Cells, Genes & Molecules

Fluorescence imaging reveals the colourful phenolic world of Parma violet

Can fluorescence microscopy be used as a rapid tool to quantitatively assess the phenolic profiles of living plant tissues?

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The Viola genus comprises over 500 species of violets and pansies whose fragrant and medicinal properties have long attracted botanists and herbalists. The Parma violet is distinguished from other violets by the presence of double chasmogame flowers, each harbouring up to 40 petals. While likely originating in Turkey, the species was first reported in the Parma region of Italy in the 16th century, before being described in a few other regions of Europe and North America in the 19th century. Despite being a valuable resource for the phytomedicine and cosmetic industries, there was no chemical analysis of this violet until the recent report that leaf extracts of this plant are a promising source of antioxidant phenolic compounds.

In their new study published in AoBP, Khatib et al. take advantage of the autofluorescence properties of phenolic compounds to gain insights on their location in situ in Parma violet (Viola alba subsp. dehnhardtii). The phenolic imprint of flower, leaf and root tissues was characterized using a dual approach that combines fluorescence imaging of living tissues with chemical analysis of pulverised plant material using ultra-high-performance liquid chromatography–high-resolution mass spectrometry.

Overall views and transverse sections of leaf blade (A–I), petal (J–P) and root (Q) of Parma violet captured using fluorescence microscopy. Image credit: Khatib et al.

In the study, strong blue to red fluorescence displayed by living tissues upon illumination was indicative of their richness in phenolic compounds. In terms of localisation, phenolic acids were found in all tissues, flavonoids were confined to aerial organs, whilst anthocyanidins were restricted to the petals. The chemical quantification of phenolics in plant extracts confirmed their tissue-specific distribution and abundance. A key finding of the study was that the spectral signatures obtained through confocal microscopy of endogenous fluorophores in living tissues and their counterpart extracts share the same fluorescence patterns, highlighting the potential of fluorescence imaging of intact organs to accurately estimate phenolic content.

From the results of their study, Khatib et al. conclude that fluorescence microscopy can be used as a rapid, non-destructive method to assess the phenolic pattern of plant organs and tissues and use this as a marker of plant responses to environmental constraints. Together with the recent evidence of localisation of defence peptides in another violet species, this study sheds new light on the immune potential of violets to fight adverse conditions.

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