The Immunological And Biochemical Mechanism Of Enhanced Infectivity Of Zoophthora Radicans Reisolates Against Plutella Xylostella

A Pieris brassicae-derived Zoophthora radicans isolate, R0, and its reisolates obtained by repeatedly passing through Plutella xylostella larvae, R1-R5, were bioassayed against the second instar larvae of the new host P. xylostella in the laboratory. The reisolates tended to exhibit higher infectivity to the new host than the original isolate as the number of host passages increased. Based on time-concentration-mortality modeling, the LC20 (a lethal concentration to cause 20% mortality) on days 1-6 after exposure to spore showers was estimated as 14.7, 14.5, 9.0, 7.1, 6.0 and 5.5 spores/mm2 respectively for R0; 9.6, 5.0, 4.2, 3.6, 3.1 and 3.0 spores/mm2 for R1; 4.6, 2.9, 2.8. 2.5, 2.4 and 2.2 spores/mm2 for R3; and 5.2, 3.7, 3.2, 2.8, 2.6 and 2.6 sproes/mm2 for R5.The phenoloxidase activities were significantly lower in the larval hemonymph stimulated by the infection of the reisolates than that by the infection of the original isolate. There was also a positive correlation between the log10 (LC20) and the detected phenoloxidase activities among the fungal isolates (0.85 r2 0.95). Apparently, the increased infectivity to the new host may result from an ability of the original Z. radicans isolate to avoid or overcome its immune defense in the course of the repeated new host passages.The reisolates tended to exhibit higher specific activities of chitinase than the original isolate as the number of host passages increased, and this tendency was correspond to the enhanced infectivity of reisolates against P. xylostella (Ro). A positive correlation between the log10 (LC10) and the detected specific activities of chitinase among the fungal isolates in 3 day was confirmed (r2 =0.97, F =66.188,P =0.015). It is suggested that there were inherence relationship between chitinase activities excreted by Z. radicans isolates and their infectivity against P. xylostella.The PAGE analysis of soluble proteins of fungal isolates showed that there were the evident differences on the basis of constituent and concentration between original isolate (ARSEF1342) and reisolates (R1-R5). Based on above results, we inferred that the differences in the inheritance and characteristics between reisolatesand original isolate may exist.