Low concentrations of salicylic acid stimulate insect elicitor responses in Zea mays seedlings

A delayed response in phytohormone signaling and production contributes to pine susceptibility to Fusarium circinatum

BACKGROUND

Fusarium circinatum is the causal agent of pine pitch canker disease, which affects Pinus species worldwide, causing significant economic and ecological losses. In Spain, two Pinus species are most affected by the pathogen; Pinus radiata is highly susceptible, while Pinus pinaster has shown moderate resistance. In F. circinatum-Pinus interactions, phytohormones are known to play a crucial role in plant defense. By comparing species with different degrees of susceptibility, we aimed to elucidate the fundamental mechanisms underlying resistance to the pathogen. For this purpose, we used an integrative approach by combining gene expression and metabolomic phytohormone analyses at 5 and 10 days post inoculation.

RESULTS

Gene expression and metabolite phytohormone contents suggested that the moderate resistance of P. pinaster to F. circinatum is determined by the induction of phytohormone signaling and hormone rearrangement beginning at 5 dpi, when symptoms are still not visible. Jasmonic acid was the hormone that showed the greatest increase by 5 dpi, together with the active gibberellic acid 4 and the cytokinin dehydrozeatin; there was also an increase in abscisic acid and salicylic acid by 10 dpi. In contrast, P. radiata hormonal changes were delayed until 10 dpi, when symptoms were already visible; however, this increase was not as high as that in P. pinaster. Indeed, in P. radiata, no differences in jasmonic acid or salicylic acid production were found. Gene expression analysis supported the hormonal data, since the activation of genes related to phytohormone synthesis was observed earlier in P. pinaster than in the susceptible P. radiata.

CONCLUSIONS

We determine that the moderate resistance of P. pinaster to F. circinatum is in part a result of early and strong activation of plant phytohormone-based defense responses before symptoms become visible. We suggest that jasmonic acid signaling and production are strongly associated with F. circinatum resistance. In contrast, P. radiata susceptibility was attributed to a delayed response to the fungus at the moment when symptoms were visible. Our results contribute to a better understanding of the phytohormone-based defense mechanism involved in the Pinus-F. circinatum interactions and provide insight into the development of new strategies for disease mitigation.

Salicylic acid positively regulates maize defenses against lepidopteran insects

In response to insect attack, plants use intricate signaling pathways, including phytohormones, such as jasmonate (JA), ethylene (ET), and salicylic acid (SA), to activate defenses. Maize (Zea mays) is one of the most important staple food crops around the world. Previous studies have shown that the JA and ET signaling play important roles in maize defense against insects, but little is known about whether and how SA regulates maize resistance to insect herbivores. In this study, we ectopically expressed the NahG (salicylate hydroxylase) gene in maize plants (NahG maize) to block the accumulation of SA. It was found that compared with the wild-type (WT) maize, the NahG maize exhibited decreased resistance to the generalist insects S podoptera litura and Spodoptera frugiperda and the specialist Mythimna separata, and the compromised resistance in the NahG maize was associated with decreased levels of defensive metabolites benzoxazinoids (Bxs) and chlorogenic acid (CA). Quantification of simulated S. litura feeding-induced JA, JA-isoleucine conjugate (JA-Ile), and ET in the WT and NahG maize indicated that SA does not regulate JA or JA-Ile, but positively controls ET. We provide evidence suggesting that the SA pathway does not crosstalk with the JA or the ET signaling in regulating the accumulation of Bxs and CA. Transcriptome analysis revealed that the bHLH, ERF, and WRKY transcription factors might be involved in SA-regulated defenses. This study uncovers a novel and important phytohormone pathway in maize defense against lepidopterous larvae.

