Phytohormone Mediation of Interactions Between Herbivores and Plant Pathogens

Metabolomic Response to Huanglongbing: Role of Carboxylic Compounds in Citrus sinensis Response to 'Candidatus Liberibacter asiaticus' and Its Vector, Diaphorina citri

Huanglongbing, a destructive disease of citrus, is caused by the fastidious bacterium 'Candidatus Liberibacter asiaticus' and transmitted by Asian citrus psyllid, Diaphorina citri. The impact of 'Ca. L. asiaticus' infection or D. citri infestation on Valencia sweet orange (Citrus sinensis) leaf metabolites was investigated using gas chromatography mass spectrometry, followed by gene expression analysis for 37 genes involved in jasmonic acid (JA), salicylic acid (SA), and proline-glutamine pathways. The total amino acid abundance increased after 'Ca. L. asiaticus' infection, while the total fatty acids increased dramatically after infestation with D. citri, compared with control plants. Seven amino acids (glycine, l-isoleucine, l-phenylalanine, l-proline, l-serine, l-threonine, and l-tryptophan) and five organic acids (benzoic acid, citric acid, fumaric acid, SA, and succinic acid) increased in 'Ca. L. asiaticus'-infected plants. On the other hand, the abundance of trans-JA and its precursor α-linolenic increased in D. citri-infested plants. Surprisingly, the double attack of both D. citri infestation and 'Ca. L. asiaticus' infection moderated the metabolic changes in all chemical classes studied. In addition, the gene expression analysis supported these results. Based on these findings, we suggest that, although amino acids such as phenylalanine are involved in citrus defense against 'Ca. L. asiaticus' infection through the activation of an SA-mediated pathway, fatty acids, especially α-linolenic acid, are involved in defense against D. citri infestation via the induction of a JA-mediated pathway.

Combined biotic stresses trigger similar transcriptomic responses but contrasting resistance against a chewing herbivore in Brassica nigra

BACKGROUND

In nature, plants are frequently exposed to simultaneous biotic stresses that activate distinct and often antagonistic defense signaling pathways. How plants integrate this information and whether they prioritize one stress over the other is not well understood.

RESULTS

We investigated the transcriptome signature of the wild annual crucifer, Brassica nigra, in response to eggs and caterpillars of Pieris brassicae butterflies, Brevicoryne brassicae aphids and the bacterial phytopathogen Xanthomonas campestris pv. raphani (Xcr). Pretreatment with egg extract, aphids, or Xcr had a weak impact on the subsequent transcriptome profile of plants challenged with caterpillars, suggesting that the second stress dominates the transcriptional response. Nevertheless, P. brassicae larval performance was strongly affected by egg extract or Xcr pretreatment and depended on the site where the initial stress was applied. Although egg extract and Xcr pretreatments inhibited insect-induced defense gene expression, suggesting salicylic acid (SA)/jasmonic acid (JA) pathway cross talk, this was not strictly correlated with larval performance.

CONCLUSION

These results emphasize the need to better integrate plant responses at different levels of biological organization and to consider localized effects in order to predict the consequence of multiple stresses on plant resistance.

Transcriptional analysis of defense mechanisms in upland tetraploid switchgrass to greenbugs

BACKGROUND

Aphid infestation of switchgrass (Panicum virgatum) has the potential to reduce yields and biomass quality. Although switchgrass-greenbug (Schizaphis graminum; GB) interactions have been studied at the whole plant level, little information is available on plant defense responses at the molecular level.

RESULTS

The global transcriptomic response of switchgrass cv Summer to GB was monitored by RNA-Seq in infested and control (uninfested) plants harvested at 5, 10, and 15 days after infestation (DAI). Differentially expressed genes (DEGs) in infested plants were analyzed relative to control uninfested plants at each time point. DEGs in GB-infested plants induced by 5-DAI included an upregulation of reactive burst oxidases and several cell wall receptors. Expression changes in genes linked to redox metabolism, cell wall structure, and hormone biosynthesis were also observed by 5-DAI. At 10-DAI, network analysis indicated a massive upregulation of defense-associated genes, including NAC, WRKY, and MYB classes of transcription factors and potential ancillary signaling molecules such as leucine aminopeptidases. Molecular evidence for loss of chloroplastic functions was also detected at this time point. Supporting these molecular changes, chlorophyll content was significantly decreased, and ROS levels were elevated in infested plants 10-DAI. Total peroxidase and laccase activities were elevated in infested plants at 10-DAI relative to control uninfested plants. The net result appeared to be a broad scale defensive response that led to an apparent reduction in C and N assimilation and a potential redirection of nutrients away from GB and towards the production of defensive compounds, such as pipecolic acid, chlorogenic acid, and trehalose by 10-DAI. By 15-DAI, evidence of recovery in primary metabolism was noted based on transcript abundances for genes associated with carbon, nitrogen, and nutrient assimilation.

