Clare L. Casteel

Herbivory in the Previous Generation Primes Plants for Enhanced Insect Resistance

Inducible defenses, which provide enhanced resistance after initial attack, are nearly universal in plants. This defense signaling cascade is mediated by the synthesis, movement, and perception of jasmonic acid and related plant metabolites. To characterize the long-term persistence of plant immunity, we challenged Arabidopsis (Arabidopsis thaliana) and tomato (Solanum lycopersicum) with caterpillar herbivory, application of methyl jasmonate, or mechanical damage during vegetative growth and assessed plant resistance in subsequent generations. Here, we show that induced resistance was associated with transgenerational priming of jasmonic acid-dependent defense responses in both species, caused caterpillars to grow up to 50% smaller than on control plants, and persisted for two generations in Arabidopsis. Arabidopsis mutants that are deficient in jasmonate perception (coronatine insensitive1) or in the biogenesis of small interfering RNA (dicer-like2 dicer-like3 dicer-like4 and nuclear RNA polymerase d2a nuclear RNA polymerase d2b) do not exhibit inherited resistance. The observation of inherited resistance in both the Brassicaceae and Solanaceae suggests that this trait may be more widely distributed in plants. Epigenetic resistance to herbivory thus represents a phenotypically plastic mechanism for enhanced defense across generations.

Anthropogenic increase in carbon dioxide compromises plant defense against invasive insects

Elevated levels of atmospheric carbon dioxide (CO2), a consequence of anthropogenic global change, can profoundly affect the interactions between crop plants and insect pests and may promote yet another form of global change: the rapid establishment of invasive species. Elevated CO2 increased the susceptibility of soybean plants grown under field conditions to the invasive Japanese beetle (Popillia japonica) and to a variant of western corn rootworm (Diabrotica virgifera virgifera) resistant to crop rotation by down-regulating gene expression related to defense signaling [lipoxygenase 7 (lox7), lipoxygenase 8 (lox8), and 1-aminocyclopropane-1-carboxylate synthase (acc-s)]. The down-regulation of these genes, in turn, reduced the production of cysteine proteinase inhibitors (CystPIs), which are specific deterrents to coleopteran herbivores. Beetle herbivory increased CystPI activity to a greater degree in plants grown under ambient than under elevated CO2. Gut cysteine proteinase activity was higher in beetles consuming foliage of soybeans grown under elevated CO2 than in beetles consuming soybeans grown in ambient CO2, consistent with enhanced growth and development of these beetles on plants grown in elevated CO2. These findings suggest that predicted increases in soybean productivity under projected elevated CO2 levels may be reduced by increased susceptibility to invasive crop pests.

Transcriptional profiling reveals elevated CO2 and elevated O3 alter resistance of soybean (Glycine max) to Japanese beetles (Popillia japonica)

The accumulation of CO2 and O3 in the troposphere alters phytochemistry which in turn influences the interactions between plants and insects. Using microarray analysis of field-grown soybean (Glycine max), we found that the number of transcripts in the leaves affected by herbivory by Japanese beetles (Popillia japonica) was greater when plants were grown under elevated CO2, elevated O3 and the combination of elevated CO2 plus elevated O3 than when grown in ambient atmosphere. The effect of herbivory on transcription diminished strongly with time (<1% of genes were affected by herbivory after 3 weeks), and elevated CO2 interacted more strongly with herbivory than elevated O3. The majority of transcripts affected by elevated O3 were related to antioxidant metabolism. Constitutive levels and the induction by herbivory of key transcripts associated with defence and hormone signalling were down-regulated under elevated CO2; 1-aminocyclopropane-1-carboxylate (ACC) synthase, lipoxygenase (LOX), allene oxide synthase (AOS), allene oxide cyclase (AOC), chalcone synthase (CHS), polyphenol oxidase (PPO) and cysteine protease inhibitor (CystPI) were lower in abundance compared with levels under ambient conditions. By suppressing the ability to mount an effective defence, elevated CO2 may decrease resistance of soybean to herbivory.

Manipulation of plant defense responses by the tomato psyllid (Bactericerca cockerelli) and its associated endosymbiont Candidatus Liberibacter psyllaurous.

Some plant pathogens form obligate relationships with their insect vector and are vertically transmitted via eggs analogous to insect endosymbionts. Whether insect endosymbionts manipulate plant defenses to benefit their insect host remains unclear. The tomato psyllid, Bactericerca cockerelli (Sulc), vectors the endosymbiont “Candidatus Liberibacter psyllaurous” (Lps) during feeding on tomato (Solanum lycopersicum L.). Lps titer in psyllids varied relative to the psyllid developmental stage with younger psyllids harboring smaller Lps populations compared to older psyllids. In the present study, feeding by different life stages of B. cockerelli infected with Lps, resulted in distinct tomato transcript profiles. Feeding by young psyllid nymphs, with lower Lps levels, induced tomato genes regulated by jasmonic acid (JA) and salicylic acid (SA) (Allene oxide synthase, Proteinase inhibitor 2, Phenylalanine ammonia-lyase 5, Pathogenesis-related protein 1) compared to feeding by older nymphs and adults, where higher Lps titers were found. In addition, inoculation of Lps without insect hosts suppressed accumulation of these defense transcripts. Collectively, these data suggest that the endosymbiont-like pathogen Lps manipulates plant signaling and defensive responses to benefit themselves and the success of their obligate insect vector on their host plant.

