Secondary metabolites expressed in the fruits of Rubus glaucus Benth grown in Colombia as a defense mechanism against Peronospora sparsa

Abstract

In Colombia, Rubus glaucus Benth is of great economic importance; however, this crop is severely affected by the Peronospora fungus, which causes downy mildew disease. In the plant-pathogen interaction, the pathogen causes biotic stress conditions, for which plants have developed different defense mechanisms, where the increase in the biosynthesis of certain secondary metabolites is highlighted. The objective of this research was to establish the taxonomic identity of Peronospora species that affect blackberry crops, to evaluate antioxidant capacity and total phenol content, and to identify some of the secondary metabolites as indicators of the response to biotic stress. The taxonomic identification of the fungus Peronospora sp. was carried out by means of amplification by polymerase chain reaction (PCR). The antioxidant activity was analyzed by the 2,2-diphenyl-1-picrylhydrazyl (DPPH), 2,2′-azino-bis-(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS) and ferric ion reducing antioxidant power (FRAP) methods, and the total phenol content was analyzed by the Folin-Ciocalteu method. In addition, GC–MS analysis was performed for the identification of phytosterols, and UHPLC‒MSMS/MS was used for the identification of phenolic compounds. P. sparsa was confirmed as the cause of the disease in this crop. Furthermore, in the R. glaucus Benth fruits affected by this microorganism, there was a significant increase in the antioxidant capacity with values ranging from 53.975 ± 1.312 - 74.758 ± 0.058 µmol TE g − 1 fresh sample by the DPPH method; 15.626 ± 0.838 - 19.116 ± 0.086 µmol TE g − 1 fresh sample by the ABTS method and 61.050 ± 1.274 - 124.271 ± 1.148 mmol Trolox g − 1 fresh sample by the FRAP method. For healthy fruits, presented values between 38.475 ± 0.295 - 50.429 ± 0.058 µmol TE g − 1 fresh sample by DPPH method; 14.185 ± 0. 112 - 17.984 ± 0.031 µmol TE g-1 fresh sample by ABTS method and 56.837 ± 0.480 - 85.559 ± 0.411 mmol Trolox g-1 fresh sample by FRAP method. The content of phenolic compounds in affected fruits showed values between 8.322 ± 0.879 and 13.372 ± 0.18 mg g − 1 of sample, while for healthy fruits a lower concentration with values between 7.88 ± 0.172 and 10.519 ± 0.071 mg g − 1 of sample.A higher presence of β-sitosterol and campesterol was identified. Phenolic-type compounds were tentatively identified, including derivatives of phenolic acids such as ellagic acid and gallic acid, as well as tannins and flavonoids. Finally, this study allows us to understand the increase in some secondary metabolites in blackberry fruits as a defense mechanism against the pathogen Peronospora sparsa that affects this crop. On the other hand, it is necessary to extend the study to other types of metabolites associated with this plant-pathogen interaction.