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Review
. 2019 Jul 1;60(7):1405-1419.
doi: 10.1093/pcp/pcz076.

The Age of Coumarins in Plant-Microbe Interactions

Ioannis A Stringlis  1 , Ronnie de Jonge  1 , Cornï M J Pieterse  1
Affiliations
Review

The Age of Coumarins in Plant-Microbe Interactions

Ioannis A Stringlis et al. Plant Cell Physiol. .

Abstract

Coumarins are a family of plant-derived secondary metabolites that are produced via the phenylpropanoid pathway. In the past decade, coumarins have emerged as iron-mobilizing compounds that are secreted by plant roots and aid in iron uptake from iron-deprived soils. Members of the coumarin family are found in many plant species. Besides their role in iron uptake, coumarins have been extensively studied for their potential to fight infections in both plants and animals. Coumarin activities range from antimicrobial and antiviral to anticoagulant and anticancer. In recent years, studies in the model plant species tobacco and Arabidopsis have significantly increased our understanding of coumarin biosynthesis, accumulation, secretion, chemical modification and their modes of action against plant pathogens. Here, we review current knowledge on coumarins in different plant species. We focus on simple coumarins and provide an overview on their biosynthesis and role in environmental stress responses, with special attention for the recently discovered semiochemical role of coumarins in aboveground and belowground plant-microbe interactions and the assembly of the root microbiome.

Keywords: Coumarins; Iron homeostasis; Microbiome; Plant–microbe interactions; Scopoletin; Secondary metabolism.

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Figures

Fig. 1
Fig. 1
(a) Visualization of fluorescent coumarins produced by roots of Fe-starved Arabidopsis thaliana Col-0 plants. (b) Chemical structures of representative plant-derived simple coumarins and of coumarin ayapin, whose role is discussed in this review.
Fig. 2
Fig. 2
Coumarin accumulation and regulation in unelicited and elicited leaves with coumarins scopolin and scopoletin used as an example. In healthy leaves, scopolin and scopoletin accumulate to low basal levels. Due to the activity of glucosyltransferases, unstable and toxic scopoletin is converted to the glycosylated form scopolin. Scopolin is transferred within cells and stored in the vacuoles, spatially separated from β-glucosidases. Following defense activation in leaves by a pathogen or elicitor, scopoletin accumulates in the infected tissue and scopolin in the surrounding tissue (a). When scopolin is released from the vacuoles, it is subjected to the activity of the β-glucosidases that convert it to scopoletin (b). Then scopoletin exerts its antimicrobial activity and scavenges H2O2 in the infected tissues therewith restricting cell death (c). In infected Arabidopsis, MYB15 regulates F6′H1 activity and the subsequent accumulation of lignin and scopoletin (d). MPK3 is also found to be required for scopoletin accumulation in infected tissues. Produced scopoletin becomes oxidized by H2O2, which is generated by the activity of AtRbohD. Depending on the environmental cues, plants can control scopoletin levels by converting it to scopolin via the activity of glycosyltransferases and converting scopolin back to scopoletin by β-glucosidases (d).
Fig. 3
Fig. 3
Production of coumarins during belowground plant–microbe interactions, as suggested by studies in Arabidopsis and tobacco. Coumarins scopolin, scopoletin, esculin and esculetin are present at low basal levels in roots and exudates of healthy, unelicited Arabidopsis plants. In the case of root infection by a pathogen, scopolin levels decrease while scopoletin (and fraxetin) accumulates inside the roots and in root exudates. In roots colonized by beneficial MYB72-inducing microbes, scopolin production increases. Due to the activity of BGLU42, scopolin is converted to scopoletin, which is then released into the rhizosphere, where it may be further processed to fraxetin and sideretin. Coumarin biosynthesis relies on a functional F6′H1 and expression of F6′H1 in the cortex (C) suggests that biosynthesis of this coumarin predominantly takes place in this cell layer (Schmid et al. 2014). Subsequently, scopoletin can be transferred to the epidermal cell layer (E) where due to the activity of S8H and CYP82C4 it can be converted to fraxetin or sideretin, respectively (Rajniak et al. 2018). In the rhizosphere, scopoletin either favors or inhibits the proliferation of different microbiome members (Stringlis et al. 2018b) but the role of fraxetin and sideretin in this context is unknown. Coumarins can negatively affect microbial growth by repressing motility, suppressing the activation of the type III secretion system (T3SS) or by disrupting microbial cell membranes.
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