CONCLUSIONS

Extensive remodeling of the plant transcriptome and the production of ROS and several defensive metabolites in an upland switchgrass cultivar were observed in response to GB feeding. The early loss and apparent recovery in primary metabolism by 15-DAI would suggest that these transcriptional changes in later stages of GB infestation could underlie the recovery response categorized for this switchgrass cultivar. These results can be exploited to develop switchgrass lines with more durable resistance to GB and potentially other aphids.

A Plant-Feeding Nematode Indirectly Increases the Fitness of an Aphid

Plants suffer multiple, simultaneous assaults from above and below ground. In the laboratory, pests and/or pathogen attack are commonly studied on an individual basis. The molecular response of the plant to attack from multiple organisms and the interaction of different defense pathways is unclear. The inducible systemic responses of the potato (Solanum tuberosum L.) host plant were analyzed to characterize the plant-mediated indirect interactions between a sedentary, endoparasitic nematode (Globodera pallida), and a phloem-sucking herbivore (Myzus persicae). The reproductive success of M. persicae was greater on potato plants pre-infected with G. pallida compared to control plants. Salicylic acid (SA) increased systemically in the leaves of potato plants following nematode and aphid infection singly with a corresponding increase in expression of SA-mediated marker genes. An increase in jasmonic acid associated with aphid infection was suppressed when plants were co-infected with nematodes. Our data suggests a positive, asymmetric interaction between a sedentary endoparasitic nematode and a sap-sucking insect. The systemic response of the potato plant following infection with G. pallida indirectly influences the performance of M. persicae. This work reveals additional secondary benefits of controlling individual crop pests.

The plant response induced in wheat ears by a combined attack of Sitobion avenae aphids and Fusarium graminearum boosts fungal infection and deoxynivalenol production

The pathogen Fusarium graminearum, producer of the mycotoxin deoxynivalenol, and Sitobion avenae aphids both reside on wheat ears. We explored the influence of an earlier aphid infestation on the expression profile of specific molecular markers associated with F. graminearum infection. Using reverse transcription-quantitative polymerase chain reaction analysis, we followed the expression of wheat defence genes on S. avenae infestation and explored the effect on a subsequent F. graminearum infection. This was performed by the assessment of disease symptoms, fungal biomass, mycotoxin production and number of aphids at several time points during disease progress. Wheat ears infected with F. graminearum showed more disease symptoms and higher deoxynivalenol levels when ears were pre-exposed to aphids relative to a sole inoculation with F. graminearum. Aphids induced defence genes that are typically induced on F. graminearum infection. Other defence genes showed earlier and/or enhanced transcription after exposure to both aphids and F. graminearum. In the discussion, we link the symptomatic and epidemiological parameters with the transcriptional induction pattern in the plant. Our study suggests that pre-exposure of wheat ears to aphids affects the plant response, which plays a role in the subsequent attack of F. graminearum, enabling the fungus to colonize wheat ears more rapidly.

Identification of an orthologous clade of peroxidases that respond to feeding by greenbugs (Schizaphis graminum) in C4 grasses

Knowledge of specific peroxidases that respond to aphid herbivory is limited in C4 grasses, but could provide targets for improving defence against these pests. A sorghum (Sorghum bicolor (L.) Moench) peroxidase (SbPrx-1; Sobic.002G416700) has been previously linked to biotic stress responses, and was the starting point for this study. Genomic analyses indicated that SbPrx-1 was part of a clade of five closely related peroxidase genes occurring within a ~30kb region on chromosome 2 of the sorghum genome. Comparison of this ~30-kb region to syntenic regions in switchgrass (Panicum virgatum L.) and foxtail millet (Setaria italica L.) identified similar related clusters of peroxidases. Infestation of a susceptible sorghum cultivar with greenbugs (Shizaphis graminum Rondani) induced three of the five peroxidases. Greenbug infestation of switchgrass and foxtail millet plants showed similar inductions of peroxidases. SbPrx-1 was also induced in response to aphid herbivory in a greenbug-resistant sorghum line, Cargill 607E. These data indicate that this genomic region of C4 grasses could be valuable as a marker to assess potential insect resistance in C4 grasses.

Recognizing Plant Defense Priming

Defense priming conditions diverse plant species for the superinduction of defense, often resulting in enhanced pest and disease resistance and abiotic stress tolerance. Here, we propose a guideline that might assist the plant research community in a consistent assessment of defense priming in plants.