Jasmonates and salicylic acid: Evolution of defense hormones in land plants

The emergence of plant hormone signaling pathways is deeply intertwined with land plant evolution. In angiosperms, two plant hormones, salicylic Acid (SA) and Jasmonates (JAs), play a key role in plant defense, where JAs-mediated defenses are typically activated in response to herbivores and necrotrophic pathogens, whereas SA is prioritized against hemi/biotrophic pathogens. Thus, studying the evolution of SA and JAs and their crosstalk is essential to understand the evolution of molecular plant-microbe interactions (EvoMPMI) in land plants. Recent advances in the evolution of SA and JAs biosynthesis, signaling, and crosstalk in land plants illustrated that the insight gained in angiosperms does not necessarily apply to non-seed plant lineages, where the receptors perceive different ligands and the hormones activate pathways independently on the canonical receptors. In this review, recent findings on the two main defense hormones (JAs and SA) in non-seed plants, including functional studies in the bryophyte model Marchantia polymorpha, will be discussed.

Sorghum defense responses to sequential attack by insect herbivores of different feeding guilds

MAIN CONCLUSION

Insect herbivores of different feeding guilds induced sorghum defenses through differential mechanisms, regardless of the order of herbivore arrival on sorghum plants. Sorghum, one of the world's most important cereal crops, suffers severe yield losses due to attack by insects of different feeding guilds. In most instances, the emergence of these pests are not secluded incidents and are followed by another or can also co-infest host plants. Sugarcane aphid (SCA) and fall armyworm (FAW) are the two most important destructive pests of sorghum, which belongs to sap-sucking and chewing feeding guilds, respectively. While the order of the herbivore arriving on the plants has been found to alter the defense response to subsequent herbivores, this is seldom studied with herbivores from different feeding guilds. In this study, we investigated the effects of sequential herbivory of FAW and SCA on sorghum defense responses and their underlying mechanism(s). Sequential feeding on the sorghum RTx430 genotype by either FAW primed-SCA or SCA primed-FAW were monitored to unravel the mechanisms underlying defense priming, and its mode of action. Regardless of the order of herbivore arrival on sorghum RTx430 plants, significant defense induction was observed in the primed state compared to the non-primed condition, irrespective of their feeding guild. Additionally, gene expression and secondary metabolite analysis revealed differential modulation of the phenylpropanoid pathway upon insect attack by different feeding guilds. Our findings suggest that priming in sorghum plants upon sequential herbivory induces defense by the accumulation of the total flavonoids and lignin/salicylic acid in FAW primed-SCA and SCA primed-FAW interaction, respectively.

Plant hormone cytokinin at the crossroads of stress priming and control of photosynthesis

To cope with biotic and abiotic stress conditions, land plants have evolved several levels of protection, including delicate defense mechanisms to respond to changes in the environment. The benefits of inducible defense responses can be further augmented by defense priming, which allows plants to respond to a mild stimulus faster and more robustly than plants in the naïve (non-primed) state. Priming provides a low-cost protection of agriculturally important plants in a relatively safe and effective manner. Many different organic and inorganic compounds have been successfully tested to induce resistance in plants. Among the plethora of commonly used physicochemical techniques, priming by plant growth regulators (phytohormones and their derivatives) appears to be a viable approach with a wide range of applications. While several classes of plant hormones have been exploited in agriculture with promising results, much less attention has been paid to cytokinin, a major plant hormone involved in many biological processes including the regulation of photosynthesis. Cytokinins have been long known to be involved in the regulation of chlorophyll metabolism, among other functions, and are responsible for delaying the onset of senescence. A comprehensive overview of the possible mechanisms of the cytokinin-primed defense or stress-related responses, especially those related to photosynthesis, should provide better insight into some of the less understood aspects of this important group of plant growth regulators.

