Root exudate influence rhizosphere microbial communities. The interaction between plant root and soil organisms takes place in rhizosphere mediated by root exudate (Walker et al., 2003).
Pathogen attack may lead to de novo synthesis of flavonoid phytoalexin that exhibits antifungal and antibacterial activities (Hassan and Mathesius 2012). Spatial and physical localization of the sites of exudation in the root will elucidate plant-microbe and plant-plant interaction for instance the external signal from pathogen and invasive plant may determine the exact location of the root releasing exudate (Walker et al., 2003). Plant root release a mixture of organic compounds into the rhizosphere some of which are flavonoids. These secondary metabolites have antioxidant property. The accumulation of root associated components in the rhizosphere is termed rhizodeposit which affect plant growth and soil ecology. The molecular communication in rhizosphere specifically the signaling pathway allows microorganisms to sense diverse plant signal and plants to respond to microbial infection. An example of dynamic signal exchange and its biological significance is symbiosis and pathogenesis (Faure et al., 2009). The root-secreted defense compounds and the role of transport protein modulating their release has been discussed by Baetz and Martinoia (2014). Flavonoids affect nutrient availability through soil chemical changes (Cesco et al., 2010). Reduced form of glutathione elicits the formation of pterocarpan phytoalexins medicarpin and maackiain but also that of the isoflavones biochanin A and formononetin (Armero et al., 2001). These glutathione induced secondary metabolites that are just not accumulated in root tissues but are released into the surroundings. Pterocarpons and isoflavonoids show antifungal activity against the pathogenic fungi (VanEtten 1976). Coumestrol and medicarpin present in the exudates possess nod gene repressing activity (Zuanazzi et al., 1998). Quercetin 3-methyl ether and its glycosides inhibited conidial germination of Neurospora crassa. The presence of methyl group in flavonoid nucleus show inhibiting effect towards Arabidopsis thaliana and Neurospora crassa (Parvez et al., 2004).
Flavonoids are exuded from roots often in response to elicitors (Schmidt et al., 1994; Armero et al., 2001) and are located in vacuoles of cortical cells, cell wall, cell membranes, nucleus and the cytoplasm (Hassan and Mathesius 2012). The transportation of flavonoids within and between plant tissues and cells are by specific transport proteins or transporters and are released into the rhizosphere by roots where they are involved in various interaction including allelopathy (Weston and Mathesius 2014). Flavonoids and other phenolics have been found to inhibit root pathogens and pests including bacteria, fungi and insects (Rao 1990). Shaw et al.(2006) hypothesize that flavonoids act to shape rhizosphere microbial community structure due to potential source of carbon. Low flavonoid concentrations exhibit slight antimicrobial properties against Fusarium oxysporum f. sp. lycopersici (Steinkellner and Mammerler 2007). Exudation of flavonoid may increase microbial species that use flavonoid as carbon source while inhibiting the growth of others by phytoalexins production (Hassan and Mathesius 2012). It may also alter the soil by acting as antioxidant and metal chelators. Both aglycones and glycosides of flavonoids are found in soil solution and are released by root exudation or tissue degradation (Weston and Mathesius 2013).
Plant can select a subset of microbes at different stages of development for specific function for example gene involved in streptomycin synthesis was induced during flowering stage for disease suppression (Chaparro et al., 2014). Rhizobia only infects roots close to the root tip so flavonoid exudation would be most effective in that region (Hassan and Mathesius 2012). The nod gene inducing activity of exudate is determined not simply by the nod gene inducing flavonoids but rather by both inducing and repressing flavonoids (Zuanazzi et al., 1998). Interestingly some flavonoid show nod gene repressing activity for certain rhizobia (Hassan and Mathesius 2012).
Flavonoids consist of more than ten thousand structurally diverse compounds. They accumulate in plant vacuoles as glycosides while few are released by roots in the rhizopshere (Sugiyama and Yazaki 2014). Flavonoids play important role in the transport of auxin, root and shoot development, pollination, modulation of reactive oxygen species and signalling of symbiotic bacteria in the legume. Both aglycones and glycosides of flavonoids and other bioactive secondary metabolites are found in rhizosphere either released by root as exudate or by tissue degradation (Weston and Mathesius 2014).
Plant respond to stresses and change their exudation that is when plant does not experience any biotic stress and have access to nutrient they release exudate that maintains a balance between pathogenic and beneficial microorganisms in the rhizosphere but upon pathogenic invasion the exudation profile of root changes and stress-induced exudates aid the plant in inhibiting pathogenic growth in the rhizosphere. Some of the beneficial microbes in the rhizosphere may trigger induced systemic response protecting plant from infection (Pascale et al., 2019). The chemical succession in the rhizosphere interact with microbial metabolite substrate. The combination of plant exudation traits and microbial substrate uptake traits yield the pattern of microbial community observed in the rhizosphere (Zhalnina et al., 2018). It has become evident that the profile of defense root exudates is not only diverse in composition but also dynamics.
Understanding the complexity of chemical composition in the rhizosphere and how the rhizosphere microorganisms assist defense mechanisms in control of pathogens is of importance.
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