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Txp40, a protein from Photorhabdus akhurstii conferred potent insecticidal activity against the larvae of Helicoverpa armigera, Spodoptera litura and S. exigua.

BACKGROUND
Txp40, a 37 kDa protein, previously characterized from the gram-negative bacterium, Photorhabdus akhurstii (symbiotically associates with insect-parasitic nematode, Heterorhabditis indica), conferred insecticidal activity against Galleria mellonella. Herein, biological activity of Txp40 was evaluated against economically important insects including Helicoverpa armigera, Spodoptera litura and S. exigua.


RESULTS
When both intra-hemocoel injected and orally fed to test insects, comparatively greater oral LD50 (187.7-522 ng/g) than injection LD50 (32.33-150.6 ng/g) was obtained with Txp40 derived from P. akhurstii strain IARI-SGMG3. Injection of purified Txp40 caused dose-dependent reduction in total circulatory hemocytes and hemocyte viability of fourth instar larvae of test insects at 12 h post incubation; unlike healthy cells toxin-treated ones displayed aggregated distribution. Injection of Txp40 significantly elevated the phenoloxidase activity of insect hemolymph which potentially led to unrestrained melanization reaction and ultimately larval death. Histological analyses showed the primary site of action of Txp40 in the insect midgut. An extensive damage to midgut epithelium 24 h after injection of the Txp40 explains the access of the toxin from hemocoel to midgut via leaky septate junctions. In silico analyses suggested that Txp40 can potentially interact with H. armigera midgut receptor proteins cadherin, ATP-binding cassettes, aminopeptidase N1 and alkaline phosphatase, to exert toxicity.


CONCLUSION
We propose Txp40 as an attractive alternative to Cry toxins of Bacillus thuringiensis, the transgenic expression of which is continually being reported to cause resistance development in insects. This article is protected by copyright. All rights reserved.