Enhanced volatile emissions and anti-herbivore functions mediated by the synergism between jasmonic acid and salicylic acid pathways in tea plants

The interaction between jasmonic acid (JA) and salicylic acid (SA) pathways, which affects plant stress resistance, is mainly considered to be antagonistic. Using an established theoretical model, we investigated how tea plant (Camellia sinensis) volatiles induced by exogenous elicitors of the JA and SA pathways are affected by the sequence of elicitor application, elicitor identity, and the applied concentrations. We also examined the effects of the volatiles mediated by the JA-SA synergistic interaction on the behaviors of a tea leaf-chewing herbivore (Ectropis grisescens) and its parasitic wasp (Apanteles sp.). The JA and SA pathway interactions were almost always reciprocally synergistic when the two pathways were elicited at different times, except at high JA elicitor concentrations. However, the JA pathway antagonized the SA pathway when they were elicited simultaneously. The elicitor identity affected the degree of JA-SA interaction. The volatiles induced by the JA pathway in the JA-SA reciprocal synergism treatments included up to 11 additional compounds and the total amount of volatiles was up to 7.9-fold higher. Similarly, the amount of emitted volatiles induced by the SA pathway in the reciprocal synergism treatments increased by up to 4.2-fold. Compared with the volatiles induced by either pathway, the enriched volatiles induced by the JA-SA reciprocal synergism similarly repelled E. grisescens, but attracted Apanteles sp. more strongly. Thus, non-simultaneous activation is important for optimizing the JA-SA reciprocal synergism. This reciprocal synergism enables plants to induce multifarious responses, leading to increased biotic stress resistance.

Herbivore-Induced (Z)-3-Hexen-1-ol is an Airborne Signal That Promotes Direct and Indirect Defenses in Tea (Camellia sinensis) under Light

Tea (Camellia sinensis) is the most popular nonalcoholic beverage worldwide. During cultivation, tea plants are susceptible to herbivores and pathogens, which can seriously affect tea yield and quality. A previous report showed that (Z)-3-hexenol is a potentially efficient defensive substance. However, the molecular mechanism mediating (Z)-3-hexenol signaling in tea plants and the resulting effects on plant defenses remain uncharacterized. To clarify the signaling mechanisms in which (Z)-3-hexenol and light are involved, the gene transcription and metabolite levels were assessed, respectively. This study demonstrated that tea plants rapidly and continuously release (Z)-3-hexen-1-ol in response to an insect infestation. (Z)-3-Hexen-1-ol absorbed by adjacent healthy plants would be converted into three insect defensive compounds: (Z)-3-hexenyl-glucoside, (Z)-3-hexenyl-primeveroside, and (Z)-3-hexenyl-vicianoside identified with laboratory-synthesized standards. Moreover, (Z)-3-hexen-1-ol also activates the synthesis of jasmonic acid to enhance the insect resistance of tea plants. Additionally, a continuous light treatment induces the accumulation of (Z)-3-hexenyl-glycosides. Hence, (Z)-3-hexenol serves as a light-regulated signaling molecule that activates the systemic defenses of adjacent plants. Our study reveals the molecular mechanisms by which biotic and abiotic factors synergistically regulate the signaling functions of herbivore-induced plant volatiles in plants, providing valuable information for future comprehensive analyses of the systemic defense mechanisms in plants.

Exogenous salicylic acid improves resistance of aphid-susceptible wheat to the grain aphid, Sitobion avenae (F.) (Hemiptera: Aphididae)

Salicylic acid (SA), a phytohormone, has been considered to be a key regulator mediating plant defence against pathogens. It is still vague how SA activates plant defence against herbivores such as chewing and sucking pests. Here, we used an aphid-susceptible wheat variety to investigate Sitobion avenae response to SA-induced wheat plants, and the effects of exogenous SA on some defence enzymes and phenolics in the plant immune system. In SA-treated wheat seedlings, intrinsic rate of natural increase (rm), fecundity and apterous rate of S. avenae were 0.25, 31.4 nymphs/female and 64.4%, respectively, and significantly lower than that in the controls (P < 0.05). Moreover, the increased activities of phenylalanine-ammonia-lyase, polyphenol oxidase (PPO) and peroxidase in the SA-induced seedlings obviously depended on the sampling time, whereas activities of catalase and 4-coumarate:CoA ligase were suppressed significantly at 24, 48 and 72 h in comparison with the control. Dynamic levels of p-coumaric acid at 96 h, caffeic acid at 24 and 72 h and chlorogenic acid at 24, 48 and 96 h in wheat plants were significantly upregulated by exogenous SA application. Nevertheless, only caffeic acid content was positively correlated with PPO activity in SA-treated wheat seedlings (P = 0.031). These findings indicate that exogenous SA significantly enhanced the defence of aphid-susceptible wheat variety against aphids by regulating the plant immune system, and may prove a potential application of SA in aphid control.

