Abstract
Black dot is a blemish disease of potato tubers caused by the phytopathogenic fungus Colletotrichum coccodes. Qualitative resistance (monogenic) that leads to the hypersensitive response has not been reported against black dot, but commercial potato cultivars show different susceptibility levels to the disease, indicating that quantitative resistance (polygenic) mechanisms against this pathogen exist. Cytological studies are essential to decipher pathogen colonization of the plant tissue, and untargeted metabolomics has been shown effective in highlighting resistance-related metabolites in quantitative resistance. In this study, we used five commercial potato cultivars with different susceptibility levels to black dot, and studied the structural and biochemical aspects that correlate with resistance to black dot using cytological and untargeted metabolomics methods. The cytological approach using semithin sections of potato tuber periderm revealed that C. coccodes colonizes the tuber periderm, but does not penetrate in cortical cells. Furthermore, skin thickness did not correlate with disease susceptibility, indicating that other factors influence quantitative resistance to black dot. Furthermore, suberin amounts did not correlate with black dot severity, and suberin composition was similar between the five potato cultivars studied. On the other hand, the untargeted metabolomics approach allowed highlighting biomarkers of infection, as well as constitutive and induced resistance-related metabolites. Hydroxycinnamic acids, hydroxycinnamic acid amides and steroidal saponins were found to be biomarkers of resistance under control conditions, while hydroxycoumarins were found to be specifically induced in the resistant cultivars. Notably, some of these biomarkers showed antifungal activity in vitro against C. coccodes. Altogether, our results show that quantitative resistance of potatoes to black dot involves structural and biochemical mechanisms, including the production of specialized metabolites with antifungal properties.
Highlights
IntroductionHughes (Wallroth, 1833; Hughes, 1958) is a ubiquitous phytopathogenic fungus with multiple host plants, including weeds and several crops
It is possible that a higher inoculum than that found in the field was applied to the greenhouse experiment, since soil inoculum correlates with disease severity (Lees et al, 2010) inoculum concentration correlation with disease severity is genotype dependent (Alonso-Villaverde et al, 2011), and both the pathogen strain and the plant genotype influence host resistance in other phytopathogenic interactions (Ors et al, 2018; Samain et al, 2019)
These results suggest that a high disease pressure as applied in the greenhouse might negatively influence the resistance of Cheyenne and Lady Christl to black dot
Summary
Hughes (Wallroth, 1833; Hughes, 1958) is a ubiquitous phytopathogenic fungus with multiple host plants, including weeds and several crops It is the responsible for anthracnose in peppers, tomatoes, and onions, and causes black dot in potatoes (Solanum tuberosum L.). Black dot can affect the yield of potato production (Tsror (Lahkim) et al, 1999), and contamination of tubers with C. coccodes results in lesions on the skin of potato tubers and water losses during storage (Lees and Hilton, 2003). Qualitative resistance to black dot has not been reported in potatoes, but different susceptibility levels have been observed among commercially available potato cultivars (Read, 1991; Tsror (Lahkim) et al, 1999; Brierley et al, 2015). The use of existing cultivars with resistance to black dot is of interest, because it does not require chemical fungicide application
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