Feeding by Hessian Fly (Mayetiola destructor [Say]) Larvae on Wheat Increases Levels of Fatty Acids and Indole-3-Acetic Acid but not Hormones Involved in Plant-Defense Signaling

Gall-inducing insects exert a unique level of control over the physiology of their host plants. This control can extend to host–plant defenses so that some, if not most, gall-inducing species appear to avoid or modify host plant defenses to effect production of their gall. Included among gall insects is Hessian fly (Mayetiola destructor [Say], Diptera: Cecidomyiidae), a damaging pest of wheat (Triticum aestivum L.) and an emerging model system for studying plant–insect interactions. We studied the dynamics of some defense-related phytohormones and associated fatty acids during feeding of first instar Hessian fly larvae on a susceptible variety of wheat. We found that Hessian fly larvae significantly elevated in their host plants’ levels of linolenic and linoleic acids, fatty acids that may be nutritionally beneficial. Hessian fly larvae also elevated levels of indole-3-acetic acid (IAA), a phytohormone hypothesized to be involved in gall formation, but not the defense-related hormones jasmonic (JA) and salicylic acids. Moreover, we detected in Hessian fly-infested plants a significant negative relationship between IAA and JA that was not present in control plants. Our results suggest that Hessian fly larvae may induce nutritionally beneficial changes while concomitantly altering phytohormone levels, possibly to facilitate plant-defense avoidance.     

Production and function of jasmonates in nodulated roots of soybean plants inoculated with Bradyrhizobium japonicum

Little is known regarding production and function of endogenous jasmonates (JAs) in root nodules of soybean plants inoculated with Bradyrhizobium japonicum. We investigated (1) production of jasmonic acid (JA) and 12-oxophytodienoic acid (OPDA) in roots of control and inoculated plants and in isolated nodules; (2) correlations between JAs levels, nodule number, and plant growth during the symbiotic process; and (3) effects of exogenous JA and OPDA on nodule cell number and size. In roots of control plants, JA and OPDA levels reached a maximum at day 18 after inoculation; OPDA level was 1.24 times that of JA. In roots of inoculated plants, OPDA peaked at day 15, whereas JA level did not change appreciably. Shoot dry matter of inoculated plants was higher than that of control at day 21. Chlorophyll a decreased more abruptly in control plants than in inoculated plants, whereas b decreased gradually in both cases. Exogenous JA or OPDA changed number and size of nodule central cells and peripheral cells. Findings from this and previous studies suggest that increased levels of JA and OPDA in control plants are related to senescence induced by nutritional stress. OPDA accumulation in nodulated roots suggests its involvement in "autoregulation of nodulation."     

Changes in C12:0, C18:1, C18:2 and C20:2 fatty acid content in wheat treated with resistance inducers and infected by powdery mildew

This work presents a global investigation of total fatty acid (FA) content in wheat in relation to treatment with four inducers of resistance and to powdery mildew infection. Linolenic acid (C18:3), linoleic acid (C18:2) and palmitic acid (16:0) were the most abundant FAs in wheat leaves. We investigated the effect of the following inducers of resistance: Iodus40, heptanoyl salicylic acid (HSA), Milsana and trehalose on FA accumulation. Previous studies established that lipid metabolism is altered by these compounds, and we therefore aimed to characterise their impact at the FA level. During a time course experiment, content (quantitative analysis) and percentage (qualitative analysis) of FAs were compared in treated plants and in controls, as well as in plants inoculated with Blumeria graminis f. sp. tritici (i) and non-inoculated (ni) plants. No change in C18:3 content was observed. C18:1 in Iodus 40-treated (ni) plants showed a quantitative 1.2-fold increase. Lauric acid (C12:0) content quantitatively increased after Iodus 40 (2.8-fold), Milsana (4.8-fold) and trehalose (4.0-fold) treatment in (i) plants. However, eicosadienoic acid (C20:2) quantitatively decreased in (ni) plants after Iodus 40 (1.5-fold) and Milsana (2.3-fold) treatment. The amount of C18:2 increased (1.6-fold) after HSA treatment in (i) plants. All these variations in FA content were correlated with variations in the corresponding relative percentages. Our work provides the first evidence for alterations in C12:0, C18:1, C18:2 and C20:2 FA content caused by four resistance inducers. We also compared the amount and percentage of each FA in untreated (i) and (ni) plants. In (i) plants, eicosadienoic acid (C20:2) increased and C18:2 decreased slightly. The potential involvement of these FAs during induced resistance and infection is discussed.     

