Mass spectrometry-based metabolomic discrimination of Cercospora leaf spot resistant and susceptible sugar beet germplasms

Abstract

A better understanding of the plant metabolites produced in response to disease infection may be useful for the development of disease-resistant crop varieties. In the present study, ultra high-performance liquid chromatography coupled to quadrupole-time-of-flight mass spectrometry (QTOF-MS) was used to identify differentially accumulated metabolites in a subset of sugar beet genotypes harbouring different levels of resistance to Cercospora leaf spot (CLS), a disease caused by the fungal pathogen Cercospora beticola. Leaves of three susceptible (S1, S2 and S3) and two resistant (R1 and R2) genotypes were subjected to QTOF-MS for metabolite profiling. A wide range of metabolites was identified in sugar beet genotypes using metabolomics. Results of Partial Least Squares-Discriminant Analysis indicated that 15 metabolites could better discriminate resistant and susceptible genotypes. A Volcano Plot analysis indicated that the flavonoid quercetin 3-O-(6″-O-p-coumaroyl)-glucoside and gibberellin A51 with the highest absolute fold change (FC = 16), were repressed in resistant samples. Among the 3 metabolites (isovitexin-7-O-xyloside, 3-demethylubiquinol-8 and apigenin 7-O-d-glucoside) showing significant up accumulation in CLS-resistant samples, the flavonoid isovitexin-7-O-xyloside (FC = 4825.634) is associated with resistance to infection with fungal species causing the disease in other crops. Although further studies are still necessary to better elucidate the mechanism of resistance, our results suggest that breeders might exclude susceptible plants based on discriminating metabolites without the need for field inoculation tests. The results also create a solid basis for metabolite-associated reverse genetics and single nucleotide polymorphism discovery based on significantly differentially accumulated metabolites, whose identification is a next strategic priority. The results obtained also underline the role of metabolic signature in CLS resistance mechanisms and provide a platform for the metabolic engineering of sugar beet with higher resistance against C. beticola pathogen.