Resistance to insects

The commercial production of eggplants is frequently hampered in western countries by attacks by Colorado potato beetle (CPBL) (eptinotarsa decemlineata Say). Because the eggplant germplasm lacks an effective resistance gene to CPB, improvement of insect resistance via biotechnology represents the only alternative for the generation of insect-resistant eggplants. The insecticidal crystal protein genes (Cry) of Bacillus thuringiensis Berl. represent an important gene family for the biotechnological improvement of resistance to certain insects (mainly lepidoptera and coleoptera) in cultivated plants.⁷⁴ The native Cry IIIB gene was first used to produce eggplants resistant to the CPB⁷¹ following the transformation protocol described by Billings et al.⁶⁹ A study of the seedlings of eight independent transgenic lines revealed the absence of any significant resistance to the first and second in star larvae of the CPB. Using amodified synthetic Cry IIIA gene⁷² several lines of transgenic eggplants were generated that now showed resistance to neonate larvae and adult CPB under field conditions. These lines showed higher expression of the Cry IIIA gene than the previously tested native Cry IIIB gene.⁷¹ A mutagenised version of the Cry IIIB gene has been transferred by Arpaia et al.⁷⁵ to the female parent of the commercial eggplant F1 hybrid ‘Rimina’. Over 150 transgenic plants were produced among which 23 showed high expression of the toxin and significant insecticidal activity on neonate CPB larvae. Further tests were done on selfed transgenic progeny showing significant resistance to CBP and higher yields under natural infestation in field conditions.⁷⁶

In Asian countries, the most devastating insect of eggplants is a lepidopteran fruit and stem borer (Leucinodes orbonalis Guenee) . Kumar et al.,⁷⁷ using the transformation method of Rotino and Gleddie,⁶⁷ have generated transgenic Brinjal type eggplants expressing a synthetic CryIAb gene. Plants strongly expressing the toxin showed significant insecticidal activity against the larvae in bioassay studies. Glasshouse and field evaluations remain to be performed to critically evaluate the practical usefulness of the strategy. Altogether, these results demonstrate that recovery of a high level of CPB resistance can be achieved by using modified Cry Bt genes rather than native ones and that biotechnology is an efficient strategy for the control of CPB in eggplants. Another type of gene for plant resistance to insects has been tested.⁷⁰ It encodes proteinase inhibitors that prevent digestion of plant proteins by some coleopteran and hemipteran insects.⁷⁸ La Porta et al.⁷⁰ have transformed eggplant using an optimised protocol of Rotino and Gleddie⁶⁷ with a soybean gene encoding a cysteine inhibitor of proteases, but information on the resistance to insects of the transgenic plants generated is lacking.