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 resistance mediated through latex 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 feeding by D. balteata adults than latex from undamaged plants when applied onto the surface of artificial diet under choice conditions; no such difference was found in choice tests using latex from damaged and undamaged Tall Guzmaine plants. 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 level of phenylalanine ammonia lyase and polyphenol oxidase was also significantly higher in the Valmaine latex than in Tall Guzmaine latex. These studies suggest that 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, demonstrated previously in Valmaine latex.