Abstract
Genus Ocimum contains a reservoir of diverse secondary metabolites, which are known for their defense and medicinal value. However, the defense-related metabolites from this genus have not been studied in depth. To gain deeper insight into inducible defense metabolites, we examined the overall biochemical and metabolic changes in Ocimum kilimandscharicum that occurred in response to the feeding of Helicoverpa armigera larvae. Metabolic analysis revealed that the primary and secondary metabolism of local and systemic tissues in O. kilimandscharicum was severely affected following larval infestation. Moreover, levels of specific secondary metabolites like camphor, limonene and β-caryophyllene (known to be involved in defense) significantly increased in leaves upon insect attack. Choice assays conducted by exposing H. armigera larvae on O. kilimandscharicum and tomato leaves, demonstrated that O. kilimandscharicum significantly deters larval feeding. Further, when larvae were fed on O. kilimandscharicum leaves, average body weight decreased and mortality of the larvae increased. Larvae fed on artificial diet supplemented with O. kilimandscharicum leaf extract, camphor, limonene and β-caryophyllene showed growth retardation, increased mortality rates and pupal deformities. Digestive enzymes of H. armigera - namely, amylase, protease and lipase- showed variable patterns after feeding on O. kilimandscharicum, which implies striving of the larvae to attain required nutrition for growth, development and metamorphosis. Evidently, selected metabolites from O. kilimandscharicum possess significant insecticidal activity.
Highlights
Members of genus Ocimum have a unique blend of secondary metabolites which imparts them great medicinal properties as well as a peculiar flavor and taste [1]
Feeding- choice assays showed that H. armigera larvae consumed significantly less O. kilimandscharicum leaf tissue than tomato (Fig. 1A)
By the end of the fourth day, larvae had consumed all the tomato leaves and showed lower preference for O. kilimandscharicum leaves. Such resistance to feeding on O. kilimandscharicum leaves by H. armigera larvae clearly indicates the presence of defense compounds, which strongly deter larval feeding
Summary
Members of genus Ocimum have a unique blend of secondary metabolites which imparts them great medicinal properties as well as a peculiar flavor and taste [1]. Defense metabolites from these species have not been characterized, the insecticidal activity of the plant leaves against storage pests is reported [8]. Different species of Ocimum greatly differ in the composition of their secondary metabolites and may offer variable levels of resistance to specific insect pests. Known as camphor basil, is a relatively unexplored tropical plant species widely distributed in East Africa, India and Thailand. The species possesses a rich reservoir of secondary metabolites such as camphor, eucalyptol, limonene, geramacrene D and b-caryophyllene. These metabolites are reported to have insecticidal properties [9,10,11]. O. kilimandscharicum is an attractive system for studying potential insecticidal molecules
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