Abstract
When plants are under insect herbivore attack defensive measures are activated not only locally, but also in distant and systemic tissues. While insect elicitors (IE) abundant in the oral secretions of the attacking herbivore are essential in the regulation of induced defenses, little is known about their effects on systemic defense signaling in maize (Zea mays). The goal of this study was therefore to identify genetic markers that can be used to further characterize local and systemic signaling events induced by IE or mechanical wounding (MW). We selected genes for this study based on their putative involvement in signaling (allene oxide synthase), regulation of gene expression (transcription factor MYC7), and in direct defenses (ribosome inactivating protein) and analyzed their expression in different sections of the treated leaf as well as in systemic parts of the same plant. We found the most significant transcript accumulation of the selected genes after treatment with insect elicitors in those parts with increased JA levels. Additionally, treatment with IE did also induce the accumulation of MYC7 transcripts in basal parts of the treated leaf and systemically. MW, in contrast, did induce RIP and AOS only locally, but not MYC7. This local suppression of MYC7 was further studied by adding glutathione (GSH) as an electron donor to MW plants to quench putative α, β-unsaturated carbonyls, which build up to significant levels around the damage site. Indeed, GSH-treated MW plants accumulated MYC7 at the damage site and also produced more volatiles, suggesting a putative redox-regulatory element being involved in the suppression of MYC7. The results presented herein provide evidence for the specific induction of distant signaling events triggered by IE, most likely through electric signaling. Additionally, a putative role for MW-induced α, β-unsaturated carbonyls in the transcriptional regulation of defense genes was discovered.
Highlights
Plants in natural settings and agriculture are constantly exposed to a multitude of biotic stresses mainly caused by pathogen infection and insect herbivore attack
For insect herbivory signaling mechanisms that activate anti-herbivore defenses include the recognition of movement, mechanical damage, and compounds in the oral secretions of insect herbivores, all leading to the production of inducible defenses like toxic secondary metabolites and proteins that inhibit the digestion of nutrients in the insect gut system [1,2,3]
In a comprehensive study by Schmelz and coworkers [4] it was shown that volicitin and N-linolenoylglutamine had the highest biological activity in maize seedlings when analyzed as accumulation of jasmonic acid (JA)
Summary
Plants in natural settings and agriculture are constantly exposed to a multitude of biotic stresses mainly caused by pathogen infection and insect herbivore attack. Insect elicitors (IE) abundant in the oral secretions of many insect herbivores were shown to induce defense responses in maize seedlings that were comparable to those observed after real caterpillar damage [4,5]. Volicitin is composed of linolenic acid, which conjugated to glutamine. Further analyses of insect oral secretions revealed the abundance of related compounds that exhibit elicitor activities like linolenoyl-glutamine and linolenoyl-gluatmate [7] and were found in crickets and fruit flies [7,8,9]. In a comprehensive study by Schmelz and coworkers [4] it was shown that volicitin and N-linolenoylglutamine had the highest biological activity in maize seedlings when analyzed as accumulation of jasmonic acid (JA)
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