Insects take a bigger bite out of plants in a warmer, higher carbon dioxide world

Carbon dioxide is a potent “greenhouse” gas. The dramatic increase in its concentration in the atmosphere as a result of human activities, beginning with accelerated fossil fuels combustion in the late 18th century, and perhaps even earlier, with modern agricultural expansion 8,000 years ago (1, 2), is driving a striking rise in global temperature (3). For the past 650,000 years, until relatively recently, the concentration of CO2 in the atmosphere was 280 ppm or less; however, the current concentration exceeds 380 ppm and, on its present trajectory, will surpass 550 ppm by 2050 (3). The accumulation of CO2 and other greenhouse gases in the atmosphere is forcing an elevation of global mean temperature; during the lifetime of child born today, the average temperature of the earth will increase by as much as ≈6°C (3). Working in concert, elevated temperature and CO2 are redistributing plant and animal communities on the surface of the earth (4). Because of the direct effect of CO2 and temperature on global food supplies, the influence of these changes on plant physiology and ecology is being actively studied (4–7). How these elements of global change may alter the interactions between plants and the insects that feed on them is relatively unknown. By bringing to light secrets contained in the fossil record, Currano et al. (8), published in this issue of PNAS, found that the amount and diversity of insect damage to plants increased in association with an abrupt rise in atmospheric CO2 and global temperature that occurred >55 million years ago. If the past is indeed a window to the future, their findings suggest that increased insect herbivory will be one more unpleasant surprise arising from anthropogenic climate change.

The NIa‐Pro protein of Turnip mosaic virus improves growth and reproduction of the aphid vector, Myzus persicae (green peach aphid)

Many plant viruses depend on aphids and other phloem-feeding insects for transmission within and among host plants. Thus, viruses may promote their own transmission by manipulating plant physiology to attract aphids and increase aphid reproduction. Consistent with this hypothesis, Myzus persicae (green peach aphids) prefer to settle on Nicotiana benthamiana infected with Turnip mosaic virus (TuMV) and fecundity on virus-infected N. benthamiana and Arabidopsis thaliana (Arabidopsis) is higher than on uninfected controls. TuMV infection suppresses callose deposition, an important plant defense, and increases the amount of free amino acids, the major source of nitrogen for aphids. To investigate the underlying molecular mechanisms of this phenomenon, 10 TuMV genes were over-expressed in plants to determine their effects on aphid reproduction. Production of a single TuMV protein, nuclear inclusion a-protease domain (NIa-Pro), increased M. persicae reproduction on both N. benthamiana and Arabidopsis. Similar to the effects that are observed during TuMV infection, NIa-Pro expression alone increased aphid arrestment, suppressed callose deposition and increased the abundance of free amino acids. Together, these results suggest a function for the TuMV NIa-Pro protein in manipulating the physiology of host plants. By attracting aphid vectors and promoting their reproduction, TuMV may influence plant-aphid interactions to promote its own transmission.

Disruption of Ethylene Responses by Turnip mosaic virus Mediates Suppression of Plant Defense against the Green Peach Aphid Vector

A plant virus suppresses plant defense against insect vectors by modulating ethylene responses. Plants employ diverse responses mediated by phytohormones to defend themselves against pathogens and herbivores. Adapted pathogens and herbivores often manipulate these responses to their benefit. Previously, we demonstrated that Turnip mosaic virus (TuMV) infection suppresses callose deposition, an important plant defense induced in response to feeding by its aphid vector, the green peach aphid (Myzus persicae), and increases aphid fecundity compared with uninfected control plants. Further, we determined that production of a single TuMV protein, Nuclear Inclusion a-Protease (NIa-Pro) domain, was responsible for changes in host plant physiology and increased green peach aphid reproduction. To characterize the underlying molecular mechanisms of this phenomenon, we examined the role of three phytohormone signaling pathways, jasmonic acid, salicylic acid, and ethylene (ET), in TuMV-infected Arabidopsis (Arabidopsis thaliana), with or without aphid herbivory. Experiments with Arabidopsis mutants ethylene insensitive2 and ethylene response1, and chemical inhibitors of ET synthesis and perception (aminoethoxyvinyl-glycine and 1-methylcyclopropene, respectively), show that the ET signaling pathway is required for TuMV-mediated suppression of Arabidopsis resistance to the green peach aphid. Additionally, transgenic expression of NIa-Pro in Arabidopsis alters ET responses and suppresses aphid-induced callose formation in an ET-dependent manner. Thus, disruption of ET responses in plants is an additional function of NIa-Pro, a highly conserved potyvirus protein. Virus-induced changes in ET responses may mediate vector-plant interactions more broadly and thus represent a conserved mechanism for increasing transmission by insect vectors across generations.

