Right track. Genes are switched off by increased methylation or decreased acetylation of histones (for the reasons you mentioned).
Here, something is prompting the leaves to start producing this protein (question says a chemical in the saliva makes leaves start to produce the protein). So, it means a gene is switched on and transcription/translation is taking place (also look at transcription/translation regulators, i.e. transcription factors (activators and repressors)).