Plant Defense against Insect Herbivores

Abstract

Plants have been interacting with insects for several hundred million years, leading to complex defense approaches against various insect feeding strategies. Some defenses are constitutive while others are induced, although the insecticidal defense compound or protein classes are often similar. Insect herbivory induce several internal signals from the wounded tissues, including calcium ion fluxes, phosphorylation cascades and systemic- and jasmonate signaling. These are perceived in undamaged tissues, which thereafter reinforce their defense by producing different, mostly low molecular weight, defense compounds. These bioactive specialized plant defense compounds may repel or intoxicate insects, while defense proteins often interfere with their digestion. Volatiles are released upon herbivory to repel herbivores, attract predators or for communication between leaves or plants, and to induce defense responses. Plants also apply morphological features like waxes, trichomes and latices to make the feeding more difficult for the insects. Extrafloral nectar, food bodies and nesting or refuge sites are produced to accommodate and feed the predators of the herbivores. Meanwhile, herbivorous insects have adapted to resist plant defenses, and in some cases even sequester the compounds and reuse them in their own defense. Both plant defense and insect adaptation involve metabolic costs, so most plant-insect interactions reach a stand-off, where both host and herbivore survive although their development is suboptimal.

Alteration of plant primary metabolism in response to insect herbivory

“…Metabolites and energy can be allocated more efficiently if there is not only recognition of herbivory, but also reprogramming over time (Mithöfer and Boland, 2012). Within an hour after exposure to herbivore feeding, expression of genes encoding diverse plant metabolic pathways is altered (Fürstenberg-Hägg et al, 2013). However, most published research on plant responses to herbivory has been focused on longer time periods ranging from 6 hours to several days after the start of infestation.…”

The role of L-DOPA in plants

“…22,23 Nonprotein amino acids accumulate massively in many plants and seem to play an important role in resistance to herbivores. 24 Velvet bean seeds, as aforementioned, are known to accumulate large amounts of L-DOPA and are not attacked by small mammals or insects, suggesting a feeding repellent property. Southern armyworm larvae fed a diet containing seeds of velvet bean or synthetic L-DOPA showed an increased mortality.…”

Two Herbivore-Induced Cytochrome P450 Enzymes CYP79D6 and CYP79D7 Catalyze the Formation of Volatile Aldoximes Involved in Poplar Defense

“…They produce toxins and feeding deterrents that serve as direct defenses to herbivores and release volatile compounds that repel the herbivore or even attract herbivore enemies as indirect defenses. Among the many classes of defense chemicals, some may participate in direct and indirect defense (Fürstenberg-Hägg et al, 2013).…”