Constitutive and induced insect resistance in RNAi-mediated ultra-low gossypol cottonseed cotton

Abstract

Background: Besides fibers, cotton plants also produce a large amount of seeds with a high oil and protein content. The use of these seeds is restricted by their high contents of the terpenoid gossypol, which is harmful to humans and livestock. Using a genetic engineering approach, "Ultra-low gossypol cottonseed" (ULGCS) plants were produced by knocking down an enzyme that catalyzes the formation of a precursor of gossypol. This was accomplished via RNAi-mediated silencing of the target gene using a seed-specific α-globulin promotor. Since gossypol is also a crucial defense mechanism against leaf-feeding herbivores, ULGCS plants might possess lower herbivore resistance than non-engineered plants. Therefore, we tested the constitutive and inducible direct insect resistance of two ULGCS cotton lines against the African cotton leafworm, Spodoptera littoralis.

Result: The herbivore was equally affected by both ULGCS lines and the control (Coker 312) line when feeding on fully expanded true leaves from undamaged plants and plants induced by jasmonic acid. When plants were induced by caterpillar-damage, however, S. littoralis larvae performed better on the ULGCS plants. Terpenoid analyses revealed that the ULGCS lines were equally inducible as the control plants. Levels of terpenoids were always lower in one of the two lines. In the case of cotyledons, caterpillars performed better on ULGCS cotton than on conventional cotton. This was likely caused by reduced levels of gossypol in ULGCS cotyledons.

Conclusion: Despite those effects, the insect resistance of ULGSC cotton can be considered as largely intact and the plants may, therefore, be an interesting alternative to conventional cotton varieties.

Keywords: Genetically modified crops; Glandless cotton; Gossypium hirsutum; Gossypol; Spodoptera littoralis; TAM66274; ULGCS.

Fig. 1

Concentration of gossypol in cotyledons as affected by plant type. Gossypol levels (μg/mg ± SE) in cotyledons from conventional cotton (Coker 312) and two ULGCS cotton lines (RNAi 1: 66-49B and RNAi 2: 66–274). One cotyledon was collected at the start of the feeding assay (day 1). The second cotyledon was collected on day four of the assay. Different capital letters over bars from the same sample day indicate a significant difference between plant types (Tukey HSD test), n = 32–36

Fig. 2

Impact of plant type and plant induction on concentration of terpenoids in cotton. Terpenoid levels (μg/mg ± SE) in the youngest leaves collected from conventional cotton (Coker 312) and two ULGCS cotton lines (RNAi 1: 66-49B and RNAi 2: 66–274). A) Cotton plants were untreated. B) Cotton plants had been exposed to three 2nd instar S. littoralis. C) Cotton plants were treated with jasmonic acid (4.8 μmol). Within induction treatments, different letters over the same chemical compound indicate a significantly difference between plant types (upper case letter: hemigossypolone; upper case letter italics: heliocide H1 + H4; lower case letter: gossypol) (Tukey HSD test), n = 20–25