Abstract
The discovery of barley Mlo demonstrated that filamentous pathogens rely on plant genes to achieve entry and lifecycle completion in barley leaves. While having a dramatic effect on foliar pathogens, it is unclear whether overlapping or distinct mechanisms affect filamentous pathogen infection of roots. To remove the bias connected with using different pathogens to understand colonization mechanisms in different tissues, we have utilized the aggressive hemibiotrophic oomycete pathogen Phytophthora palmivora. P. palmivora colonizes root as well as leaf tissues of barley (Hordeum vulgare). The infection is characterized by a transient biotrophy phase with formation of haustoria. Barley accessions varied in degree of susceptibility, with some accessions fully resistant to leaf infection. Notably, there was no overall correlation between degree of susceptibility in roots compared with leaves, suggesting that variation in different genes influences host susceptibility above and below ground. In addition, a developmental gradient influenced infection, with more extensive colonization observed in mature leaf sectors. The mlo5 mutation attenuates P. palmivora infection but only in young leaf tissues. The barley-P. palmivora interaction represents a simple system to identify and compare genetic components governing quantitative colonization in diverse barley tissue types.
Highlights
The discovery of barley Mlo demonstrated that filamentous pathogens rely on plant genes to achieve entry and lifecycle completion in barley leaves
Important monocot crops such as barley and wheat are colonized by fungal leaf pathogens, including Blumeria spp. and Puccinia spp. as well as Zymoseptoria tritici and the root-killing Gaeumannomyces graminis
Assessment of P. palmivora biomass relative to barley biomass by quantitative reverse transcription-polymerase chain reaction of PpEF1a showed a significant increase in biomass from 48 h postinoculation onward (Fig. 2A), which correlated with increased root-browning symptoms (Fig. 1A; Supplementary Fig. S2B)
Summary
The discovery of barley Mlo demonstrated that filamentous pathogens rely on plant genes to achieve entry and lifecycle completion in barley leaves. Removal of pathogen-essential plant genes (termed susceptibility genes) can provide long-lasting resistance (van Schie and Takken 2014) This has been shown through mutation of MLO, a member of a seven–transmembrane protein family, that provides barley with resistance to all races of powdery mildew fungus and is essential for wheat powdery mildew (Buschges et al 1997; Elliott et al 2002; Wang et al 2014). Besides its role in the barley leaf epidermis, Mlo plays a role in root cortex colonization by mycorrhizal fungi (Ruiz-Lozano et al 1999) Often it is unclear whether an identified loss-of-susceptibility to a pathogen in leaves would be effective in roots. By contrast to obligate biotrophic fungal pathogens, it is possible to cultivate Phytophthora on nutrient agar and to transform it with fluorescent proteins (Huitema et al 2011; Rey et al 2015) making it an attractive system for laboratory studies
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