Disruption of ethylene responses by Turnip mosaic virus mediates suppression of plant defense against the aphid vector, Myzus persicae

A plant virus suppresses plant defense against insect vectors by modulating ethylene responses. Plants employ diverse responses mediated by phytohormones to defend themselves against pathogens and herbivores. Adapted pathogens and herbivores often manipulate these responses to their benefit. Previously, we demonstrated that Turnip mosaic virus (TuMV) infection suppresses callose deposition, an important plant defense induced in response to feeding by its aphid vector, the green peach aphid (Myzus persicae), and increases aphid fecundity compared with uninfected control plants. Further, we determined that production of a single TuMV protein, Nuclear Inclusion a-Protease (NIa-Pro) domain, was responsible for changes in host plant physiology and increased green peach aphid reproduction. To characterize the underlying molecular mechanisms of this phenomenon, we examined the role of three phytohormone signaling pathways, jasmonic acid, salicylic acid, and ethylene (ET), in TuMV-infected Arabidopsis (Arabidopsis thaliana), with or without aphid herbivory. Experiments with Arabidopsis mutants ethylene insensitive2 and ethylene response1, and chemical inhibitors of ET synthesis and perception (aminoethoxyvinyl-glycine and 1-methylcyclopropene, respectively), show that the ET signaling pathway is required for TuMV-mediated suppression of Arabidopsis resistance to the green peach aphid. Additionally, transgenic expression of NIa-Pro in Arabidopsis alters ET responses and suppresses aphid-induced callose formation in an ET-dependent manner. Thus, disruption of ET responses in plants is an additional function of NIa-Pro, a highly conserved potyvirus protein. Virus-induced changes in ET responses may mediate vector-plant interactions more broadly and thus represent a conserved mechanism for increasing transmission by insect vectors across generations.

Vector-Borne Bacterial Plant Pathogens: Interactions with Hemipteran Insects and Plants

Hemipteran insects are devastating pests of crops due to their wide host range, rapid reproduction, and ability to transmit numerous plant-infecting pathogens as vectors. While the field of plant–virus–vector interactions has flourished in recent years, plant–bacteria–vector interactions remain poorly understood. Leafhoppers and psyllids are by far the most important vectors of bacterial pathogens, yet there are still significant gaps in our understanding of their feeding behavior, salivary secretions, and plant responses as compared to important viral vectors, such as whiteflies and aphids. Even with an incomplete understanding of plant–bacteria–vector interactions, some common themes have emerged: (1) all known vector-borne bacteria share the ability to propagate in the plant and insect host; (2) particular hemipteran families appear to be incapable of transmitting vector-borne bacteria; (3) all known vector-borne bacteria have highly reduced genomes and coding capacity, resulting in host-dependence; and (4) vector-borne bacteria encode proteins that are essential for colonization of specific hosts, though only a few types of proteins have been investigated. Here, we review the current knowledge on important vector-borne bacterial pathogens, including Xylella fastidiosa, Spiroplasma spp., Liberibacter spp., and ‘Candidatus Phytoplasma spp.’. We then highlight recent approaches used in the study of vector-borne bacteria. Finally, we discuss the application of this knowledge for control and future directions that will need to be addressed in the field of vector–plant–bacteria interactions.

Abscisic acid deficiency increases defence responses against Myzus persicae in Arabidopsis

Comparison of Arabidopsis thaliana (Arabidopsis) gene expression induced by Myzus persicae (green peach aphid) feeding, aphid saliva infiltration and abscisic acid (ABA) treatment showed a significant positive correlation. In particular, ABA-regulated genes are over-represented among genes that are induced by M. persicae saliva infiltration into Arabidopsis leaves. This suggests that the induction of ABA-related gene expression could be an important component of the Arabidopsis-aphid interaction. Consistent with this hypothesis, M. persicae populations induced ABA production in wild-type plants. Furthermore, aphid populations were smaller on Arabidopsis aba1-1 mutants, which cannot synthesize ABA, and showed a significant preference for wild-type plants compared with the mutant. Total free amino acids, which play an important role in aphid nutrition, were not altered in the aba1-1 mutant line, but the levels of isoleucine (Ile) and tryptophan (Trp) were differentially affected by aphids in wild-type and mutant plants. Recently, indole glucosinolates have been shown to promote aphid resistance in Arabidopsis. In this study, 4-methoxyindol-3-ylmethylglucosinolate was more abundant in the aba1-1 mutant than in wild-type Arabidopsis, suggesting that the induction of ABA signals that decrease the accumulation of defence compounds may be beneficial for aphids.