Enzyme induction as a possible mechanism for latex-mediated insect resistance in romaine lettuce

Plant latex is a known storehouse of various secondary metabolites with demonstrated negative impact on insect fitness. A romaine lettuce cultivar, "Valmaine", possesses a high level of latex-mediated resistance against the banded cucumber beetle, Diabrotica balteata LeConte (Coleoptera: Chrysomelidae), compared to a closely related cultivar "Tall Guzmaine". Latex from damaged Valmaine plants was much more deterrent to adult D. balteata feeding than latex from undamaged plants when applied to the surface of artificial diet under choice conditions; no such difference was found in choice tests with latex from damaged and undamaged Tall Guzmaine plants. The intensities of whiteness and browning were significantly higher in Valmaine latex than in Tall Guzmaine latex. The activities of three enzymes (phenylalanine ammonia lyase, polyphenol oxidase, and peroxidase) significantly increased over time in latex from damaged Valmaine plants (i.e., 1, 3, and 6 days after feeding initiation), but they remained the same in Tall Guzmaine latex. The constitutive levels of phenylalanine ammonia lyase and polyphenol oxidase also were significantly higher in Valmaine latex than in Tall Guzmaine latex. These studies suggest that Valmaine latex chemistry may change after plant damage due to increased activity of inducible enzymes and that inducible resistance appears to act synergistically with constitutive resistance against D. balteata.