ROOT EXUDATE COMPOSITION AND PLANT DEFENSE RESPONSES

Plant can exploit microbial consortia from soil for protection against infections. Mendes et al.(2011) detected 33,000 bacterial and archaeal species with Proteobacteria, Firmicutes and Actinobacteria associated with disease suppression.  Pathogenic microorganisms colonize the rhizosphere, try to invade and overcome the innate plant defense mechanism in order to cause disease. The genes required for the biosynthesis of secondary metabolite are clustered in fungi and the condition under which this gene cluster is naturally expresses is not known. One available technique is to stimulate the natural habitat by co-cultivation of microorganisms from the same ecosystem. This strategy led to activation of silent gene clusters and the identification of novel compound in Aspergillus nidulans (Brakhage and Schroeckh 2011).   Identification of the exudates, signals and specific species in rhizosphere microbiome will provide chemical and microbial marker to elucidate how plant recruit and stimulate beneficial microorganisms (Mendes et al., 2013). Specific suppressiveness of soil is observed towards different fungal pathogen causing disease (Weller et al., 2002).

Chemical composition of root exudates have a direct effect on rhizosphere communities and specific plant species use these compounds to select soil microbe communities (Mhlongo et al., 2018). Photosynthates comprise of carbon compound, electron, proton, water and inorganic ions which enters the rhizosphere as root exudate. Besides photosynthates, phytosiderophores and polysaccharides form a large part of rhizospheric deposits (Olanrewaju et al., 2019). Plant and rhizosphere microorganisms influence each other via secretion and detection of signaling compounds. Plant molecule such as flavonoids, strigolactones, cutin monomers and unidentified low molecular weight compounds are recognized as signals which are sensed by microorganisms and microorganisms produce signals which may induce plant systemic resistance (Venturi and Keel 2016).

Bais et al.(2005) show that exudation of antimicrobial metabolites by Arabidopsis thaliana confers tissue-specific to a range of bacterial pathogens. Plant growth promoting rhizobacteria (PGPR) may function in synthesizing particular compounds in plants, facilitate uptake of nutrient from the soil and prevent plant from diseases (Hayat et al., 2010).  PGPR are biocontrol agent of soil-borne plant pathogens (Babalola 2010). The other mechanism by which PGPR can inhibit phytopathogen is the production of Hydrogen cyanide (HCN) and/or fungal cell wall degrading enzymes example chitinase and β-1,3-glucanase, antibiotics, metabolites and phytohormones (Babalola 2010; Hayat et al., 2010).  Several antagonistic rhizobacteria produce siderophore and antibiotics. Rhizobacteria belonging to genera Pseudomonas and Bacillus are known for their antagonist effect and their ability to trigger induced systemic resistance (ISR) (Beneduzi et al., 2012). Phytosiderophores enhance microbial activities in the soil. They relieve stress due to iron and zinc deficiencies through the acquisition of required iron and zinc for plant use. Siderophore are synthesized by microorganisms under iron limiting conditions. Pseudomonas sp. from the rhizosphere of Chickpea plant produces siderophore (Tank et al., 2012). Pseudomonas strainsshowed that the antifungal activity depends on the sugar and organic acid composition of root exudates of tomato plant (Kravchenko et al., 2003).

Root exudate are not only plant nutrient but may also act as signal molecules mediating interactions in the rhizobiome. The exudate may contain sugars including monosaccharides (fructose, mannose and glucose), disaccharides (maltose), five carbon sugar (arabinose) and oligosaccharide, amino acid  such as aspartate, asparagine, glutamine, arginine and cysteine; organic acid like ascorbic, acetic, benzoic, ferulic and malic acids, phenolic compounds such as coumarine, flavonoids, enzymes, fatty acid growth hormones etc. (Olanrewaju et al., 2019). Type of plant metabolism can affect the exudation response of plant to different factors. The proportion of sugars and organic acid may differ in C3 and C4 plants. Mannose, maltose and ribose were identified as dominant sugars of C3 whereas, sugars in C4 plants are inositol, erythritol or ribitol. C3 plants have a wide spectrum of organic acid and amino acids in root exudate as compared to C4 plants (Vranova et al., 2013). Amino acid found in the rhizosphere as a result of lysis or cellular efflux from plants and microbes and proteolysis of existing peptides, can serve as a biological source of both carbon and nitrogen. Amino acid may alter key phenotype related to plant root growth and microbial colonization, symbiotic interactions and pathogenesis in the rhizosphere (Moe 2013).

Polysaccharides in plant is starch which is more complex than simple sugars and is a form of energy storage. Photosynthetically fixed carbon is secreted by plants as root exudate as well as antimicrobial compounds such as phytoanticipins and phytoalexins, it is also a carbon cost to the plant. Glyceollin phytoalexin is produced by soy plants in response to fungal infection (Bamji and Corbitt 2017). Therefore plant exudation in form of phytochemicals and rhizodeposit requires regulation (Baetz and Martinoia 2014). Exudate secretion of defensive compound is a regulated process that are in response to endogenous and exogenous stimuli. Regulated processes and stimuli alteration of root exudate demonstrate the complexity in plant rhizosphere defense mechanisms (Olanrewaju et al., 2019).   

The biocontrol activity of plant growth promoting bacteria (PGPB) is its ability to synthesize antibiotics, fungal cell wall degrading enzymes such as chitinase, siderophore activity or induction of systemic resistance within plant. Any specific PGPB may possess one or several of these activities (Gamalero and Glick 2011). Antibiotics act as growth inhibitors by inhibiting enzymes that are involved in cell wall biosynthesis, nucleic acid metabolism and repair, disrupt protein synthesis, aid in the disruption of membrane structure or to be mediators of cellular signals depending on their concentration (Olanrewaju et al., 2019).

The root exudates provide information on nutrient absorption and plant defense responses against plant pathogens.

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