Focus on Metabolism: Something Old,Something New: Conserved Enzymes and the Evolution of Novelty in Plant Specialized Metabolism

Plants produce hundreds of thousands of small molecules known as specialized metabolites, many of which are of economic and ecological importance. This remarkable variety is a consequence of the diversity and rapid evolution of specialized metabolic pathways. These novel biosynthetic pathways originate via gene duplication or by functional divergence of existing genes, and they subsequently evolve through selection and/or drift. Studies over the past two decades revealed that diverse specialized metabolic pathways have resulted from the incorporation of primary metabolic enzymes. We discuss examples of enzyme recruitment from primary metabolism and the variety of paths taken by duplicated primary metabolic enzymes toward integration into specialized metabolism. These examples provide insight into processes by which plant specialized metabolic pathways evolve and suggest approaches to discover enzymes of previously uncharacterized metabolic networks.The plant kingdom collectively produces hundreds of thousands of low molecular weight organic molecules traditionally known as secondary metabolites, some of which have been shown to play roles in abiotic and biotic stress responses (e.g. herbivory defense), beneficial insect interactions (e.g. pollinator attraction), and communication with other plant and nonplant species (e.g. allelopathy and legume-rhizobia interactions; Saito and Matsuda, 2010; Pichersky and Lewinsohn, 2011; Wink, 2011). These metabolites have been widely used throughout the course of human history as medicines, spices, perfumes, cosmetics, and pest-control agents as well as in religious and cultural rituals. For the past 150 years, there has been a strong focus on documenting the chemical diversity of secondary metabolites in the plant kingdom, leading to the discovery of diverse classes of compounds such as terpenes, flavonoids, alkaloids, phenylpropanoids, glucosinolates, and polyketides. These secondary compounds were historically differentiated from products of primary metabolism, such as sugars, amino acids, nucleic acids, and fatty acids, as being nonessential for plant survival (Sachs, 1874; Kossel, 1891; Hartmann, 2008). However, by the 1980s, important functional roles began to be elucidated for metabolites previously classified as secondary, such as the phenolics (e.g. plant-microbe interactions and UV-B light protection; Bolton et al., 1986; Peters et al., 1986; Li et al., 1993; Landry et al., 1995), alkaloids (defense against herbivory; Steppuhn et al., 2004), and terpenes (defense against herbivory, antimicrobial activities, and volatile pollinator attractants; Papadopoulou et al., 1999; Schiestl and Ayasse, 2001; Erbilgin et al., 2006; Nieuwenhuizen et al., 2009). This change in our understanding of their roles has led to the coining of a new term, specialized metabolites, for these compounds, both to acknowledge their importance and to reflect the fact that many of them are phylogenetically restricted (Pichersky et al., 2006; Pichersky and Lewinsohn, 2011).Although the structural diversity of specialized metabolites far exceeds that of primary metabolites, all specialized metabolite classes are ultimately derived from primary metabolic precursors (Wink, 2011). For example, phenylpropanoids are derived from the amino acid Phe (Vogt, 2010), while the biosynthetic blocks of terpenes, isopentenyl diphosphate and dimethylallyl diphosphate, originate from mevalonate, a sterol precursor, and alternatively from methylerythritol phosphate, which is derived from glycolytic pathway precursors (Kirby and Keasling, 2009). Nitrogen-containing alkaloids are derived from a variety of primary metabolites, including amino acids and purine nucleosides (Facchini, 2001). Over the past 20 years, an increasing number of specialized metabolic enzymes have also been found to have their origins in primary metabolic pathways (Weng, 2014). Such shifts in enzyme function are made possible primarily by the process of gene duplication, which is very common in plants.     

Regulation of a Chemical Defense against Herbivory Produced by Symbiotic Fungi in Grass Plants