Induced tolerance to abiotic and biotic stresses of broccoli and Arabidopsis after treatment with elicitor molecules

The plant hormones salicylic acid (SA) and jasmonic acid (JA) regulate defense mechanisms capable of overcoming different plant stress conditions and constitute distinct but interconnected signaling pathways. Interestingly, several other molecules are reported to trigger stress-specific defense responses to biotic and abiotic stresses. In this study, we investigated the effect of 14 elicitors against diverse but pivotal types of abiotic (drought) and biotic (the chewing insect Ascia monuste, the hemibiotrophic bacterium Pseudomonas syringae DC 3000 and the necrotrophic fungus Alternaria alternata) stresses on broccoli and Arabidopsis. Among the main findings, broccoli pre-treated with SA and chitosan showed the highest drought stress recovery in a dose-dependent manner. Several molecules led to increased drought tolerance over a period of three weeks. The enhanced drought tolerance after triggering the SA pathway was associated with stomata control. Moreover, methyl jasmonate (MeJA) reduced A. monuste insect development and plant damage, but unexpectedly, other elicitors increased both parameters. GUS reporter assays indicated expression of the SA-dependent PR1 gene in plants treated with nine elicitors, whereas the JA-dependent LOX2 gene was only expressed upon MeJA treatment. Overall, elicitors capable of tackling drought and biotrophic pathogens mainly triggered the SA pathway, but adversely also induced systemic susceptibility to chewing insects. These findings provide directions for potential future in-depth characterization and utilization of elicitors and induced resistance in plant protection.

The Role of Salicylic Acid in Plants Exposed to Heavy Metals

Salicylic acid (SA) is a very simple phenolic compound (a C₇H₆O₃ compound composed of an aromatic ring, one carboxylic and a hydroxyl group) and this simplicity contrasts with its high versatility and the involvement of SA in several plant processes either in optimal conditions or in plants facing environmental cues, including heavy metal (HM) stress. Nowadays, a huge body of evidence has unveiled that SA plays a pivotal role as plant growth regulator and influences intra- and inter-plant communication attributable to its methyl ester form, methyl salicylate, which is highly volatile. Under stress, including HM stress, SA interacts with other plant hormones (e.g., auxins, abscisic acid, gibberellin) and promotes the stimulation of antioxidant compounds and enzymes thereby alerting HM-treated plants and helping in counteracting HM stress. The present literature survey reviews recent literature concerning the roles of SA in plants suffering from HM stress with the aim of providing a comprehensive picture about SA and HM, in order to orientate the direction of future research on this topic.

The Role of Salicylic Acid in Plants Exposed to Heavy Metals

Salicylic acid (SA) is a very simple phenolic compound (a C7H6O3 compound composed of an aromatic ring, one carboxylic and a hydroxyl group) and this simplicity contrasts with its high versatility and the involvement of SA in several plant processes either in optimal conditions or in plants facing environmental cues, including heavy metal (HM) stress. Nowadays, a huge body of evidence has unveiled that SA plays a pivotal role as plant growth regulator and influences intra- and inter-plant communication attributable to its methyl ester form, methyl salicylate, which is highly volatile. Under stress, including HM stress, SA interacts with other plant hormones (e.g., auxins, abscisic acid, gibberellin) and promotes the stimulation of antioxidant compounds and enzymes thereby alerting HM-treated plants and helping in counteracting HM stress. The present literature survey reviews recent literature concerning the roles of SA in plants suffering from HM stress with the aim of providing a comprehensive picture about SA and HM, in order to orientate the direction of future research on this topic.

