The effect of ethylene in developing fruit on herbivore performance

Background/Question/Methods

Plant hormones play key roles in mediating host plant defense against herbivores. The phytohormone ethylene is commonly emitted by plants in response to herbivory and has been demonstrated to facilitate the production of a variety of defensive proteins. However, the role of ethylene in plant-herbivore interactions is considerably understudied compared to hormones jasmonic acid and salicylic acid. We investigated the effect of ethylene production by host plant, Gossypium hirsutum (cotton) (Malvaceae) on herbivore, cotton fleahopper, Psedatomoscelis seriatus, (Hemiptera: Miridae), performance. The study was completed using cotton fruit at various stages of development and we experimentally manipulated the ethylene evolution of cotton using an ethylene precursor, 1-Aminocyclopropane-1-carboxylicacid (ACC), and an ethylene perception inhibitor, 1-methylcyclopentane (1-MCP). We measured herbivore survival, fecundity and ability to induce ethylene emission and defensive proteins (polyphenoloxidase (PPO) and peroxidase (POD)) in cotton.  To test survival, we measured daily mortality of caged fleahoppers on treated or untreated cotton fruit and to measure fecundity we determined the daily number of eggs laid during a week on treated or untreated whole cotton plants by caged (2 male and 1 female) cotton fleahoppers. Ethylene, PPO, and POD levels were measured after 2 days of fleahopper infestation on treated or untreated cotton fruit.

Results/Conclusions

There was no difference in fleahopper survival and fecundity between treatments on cotton up to 3 weeks old.  However, on older cotton, 1-MCP treatments significantly increased fleahopper survival and fecundity compared to control and ACC treatments. As expected, a single fleahopper induced significantly more ethylene in fruit of all ages than in fleahopper free controls. Application of 1-MCP effectively inhibited ethylene perception. Fleahopper, ACC, and 1-MCP treatments did not significantly induce defensive PPO and POD proteins in young cotton fruit.  This suggests that early season cotton does not invest in ethlyene-mediated protein defenses because of its ability to compensate for early season fruit loss. In contrast, fleahopper and ACC treatments significantly induced, while 1-MCP depressed, PPO and POD production in fruit greater than 3 weeks. This suggests cotton’s investment in defense of its precious, mature reproductive structures.  Thus, ethlyene mediated defense responses by plants to facilitate tolerance have been demonstrated to be temporally plastic.