OPDA regulates maize defense against aphids

Copyright © 2019 American Society of Plant Biologists. DOI:10.1104/pp.18.0147

Root-expressed maize lipoxygenase 3 negatively regulates induced systemic resistance to Colletotrichum graminicola in shoots

We have previously reported that disruption of a maize root-expressed 9-lipoxygenase (9-LOX) gene, ZmLOX3, results in dramatic increase in resistance to diverse leaf and stalk pathogens. Despite evident economic significance of these findings, the mechanism behind this increased resistance remained elusive. In this study, we found that increased resistance of the lox3-4 mutants is due to constitutive activation of induced systemic resistance (ISR) signaling. We showed that ZmLOX3 lacked expression in leaves in response to anthracnose leaf blight pathogen Colletotrichum graminicola, but was expressed constitutively in the roots, thus, prompting our hypothesis: the roots of lox3-4 mutants are the source of increased resistance in leaves. Supporting this hypothesis, treatment of wild-type plants (WT) with xylem sap of lox3-4 mutant induced resistance to C. graminicola to the levels comparable to those observed in lox3-4 mutant. Moreover, treating mutants with the sap collected from WT plants partially restored the susceptibility to C. graminicola. lox3-4 mutants showed primed defense responses upon infection, which included earlier and greater induction of defense-related PAL and GST genes compared to WT. In addition to the greater expression of the octadecanoid pathway genes, lox3-4 mutant responded earlier and with a greater accumulation of H(2)O(2) in response to C. graminicola infection or treatment with alamethicin. These findings suggest that lox3-4 mutants display constitutive ISR-like signaling. In support of this idea, root colonization by Trichoderma virens strain GV29-8 induced the same level of disease resistance in WT as the treatment with the mutant sap, but had no additional resistance effect in lox3-4 mutant. While treatment with T. virens GV29 strongly and rapidly suppressed ZmLOX3 expression in hydroponically grown WT roots, T. virens Δsml mutant, which is deficient in ISR induction, was unable to suppress expression of ZmLOX3, thus, providing genetic evidence that SM1 function in ISR, at least in part, by suppressing host ZmLOX3 gene. This study and the genetic tools generated herein will allow the identification of the signals regulating the induction of resistance to aboveground attackers by beneficial soil microorganisms in the future

Maize defense elicitor, 12-oxo-phytodienoic acid, prolongs aphid salivation

12-Oxo-phytodienoic acid (OPDA), an intermediate in the jasmonic acid (JA) biosynthesis pathway, regulates diverse signaling functions in plants, including enhanced resistance to insect pests. We previously demonstrated that OPDA promoted enhanced callose accumulation and heightened resistance to corn leaf aphid (CLA; Rhopalosiphum maidis), a phloem sap-sucking insect pest of maize (Zea mays). In this study, we used the electrical penetration graph (EPG) technique to monitor and quantify the different CLA feeding patterns on the maize JA-deficient 12-oxo- phytodienoic acid reductase (opr7opr8) plants. CLA feeding behavior was unaffected on B73, opr7opr8 control plants (- OPDA), and opr7opr8 plants that were pretreated with OPDA (+ OPDA). However, exogenous application of OPDA on opr7opr8 plants prolonged aphid salivation, a hallmark of aphids’ ability to suppress the plant defense responses. Collectively, our results indicate that CLA utilizes its salivary secretions to suppress or unplug the OPDA-mediated sieve element occlusions in maize

Nonsensical choices? Fall armyworm moths choose seemingly best or worst hosts for their larvae, but neonate larvae make their own choices.

Selecting optimal host plants is critical for herbivorous insects, such as fall armyworm (Spodoptera frugiperda), an important maize pest in the Americas and Africa. Fall armyworm larvae are presumed to have limited mobility, hence female moths are presumed to be largely responsible for selecting hosts. We addressed host selection by fall armyworm moths and neonate and older (3rd-instar) larvae, as mediated by resistance and herbivory in maize plants. Thus, we compared discrimination among three maize cultivars with varying degrees of resistance to fall armyworm, and between plants subjected or not to two types of herbivory. The cultivars were: (i) susceptible, and deficient in jasmonic acid (JA) production and green leaf volatiles (GLV) emissions (inbred line B73-lox10); (ii) modestly resistant (B73), and; (iii) highly resistant (Mp708). The herbivory types were: (i) ongoing (= fall armyworm larvae present), and; (ii) future (= fall armyworm eggs present). In choice tests, moths laid more eggs on the highly resistant cultivar, and least on the susceptible cultivar, though on those cultivars larvae performed poorest and best, respectively. In the context of herbivory, moths laid more eggs: (i) on plants subject to versus free of future herbivory, regardless of whether plants were deficient or not in JA and GLV production; (ii) on plants subject versus free of ongoing herbivory, and; (iii) on plants not deficient in compared to deficient in JA and GLV production. Neonate larvae dispersed aerially from host plants (i.e. ballooned), and most larvae colonized the modestly resistant cultivar, and fewest the highly resistant cultivar, suggesting quasi-directional, directed aerial descent. Finally, dispersing older larvae did not discriminate among the three maize cultivars, nor between maize plants and (plastic) model maize plants, suggesting random, visually-oriented dispersal. Our results were used to assemble a model of host selection by fall armyworm moths and larvae, including recommendations for future research

