perceive and respond to biotic stress through a complex series of
transcriptional, proteomic and metabolic changes which often improve subsequent
resistance. On going CMAVE projects in maize include the investigation of
effectors present in insect and pathogens (defense modulators), activation of
phytohormone cascades, role of Plant Elicitor Peptides as endogenous signal
amplifiers, and induction of terpenoid-based plant defenses. Significant
overlap in insect and pathogen-induced maize defense responses occur during
stem attack by the European corn borer (ECB; Ostrinia nubilalis).
Investigation of this system lead to the recent discovery of a complex series
of acidic ent-kaurene-related diterpenoid and b-macrocarpene-derived
sesquiterpenoid phytoalexins. Despite broad activation of maize stem defenses,
subsequent ECB larval growth actually improves. This relationship strongly
contrasts the expectation of induced plant resistance. Improved performance on
previously attacked stem tissues is attributable to greatly increased total
protein levels and large scale metabolic changes. Unlike other Lepidoptera
pests examined, European corn borer larvae excrete high levels of the
phytohormone indole-3-acetic acid (IAA) into the feeding tunnel through frass contact.
While maize plants rapidly conjugate and remove exogenous IAA, stem tissue
concentrations remain elevated during European corn borer attack. Consistent
with an effector that promotes plant susceptibility, application of stable IAA
analogs trigger significant increases in stem protein levels. The tailoring of
plant responses to any biotic attacker is limited in specificity and vulnerable
to manipulation by specialists; however, mechanistic biochemical knowledge
creates testable approaches for targeted improvements in plant resistance.