Specialized Metabolites from the Allelopathic Plant Retama raetam as Potential Biopesticides

Abstract

To cope with the rising food demand, modern agriculture practices are based on the indiscriminate use of agrochemicals. Although this strategy leads to a temporary solution, it also severely damages the environment, representing a risk to human health. A sustainable alternative to agrochemicals is the use of plant metabolites and plant-based pesticides, known to have minimal environmental impact compared to synthetic pesticides. Retama raetam is a shrub growing in Algeria’s desert areas, where it is commonly used in traditional medicine because of its antiseptic and antipyretic properties. Furthermore, its allelopathic features can be exploited to effectively control phytopathogens in the agricultural field. In this study, six compounds belonging to isoflavones and flavones subgroups have been isolated from the R. raetam dichloromethane extract and identified using spectroscopic and optical methods as alpinumisoflavone, hydroxyalpinumisoflavone, laburnetin, licoflavone C, retamasin B, and ephedroidin. Their antifungal activity was evaluated against the fungal phytopathogen Stemphylium vesicarium using a growth inhibition bioassay on PDA plates. Interestingly, the flavonoid laburnetin, the most active metabolite, displayed an inhibitory activity comparable to that exerted by the synthetic fungicide pentachloronitrobenzene, in a ten-fold lower concentration. The allelopathic activity of R. raetam metabolites against parasitic weeds was also investigated using two independent parasitic weed bioassays to discover potential activities on either suicidal stimulation or radicle growth inhibition of broomrapes. In this latter bioassay, ephedroidin strongly inhibited the growth of Orobanche cumana radicles and, therefore, can be proposed as a natural herbicide.

Mediterranean Plants as Potential Source of Biopesticides: An Overview of Current Research and Future Trends

“…It is a continuous process that ends after the isolation of the compound of interest and demands several solvents, which are added based on their polarity (from less to more polar) [ 126 , 135 ]. Fractionation has been used for the isolation of alkaloids from Sophora alopecuroides L. extract [ 138 ], phenolic compounds from Humulus lupulus L. [ 95 ], and isoflavones and flavones from Retama raetam [ 99 ].…”

“…Different concentrations can be used to determine the potency of the antifungal effect by measuring certain indices, such as half maximal effective concentration (EC 50 ) [ 76 ], the minimum inhibitory concentration (MIC), or the half inhibitory concentration (IC 50 ) of the extract/essential oil [ 30 , 95 ]. Variations of the agar dilution method have been successfully employed to test the antifungal capacity of various extracts against plant pathogenic fungi, such as Verticillium dahliae in olives [ 71 ]; Zymoseptoria tritici in wheat [ 30 , 95 ]; Sclerotinia sclerotiorum [ 76 ], Fusarium oxysporum , Alternaria solani , and Pythium ultimum in tomato [ 106 , 117 ]; and Botrytis cinerea [ 116 ], Penicillium allii [ 111 ], Stemphylium vesicarium [ 99 ], and Geotrichum candidum var. citri-aurantii in decayed mandarin fruit [ 54 ].…”

Iridoid Glycosides Isolated from Bellardia trixago Identified as Inhibitors of Orobanche cumana Radicle Growth

“…A total of 3.0 g of each B. trixago organs were extracted, following a previously reported protocol often used for the extraction of plant material [53,54], in order to perform a preliminary activity screening against parasitic plants. In particular, the flowers, green organs and roots of each population were extracted separately by H 2 O/MeOH (1/1, v/v), under stirred conditions at room temperature for 24 h. The hydroalcoholic suspensions were centrifuged at 7000 rpm and extracted with n-hexane (3 × 50 mL), CH 2 Cl 2 (3 × 50 mL), and after removing methanol under reduced pressure, with EtOAc (3 × 50 mL).…”

Extracts of Senecio brasiliensis and Solanum viarum as Potential Antifungal and Bioherbicidal Agents

“…Contextually, some secondary metabolites can interfere in the growth and development of biological systems, being thus considered allelopathic compounds. The action of allelopathic compounds capable of interfering in biological systems is usually attributed to different mechanisms that can range from inhibiting the growth of weeds and pests through the interruption of metabolic processes capable of causing biochemical disturbances that affect their survival and growth to even altering the soil microbiome, influencing the competition between beneficial and pathogenic microorganisms [14][15][16]. Therefore, these allelochemical compounds can be used directly for the formulation of pesticides or altered to improve their biological action.…”