Plants synthesize range of secondary metabolite which are toxic or deterrent to herbivore or pathogens. The diverse range of secondary metabolite include terpenes, phenolics, sulfur-containing compounds, saponins and alkaloid. Plant recognize elicitors/effectors produced by the pathogens and activate defense responses Concurrently several secondary messenger such as calcium ion, reactive oxygen species (ROS) and plant hormone (salicylic acid, jasmonic acid and ethylene) mediated pathway are activated resulting in hypersensitive response (HR). Plant employ resistance (R) protein to detect effectors. Nucleotide binding-leucine rich repeat (NB-LRR) protein function as intracellular receptor for detection of pathogen effector (DeYoung et al., 2012). The leucine-rich repeats receptor-like kinases (LRR-RLK) constitute largest receptor-like kinases family (Fischer et al., 2016). The overall signal perceived by pattern recognition receptors (receptor like kinases and receptor-like proteins) are transmitted through cytosolic protein kinases [Mitogen-activated protein kinase (MAPK)] to activate an array of transcription factors which regulates several R genes to produce resistance-related metabolites and resistance-related proteins. The phytoanticipins and phytoalexins or their conjugate products are deposited to enforce the secondary cell wall thus limiting the spread of the pathogen (Kushalappa et al., 2016).
The biological property of phenolic compound is their antimicrobial activity. They act as protective compounds against fungi, bacteria and viruses (Friend 1979). Phenolic compounds are of two types flavonoid and non-flavonoid (Dzialo et al., 2016; Aleixandre-Tudo et al., 2017):
- Flavonoid compounds are: Flavonols, Flavones, Flavanones, Flavon-3-ols, Isoflavones and Aanthocyanidins
- Non-flavonoid compounds are: Phenolic acids (hydroxycinnamic acid and hydroxybenzoic acid), Lignans, Stilbenes, Tannins and Lignins
Accumulation of phenolic compounds at the infection site is a common feature of cell wall reinforcement in plant-microbe interactions (Field et al., 2006). Daayf et al. (2012) have studied the role of phenolics in plant defense against Phytophthora infestans and soil-borne pathogen Verticillium dahlia as well as the counter mechanisms developed by these pathogens. The phenolics that are accumulated in response to several pathogen include scopoletin (coumarin), p-coumaric acid methyl ester (a hydroxycinnamic acid derivative) or rutin (a flavonoid). Matern and Kneusel (1988) propose that the first line of defense against infection is rapid synthesis of phenolics and their polymerization into cell wall. Cell wall reinforcements are accompanied by localized production of ROS which drive cell wall cross-linking, induces antimicrobial activity and triggers localized hypersensitive response (Bradley et al., 1992; Levine et al., 1994).
Coumarins: Plant secondary metabolites coumarins display antimicrobial and antiviral activities. Coumarins secreted by plant roots mobilizes iron through reduction and chelation and aid in iron uptake from iron limited soil (Tsai and Schmidt 2017; Stringlis et al., 2019). Petroselinum crispum and Ammi majus produce coumarin phytoalexins upon treatment with fungal elicitors and concomitantly reinforce their cell walls by ferulic ester incorporation (Matern 1991). The structural core of coumarins is 2H-1-benzopyran-2-one. Based on modification of this core coumarins may be classified into complex and simple coumarins. Complex coumarins are produced by the addition of heterocyclic compound on the basic core and further classified into furanocoumarins, pyranocoumarin, phenylcoumarins, dihydrofurocoumarin and biscoumarin (Medina et al., 2015). Simple coumarins include scopolin, scopoletin, esculin, esculetin, umbelliferone, fraxetin and sideretin which has a role in the interaction of plants and biological activities. Pathogens or elicitors activate plant defense responses leading to scopoletin accumulation in the infected tissue while scopolin is more abundant in the adjacent uninfected tissue. The scopolin when released from vacuole, is subjected to the activity of the β-glucosidases converting it to scopoletin. The scopoletin exerts its antimicrobial activity and scavenges hydrogen per oxide (H2O2) in the infected tissues therewith restricting cell death. The plants can control scopoletin levels by converting it to scopolin via the activity of glycosyltransferases (Stringlis et al., 2019). Development and virulence of plant pathogenic fungi undergoes an oxidative burst on its own during plant infection (Egan et al., 2007). Because scopoletin is an antioxidant may scavenge ROS thereby inhibiting the growth of fungal pathogen (Beyer et al., 2019). Stringlis et al. (2018) found that transcription factor MYB72 regulates the excretion of the coumarin scopoletin, an iron mobilizing phenolic compound that shapes the root-associated microbial community. Coumarin interplays between the iron deficiency responses and induced systemic resistance (ISR) triggered upon colonization of roots by beneficial microorganisms in the rhizosphere (Pieterse et al., 2014; Verbon et al., 2017; Stringlis et al., 2018). The antibacterial activity of coumarins was found to depend on the structural requirement (Kayser and Kolodziej 1999). The presence of two methoxy and one additional phenolic group reflects highly oxygenated coumarins, this structural feature provides it the antibacterial activity. In case of scopoletin, the toxicity depends on the presence of a methoxy (O-CH3) and a hydroxy group (-OH) in the benzene ring (Stringlis et al., 2019).
Plant derived coumarin may be explored for its natural pesticidal property against plant pathogens.
See Part III
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References:
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BIOLOGICAL PROCESSES- A NATURE'S GIFT 