## 2 Naturally Occurring Coumarins and Their Role in Plants  
### Biosynthesis and Distribution of Coumarins in Nature  
As a family of benzopyrones (1,2-benzopyrones or 2\(H\)-1-benzopyran-2-ones), coumarins are widely distributed throughout nature. The benzopyrone framework is an electron-rich system with favorable charge-transport properties. Therefore, they are characterized by UV light absorption, resulting in a characteristic blue fluorescence. Besides their role in iron mobilization and uptake by plant roots, natural coumarins have a role in environmental stress responses. Also, they participate in the defense against plant pathogens, acting as phytoanticipins, or phytoalexins, which are produced upon infection and are typically not present in healthy tissues. Their increased accumulation on plant tissue is a response to the application of a molecule that triggers the hypersensitivity response in the plant (elicitor) or plant hormones [60]. Since coumarins act as signaling molecules that regulate the interaction between commensals, pathogens, and plants, they could be used as biopesticides. Endophytes such as bacteria or fungi have the ability to produce some of the secondary metabolites. Thus, coumarin isofrazidin was synthesized by the fungal endophyte _Bisocynia uxiayclin-drospora_, and identified in host plants Siberian ginseng (_Acanthopanax senticosus_) and _Apium graveolus_[61]. Infection by pathogenic bacteria can induce the synthesis of coumarin compounds around plant roots and stems to immunize the plant against pathogen invasion and propagation. Inoculation of _Arabidopsis thaliana_ by the plant pathogen _Dickeya_ spp. strains induced coumarin accumulation and plant resistance to pathogens [62]. Treatment of ryegrass (_Lolium multiform_ Lam.) with coumarins stimulates the colonization of beneficial flora in the root rhizospheric microbial community. Rhizosphere microorganisms enhance plant nutrient absorption, coordinate growth, and improve environmental adaptability [63]. The secretion of coumarins from _Arabidopsis thaliana_ roots under soil iron deprivation stimulates the bacterial root microbiota to improve plant adaptation to iron-limiting soils [64]. A strain of Aspergillus synthesizes 4-hydroxycoumarin and dicoumarol [65; 66]. Naturally occurring coumarins are mostly distributed in plants seeds, flowers, leaves, roots, and stems in more than 40 different families including _Apiaceae_, _Rutaceae_, _Asteraceae_, _Fabaceae_, _Oleaceae_, _Moraceae_, and _Thymelaaceae_[67]. The natural coumarins are derivatives of 2\(H\)-1-benzopyran-2-one, and they are classified into six groups: simple coumarins; furanocumarins (linear and angular type); dihidrofuranocoumarins; pyranocoumarins (linear and angular type); phenylcumarins; and bicumarins (Figure 1) [68]. Angiospermaes are rich in simple comumarins, followed by funcoumarins and pyranocoumarins. The most diverse sources of coumarins are plants families Apiaceae and Rutaceae containing five different types of coumarin derivatives (simple coumarins, lineal furocoumarins, angular furrocoumarins, lineal pyranocoumarins, and angular pyranocoumarins) [30]. Simple coumarins are derived by biosynthesis from shikimic acid, via cinnamic acid. They are the most common in all angiosperms, especially in _Oleaceae_ and _Asteraceae_. A key step in the biosynthesis of simple coumarins is ortho-hydroxylation of cinnates that branch off from lignin biosynthesis. The gene required for the production of fenuloyl coenzyme A (CoA) is CCoAOMT1. It also participates in the biosynthesis of lignin and simple coumarin scopoletin in Arabidopsis roots. A key enzyme involved in the biosynthesis of simple coumarins is 2-oxoglutate-dependent dioxygenase (2 OGD), which is encoded by 2 OGD genes. Thus, the gene AtF6_H1 encodes otho-hydroxyalice activity to feruloyl coenzyme A, and its deficient mutation causes a significant reduction in scopolin accumulation. 2 OGD gene RgC2'H formation of funcoumarins in _Ruta graveolenes_[69].