Abscisic acid (ABA) is an isoprenoid-derived plant signaling molecule involved in a wide variety of plant processes, including facets of growth and development as well as responses to abiotic and biotic stress. ABA had previously been reported in a wide variety of animals, including insects and humans. We used high-performance liquid chromatography-electrospray ionization tandem mass spectrometry (HPLC-(ESI)-MS/MS) to examine concentrations of ABA in 17 species of phytophagous insects, including gall- and non-gall-inducing species from all insect orders with species known to induce plant galls: Thysanoptera, Hemiptera, Lepidoptera, Coleoptera, Diptera, and Hymenoptera. We found ABA in insect species in all six orders, in both gall-inducing and non-gall-inducing species, with no tendency for gall-inducing insects to have higher concentrations. The concentrations of ABA in insects often markedly exceeded those typically found in plants, suggesting it is highly improbable that insects obtain all their ABA from their host plant via consumption and sequestration. As a follow-up, we used immunohistochemistry to determine that ABA localizes to the salivary glands in the larvae of the gall-inducing Eurosta solidaginis (Diptera: Tephritidae). The high concentrations of ABA, combined with its localization to salivary glands, suggest that insects are synthesizing and secreting ABA to manipulate their host plants. The pervasiveness of ABA among both gall- and non-gall-inducing insects and our current knowledge of the role of ABA in plant processes suggest that insects are using ABA to manipulate source-sink mechanisms of nutrient allocation or to suppress host-plant defenses. ABA joins the triumvirate of phytohormones, along with cytokinins (CKs) and indole-3-acetic acid (IAA), that are abundant, widespread, and localized to glandular organs in insects and used to manipulate host plants.
Cytokinins are a class of phytohormones which play an important role in a variety of plant processes, such as cell growth and differentiation, formation of roots and shoots, and the reallocation of nutrients to form mobilizing sinks. Exogenous cytokinins found in plant‐manipulating insects are known to contribute to many effects on host plants. Perhaps the most apparent effect is that cytokinins, in the presence of auxin, leads to cell division and the proliferation of plant tissue; consequently, plant galls or tumors are formed. Cytokinins have also been implicated in the reallocation of sugars and nutrients for the oviposition site to act as a mobilizing sink, where the intention may be gall induction or sustenance for the organism. There are two known pathways for the biosynthesis of isoprenoid cytokinins. The first is the tRNA‐ipt pathway, in which an adenine residue is modified by the addition of an isoprenoid chain by the tRNA‐ipt gene. In plant plastids and bacteria, the methyl erythritol (MEP) pathway is the main source of the prenyl group, but in the cytosol of plants and insects, the prenyl group is synthesized via the mevalonate (MVA) pathway. Cytokinins are produced upon the degradation of the prenylated tRNA. To further investigate the role of cytokinins in gall induction by insects, I performed gene expression experiments using the gall‐inducing Eurosta solidaginis and its bacterial symbiont Wolbachia sp. I also performed targeted gene expression analyses in order to examine the expression of genes in the MEP, MVA, and tRNA‐ipt pathways. Up‐regulation of these pathways in the salivary gland implies that cytokinins are being synthesized in the salivary glands by either the insect and/or its bacterial symbiont. Along with the known widespread distribution of cytokinins in insects, these further imply that cytokinin production and secretion has been amplified and is used by the insects to manipulate their host plants. However, the genes involved in cytokinin production have also been found in other insect groups, so this plant manipulation may not just be exclusive to gall‐inducing insects, suggesting widespread hostplant manipulation among plant‐eating insects. Support or Funding Information NIH MBRS‐RISE: R25‐GM059298
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