Plants are able to structure the rhizosphere microbial population as different plant species growing on the same soil support host specific microbial community (Berendsen et al., 2012). Upon pathogen attack plants enhance the activity of those microorganism that can suppress pathogen attack in the rhizosphere. Most soil borne plant pathogen need to grow saprophytically to reach the host, maintain their population before they can invade the host tissue and cause infection. The success of the pathogen is influenced by the microbial community of that soil. Flavonoid and saponins are plant- specialized metabolites secreted in the rhizosphere (Sugiyama, 2021).
Isoflavone plays a role in plant-microbe interactions in rhizosphere. Soybean isoflavones daidzein and genistein that are released in rhizosphere acts as allelochemicals. Continuous monocropping of soybean plant led to change in the microbial community due to change of rhizosphere isoflavones (Guo et al., 2011). Beside isoflavones and saponins soybean roots secrete diverse range of metabolites in the rhizosphere (Sugiyama 2019). The Trifolium microbial community was dominated by nitrogen fixing bacteria Rhizobia that may protect plant from disease causing agents (Hartman et al., 2017). Coumarins inhibits the fungal pathogen Fusarium oxysporum f. sp. raphanin and Verticillium dahlia JR2. Beneficial microorganisms induce exudation of coumarins in the rhizosphere protecting plant against pathogenic attack (Lundberg and Teixeira 2018). Isoflavone diadzein serve as a precursor for the biosynthesis of glyceollins and phytoalexins having antimicrobial property and are induced upon infection by pathogen such as Phytophthora sojae and Macrophomina phaseolina (Tzi et al., 2011). Coumarin have antimicrobial and antiviral properties beside their role in iron uptake (Stringlis et al., 2019).
Hyphal germlings respond to diadzein and genistein chemotropically suggesting hyphal tip from zoospores of Phytophthora sojae encyst near the root to utilize specific host isoflavones to locate host (Morris et al., 1998). The fungal plant pathogens Phytophthora and Pythium and a nitrogen-fixing bacterial symbiont identify their host by recognizing the same chemical (Morris and Ward 1992). It is isoflavone daidzein and genistein functioning as chemical signals in early stages of infection (Morris et al., 1998). Isoflavone cause rapid encystment and germination of swimming zoospores are also inducers of nodulation genes in Bradyrhizobium japonicum. Isoflavone diadzein and genistein which occur in soybean root exudate are effective chemoattractant for zoospores of Phytophthora sojae (Morris and Ward 1992). Diadzein and genistein secreted by soybean roots induce the symbiotic interaction with rhizobia and may modulate rhizosphere interactions with microbes. Slow rate of degradation of diadzein enabled accumulation of isoflavones in the rhizosphere (Sugiyama et al., 2017).
Saponins produced by plant have a role in plant defense they counteract pathogens and herbivores (Augustin et al., 2011). Saponins act by permeabilising plasma membranes. Tomatine caused disruption only if membrane contained sterol. The sterol complexing ability of tomatine was similar to steroidal saponin, digitonin (Roddick 1979). The extent of membrane damage depends on sterol concentration in liposomes but not with the nature of the sterol or of the phospholipid (Roddick and Drysdale 1984). Saponins have antifungal properties and protect plants against the fungal attack (Papadopoulou et al., 1999). Oats produce both steroidal and triterpenoid saponins (Osbourn 2003). Triterpene avenacins and steroidal avenacosides are present in roots and leaves (Faizal and Geelen 2013). Saponins are classified as triterpenoids, steroids and steroidal glycoalkaloids based on aglycone structure from which they are derived (Moses et al., 2014).
Saponins are released into soil by secretion from roots and /or leaching from living or decaying plant material. Some saponin are produced independent of external signals and contribute to the innate immunity. These saponins are phytoanticipins as they are present in constitutively in healthy plant or are produced in response to pathogen attack referred to as phytoalexins (Morrissey and Osbourn 1999; Faizal and Geelen 2013). Preformed pre-infection compound present in plant create effective chemical barrier to suppress plant pathogens. The saponins may control rhizosphere bacteria in soil through rhizodeposition mechanisms (Fons et al., 2003). On pathogen invasion the saponin content can change due to partial or complete hydrolysis of these compound as a plant defense response or their degradation by effective pathogen (Szakiel et al., 2011). Saponins can be stored in vacuoles of plant cells as inactive precursors readily converted to biologically active antibiotics by plant enzymes normally separated from their substrates by cell compartments. Few fungal pathogens colonize plant tissue by detoxifying the antifungal saponins present in plants to its less toxic molecules (Szakiel et al., 2011). Number of fungi produce saponin detoxifying enzymes. Avenacinase produced by Gaeumannomyces graminis is closely related to another saponin-detoxifying enzyme tomatinase which is produced by the tomato pathogen Septoria lycopersici suggesting saponin-detoxifying mechanism is present in phytopathogenic fungi (Osbourn 1996). Synthesis and accumulation of saponins is a plant defense mechanism and the change in saponin composition and level depends on the severity of infection. Trda et al.(2019) observed that aescin triggers plant defense by activating the salicylic acid pathway and oxidative burst leading to resistance of Brassica napus against the fungus Leptosphaeria maculans. The protection is comparable to that of fungicides.
Use of residues on the soil surface may have an impact on rhizosphere microbial community and the environment around them.
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