The GroEL homolog XnGroEL protein of Xenorhabdus nematophila belongs to a highly conserved family of molecular chaperones/heat shock proteins (Hsps). XnGroEL was shown to possess oral insecticidal activity against a major crop pest Helicoverpa armigera. Under normal conditions, the Hsps/chaperones facilitate folding, assembly, and translocation of cellular proteins, while in stress conditions they protect proteins from denaturation. In this study, we describe generation of transgenic tomato plants overexpressing insecticidal XnGroEL protein and their tolerance to biotic and abiotic stresses. Presence of XnGroEL in the transgenic tomato lines conferred resistance against H. armigera showing 100% (p ≤ 0.001) mortality of neonates. In addition, XnGroEL provided thermotolerance and protection against high salt concentration to the tomato plants. Expression of XnGroEL minimized photo-oxidation of chlorophyll and reduced oxidative damage of cell membrane system of the plants under heat and salt stress. The enhanced tolerance to abiotic stresses correlated with increase in the anti-oxidative enzyme activity and reduced H2O2 accumulation in transgenic tomato plants. The variety of beneficial properties displayed by XnGroEL protein provides an opportunity for value addition and improvement of crop productivity.
Xenorhabdus nematophila is an entomopathogenic bacteria. It secretes a GroEL homolog, XnGroEL protein, toxic to its larval prey. GroEL belongs to the family of molecular chaperones and is required for proper folding of cellular proteins. Oral ingestion of insecticidal XnGroEL protein is toxic to Helicoverpa armigera, leading to cessation of growth and development of the larvae. In the present study, the insecticidal efficacy of XnGroEL against H. armigera has been evaluated in transgenic tobacco plant expressing the protein. A 1.7-kb gene encoding the 58-kDa XnGroEL protein was incorporated into the tobacco genome via Agrobacterium-mediated transformation. The stable integration of the transgene was confirmed by Southern blot analysis and its expression by RT-PCR and western blot analyses in transgenic plants. The transgenic lines showed healthy growth and were phenotypically normal. Insect bioassays revealed significant reduction of 100 % in the survival of larvae (p < 0.001) and 55-77 % reduction in plant damage (p < 0.05 and p < 0.001) compared to the untransformed and vector control plants. The results demonstrate that XnGroEL is a novel potential candidate for imparting insect resistance against H. armigera in plants.
Cocoa (Theobroma cacao L.) is an important beverage crop and commercially grown as a plantation crop. With the changing climate, the tea mosquito bug species complex, viz., Helopeltis theivora, H. bradyi and H. antonii, is emerging as a major threat to cocoa cultivation in India. Among the species of this complex H. theivora is responsible for causing considerable damage. The present investigations were carried out to find a weak link in the life cycle of H. theivora so it can be managed effectively. Specimens of the tea mosquito bug were found to first appear during the first week of September in South India. Helopeltis theivora requires on average 29.28 days to complete its life cycle on cocoa. The highest level of natural mortality was recorded in the first-instar nymph. The total developmental period of the fifth-instar nymph was significantly longer than that for the other nymphal instars. The sex ratio reflects that the population is highly female-biased, which may contribute to the dominant nature of this species in the cocoa ecosystem. Observation of the feeding and oviposition behavior of H. theivora revealed that the insect prefers to feed and oviposit on developing pods rather than on leaves and shoots. Analysis of the species distribution of tea mosquito bug at different elevations revealed that cocoa gardens situated less than 300 meter above mean sea level are dominated by H. theivora, whereas gardens situated more than 300 meter above mean sea level are dominated by H. bradyi. This separation of species across elevation may be driven either by abiotic or biotic factors.
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