Interactions Between Signaling Compounds Involved in Plant Defense

To elude or minimize the effects of disease and herbivory, plants rely on both constitutive and inducible defenses. In response to attack by pathogens or pests, plants activate signaling cascades leading to the accumulation of endogenous hormones that trigger the induction of defenses. Salicylic acid (SA), jasmonic acid (JA), and ethylene (E) are plant-specific hormones involved in communicating the attack by many pathogens and pests in a broad range of plant species. SA, JA and E signaling cascades do not activate defenses independently, but rather establish complex interactions that determine the response mounted in each condition. Deployment of defenses is energetically costly, so a trade-off between the activation of resistance against a particular pest or pathogen and down regulation of other defenses is common. Conversely, activation of broad range resistance in response to an initial attack may serve to deter opportunistic agents. Thus, the interaction among SA, JA and E defense signaling pathways can be antagonistic, cooperative or synergistic, depending on the plant species, the combination of organisms attacking the plants, and the developmental and physiological state of the plant. A characterization of the interactions among defense signaling pathways and the determination of the molecular components mediating cross-talk between the different pathways will be essential for the rational design of transgenic plants with increased resistance to disease and/or herbivores without critically compromising other agronomic traits.     

Jasmonic acid and salicylic acid activate a common defense system in rice

Jasmonic acid (JA) and salicylic acid (SA) play important roles in plant defense systems. JA and SA signaling pathways interact antagonistically in dicotyledonous plants, but, the status of crosstalk between JA and SA signaling is unknown in monocots. Our rice microarray analysis showed that more than half of the genes upregulated by the SA analog BTH are also upregulated by JA, suggesting that a major portion of the SA-upregulated genes are regulated by JA-dependent signaling in rice. A common defense system that is activated by both JA and SA is thus proposed which plays an important role in pathogen defense responses in rice.     

Probenazole induces systemic acquired resistance in Arabidopsis with a novel type of action

Probenazole (PBZ; 3-allyloxy-1,2-benzisothiazole-1,1-dioxide), which is the active ingredient in Oryzemate, has been used widely in Asia to protect rice plants against the rice blast fungus Magnaporthe grisea. To study PBZ's mode of action, we analyzed its ability, as well as that of its active metabolite 1, 2-benzisothiazol-3 (2H)-one 1,1-dioxide (BIT) to induce defense gene expression and resistance in Arabidopsis mutants that are defective in various defense signaling pathways. Wild-type Arabidopsis treated with PBZ or BIT exhibited increased expression of several pathogenesis-related genes, increased levels of total salicylic acid (SA), and enhanced resistance to the bacterial pathogen Pseudomonas syringae pv. tomato DC 3000 and the oomycete pathogen Peronospora parasitica Emco5. The role of several defense signaling hormones, such as SA, ethylene and jasmonic acid (JA), in activating resistance following PBZ or BIT treatment was analyzed using NahG transgenic plants and etr1-1 and coi1-1 mutant plants, respectively. In addition, the involvement of NPR1, a key component in the SA signaling pathway leading to defense responses, was assessed. PBZ or BIT treatment did not induce disease resistance or PR-1 expression in NahG transgenic or npr1 mutant plants, but it did activate these phenomena in etr1-1 and coi 1-1 mutant plants. Thus SA and NPR1 appear to be required for PBZ- and BIT-mediated activation of defense responses, while ethylene and JA are not. Furthermore, our data suggest that PBZ and BIT comprise a novel class of defense activators that stimulate the SA/NPR1-mediated defense signaling pathway upstream of SA.     

Modulation of defense-response gene expression in wheat during Hessian fly larval feeding

Abstract

Expression profiles of ten genes commonly up-regulated during plant defense against microbial pathogens were compared temporally during compatible and incompatible interactions with first-instar Hessian fly larvae, in two wheat lines carrying different resistance genes. Quantitative real-time PCR revealed that while a lipoxygenase gene (WCI-2) was strongly up-regulated during the incompatible interactions, genes encoding β-1,3 endoglucanase (GNS) and an integral membrane protein (WIR1) were moderately responsive. Genes for thionin-like protein (WCI-3), PR-17-like protein (WCI-5), MAP kinase (WCK-1), phenylalanine ammonia-lyase (PAL), pathogenesis-related protein-1 (PR-1), receptor-like kinase (LRK10) and heat shock protein 70 (HSP70) were minimally responsive. The application of signaling molecules, salicylic acid (SA), methyl jasmonate (MJ) and abscisic acid (ABA), to insect-free plants demonstrated association of these genes with specific defense-response pathways. SA-induced up-regulation of a gene related to lipoxygenases that are involved in jasmonic acid (JA)-biosynthesis is suggestive of positive cross-talk between SA- and JA-mediated signaling pathways. Data suggest that alternative mechanisms may be involved since few of these classical defense-response genes are significantly up-regulated during incompatible interactions between wheat and Hessian fly.     

