Background

Fusarium graminearum (previously also called Gibberella zeae) is a destructive pathogen that upon infection is responsible for causing Fusarium head blight (FHB) and seedling blight on cereal crops, as well as stalk and ear rot on maize (Dal Bello et al., 2002; Bai and Shaner, 2004; Kazan et al., 2012). Extensive molecular and genetic studies have been performed to investigate the interaction between F. graminearum and wheat. Given the availability of an efficient genetic transformation system and the well annotated genome, hundreds of F. graminearum genes have been investigated for their roles in vegetative growth, sexual development, secondary metabolism, stress responses and even virulence on host (Jia and Tang, 2015). However, only a few fungal effectors (e.g., FGL1 and deoxynivalenol) and host resistance genes (e.g., wheat Fhb1) have been identified (Proctor et al., 1995; Blümke et al., 2014; Rawat et al., 2016).

F. graminearum has been reported to lack pathogen-specialized patterns that typically induce gene-for-gene-mediated resistance in the host (van Eeuwijk et al., 1995). Furthermore, gene redundancy and functional complementation make assigning definitive virulence roles to pathogen genes achallenge. In addition, the traditional wheat head infection assay is limited due to seasonal, temporal and spatial factors. The distinct structures (rachis, paleas, lemmas, caryopses and glumes) and diverse features of wheat florets also make it difficult to track the infection progress of F. graminearum.

Previously, we used a modified wheat coleoptile infection assay and microscopic inspection to study F. graminearum infection inside host tissue (Zhang et al., 2012). Unlike the wheat head infection assay, there are few temporal and spatial constraints for the seedling infection system. The wheat coleoptile infection assay is performed in a growth chamber that can hold up to two hundred 24-well plates, which means more than 100 genes can be evaluated for their roles in virulence (three independent transgenic lines for each tested gene, and at least twelve seedlings for inoculation of each fungal strain). The time required to complete the assay is short: ten days for seed germination (Figure 1), inoculation (Figure 2) and examination of lesion size (Figure 3). The structure of wheat coleoptile is simple: the annular coleoptile comprises similar cells and two vascular bundles (Figure 2D) and is easy to inspect microscopically (Zhang et al., 2012). Seven genes were identified required for full virulence of F. graminearum on wheat coleoptile, and several of which were also required for wheat head infection (Zhang et al., 2012) and even maize stalk infection (Zhang et al., 2016).