Plant take up most of the mineral nutrient from rhizosphere. Root exudate, root border cells and rhizodeposits are component of rhizosphere that affect microbial colonization, multiplication and secretion of organic bioactive compounds. Rhizodeposits released by plant root in rhizosphere include root exudate, mucilage, lysate produced and contain monopolysaccharides, organic acids, phenolic compounds, amino acid and protein (Dennis et al., 2010; Park and Ryu 2021). Rhizosphere microorganisms release extracellular enzymes for degradation of different polymeric compound and may suppress plant pathogenic fungi (Egamberdieva et al., 2011). The root exudation profiles can shape the root microbiome of diseased plant. Plant depute beneficial rhizosphere communities by modifying the exudation pattern in response to above ground pathogens to benefit plant (Yuan et al., 2018). The study was made on the influences of root exudate changes mediated by biocontrol agent Bacillus cereus AR156 while controlling tomato bacterial wilt caused by Ralstonia solanacearum. It was observed that B cereus AR156 induce specific components in plant root exudate (Wang et al., 2019). Phytochemicals repel, inhibit or kill pathogenic microorganisms in the rhizosphere (Baetz and Martinoia 2014). The roots of nitrogen-fixing legumes exude phenolics and aldonic acids that serves as a signal to attract Rhizobiaceae bacteria. An acidic environment can pose threat to acquisition of nutrient by plant roots and threaten the survival of beneficial microorganisms and root. Rooibos tea (Aspalathus linearis L.) modify rhizosphere pH by extruding OH- and HCO3- to facilitate growth in pH 3 – 5 (Dakora and Phillips 2002).
More rhizosphere-enhanced metabolites (REMs) and rhizosphere-abated metabolites (RAMs) were detected in sand and clay substrate compared to soil substrate. Miller et al.(2019) study demonstrated that substrate influence the root exudate profiles. Exometabolites can have nutritional value and signaling functions. Few microbes can migrate from the rhizosphere into rhizoplane and from there into the root where they become endophytes. The mature cluster root secrete organic acid and also exude isoflavonoids and fungal cell wall degrading enzymes, leading to decrease in bacterial abundance (Weisskopf et al., 2006).
The exometabolites has to cross atleast one membrane to transit from the cytoplasm of root cells into rhizosphere (Joelle et al., 2017).
- Small hydrophilic compounds could diffuse from the root into the rhizosphere driven by the large concentration gradient. The carbon and nitrogen flow at the soil-root interface is bidirectional with carbon and nitrogen lost from root and taken up from soil simultaneously (Jones et al., 2009).
- Channel proteins could facilitate such diffusion
- Active (ATP-driven) or secondary active (proton gradient driven) transporters could shuttle compounds across membranes against a concentration gradient.
- Diffusion of compounds is possible only in young root tissue, which is still devoid of Casparian strips or suberized endodermis as both block apoplasmic flow in adult tissues (Joelle et al., 2017).
The root tip having apical and root cap meristem is resistant to infection caused by number of pathogens. Wen et al.(2009) report extracellular DNA (exDNA) is a component of root cap slime and that exDNA degradation during inoculation by a fungal pathogen results in loss of root tip resistance to infection.
Root derived antimicrobial compound include inodole, terpenoid, benzoxazinone, flavonoid, isoflavonoid and phenolics (Lanoue et al., 2009). Terpenes are natural products with diverse structure and biological activity. Arabidopsis thaliana mutant that lacks diterpene rhizathalene is susceptible to opportunistic root herbivore fungus gnat (Bradysia sp.) and suffers substantial damage of peripheral tissue at the larval feeding sites (Huang and Osbourn 2019).
Maize sesquiterpenoid phytoalexin zealexin is accumulated in Fusarium graminearum infected tissue. Zealexins exhibits antifungal activity against number of plant pathogenic fungi (Huffaker et al., 2011). Triterpenoids are the most potent antifungal defense compound released by plant root. Roots of Arabidopsis thaliana produce triterpenoids such as tricyclic triterpene diol and arabidiol. In a degradation reaction induced by Pythium irregular infection, arabidol is cleaved to homoterpene (E)-4,8-dimethyl-1,3,7 –nonatriene (DMNT) which has a role in defense against Pythium irregulare infection (Sohrabi and Ali 2017). Diterpenoid kauralexins accumulates in response to fungal attack (Schmelz et al., 2011). Soybean secretes large amount of soyasaponin (triterpenoid glycosides) as root exudates (Tsuno et al., 2018). Quinonemethide triterpenoid 22β-hydroxy-maytenin and maytenin present in root cap and near vascular cylinder of Peritassa laevigata in-vitro root suggest role in plant defense against infection by microorganisms as well as in the root exudation process. The root culture obtained from P. laevigata may accumulate secondary metabolites which are cytotoxic such as quinonemethide triterpenes(Pina et al., 2016).
Plants modify the soil properties by modulating the composition of the root exudates to ensure survival under adverse condition.
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