Abstract:

The prophenoloxidase (proPO) activating system (proPO-AS) involved in crude extracts of rice planthopper Nilaparvata lugens (Stal) and green peach aphid Myzus persicae (Sulzer) was investigated in relation to potentially influential agents. The results showed that as Ca²⁺ concentration increased from 0.1 mmol/L to 100 mmol/L, the activities of phenoloxidase (PO) in the crude extracts of N. lugens and M. persicae were concentration dependent. The activation of the proPO peaked at 30 mmol/L of the Ca²⁺ concentration. However, a further increase of the Ca²⁺ to 100 mmol/L resulted in a decrease in proPO activation. Incubation of the extracts in conjunction with different concentrations of laminarin (from 10^-4 mg/mL to 10 mg/mL) witnessed the levels of proPO activation entirely depending on the laminarin concentrations. When different glucans were used as activating agents, laminarin and zysoman significantly enhanced the PO activities whereas the effect of curdlan was not significant compared to a blank control. In contrast, the PO activities were reduced when mannan, dextran or cellulose was added. These results indicated that the β-1,3-glucan effectively activated the proPO present in the extracts of the two insect species with the proPO-AS selectively responding to different glucans. The sequence of adding Ca²⁺ and phenylmethyl sulfonyl fluorid (a serine protease inhibitor) in the extracts also affected the PO activities.

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Key words: Nilaparvata lugens, Myzus persicae, prophenoloxidaseactivating system, prophenoloxidase, insect immunity