Priming for JA-dependent defenses using hexanoic acid is an effective mechanism to protect Arabidopsis against B. cinerea

Soil drench treatments with hexanoic acid can effectively protect Arabidopsis plants against Botrytis cinerea through a mechanism based on a stronger and faster accumulation of JA-dependent defenses.Plants impaired in ethylene, salicylic acid, abscisic acid or glutathion pathways showed intact protection by hexanoic acid upon B. cinerea infection. Accordingly, no significant changes in the SA marker gene PR-1 in either the SA or ABA hormone balance were observed in the infected and treated plants. In contrast, the JA signaling pathway showed dramatic changes after hexanoic acid treatment, mainly when the pathogen was present. The impaired JA mutants, jin1-2 and jar1, were unable to display hexanoic acid priming against the necrotroph. In addition, hexanoic acid-treated plants infected with B. cinerea showed priming in the expression of the PDF1.2, PR-4 and VSP1 genes implicated in the JA pathways. Moreover, JA and OPDA levels were primed at early stages by hexanoic acid. Treatments also stimulated increased callose accumulation in response to the pathogen. Although callose accumulation has proved an effective IR mechanism against B. cinerea, it is apparently not essential to express hexanoic acid-induced resistance (HxAc-IR) because the mutant pmr4.1 (callose synthesis defective mutant) is protected by treatment.We recently described how hexanoic acid treatments can protect tomato plants against B. cinerea by stimulating ABA-dependent callose deposition and by priming OPDA and JA-Ile production. We clearly demonstrate here that Hx-IR is a dependent plant species, since this acid protects Arabidopsis plants against the same necrotroph by priming JA-dependent defenses without enhancing callose accumulation.     

Restoration of defective cross talk in ssi2 mutants: role of salicylic acid, jasmonic acid, and fatty acids in SSI2-mediated signaling

The Arabidopsis mutants ssi2 and fab2 are defective in stearoyl ACP desaturase, which causes altered salicylic acid (SA)- and jasmonic acid (JA)-mediated defense signaling. Both ssi2 and fab2 plants show spontaneous cell death, express PR genes constitutively, accumulate high levels of SA, and exhibit enhanced resistance to bacterial and oomycete pathogens. In contrast to constitutive activation of the SA pathway, ssi2 and fab2 plants are repressed in JA-mediated induction of the PDF1.2 gene, which suggests that the SSI2-mediated signaling pathway modulates cross talk between the SA and JA pathways. In this study, we have characterized two recessive nonallelic mutants in the ssi2 background, designated as rdc (restorer of defective cross talk) 2 and rdc8. Both ssi2 rdc mutants are suppressed in constitutive SA signaling, show basal level expression of PR-1 gene, and induce high levels of PDF1.2 in response to exogenous application of JA. Interestingly, while the rdc8 mutation completely abolishes spontaneous cell death in ssi2 rdc8 plants, the ssi2 rdc2 plants continue to show some albeit reduced cell death. Fatty acid (FA) analysis showed a reduction in 16:3 levels in ssi2 rdc8 plants, which suggests that this mutation may limit the flux of FAs into the prokaryotic pathway of glycerolipid biosynthesis. Both rdc2 and rdc8 continue to accumulate high levels of 18:0, which suggests that 18:0 levels were responsible for neither constitutive SA signaling nor repression of JA-induced expression of the PDF1.2 gene in ssi2 plants. We also analyzed SA and JA responses of the fab2-derived shs1 mutant, which accumulates levels of 18:0 over 50% lower than those in the fab2 plants. Even though fab2 shs1 plants were morphologically bigger than fab2 plants, they expressed PR genes constitutively, showed HR-like cell death, and accumulated elevated levels of SA. However, unlike the ssi2 rdc plants, fab2 shs1 plants were unable to induce high levels of PDF1.2 expression in response to exogenous application of JA. Together, these results show that defective cross talk in ssi2 can be restored by second site mutations and is independent of morphological size of the plants, cell death, and elevated levels of 18:0.     

Role of cis-12-oxo-phytodienoic acid in tomato embryo development

Oxylipins including jasmonates are signaling compounds in plant growth, development, and responses to biotic and abiotic stresses. In Arabidopsis (Arabidopsis thaliana) most mutants affected in jasmonic acid (JA) biosynthesis and signaling are male sterile, whereas the JA-insensitive tomato (Solanum lycopersicum) mutant jai1 is female sterile. The diminished seed formation in jai1 together with the ovule-specific accumulation of the JA biosynthesis enzyme allene oxide cyclase (AOC), which correlates with elevated levels of JAs, suggest a role of oxylipins in tomato flower/seed development. Here, we show that 35S::SlAOC-RNAi lines with strongly reduced AOC in ovules exhibited reduced seed set similarly to the jai1 plants. Investigation of embryo development of wild-type tomato plants showed preferential occurrence of AOC promoter activity and AOC protein accumulation in the developing seed coat and the embryo, whereas 12-oxo-phytodienoic acid (OPDA) was the dominant oxylipin occurring nearly exclusively in the seed coat tissues. The OPDA- and JA-deficient mutant spr2 was delayed in embryo development and showed an increased programmed cell death in the developing seed coat and endosperm. In contrast, the mutant acx1a, which accumulates preferentially OPDA and residual amount of JA, developed embryos similar to the wild type, suggesting a role of OPDA in embryo development. Activity of the residual amount of JA in the acx1a mutant is highly improbable since the known reproductive phenotype of the JA-insensitive mutant jai1 could be rescued by wound-induced formation of OPDA. These data suggest a role of OPDA or an OPDA-related compound for proper embryo development possibly by regulating carbohydrate supply and detoxification.