Root exude a diverse repertoire of metabolites. Root exudate can change the pH of soil to solubilize nutrient or chelate toxic compounds or release toxic substances to control pathogen growth. Plant can modify the soil properties by regulating the composition of root exudate to ensure their survival under adverse condition (Vives-Peris et al., 2020). Rhizosphere is the zone of soil surrounding root and is affected by it. Root exudate function not only as nutrients for soil microbes but also as signal molecules in plant-microbe interactions. Root border cells surround the root apices of plants and are involved in root exudate production. Border cells can rapidly attract and stimulate growth of some microorganisms and repel or inhibit the growth of others (Hawes et al., 1998). Root exudate initiate and manipulate biological and physical interactions between roots and soil organisms and thus play an active role in root-root and root-microbe communication (Bais et al., 2004a). The same chemical signal may elicit dissimilar responses from different recipient (Bais et al., 2006). An example of diverse meaning for a chemical signal is the secretion of isoflavones from soybean roots, which attracts a mutualist (Bradyrhizobium japonicum) and a pathogen (Phytophthora sojae) (Morris et al., 1998). Plant defense induced by root exudates reduces susceptibility to pathogen infection. In other cases may initiate production and release of volatiles from leaf that attract predator of plant enemies (Bais et al., 2006).
Rhizobacteria create suppressive soil by controlling plant diseases caused by soil fungi and bacteria. The mechanisms include competition for nutrients, niche exclusion, induced systemic resistance (ISR) and production of antifungal metabolites (Bais et al., 2006). Pathogen-activated plant defense may result in secretion of antimicrobial compound by root (Doornbos et al., 2012). Upon root colonization, Bacillus subtilis 6051 forms a stable, extensive biofilm and secretes surfactin which protects plant from pathogenic bacteria attack (Bais et al., 2004). Rhizosphere bacteria Pseudomonas spp. possess variety of trait that are relevant for the biological control of diseases. One such trait is the flagellar motility towards substrate like amino acids secreted by plant roots (De Weert et al., 2007).
Flavonoid inhibits fungal growth in cooperation with cyanide (Seo et al., 2011). Hydrogen cyanide (HCN) is produced by rhizobacteria (Rijavec and Lapanje 2016). Pseudomonas fluorescens CHA0produces several metabolites with antifungal property and one such metabolite is HCN. The bacterial cyanide helps in supressing the fungus Thielaviopsis basicola causing black root rot in tobacco (Voisard et al., 1989). Plant defense mechanism consists of variety of antimicrobial compounds. HCN is a secondary metabolite produced by many antagonistic Pseudomonas species and has antifungal property (Michelsen and Stougaard 2012). Seo et al.(2011) suggest that cyanide produced during hypersensitive response (HR) provides blast resistance in rice by inhibiting fungal growth. The plant growth- promoting rhizobacteria (PGPR) strains when applied to seed can induce systemic resistance (ISR) to Colletotrichum orbiculare (Wei et al., 1991). It was also observed that PGPR strain that induced resistance in cucumber produced HCN in vitro. ISR is an effective defense mechanism in plants for biocontrol of plant disease. Rhizobacteria-mediated ISR has been investigated against pathogens in which inducing bacteria and the challenging pathogen remain spatially separated. Plants show variation in expression of ISR when induced by specific bacterial strain (van Loon et al., 1998). Biological control of Fusarium oxysporum f. sp. dianthi by Pseudomonas sp. strain WCS417r is not due to the competition between pathogen and the strain WCS417r but is ascribed to induced resistance (van Peer et al., 1991).
The pathogenic fungus Fusarium oxysporum f. sp. radicis-lycopersici causing severe foot and root rot disease in tomato decreased when seeds were treated with Pseudomonas fluorescens WCS365. Analysis of root exudate revealed that the presence of F. oxysporum f. sp. radicis-lycopersici did not alter the amount of organic acid however, the amount of citric acid decreased and succinic acid was increased. Whereas, in presence of biocontrol strain Pseudomonas fluorescensWCS365 the total amount of organic acid increased due to increase in the amount of citric acid and dramatic decrease in the amount of succinic acid (Kamilova et al., 2007). Rosmarinic acid is a constitutive antimicrobial compound and is released in the rhizosphere upon microbe challenge (Bais et al., 2002).
Plant defense has the ability to affect bacterial population in the rhizosphere either by recruiting beneficial microorganisms or by suppressing proliferation of pathogen (Doornbos et al., 2012). Example: Biological control of the fungus Gaeumannomyces graminis var. tritici (Ggt) causing take-all disease in wheat. When wheat is grown continuously in the same field for several years, pathogen population builds up resulting in severe outbreak of the disease. A decrease in take-all disease, the phenomenon known as take-all decline (TAD) is associated with the growth of specific strain of fluorescent Pseudomonas spp. in wheat rhizosphere that produces antibiotic 2,4-diacetylphloroglucinol (DAPG) suppressing the fungus Ggt (Weller et al., 2002; Kwak et al., 2009; Doornbos et al., 2012).
Root exude chemicals of different composition and concentration affecting plant microbe interaction.
See Part A for further information ………
References:
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BIOLOGICAL PROCESSES- A NATURE'S GIFT 