The transcriptome of Pinus pinaster under Fusarium circinatum challenge

BACKGROUND

Fusarium circinatum, the causal agent of pitch canker disease, poses a serious threat to several Pinus species affecting plantations and nurseries. Although Pinus pinaster has shown moderate resistance to F. circinatum, the molecular mechanisms of defense in this host are still unknown. Phytohormones produced by the plant and by the pathogen are known to play a crucial role in determining the outcome of plant-pathogen interactions. Therefore, the aim of this study was to determine the role of phytohormones in F. circinatum virulence, that compromise host resistance.

RESULTS

A high quality P. pinaster de novo transcriptome assembly was generated, represented by 24,375 sequences from which 17,593 were full length genes, and utilized to determine the expression profiles of both organisms during the infection process at 3, 5 and 10 days post-inoculation using a dual RNA-sequencing approach. The moderate resistance shown by Pinus pinaster at the early time points may be explained by the expression profiles pertaining to early recognition of the pathogen, the induction of pathogenesis-related proteins and the activation of complex phytohormone signaling pathways that involves crosstalk between salicylic acid, jasmonic acid, ethylene and possibly auxins. Moreover, the expression of F. circinatum genes related to hormone biosynthesis suggests manipulation of the host phytohormone balance to its own benefit.

CONCLUSIONS

We hypothesize three key steps of host manipulation: perturbing ethylene homeostasis by fungal expression of genes related to ethylene biosynthesis, blocking jasmonic acid signaling by coronatine insensitive 1 (COI1) suppression, and preventing salicylic acid biosynthesis from the chorismate pathway by the synthesis of isochorismatase family hydrolase (ICSH) genes. These results warrant further testing in F. circinatum mutants to confirm the mechanism behind perturbing host phytohormone homeostasis.

Fighting on two fronts: Elevated insect resistance in flooded maize

To grow and thrive plants must be able to adapt to both adverse environmental conditions and attack by a variety of pests. Elucidating the sophisticated mechanisms plants have developed to achieve this has been the focus of many studies. What is less well understood is how plants respond when faced with multiple stressors simultaneously. In this study, we assess the response of Zea mays (maize) to the combinatorial stress of flooding and infestation with the insect pest Spodoptera frugiperda (fall armyworm). This combined stress leads to elevated production of the defence hormone salicylic acid, which does not occur in the individual stresses, and the resultant salicylic acid-dependent increase in S. frugiperda resistance. Remodelling of phenylpropanoid pathways also occurs in response to this combinatorial stress leading to increased production of the anti-insect C-glycosyl flavones (maysins) and the herbivore-induced volatile phenolics, benzyl acetate, and phenethyl acetate. Furthermore, changes in cellular redox status also occur, as indicated by reductions in peroxidase and polyphenol oxidase activity. These data suggest that metabolite changes important for flooding tolerance and anti-insect defence may act both additively and synergistically to provide extra protection to the plant.

Root exudate metabolites drive plant-soil feedbacks on growth and defense by shaping the rhizosphere microbiota

By changing soil properties, plants can modify their growth environment. Although the soil microbiota is known to play a key role in the resulting plant-soil feedbacks, the proximal mechanisms underlying this phenomenon remain unknown. We found that benzoxazinoids, a class of defensive secondary metabolites that are released by roots of cereals such as wheat and maize, alter root-associated fungal and bacterial communities, decrease plant growth, increase jasmonate signaling and plant defenses, and suppress herbivore performance in the next plant generation. Complementation experiments demonstrate that the benzoxazinoid breakdown product 6-methoxy-benzoxazolin-2-one (MBOA), which accumulates in the soil during the conditioning phase, is both sufficient and necessary to trigger the observed phenotypic changes. Sterilization, fungal and bacterial profiling and complementation experiments reveal that MBOA acts indirectly by altering root-associated microbiota. Our results reveal a mechanism by which plants determine the composition of rhizosphere microbiota, plant performance and plant-herbivore interactions of the next generation.

