A Snake in the Grass: The Serpin gene family in wheat
To combat attack by fungal pathogens, wheat (Triticum aestivum) has evolved a repertoire of defence proteins capable of halting or suppressing fungal effectors that augment fungal virulence. Serine protease inhibitors (serpins) are the largest and most widely found family of protease inhibitors, and are involved in plant/fungus interactions, both endophytic and pathogenic. In the latter scenario, serine proteases are secreted by invading pathogens as fungal effectors, to which the plant responses with serpins to inhibit the proteolytic activity of the fungal effectors. There is a wealth of evidence to state that serpin genes are important for plant defence against pathogens in soybean, Arabidopsis thaliana, Rice, and Barley but unfortunately these genes are understudied in wheat. Two of the most important fungal diseases of wheat worldwide are Septoria tritici blotch, caused by Zymoseptoria tritici, and Fusarium head blight, caused by Fusarium graminearum. In the case of Z. tritici, the fungus displays high protease activity during early infection, and up to 11 days post infection, and wheat serpin genes have been found to inhibit protease activity in the apoplast of infected wheat leaves. In the case of Fusarium infection, protease activity has not been well studied but another Fusarium species, F. culmorum (itself a pathogen of wheat), has well-studied protease activity linked to grain damage and yield loss. Five serpin genes have been found to be upregulated in a resistant wheat cultivar, in response to Fusarium infection. Either directly implicated in pathogenicity or loss of grain yield and quality, and regardless of whether the fungus is secreting protease enzymes to support its virulence, trophic adaptation or pathogenicity, the consequences of protease activity by invading pathogens are dire. We present a review on serpin activity in wheat, and their involvement in resistance to fungal diseases, with the aim of shining a spotlight on this potential reservoir of important regulators of disease. Subsequently, we show that transiently silencing a wheat serpin gene causes a reduction in resistance to Septoria tritici blotch, and that this gene is interacting with small secreted effector proteins from the invading pathogenic fungus; Zymoseptoria tritici.