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Abstract
Corn plants have a variety of defense mechanisms against insects, including benzoxazinoids, major secondary metabolites in the plant, and a number of oxidized fatty acids mediating corn defense responses. The effects of benzoxazinoids on the specialist insect western corn rootworm (WCR, Diabrotica virgifera virgifera LeConte) and the congeneric generalist, southern corn rootworm (SCR, Diabrotica undecimpunctata howardi Barber) were determined by using a benzoxazinoid-deficient corn line, 428G, that carries a homozygous mutant bx1 gene, and H88, the wild-type parental line of 428G with normal benzoxazinoid production. Unlike SCR, WCR gained significantly more weight on H88 than 428G plants, suggesting that specialists and generalists respond distinctly to benzoxazinoids. A small amount of benzoxazinoids was found in roots of 428G plants. Inducibility of the biosynthesis of benzoxazinoids in roots of 428G plants by insect feeding was analyzed by comparing the concentration of benzoxazinoids in insect-infested roots and the unifested roots. The results showed that benzoxazinoids are constitutively synthesized in 428G plants.
The functions of oxylipin-mediated defense responses to either WCR or SCR were analyzed by using corn plants defective in either two oxo-phytodienoate reductase genes, opr7 and opr8 (Zmopr7/8), 9-Lipoxygenase 3 (Zmlox3) or 13-Lipoxygenase 10 (Zmlox10) and their corresponding wild types. No significant differences were observed in the performance of SCR fed on JA-deficient and JA-containing corn plants and the corresponding wild types, suggesting that JA might not be involved in the corn defense response against SCR. Unlike WCR, SCR grew better on Zmlox10 mutant plants which had a significantly reduced amount of dinor-OPDA (dnOPDA) and 10-OPEA. Taken together these studies advance our understanding about the interaction between specialist and generalist corn rootworms and benzoxazinoids, and provide evidence that LOX10 is involved in corn defense against southern corn rootworm larvae. The results also provide physiological bases for further research on the defense compounds involved in each of the signaling cascade.
