Abstract
Feeding by chewing insects induces chemical defenses in plants that are regulated by the jasmonic acid (JA) pathway. Jasmonates are usually quantified by liquid chromatography–mass spectrometry (LC-MS) analysis of precursors and products in the biosynthetic pathway or inferred from the extraction and expression of genes known to respond to elevated levels of JA. Both approaches are costly and time consuming. To address these limitations, we developed a rapid reporter for the synthesis of JA based on the OPR3promoter:YFP-PTS1. Yellow fluorescent protein (YFP) fluorescence was increased by mechanical wounding and methyl jasmonate (MeJA) treatment and by caterpillar feeding. To develop an optimal sampling time for a quantitative bioassay, OPR3promoter:YFP-PTS1 plants were sampled at 1, 2, 3, and 24 h after treatment with 115 µM MeJA. The first increase in YFP fluorescence was detected at 2 h and remained elevated 3 and 24 h later; as a result, 3 h was chosen as the sampling time for a quantitative bioassay of jasmonate response to insect attack. Feeding by Pieris rapae caterpillars induced a 1.8-fold increase in YFP fluorescence, consistent with the known induction of JA production by this insect. We also assessed the utility of this reporter in studies of plant responses to caterpillar feeding vibrations, which are known to potentiate the JA-dependent production of chemical defenses. Pretreatment with feeding vibrations increased expression of the OPR3promoter:YFP-PTS1 in response to 14 µM MeJA. Feeding vibrations did not potentiate responses at higher MeJA concentrations, suggesting that potentiating effects of prior treatments can only be detected when plants are below a response threshold to the elicitor. The expression of OPR3 does not indicate levels of specific downstream jasmonates and quantification of specific jasmonates still requires detailed analysis by LC-MS. However, OPR3 expression does provide a rapid and inexpensive way to screen large numbers of plants for the involvement of jasmonate signaling in their response to a wide variety of treatments, and to study the induction and expression of AtOPR3.
Highlights
Plant perception of insect herbivores is complex and dynamic
A confocal laser scanning microscope was used to visualize the transient expression of Yellow fluorescent protein (YFP) fused to the promoter of the gene for the peroxisomal enzyme (Supplement S1) REDUCTASE 3 (OPR3), OPR3promoter:YFP-PTS1, in A. thaliana (Figure 1)
We developed a quick and inexpensive method to quantify YFP expression in leaves of A. thaliana plants with an OPR3promoter:YFP-PTS1 reporter and showed that it was induced by methyl jasmonate (MeJA) and caterpillar feeding within 1 h
Summary
Leaf feeding herbivores produce multiple signals of their attack, including tissue damage, oral secretions, and feeding vibrations. Vibrations of the leaf caused by insect feeding can potentiate defense responses to increase foliar and volatile defenses, independent of tissue damage and oral secretions (Appel and Cocroft 2014; Body et al, 2019). These local signals of insect attack can be transmitted to other parts of the leaf and other parts of the plant when they elicit internal chemical and electrical signals and external volatile signaling. Local tissue damage can elicit higher levels of ROS, Ca2+ signaling, electrical signaling, and production of jasmonates in other areas of the damaged leaf or other leaves on the plant (Miller et al, 2009; Mousavi et al, 2013; Koo 2018; Toyota et al, 2018)
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