Anopheline larvicidal property of T. asperellum has been found recently in medical science. The mechanism of actions exhibited by T. asperellum to infect mosquito larvae is the pivotal context of our present study. To infect an insect, entomopathogens must undergo some events of pathogenesis. We performed some experiments to find out the mechanisms of action of T. asperellum against anopheline larvae and compared its actions with other two well recognized entomopathogens like Metarhizium anisopliae and Beauveria bassiana. The methodology adopted for this includes Compound light and SE Microscopic study of host–pathogen interaction, detection of fungal spore adhesion on larval surface (Mucilage assay), detection of cuticle degrading enzymes (Spore bound pr1, chitinase and protease) by spectro-photometric method, Quantitative estimation of chitinase and protease enzymes, and determination of nuclear degeneration of hemocyte cells of ME (methanolic extract) treated larvae by T. asperellum under fluorescence microscope. Compound light microscopic studies showed spore attachment, appressorium and germ tube formation, invasion and proliferated hyphal growth of T. asperellum on epicuticle and inside of dead larvae. SEM study also supported them. After 3 h of interaction, spores were found to be attached on larval surface exhibiting pink colored outer layer at the site of attachment indicating the presence of mucilage surrounding the attached spores. The enzymatic cleavage of the 4-nitroanilide substrate yields 4-nitroaniline which indicates the presence of spore-bound PR1 protein (Pathogenecity Related 1 Protein) and it was highest (absorbance 1.298 ± 0.002) for T. asperellum in comparison with control and other two entomopathogens. T. asperellum exhibited highest enzymatic index values for both chitinase (5.20) and protease (2.77) among three entomopathogens.
Fig: Interaction of T. asperellum spore with anopheline larvae under compound microscope (Olympus CX-31). (a) Germination of fungal spore and germ tube formation on larval surface (40×). (b) Cuticle penetration by formation of appresorium and infection peg (40×). (c) Hyphal proliferation of fungus on dead larvae (10×).
Quantitative experiment showed that chitinase enzyme concentration of T. asperellum (245 µg mL−1) was better than other two M. anisopliae (134.59 µg mL−1) and B. bassiana(128.65 µg mL−1). Similarly protease enzyme concentration of this fungus was best (298.652 µg mL−1) among three entomopathogens. Here we have detected and estimated fragmentized nuclei of hemocyte cells by fluorescence microscopy in treated larvae with different ME doses of T. asperellum, and also observed that mosquito larvae exposed to 0.1 mg mL−1 dose of ME showed maximum (100%) nuclear fragmentations of hemocytes and while 20, 45, 70 and 85% of nuclear deformities were recorded at 0.02, 0.04, 0.06 and 0.08 mg mL−1concentrations of ME. The knowledge of this work certainly will help in understanding of mechanism of action of T. asperellum for anopheline larval killing and consequently in eradication of malaria vector.
Ghosh, S.K., Podder, D. & Mukherjee, A. An insight of anopheline larvicidal mechanism of Trichoderma asperellum (TaspSKGN2). Sci Rep 11, 16029 (2021). https://doi.org/10.1038/s41598-021-95310-1