Aboveground Whitefly Infestation-Mediated Reshaping of the Root Microbiota

Plants respond to various types of herbivore and pathogen attack using well-developed defensive machinery designed for self-protection. Infestation from phloem-sucking insects such as whitefly and aphid on plant leaves was previously shown to influence both the saprophytic and pathogenic bacterial community in the plant rhizosphere. However, the modulation of the root microbial community by plants following insect infestation has been largely unexplored. Only limited studies of culture-dependent bacterial diversity caused by whitefly and aphid have been conducted. In this study, to obtain a complete picture of the belowground microbiome community, we performed high-speed and high-throughput next-generation sequencing. We sampled the rhizosphere soils of pepper seedlings at 0, 1, and 2 weeks after whitefly infestation versus the water control. We amplified a partial 16S ribosomal RNA gene (V1-V3 region) by polymerase chain reaction with specific primers. Our analysis revealed that whitefly infestation reshaped the overall microbiota structure compared to that of the control rhizosphere, even after 1 week of infestation. Examination of the relative abundance distributions of microbes demonstrated that whitefly infestation shifted the proteobacterial groups at week 2. Intriguingly, the population of Pseudomonadales of the class Gammaproteobacteria significantly increased after 2 weeks of whitefly infestation, and the fluorescent Pseudomonas spp. recruited to the rhizosphere were confirmed to exhibit insect-killing capacity. Additionally, three taxa, including Caulobacteraceae, Enterobacteriaceae, and Flavobacteriaceae, and three genera, including Achromobacter, Janthinobacterium, and Stenotrophomonas, were the most abundant bacterial groups in the whitefly infested plant rhizosphere. Our results indicate that whitefly infestation leads to the recruitment of specific groups of rhizosphere bacteria by the plant, which confer beneficial traits to the host plant. This study provides a new framework for investigating how aboveground insect feeding modulates the belowground microbiome.

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.

Simultaneous analysis of defense-related phytohormones in Arabidopsis thaliana responding to fungal infection

PREMISE OF THE STUDY

Simultaneous analysis of defense-related phytohormones can provide insights into underlying biochemical interactions. Ultra-high-performance liquid chromatographic (UHPLC) techniques hyphenated to electrospray ionization mass spectrometry (ESI-MS) are powerful analytical platforms, suitable for quantitative profiling of multiple classes of metabolites.

METHODS

An efficient and simplified extraction method was designed followed by reverse-phase UHPLC for separation of seven phytohormones: salicylic acid, methyl salicylate, jasmonic acid, methyl jasmonate, absiscic acid, indole acetic acid, and the ethylene precursor 1-aminocyclopropane-1-carboxylic acid. A triple quadrupole multiple reaction monitoring (MRM) method was developed for MS quantification. The methods were applied to analyze phytohormones in Arabidopsis leaf tissue responding to biotic stresses.

RESULTS

Under the optimized conditions, the phytohormones were separated within 15 min, with good linearities and high sensitivity. Repeatable results were obtained, with the limits of detection and quantification around 0.01 ng/μL (∼9 ng/g tissue). The method was validated and applied to monitor, quantify, and compare the temporal changes of the phytohormones under biotic stress.

DISCUSSION

Quantitative changes indicate increased production of defense phytohormones from the various classes. The analytical method was useful and suitable to distinguish distinctive variations in the phytohormonal profiles and balance in A. thaliana leaves resulting from pathogen attack.

Pseudomonas syringae enhances herbivory by suppressing the reactive oxygen burst in Arabidopsis

Plant-herbivore interactions have evolved in the presence of plant-colonizing microbes. These microbes can have important third-party effects on herbivore ecology, as exemplified by drosophilid flies that evolved from ancestors feeding on plant-associated microbes. Leaf-mining flies in the genus Scaptomyza, which is nested within the paraphyletic genus Drosophila, show strong associations with bacteria in the genus Pseudomonas, including Pseudomonas syringae. Adult females are capable of vectoring these bacteria between plants and larvae show a preference for feeding on P. syringae-infected leaves. Here we show that Scaptomyza flava larvae can also vector P. syringae to and from feeding sites, and that they not only feed more, but also develop faster on plants previously infected with P. syringae. Our genetic and physiological data show that P. syringae enhances S. flava feeding on infected plants at least in part by suppressing anti-herbivore defenses mediated by reactive oxygen species.

Expression and Functional Roles of the Pepper Pathogen-Induced bZIP Transcription Factor CabZIP2 in Enhanced Disease Resistance to Bacterial Pathogen Infection

A pepper bZIP transcription factor gene, CabZIP2, was isolated from pepper leaves infected with a virulent strain of Xanthomonas campestris pv. vesicatoria. Transient expression analysis of the CabZIP2-GFP fusion protein in Nicotiana benthamiana revealed that the CabZIP2 protein is localized in the cytoplasm as well as the nucleus. The acidic domain in the N-terminal region of CabZIP2 that is fused to the GAL4 DNA-binding domain is required to activate the transcription of reporter genes in yeast. Transcription of CabZIP2 is induced in pepper plants inoculated with virulent or avirulent strains of X. campestris pv. vesicatoria. The CabZIP2 gene is also induced by defense-related hormones such as salicylic acid, methyl jasmonate, and ethylene. To elucidate the in vivo function of the CabZIP2 gene in plant defense, virus-induced gene silencing in pepper and overexpression in Arabidopsis were used. CabZIP2-silenced pepper plants were susceptible to infection by the virulent strain of X. campestris pv. vesicatoria, which was accompanied by reduced expression of defense-related genes such as CaBPR1 and CaAMP1. CabZIP2 overexpression in transgenic Arabidopsis plants conferred enhanced resistance to Pseudomonas syringae pv. tomato DC3000. Together, these results suggest that CabZIP2 is involved in bacterial disease resistance.