Development and evaluation of transgenic castor (Ricinus communis L.) expressing the insecticidal protein Cry1Aa of Bacillus thuringiensis against lepidopteran insect pests

Abstract

Castor is an industrially valued non-edible oilseed crop. Susceptibility of the crop to foliage feeders like Achaea janata (semilooper) and Spodoptera litura accounts for 30–50% of yield losses. Owing to a lack of reliable sources of resistance to these lepidopteran pests, attempts were made to develop transgenic events expressing the Bacillus thuringiensis (Bt) cry1Aa gene. Transformation of decotyledonated embryo axes through Agrobacterium tumefaciens, particle gun bombardment and in planta methods resulted in transformation frequencies of 2.4%, 1.1% and 2.1%, respectively. The presence and integration of the cry1Aa gene in the T0 plants was confirmed by polymerase chain reaction (PCR) and Southern hybridization analysis. Based on segregation for a Mendelian ratio of 3:1, eight events (AMT-894, AMT-899, AK1304-PB-1, AK1304-PB-4, AK1304-PB-785, AK1304-PB-830, AK1304-PB-837 and DTS-43) were advanced. ELISA analysis detected protein from 0.16 to 2.76 ng/mg fresh leaf tissue across events and in different generations. In laboratory insect bioassays, the mortality of S. litura and A. janata ranged from 20 to 80% in different transgenic events and the weight reduction of surviving larvae over the control larvae after 8 days of feeding was 28.4–87.2% in the case of S. litura and 27.9–78.1% for A. janata. In field bioassays, the event AMT-894 was most promising with 43% of plants showing less than 25% leaf damage. As part of the regulatory requirement to check the toxicity of the transgenic events to beneficial insects, larval bioassays against Samia cynthia ricini (eri silkworm) using three transgenic events (AK1304-PB-1, AK1304-PB-4 and AMT-894) showed a 20.2–78.5% reduction in weight.