Caterpillar- and Salivary-Specific Modification of Plant Proteins

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Though there is overlap, plant responses to caterpillar herbivory show distinct variations from mechanical wounding. In particular, effectors in caterpillar oral secretions modify wound-associated plant responses. Previous studies have focused on transcriptional and protein abundance differences in response to caterpillar herbivory. This study investigated Spodoptera exigua caterpillar-specific post-translational modification of Arabidopsis thaliana soluble leaf proteins by liquid chromatography/electrospray ionization/mass spectroscopy/mass spectroscopy (LC/ESI/MS/MS). Given that caterpillar labial saliva contains oxidoreductases, such as glucose oxidase, particular attention was paid to redox-associated modifications, such as the oxidation of protein cysteine residues. Caterpillar- and saliva-specific protein modifications were observed. Differential phosphorylation of the jasmonic acid biosynthetic enzyme, lipoxygenase 2, and a chaperonin protein is seen in plants fed upon by caterpillars with intact salivary secretions compared to herbivory by larvae with impaired labial salivary secretions. Often a systemic suppression of photosynthesis is associated with caterpillar herbivory. Of the five proteins modified in a caterpillar-specific manner (a transcription repressor, a DNA-repair enzyme, PS I P700, Rubisco and Rubisco activase), three are associated with photosynthesis. Oxidative modifications are observed, such as caterpillar-specific denitrosylation of Rubisco activase and chaperonin, cysteine oxidation of Rubisco, DNA-repair enzyme, and chaperonin and caterpillar-specific 4-oxo-2-nonenal modification of the DNA-repair enzyme.