Synthesis and extraction routes of allelochemicals from plants and microbes: A review

Abstract

Allelopathy, a complex phenomenon has unveiled both stimulatory and inhibitory effects in plant processes that are mediated by the release of certain chemical compounds commonly known as allelochemicals. Allelochemicals, a form of bioactive secondary metabolites are produced by a diverse group of plants and microbes in response to biotic and abiotic stress. It ranges from a simple hydrocarbon to complex polycyclic aromatic compounds like phenol, flavonoids, tannins, steroids, amino acids, alkaloids and quinones. These plant bioactive compounds are released into the environment via decomposition, exudation, leaching and volatilization that play a significant role in regulating the intra-specific or inter-specific relations with counterparts. A wide variety of methods has been proposed for analyzing the basic mechanism and overall effect of allelochemicals. However, the lack of a reliable and effective method to identify their molecular mode of action and their modulation in the metabolic pathway still remains as a great challenge. From a commercial perspective, these allelochemicals are deemed to be better candidates for green natural herbicides and weedicides that are proven to be environment friendly, unlike synthetic chemicals. In order to pave a way for the economic viability of these chemicals, a basic understanding of their chemistry is inevitable. This review article is focused to give an in-depth understanding of metabolic pathways genes responsible for the elicitation/secretion and the adoption of a suitable downstream process and analytical techniques that can intensify the process.

Allelopathic Activity of a Novel Compound, 5,6-Dihydrogen-11α-O-acetyl-12β-O-tigloyl-17β-marsdenin, and a Known Steroidal Glycoside from the Leaves of Marsdenia tenacissima (Roxb.) Moon

“…Bioactive secondary metabolites with allelopathic effects on other plants are termed allelochemicals, which can be found in different parts of plants: roots, leaves, stems, bark, fruits, flowers, and seeds [6]. These allelochemicals obtained from plants belong to diverse chemical groups like alcohols, fatty acids, phenolics, flavonoids, terpenoids, and steroids [7,8]. When released into the environment, allelochemicals can inhibit the germination, growth, and development of adjacent plants by disrupting physiological mechanisms including photosynthesis, respiration, membrane permeability, cell extension, cell division, and water and nutrient uptake [8,9].…”

Aqueous Extracts of Three Herbs Allelopathically Inhibit Lettuce Germination but Promote Seedling Growth at Low Concentrations

“…Several studies demonstrated that allelopathy affects plant communities and alters interspecific interactions, either by directly affecting plant growth and development or by indirectly affecting decomposition and soil fertility via soil organisms [5,[9][10][11][12]. Allelopathy modulates plants by interfering with germination, growth, reproduction, and distribution [13]. The grassland ecosystem is one of the largest in the world; it provides vital services such as livestock forage production and has been the focus of allelopathy studies [14].…”

“…Several studies reported that allelopathic compounds extracted from donor plants may significantly reduce seed germination and seedling growth in recipient plants in a concentration-dependent manner [23,[34][35][36][37]. Other authors stated that allelochemicals released by donor plants exerted positive effects on recipient plants at low concentrations, but were phytotoxic above certain threshold doses [13,34]. This pattern may be characteristic of the hormesis effect, in which the allelochemical is stimulatory at low concentrations but inhibitory at high concentrations.…”

Response of soil microbial compositional and functional heterogeneity to grazing exclusion in alpine shrub and meadows in the Qinghai–Tibet Plateau

“…Further analysis of the differences in microbial community functions revealed that fatty acid metabolic capability improved in GE plots. Many fatty acids, such as hexadecanoic acid, hexadecenoic acid, octadecanoic acid, and octadecenoic acid, are allelopathic agents ( Iqbal et al, 2019 ; Kong et al, 2019 ), which are derived from decomposed litter, root exudates, and stemflows and negatively affect shrubs and the seedlings of other plants ( Singh et al, 2021 ). GE contributed to vegetation recovery by increasing the metabolic capacity of microbes for allelochemicals, which may partly explain the invasion and expansion of shrubs after the removal of grazing pressure ( Zhang et al, 2019 ).…”