While it would be business suicide for any restauranteur to go out of their way to sicken those who eat on its premises, tomato plants depend on such a strategy for their very survival. When the larvae of herbivorous butterflies or moths begin to chew on tomato plant leaves, threonine deaminase, an enzyme commonly found in tomato leaves, changes to a form that enables it to survive the harsh conditions of the lepidopteran gut. In fact, it's the conditions of the gut that activate this Clark Kent of an enzyme into a superhero molecule that breaks the bonds of threonine, an amino acid essential for the lepidopteran larvae's survival. It may also add nails to the lepidopteran larvae's coffin by degrading another amino acid, L-serine, and by producing toxic ammonia.
Larva eating the leaf of a tomato, and the tomato's defensive response. Artist: Emily Harrington. Copyright: All rights reserved. See gallery for details.
"The growth and development of insect herbivores depends on their ability to acquire essential amino acids by digestion of plant protein. Here, we describe the biochemical and structural features of the defense-related TD2 [threonine deaminase paralog] isoform from tomato that exploits this nutritional vulnerability. TD2 appears to reduce herbivory by acting in the insect gut to degrade Thr[eonine], which is an essential and limiting nutrient for the growth of lepidopteran larvae (11)." (Gonzales-Vigil et al.:5897)