Root exudate chemical cues of an invasive plant modulate oviposition behavior and survivorship of a malaria mosquito vector.

Gravid female Anopheles gambiae mosquitoes identify suitable oviposition sites through a repertoire of cues, but the influence of allelochemicals, especially root phytochemicals in modulating this behavior and impacting subsequent progeny bionomics remains unexplored. We addressed these questions in the malaria vector Anopheles gambiae and its invasive host plant Parthenium hysterophorus. Using chemical analysis combined with laboratory behavioral assays, we demonstrate that a blend of terpenes, namely α-pinene, α-phellandrene, ß-phellandrene, 3-carene and (E)-caryophyllene emitted from P. hysterophorus root exudate treated-water attracted gravid females. However, fewer eggs (55%) hatched in this treatment than in control water (66%). The sesquiterpene lactone parthenin, identified in both the natural aquatic habitat harboring P. hysterophorus and root exudate-treated water was found to be responsible for the ovicidal effect. Moreover, larvae exposed to parthenin developed 2 to 3 days earlier but survived 4 to 5 days longer as adults (median larval survival time = 9 days (all replicates);11 to 12 days as adults) than the non-exposed control (median larval survival time = 11 days (reps 1 & 2), 12 days (rep 3); 6 to 7 days as adults). These results improve our understanding of the risk and benefits of oviposition site selection by gravid An. gambiae females and the role root exudate allelochemicals could play on anopheline bionomics, with potential implications in malaria transmission.

Derivatization increases mosquito larvicidal activity of the sesquiterpene lactone parthenin isolated from the invasive weed Parthenium hysterophorus.

BACKGROUND: Extracts of the invasive weed Parthenium hysterophorus (Asteraceae) have been shown to possess larvicidal activity against a wide range of disease vectors. However, the phytochemicals responsible for the larvicidal activity from this plant remain unidentified. Here, we isolated the major sesquiterpene lactone, parthenin (1) from the plant and synthesized two derivatives [ethylene glycol (2) and azide (3) derivatives] targeting the α,ß-unsaturated carbonyl group, previously known to account for its biological activity such as toxicity towards cells and microorganism. All three compounds were screened for larvicidal activity against the African malaria vector Anopheles gambiae. RESULTS: The larval mortality of ethylene glycol derivative (2) and 2α-azidocoronopilin (3) were approximately two-four-fold higher than that of parthenin (1) and neem oil with LC50 values of 37 and 66 mg L-1 , respectively. Parthenin (1) and the positive control, neem oil, had comparable median lethal concentration (LC50 ) values of 154 and 121 mg L-1 , respectively. In assays with binary combinations of the three compounds, larvicidal activity followed the order: parthenin (1) + 2α-azidocoronopilin (3) (LC50 = 14 mg L-1 ) > parthenin (1) + ethylene glycol derivative (2) (LC50 = 109 mg L-1 ), > blend of 2α-azidocoronopilin (3) and ethylene glycol derivative (2) (LC50 = 200 mg L-1 ). CONCLUSION: Structural modification of parthenin (1) through addition of hydroxyl groups increases its larvicidal effects. These findings advance the use of structural modification approach in the development of lead chemical molecules for potential exploitation in larval source management.

Antagonistic effect of alkaloids and saponins on bioactivity in the quinine tree (Rauvolfia caffra sond.): further evidence to support biotechnology in traditional medicinal plants.

BACKGROUND: The Quinine tree (Rauvolfia caffra) is used as a medicinal plant among traditional communities in many countries to manage tumors and other diseases associated with oxidative stress. To validate indigenous knowledge and possibly position this herb for technology uptake and utilization, we established the level of antioxidant activity in R. caffra, and probed for the presence of associated phytochemicals. METHODS: Antioxidant activity was determined on 1,1-diphenyl-2-picrylhydrazyl (DPPH) while major phytochemicals were identified by multiple tests on methanol fractions. RESULTS: R. caffra showed promise as a cure, with antioxidant activity comparable to the commercially used drug quercetin (R. caffra = 79.7% ±1.9; quercetin = 82.6% ± 2.0). However, we found two phytochemicals with possible antagonistic effect: co-occurrence of alkaloids and saponins significantly reduced antioxidant activity (alkaloids only = 63%; alkaloids plus saponins = 15%; steroids, terpenoids and cardiac glycosides = 82%), thus alkaloids and saponins should be exclusive to each other in drug formulations. CONCLUSIONS: Antagonistic relationship among phytochemicals would affect the efficacy of crude extracts as used in traditional medicine. Unlike in herbal medicine, use of modern biotechnology in extraction, purification and design of optimal combinations will ensure efficient drug formulations with optimum bioactivity and minimum toxicity. Metabolic pathway engineering under a controlled environment may optimize availability of desired compounds.