Plants can modify soil microbiota through root exudation, but how this process influences plant health in turn is often unclear. Here, Hu et al. show that maize benzoxazinoids released into the soil modify root-associated microbiota and thereby increase leaf defenses of the next plant generation.

Green leaf volatiles protect maize (Zea mays) seedlings against damage from cold stress

Although considerable evidence has accumulated on the defensive activity of plant volatile organic compounds against pathogens and insect herbivores, less is known about the significance of volatile organic compounds emitted by plants under abiotic stress. Here, we report that green leaf volatiles (GLVs), which were previously shown to prime plant defences against insect herbivore attack, also protect plants against cold stress (4 °C). We show that the expression levels of several cold stress-related genes are significantly up-regulated in maize (Zea mays) seedlings treated with physiological concentrations of the GLV, (Z)-3-hexen-1-yl acetate (Z-3-HAC), and that seedlings primed with Z-3-HAC exhibit increased growth and reduced damage after cold stress relative to unprimed seedlings. Together, these data demonstrate the protective and priming effect of GLVs against cold stress and suggest an activity of GLVs beyond the activation of typical plant defence responses against herbivores and pathogens.

Oxidative enzymes activity during abiotic and biotic stresses in Zea mays leaves and roots exposed to Cu, methyl jasmonate and Trigonotylus caelestialium

The activities of antioxidative enzymes, i.e. superoxide dismutase (SOD), catalase (CAT), ascorbate peroxidase (APX), and guaiacol peroxidase (GPX), in the leaves and roots of Zea mays L. plants exposed to abiotic (methyl jasmonate, MJ, or/and copper, Cu) and biotic (Trigonotylus caelestialium) factors were examined. The contribution of MJ as a signal molecule in the defense mechanism against abiotic and biotic stresses was studied. All plants were cultivated hydroponically and divided into three groups: not treated by abiotic factors (control), treated by MJ only (MJ) and by MJ and Cu (MJ + Cu) and in each group half of the plants were exposed to T. caelestialium attack. The enzymatic activities of SOD, CAT, APX, and GPX in the leaves were higher in the insect-treated than non-insect-treated control plants, but lower in both MJ + Cu- or MJ- and insect-treated plants. In the roots, the enzyme activities were elevated in all insect-treated plants with the highest rise in MJ + Cu, in comparison with the MJ-treated plants. The results showed that MJ and MJ + Cu were efficient in reducing the activity of the antioxidative enzymes in the leaves under the insect influence by elevating enzyme activity in the roots.

Evolution of jasmonate biosynthesis and signaling mechanisms

Jasmonates are phytohormones that modulate a wide spectrum of plant physiological processes, especially defense against herbivores and necrotrophs. The molecular mechanisms of jasmonate biosynthesis and signaling have been well characterized in model plants. In this review, we provide an in-depth analysis and overview of the origin and evolution of the jasmonate biosynthesis and signaling pathways. Furthermore, we discuss the striking parallels between jasmonate and auxin signaling mechanisms, which reveals a common ancestry of these signaling mechanisms. Finally, we highlight the importance of studying jasmonate biosynthesis and signaling in lower plants.

