## 2 Naturally Occurring Coumarins and Their Role in Plants
### Classification of Naturally Occurring Coumarins and Their Role in Plant Protection  
#### 2.2.1 Simple Coumarins  
Simple coumarins act in plants' interaction with biotic and abiotic environmental stress factors. Secretion of iron-mobilizing coumarins by plant roots is a crucial factor for improving iron bioavailability in crops, enabling them to grow in iron-depleted soils. In Arabidopsis, three coumarins with iron-mobilizing properties are found: fracretin (7,8-dihydroxy-6-methoxycoumarin), sideretin (5,7,8-trihydroxy-6-methoxycoumarin), and esculetin (6,7-dihydroxycournarin) (Figure 2 Their catechol moiety (two neighboring hydroxyl groups) is thought to be a crucial structural feature for iron mobilization in soil [75]. Scopoletin (7-hydroxy-6-methoxy coumarin) (Figure 2) is a simple coumarin that occurs in _Arabidopsis thaliana_[76], and many other plants [77]. Physiologically, scopoletin protects against stress. Thus, it was proved that scopoletin accumulates in Arabidopsis leaves after the attack of the fungus _Phakospora pachyrhizi_, which causes Asian soybean rust disease [78]. In tobacco plants, scopoletin and its \(\beta\)-glucose, scopolin, have physiological roles against stress, for example during tobacco mosaic virus infiltration [79]. Surangib B from _Mamnea longifolia_ inhibits mycelial growth of _Rhizoctonia solani_ and _Botrytis cinerea_[80]. Umbelliferone (7-hydroxycoumarin) is a phytoalexin widely distributed within the Rutaceae and Apiaceae (Umbelliferae) families. It is an important phytoalexin that protects plants from pathogenic fungi, such as _Fusarium culmorum_[81], and the aerobic, Gram-negative, plant-pathogenic bacterium _Ralstonia solanacearum_[82]. [PERSON] et al. [83] studied the antimicrobial activity of 18 natural compounds against _Ralstonia solanacearum_, the bacterium responsible for tobacco, tomatoes, and potatoes wilting in subtropical regions. The research showed that four coumarins, daphnetin, esculetin, umbelliferone, and xanthotol demonstrated stronger antibacterial effects than the standard treatment with thiadiazole and copper. A more detailed analysis showed that the enhanced antibacterial activity is due to the substitution at positions C-6, C-7, and C-8 of the coumarin nucleus. For this reason, they tested the activity of the hydroxycumarins umbelliferone, esculetin, and daphnetin in the concentration range from 10 to 100 mg/L. Daphnein (OH groups at positions C-7 and C-8) proved to be the most effective, esculetin (OH groups at positions C-6 and C-7) was somewhat weaker, while umbelliferone (OH group at positions C-7) showed the weakest activity. Thus, treatment of tobacco roots with umbelliferone prior to infection with _R. solanacearum_ significantly reduced _R. solanacearum_ biofilm formation, increasing resistance to disease [84]. Application of an elicitor of coumarin biosynthesis, salicylic acid to the roots of chamomile (_Matricaria chamonilla_) resulted in the accumulation of ambelliferone and herniarin (7-methoxy coumarin) in the leaves [85]. Herniarin suppressed _R. solanacearum_ bacterial growth by destroying the bacterial cell membrane [86]. Also, plant-derived 6-methylcoumarin showed inhibitory effects against _R. solanacearum_, and suppressed tobacco bacterial wilt [87]. Derivatives of 3,4-dihydroisocoumarin isolated from endophytic fungus _Lophiostoma_ sp., displayed antibacterial activities against _Bacillus subtilis_, _Agrobacteriumium tumefaciens_, _R. solanacearum_, and _Xanthomonas vesicatoria_[88]. Increased biosynthesis of coumarins, ayapin and scopoletin, has been observed in sunflowers (_Helianthus annuus_ L.) during the attack of the sunflower beetle, _Zygogramma exclamationis_, which resulted in distracting further feeding of the beetle [89]. The coumarin (2_H_-1-benzopyran-2-one) proved effective against the green peach aphid _Myzus persicae_ and friendly to the natural enemy of aphids, _Harmonia axyridis_ and soil invertebrates, _Eisenia fefida_.