Some plant proteins have been detected in frass of caterpillars (i.e. post-digestion), suggesting that they are particularly resistant to digestive proteases and can play a role in continuous post-ingestion defense. Some of them are JA-responsive, some of them clearly implicated in plant defense. Depletion of specific amino acids has been proposed as a plant defense mechanism, in tomato by threonine deaminase and arginase, respectively, leading to nutrient imbalances.In maize, two particular proteins found in frass suggest that depletion of the semi-essential amino acid Arg may be important in this plant too: Arg decarboxylase (forming agmatine + CO2) and agmatine deiminase. Agmatine has also some reported bioactivities, and especially can inhibit Arg decarboxylase, justifying why both enzymes are present and could be active.
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