acid-long protein, a cell wall-associated kinase (WAK). Like other WAKs involved in pathogen resistance, the Stb6-encoded WAK has an extracellular domain that controls changes to the cell wall and an intracellular domain that initiates a cascade of defense mechanisms.
The researchers then functionally characterized the gene. When Stb6 is expressed in an otherwise vulnerable wheat variety, the latter becomes resistant to STD.
Unsurprisingly, Stb6 is highly conserved in common wheat (Triticum aestivum). Just eight alternative forms of this gene (i.e., alleles) have been described from among 98 wheat accessions (farmed varieties or wild relatives of wheat that are preserved in collections). Of these eight alleles, one plays a significant role in resistance. It is present in half of the wheat varieties farmed in France and the UK. There are also several alleles involved in plant vulnerability. The resistance allele differs from one of the vulnerability-related alleles by just a single nucleotide, located in the WAK's active site. Nucleotide substitution results in a loss of activity, which seems to be the mechanistic basis for increased plant vulnerability.
The resistance-conferring allele is present in emmer wheat (Triticum turgidum dicoccon), an ancient cultivar and the ancestor of modern common wheat varieties. This fact suggests that Stb6 was already around when wheat was domesticated, which would explain its broad distribution among wheat varieties today.
Stb6 is the first resistance gene against Z. tritici to be identified and functionally validated in wheat. This research represents a major step forward for the wheat-Z. tritici pathosystem, which is of global importance. The findings will help clarify the molecular mechanisms underlying the interactions between the fungal pathogen and its plant host. This work is timely because another INRA team recently identified and functionally validated an avirulence gene in Z. tritici, AvrStb6, which encodes for a protein that is directly or indirectly recognized by Stb6. Overall, this study should help improve strategies for enhancing wheat resistance to STB.
1 Generally speaking, a host plant’s response to a given pathogen is determined by a specific resistance (R) gene. The pathogen, in turn, carries a corresponding avirulence gene (Avr), which determines the pathogen’s ability to cause disease in the host plant. Upon infection, the protein encoded by the host plant’s R gene recognizes the protein encoded by the pathogen's Avr gene, and the plant then initiates a cascade of immune defenses. This reaction limits the pathogen's development and thus protects the host against disease. This interaction is known as a gene-for-gene relationship.
Cyrille Saintenac, Wing-Sham Lee, Florence Cambon, Jason J Rudd, Robert King, William Marande, Hélène Bergès, Andy L Phillips, Cristobal Uauy, Kim E Hammond-Kosack, Thierry Langin, Kostya Kanyuka. Wheat receptor kinase-like protein Stb6 controls gene-for-gene resistance to fungal pathogen Zymoseptoria tritici Nature Genetics, 2018 doi:10.1038/s41588-018-0051-x