Allelochemicals and signaling chemicals are produced by plants. These allelochemical and signaling chemicals are released in the environment that regulates the interaction between plant and the other organisms. Allelochemicals protect plant against microbial attack, herbivore predation and/or competition with other plants notably allelopathy, which affect the establishment of competing plants (Kong et al., 2019). Chemically mediated plant-plant interactions are mediated by allelopathy and allelobiosis. Allelopathy effects the establishment and growth of the neighboring plant by allelochemicals while allelobiosis via signaling chemicals can identify or detect neighboring plant (Kong et al., 2024). Allelochemicals are chemically diverse, and are represented by phenolic compounds (phenolics, flavonoids, coumarins and quinones,) terpenoids (monoterpenes, sesquiterpenes, diterpene, triterpene and steroids), alkaloids and nitrogen-containing chemicals (non-protein amino acids, benzoxazinoids, cyanogenic glycosides) (Li et al., 2010, Kong et al., 2019, Macias et al., 2019, Xu et al., 2023). Root derived allelochemicals play an essential role in interaction among soil organisms (Bais et al., 2006). Allelochemical exuded from allelopathic rice roots can modify soil microbial community (Kong et al., 2008).
The spatiotemporal dynamics of plant metabolite released by roots as root exudate is vital in understanding the biogeochemical processes occurring at the root-microbe-soil-interface (Oburger and Jones 2018). It is difficult to identify and quantify soil-borne allelochemicals and signaling chemicals due to the complexicity of plant-soil interactions (Kong et al., 2019). Plants co-exist and therefore there are intraspecific (communication between individuals of the same species) and interspecific (communication between different species) interactions occurring which may involve competition, parasitism and incompatibility (Kong et al., 2024). Intraspecific allelopathy is an autotoxicity phenomenon, where a plant species inhibits the growth of its own kind through the release of allelochemicals (Singh et al., 1999). Intraspecific competition generally is much stronger than the interspecific competition where member of the same species interacts more intensely because of their similarity (Chesson 2000).
Allelochemicals are released from root and effect soil microorganisms, whereas soil microorganisms consume and degrade the allelochemical (Vidal and Bauman 1997). Root exudate containing wide variety of metabolites have ecological impact on soil macro and microbiota as well as on the whole plant itself (Bertin et al., 2003). The roots secrete metabolites and signal molecules into the soil at the rate significant to the soil organisms (Farrar et al., 2003). The level of phytotoxicity of allelochemicals is not only affected by single soil characteristic but by multiple effect such as retention, transport and transformation processes of allelochemicals in soil. The allelochemicals interact with the organic and inorganic soil phase as well as with soil microorganism (Scavo et al., 2019). Allelopathy is inhibition of germination, growth or metabolism of one plant due to release of organic chemicals by the second plant. This form of interference is fundamentally distinct from competition which acts through the depletion of resources (Del Moral and Cates 1971). Competition is a process whereby plant interfere with the growth of the neighboring plants by utilizing or competing for the common limiting resources including space, light, nutrients or water (Rice 1974, Craine and Dybzinski 2013). Plant detects competing plants, herbivores and pathogen by signaling chemicals and then release allelochemical as a defense response to suppress their growth. Release of allelochemicals from various plant tissue may reach the receiver plant in various ways including volatilization or leaching from the aerial parts of the plant, exudation from roots and decomposition of plant residue in soil (Kobayashi 2004, Latif et al. 2017). Co-existence with other plants is a factor affecting the growth of plant and the distribution of species. Allelochemicals in soil are adsorbed on soil solids and metabolized by chemical and biological activity and this activity is affected by soil texture, organic and inorganic matter, moisture and organisms which may affect the phytotoxic activity in soil (Kobayashi 2004). When secondary metabolites produced by plants are perceived by other organisms, they acquire ecological functions and become subjected to selective pressure (Petschenka and Agrawal 2016, Silva et al., 2018). Allelochemicals present in soil and on being released by plants may interact synergistically with each other, resulting into higher reductions of plant growth (Jilani et al., 2008).
Allelochemicals in environment are composed of water-soluble fractions and insoluble fractions, which are bound up in straw residues and are released by microbial decomposition as well as by chemical decomposition followed by microbial transformation of these chemicals (Inderjit 2005, Xiao et al., 2020). Transformation of allelochemicals by soil microbes may result into the compounds with modified biological properties. Such biotransformation affects the overall allelopathic capability of the plant in a direct manner (Jilani et al., 2008). The production of phytotoxic allelochemicals by invasive weeds is a mechanism by which these species become successful competitors (Qin et al., 2006). Plant growth promoting rhizobacteria enhance plant growth by suppressing phytopathogen by producing allelochemicals (Saraf et al., 2014).
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