Plant-Mediated Systemic Interactions Between Pathogens, Parasitic Nematodes, and Herbivores Above- and Belowground

Plants are important mediators of interactions between aboveground (AG) and belowground (BG) pathogens, arthropod herbivores, and nematodes (phytophages). We highlight recent progress in our understanding of within- and cross-compartment plant responses to these groups of phytophages in terms of altered resource dynamics and defense signaling and activation. We review studies documenting the outcome of cross-compartment interactions between these phytophage groups and show patterns of cross-compartment facilitation as well as cross-compartment induced resistance. Studies involving soilborne pathogens and foliar nematodes are scant. We further highlight the important role of defense signaling loops between shoots and roots to activate a full resistance complement. Moreover, manipulation of such loops by phytophages affects systemic interactions with other plant feeders. Finally, cross-compartment-induced changes in root defenses and root exudates extend systemic defense loops into the rhizosphere, enhancing or reducing recruitment of microbes that induce systemic resistance but also affecting interactions with root-feeding phytophages.

Defense Priming and Jasmonates: A Role for Free Fatty Acids in Insect Elicitor-Induced Long Distance Signaling

Green leaf volatiles (GLV) prime plants against insect herbivore attack resulting in stronger and faster signaling by jasmonic acid (JA). In maize this response is specifically linked to insect elicitor (IE)-induced signaling processes, which cause JA accumulation not only around the damage site, but also in distant tissues, presumably through the activation of electrical signals. Here, we present additional data further characterizing these distal signaling events in maize. Also, we describe how exposure to GLV increases free fatty acid (fFA) levels in maize seedlings, but also in other plants, and how increased fFA levels affect IE-induced JA accumulation. Increased fFA, in particular α-linolenic acid (LnA), caused a significant increase in JA accumulation after IE treatment, while JA induced by mechanical wounding (MW) alone was not affected. We also identified treatments that significantly decreased certain fFA level including simulated wind and rain. In such treated plants, IE-induced JA accumulation was significantly reduced when compared to un-moved control plants, while MW-induced JA accumulation was not significantly affected. Since only IE-induced JA accumulation was altered by changes in the fFA composition, we conclude that changing levels of fFA affect primarily IE-induced signaling processes rather than serving as a substrate for JA.

The rapidly evolving associations among herbivore associated elicitor-induced phytohormones in Nicotiana

In response to herbivore attack, plants perceive herbivore associated elicitors (HAE) and rapidly accumulate jasmonic acid (JA) and other phytohormones, which interact in complex ways, such as the crosstalk between JA and salicylic acid (SA). Although recent studies have shown that HAE-induced individual phytohormones can be highly specific among closely related species, it remains unclear how conserved and specific the relationships among HAE-induced phytohormones are. Here we analyzed the correlations among 4 different phytohormones, JA, JA-isoleucine (JA-Ile), SA, and abscisic acid (ABA) in 6 closely related Nicotiana species that were induced by 3 different HAEs. Our results showed that while no clear association between ABA and other phytohormones were found, the positive association between JA and JA-Ile is mostly conserved among closely related Nicotiana species. Interestingly, the association between JA and SA are highly variable and can be regulated by different HAEs.

Comparative analysis of antiviral responses in Brachypodium distachyon and Setaria viridis reveals conserved and unique outcomes among C3 and C4 plant defenses

Viral diseases cause significant losses in global agricultural production, yet little is known about grass antiviral defense mechanisms. We previously reported on host immune responses triggered by Panicum mosaic virus (PMV) and its satellite virus (SPMV) in the model C3 grass Brachypodium distachyon. To aid comparative analyses of C3 and C4 grass antiviral defenses, here, we establish B. distachyon and Setaria viridis (a C4 grass) as compatible hosts for seven grass-infecting viruses, including PMV and SPMV, Brome mosaic virus, Barley stripe mosaic virus, Maize mild mottle virus, Sorghum yellow banding virus, Wheat streak mosaic virus (WSMV), and Foxtail mosaic virus (FoMV). Etiological and molecular characterization of the fourteen grass-virus pathosystems showed evidence for conserved crosstalk among salicylic acid (SA), jasmonic acid, and ethylene pathways in B. distachyon and S. viridis. Strikingly, expression of PHYTOALEXIN DEFICIENT4, an upstream modulator of SA signaling, was consistently suppressed during most virus infections in B. distachyon and S. viridis. Hierarchical clustering analyses further identified unique antiviral responses triggered by two morphologically similar viruses, FoMV and WSMV, and uncovered other host-dependent effects. Together, the results of this study establish B. distachyon and S. viridis as models for the analysis of plant-virus interactions and provide the first framework for conserved and unique features of C3 and C4 grass antiviral defenses.

