Seasonal variation of phenolic compounds in Strawberry tree (Arbutus unedo L.) leaves and inhibitory potential on Phytophthora cinnamomi

Abstract

Key message

This is the first report of the anti-oomycete activity of A. unedo leaves extract against Phytophthora cinnamomi. The extract was characterized by HPLC–MS and different chemotypes identified.

Abstract

Arbutus unedo L. is a small perennial Ericaceae tree well adapted to the environmental conditions of its Mediterranean distribution area. It produces a wide range of secondary metabolites with bioactive properties and applications for food, cosmetic and pharmaceutical industries. Due to the antifungal activity of some secondary metabolites they can play a key role in plant defense. In particular, arbutin and hydroquinone, two phenolic compounds typical of the Ericaceae species, might be especially important in plant defense against pathogens. Thus, in this work we (i) established a phenolic fingerprint of wild strawberry tree leaves, (ii) evaluated the effect of genotype and seasonal variation on phenolic metabolite composition, (iii) quantified arbutin and hydroquinone, and (iv) tested the antifungal effect of arbutin, hydroquinone and a methanol extract of A. unedo leaves against common strawberry tree pathogens. A total of 54 compounds were assigned by HPLC–PDA–ESI/MSⁿ, namely, gallic acid derivatives, hydrolysable and condensed tannins, flavonoids, arbutin, hydroquinone and their derivatives, and significant differences on relative abundance of the phenolic compounds between individuals were verified. Hydroquinone and its glucoside arbutin were quantified and its antifungal and anti-oomycete effect against A. unedo pathogens evaluated. Arbutin concentrations ranged from 0.69 ± 0.05 to 22.97 ± 0.76 mg g⁻¹ fresh weight of leaves. Hydroquinone was only detected in a few samples and its concentration ranged from 0.069 ± 0.004 to 0.604 ± 0.024 mg g⁻¹ fresh weight. Both arbutin and hydroquinone induced a reduction in growth of Glomerella cingulata and both compounds were highly effective against Phytophthora cinnamomi, with a reduction in growth close to 80% at the higher arbutin concentration tested. Microplate tests showed that a methanol extract of A. unedo leaves displayed an anti-oomycete activity with MIC (minimum inhibitory concentration) against P. cinnamomi at an arbutin equivalent concentration of 0.625 mg mL⁻¹, and was more active against P. cinnamomi than the arbutin standard (MIC = 12 mg mL⁻¹), which suggests that a synergic effect occurs in the presence of other metabolites existing in the extract. Moreover, extract containing an arbutin equivalent concentration of 2.5 mg mL⁻¹ was lethal for this pathogen. Arbutin, their derivatives and/or other secondary metabolites play an important role in A. unedo defense mechanisms against biotic stress, opening the way to biotechnological applications, namely, to use A. unedo leaf extracts as a natural phytosanitary agent. A genotype with the ability to produce high amounts of arbutin was also identified. The implications of these results on agroforestry management are discussed, as well as its relevance on plant phenotyping and selection for future breeding assays.