Using a developmental comparison to decipher priming of induced defenses in maize and its effects on a generalist herbivore

Plants have a diverse array of defensive strategies that are induced by herbivory. Some of these defenses, such as herbivore-induced plant volatiles (HIPVs) are airborne and can be perceived by other plants. Priming is a strategy for plants to capitalize on their perception of HIPVs. The steps by which an undamaged plant becomes primed after perception of HIPVs are poorly understood, but the effects of priming are distinctive. Once an herbivore starts to feed, the primed plant displays a faster and stronger defense response. In maize, whether caterpillar feeding induces a primed defense response and how feeding on primed maize impacts the caterpillar remains relatively unknown. The defense profile consists of direct and indirect defenses. To characterize these defense categories, I assessed volatile production as a measure of indirect defenses, transcript levels of proteinase inhibitors as an estimate of direct defenses, and jasmonic acid (JA) levels as a general indicator of a plant defense response. To evaluate this, I performed a developmental comparison of two seedling stages, v1 with three leaves and v3 with five leaves. Here, I report that simulated and actual herbivory differentially elicit volatile production and proteinase inhibitor transcript accumulation in two maize seedling stages. Once the defense profiles from each developmental stage had been characterized, I proceeded to compare primed and non-primed defense profiles in v1 and v3 plants. To establish that the plants responded to the priming agent, a HIPV, I measured JA levels shortly after exposure to the HIPV. Both developmental stages contained elevated JA levels, which indicated that the plants responded to the priming treatment. Caterpillars feeding on primed v1 and v3 maize compared to non-primed maize did not elicit faster or higher amounts of volatiles within twenty four hours. In addition, JA levels were not higher in primed versus non-primed plants at several points during four hours of caterpillar feeding. However, bioassays indicated that the youngest-v1primed maize plants reduced larval weights. In addition, primed v1 maize plants experienced less leaf iv damage. Since the bioassays indicated that the primed v1 maize plants reduced caterpillar growth, I then assessed at a molecular level, whether transcripts of proteinase inhibitors were higher in primed plants. However, the transcripts at 6, 24, 48, and 72 hours were not higher in primed versus non-primed plants. These results suggest that priming in maize may be most effective at certain developmental stages. My findings provoke a re-assessment of defense characterization in primed maize. The ontogenic comparison reveals that different life stages of maize may prioritize different defense strategies and therefore early stages such as v1 can be expected to exhibit a stronger primed direct defense response. Later life stages such as v3 may shift their emphasis to indirect defenses. These findings deepen our knowledge of an agronomically important crop and are essential for the development of sustainable pest management strategies, which may act in concert with maize’s own manifold defense system.