Early transcriptome analyses of Z-3-Hexenol-treated zea mays revealed distinct transcriptional networks and anti-herbivore defense potential of green leaf volatiles

Green leaf volatiles (GLV), which are rapidly emitted by plants in response to insect herbivore damage, are now established as volatile defense signals. Receiving plants utilize these molecules to prime their defenses and respond faster and stronger when actually attacked. To further characterize the biological activity of these compounds we performed a microarray analysis of global gene expression. The focus of this project was to identify early transcriptional events elicited by Z-3-hexenol (Z-3-HOL) as our model GLV in maize (Zea mays) seedlings. The microarray results confirmed previous studies on Z-3-HOL -induced gene expression but also provided novel information about the complexity of Z-3-HOL -induced transcriptional networks. Besides identifying a distinct set of genes involved in direct and indirect defenses we also found significant expression of genes involved in transcriptional regulation, Ca(2+)-and lipid-related signaling, and cell wall reinforcement. By comparing these results with those obtained by treatment of maize seedlings with insect elicitors we found a high degree of correlation between the two expression profiles at this early time point, in particular for those genes related to defense. We further analyzed defense gene expression induced by other volatile defense signals and found Z-3-HOL to be significantly more active than methyl jasmonate, methyl salicylate, and ethylene. The data presented herein provides important information on early genetic networks that are activated by Z-3-HOL and demonstrates the effectiveness of this compound in the regulation of typical plant defenses against insect herbivores in maize.

Copper and herbivory lead to priming and synergism in phytohormones and plant volatiles in the absence of salicylate-jasmonate antagonism

Abiotic stress factors can interfere with the emission of herbivore-induced plant volatile organic compounds (VOCs) and thus disrupt chemical communication channels between plants and other organisms. We investigated whether copper (Cu) stress alone or in conjunction with insect damage modifies the kinetics of (1) VOCs, (2) the VOC-inducing phytohormone jasmonic acid (JA) and (3) its putative antagonist salicylic acid (SA). Hydroponically grown Zea mays exposed to 10 and 80 µM of Cu showed no increases in JA or VOC levels in the absence of herbivory. However when challenged by herbivores, Cu (80 µM) caused ROS generation in root tissues and primed for increased JA accumulation and VOC emission in leaves. SA synthesis was equally primed but higher concentrations were also apparent before insects started feeding. In contrast, plants grown at 10 µM Cu did not differ from controls. These results show that abiotic and biotic stresses result in concentration-dependent, non-additive defense responses. Further support is given to the notion that JA-SA antagonism is absent in Z. mays.

Evolution of jasmonate and salicylate signal crosstalk

The evolution of land plants approximately 470 million years ago created a new adaptive zone for natural enemies (attackers) of plants. In response to attack, plants evolved highly effective, inducible defense systems. Two plant hormones modulating inducible defenses are salicylic acid (SA) and jasmonic acid (JA). Current thinking is that SA induces resistance against biotrophic pathogens and some phloem feeding insects and JA induces resistance against necrotrophic pathogens, some phloem feeding insects and chewing herbivores. Signaling crosstalk between SA and JA commonly manifests as a reciprocal antagonism and may be adaptive, but this remains speculative. We examine evidence for and against adaptive explanations for antagonistic crosstalk, trace its phylogenetic origins and provide a hypothesis-testing framework for future research on the adaptive significance of SA-JA crosstalk.