Host plant finding and preferences of older (3^rd-instar) fall armyworm larvae for three maize inbred lines, B73-<i>lox10</i>, B73, and Mp708.

<p>Ten 3^rd-instar larvae were released at the center of a circular arena in which one seedling of each of the three maize inbred lines were positioned near the arena’s margin, and the number of larvae on each of the seedlings was recorded after 2, 15, 60, 120, 180, 240, and 300 min. While increasingly more larvae were found on seedlings over time (F_{6, 104} = 35.41, P < 0.001), differences were not detected among the numbers of larvae on seedlings of the different maize inbred lines (F_{2, 104} = 0.56, P = 0.583), and the numbers of larvae on seedlings were unaffected by any interaction between maize inbred line and time (F_{12, 104} = 1.19, P = 0.309).</p

Host finding by older (3^rd-instar) larvae in relation to host plant volatiles.

<p>The attraction of 3^rd-instar larvae to model (constructed of plastic and metal wire) or real seedlings of two maize inbred lines, B73-<i>lox10</i> and B73, was assessed in a paired choice-test, independently for each inbred line. In each trial, ten 3^rd-instar larvae were released in the center of a circular arena in which three each, model and real (B73 or B73-<i>lox10</i>) seedlings were positioned along the arena’s margin. After release, larvae were monitored for 5 min, and first-encounters of larvae with a seedling were recorded by observing then removing larvae immediately upon encountering a seedling. First-encounters of larvae with seedlings were unaffected by plant nature (model or real seedling, F_{1, 32} = 3.883, P = 0.058), maize inbred line (B73 or B73-<i>lox10</i>, F_{1, 32} = 0.083, P = 0.775), or any interaction between those variables (F_{1, 32} = 0.104, P = 0.749). In the plot, maize inbred line is indicated with open (B73-<i>lox10</i>) or closed (B73) circles, and seedling nature (real or model) is on the horizontal axis.</p

Colonization and settling on seedlings of three maize inbred lines, B73-<i>lox10</i>, B73, and Mp708 by neonate fall armyworm larvae.

<p>Two-hundred fall armyworm eggs were deployed at dusk on a seedling (maize inbred line W438) positioned at the center of a circular arena and surrounded with six seedlings (two of each: B73-<i>lox10</i>, B73, Mp708). The numbers of neonate larvae on each of the six seedlings (other than the seedling at the arena’s center) were counted at 0–24 h and 84–108 h after hatching; the larvae counted at 0–24 h were considered to have <i>colonized</i> a seedling, while those counted at 84–108 h were considered to have <i>settled</i> on a seedling. Differences in the frequency of colonization among seedlings of the three maize inbred lines <b>(A)</b>, and between colonizing and settled larvae <b>(B)</b> were detected, but no interaction was detected between maize inbred line and colonizing or settled larvae (F_{2, 24} = 0.610, P = 0.554). Different lower-case letters above means in <b>(A)</b> indicate significant differences per ANOVA (critical P = 0.05) and Tukey’s tests. The maize inbred lines in <b>(A)</b> are ordered from left to right according to the presumed strength of their host plant resistance to fall armyworm, as indicated at the top of the figure.</p

Ovipositional preference of fall armyworm females for maize plants subject to present herbivory (= exposed to conspecific larvae) or plants not subject to present herbivory (= plants not exposed to larvae) on two maize inbred lines, B73 and B73-<i>lox10</i>.

<p>Preference was assessed in a choice-test over a 24-h period, independently on each maize inbred line, and was measured as the numbers of eggs (filled circles) and egg masses (empty circles) laid by females on seedlings with (= Treatment) or without (= Control) feeding injury. Feeding injury was produced by confining three larvae on a seedling for 3 h, beginning 7–8 h prior to a trial. Overall, differences in eggs and egg masses laid per seedling were detected between seedlings with or without present herbivory <b>(A)</b>, and between B73 and B73-<i>lox10</i> maize inbred lines <b>(B)</b>, while a significant interaction between herbivory and maize inbred line was not found (F_{1, 84} ≤ 0.84, P ≥ 0.36).</p