The complexity of nitrogen metabolism and nitrogen-regulated gene expression in plant pathogenic fungi

Plant pathogens secrete effector molecules that contribute to the establishment of disease in their plant hosts. The identification of cellular cues that regulate effector gene expression is an important aspect of understanding the infection process. Nutritional status in the cell has been postulated to be a cue for effector gene expression. Several studies have shown the induction of the same effector genes during growth under nitrogen-starved conditions in vitro as during growth in planta, suggesting that a nitrogen-poor environment exists during colonization. As a consequence, it has been proposed that growth in nitrogen-starved media mimics the environment that a pathogen encounters during growth in planta. Although this relationship has been implied in several studies, there is little known regarding available nitrogen during colonization for fungal pathogens. This review focuses on the effect of nitrogen starvation on fungal effector gene expression and examines the role of fungal nitrogen regulatory genes to help gain an understanding of whether nitrogen availability within the host is an important factor in disease development.