Neotyphodium uncinatum and Neotyphodium siegelii are fungal symbionts (endophytes) of meadow fescue (MF; Lolium pratense), which they protect from insects by producing loline alkaloids. High levels of lolines are produced following insect damage or mock herbivory (clipping). Although loline alkaloid levels were greatly elevated in regrowth after clipping, loline-alkaloid biosynthesis (LOL) gene expression in regrowth and basal tissues was similar to unclipped controls. The dramatic increase of lolines in regrowth reflected the much higher concentrations in young (center) versus older (outer) leaf blades, so LOL gene expression was compared in these tissues. In MF-N. siegelii, LOL gene expression was similar in younger and older leaf blades, whereas expression of N. uncinatum LOL genes and some associated biosynthesis genes was higher in younger than older leaf blades. Because lolines are derived from amino acids that are mobilized to new growth, we tested the amino acid levels in center and outer leaf blades. Younger leaf blades of aposymbiotic plants (no endophyte present) had significantly higher levels of asparagine and sometimes glutamine compared to older leaf blades. The amino acid levels were much lower in MF-N. siegelii and MF-N. uncinatum compared to aposymbiotic plants and MF with Epichloë festucae (a closely related symbiont), which lacked lolines. We conclude that loline alkaloid production in young tissue depleted these amino acid pools and was apparently regulated by availability of the amino acid substrates. As a result, lolines maximally protect young host tissues in a fashion similar to endogenous plant metabolites that conform to optimal defense theory.Loline alkaloids (LAs; Hofmeister, 1892; Siegel et al., 1990; TePaske et al., 1993; Blankenship et al., 2001) are protective secondary metabolites produced by some Epichloë and Neotyphodium spp. (epichloae), fungi that live as systemic symbionts in many cool season grasses (Poaceae subfamily Pooideae). The lolines are active against a broad spectrum of insects (Schardl et al., 2007) and are derived from l-Pro (Pro) and l-homoserine (Hse; Blankenship et al., 2005). Mock herbivory (clipping plants) is reported to induce higher levels of lolines in several grass-epichloë symbiota (Craven et al., 2001; Bultman et al., 2004; Gonthier et al., 2008), suggesting that the epichloae have evolved to regulate their metabolism in a manner appropriate for defense of their hosts. However, little is known of the regulation of LA synthesis in symbio and whether these symbionts follow prevailing models for how plants deploy chemical defenses against herbivores (McKey, 1979; Rhoades, 1979; Barto and Cipollini, 2005).The loline-alkaloid biosynthesis (LOL) gene cluster contains nine genes likely to direct LA production (Spiering et al., 2005). Neotyphodium uncinatum contains two highly similar LOL clusters (LOL1 and LOL2), and a single LOL cluster has been found in each of the LA-producing species, Neotyphodium coenophialum, Neotyphodium siegelii, and some strains of Epichloë festucae, among others (Spiering et al., 2005; Kutil et al., 2007). Fermentation cultures of N. uncinatum produce lolines, and studies involving application of labeled precursors and intermediates have almost completely elucidated the LA biosynthetic pathway (Blankenship et al., 2005; Spiering et al., 2005; Faulkner et al., 2006; Schardl et al., 2007). Putative roles of the LOL gene products—based on sequence relationships to known enzyme classes—fit well with the pathway. Furthermore, an RNA interference knockdown of lolC reduces LA levels, and a lolP knockout prevents conversion of N-methylloline to N-formylloline (Spiering et al., 2005, 2008). Expression kinetics of the LOL genes are tightly correlated with each other and with the LA production phase in N. uncinatum cultures (Zhang et al., 2009). This finding raises the question whether and how LOL gene expression in symbio relates to changes in LA levels in response to development and stresses in host plants.LA production in symbio may be influenced by physiological differences among plant tissues and developmental stages, as well as differences in nutritional status and environmental stresses (Kennedy and Bush, 1983; Belesky et al., 1987; Justus et al., 1997; Tong et al., 2006). Given the anti-insect activity of lolines, effects of plant damage on LA levels are of particular interest. Mock herbivory (clipping of leaves) leads to apparent increases in LA concentrations in regrowth tissues of tall fescue (TF; Lolium arundinaceum) symbiotic with N. coenophialum (Bultman et al., 2004; Sullivan et al., 2007) and of meadow fescue (MF; Lolium pratense) symbiotic with N. uncinatum or N. siegelii (Craven et al., 2001). Despite the higher LA levels, however, clipping or damage of TF-N. coenophialum by the herbivore Spodoptera frugiperda (fall armyworm) was reported to elicit only minor, marginally significant (P = 0.052) effects on expression of lolC (Sullivan et al., 2007). A study of the Glyceria striata-Epichloë glyceriae symbiotum demonstrated significantly higher expression of lolC and higher LA production when the grass was artificially damaged, whereas the effect of damage by S. frugiperda on LA concentrations and lolC expression was not significant (Gonthier et al., 2008).Prevailing concepts about how plants deploy chemical defenses include the optimal defense theory (ODT; McKey, 1979; Rhoades, 1979) and the growth differentiation balance hypothesis (GDBH; Barto and Cipollini, 2005). The ODT addresses the distribution of chemical defenses in the plant, predicting that such defenses will be concentrated in tissues that have relatively little means to physically inhibit herbivory (e.g. in young tissues) and are important in the fitness of the plant. The GBDH addresses the location of biosynthesis and predicts that mature tissues are more likely to produce secondary metabolites than are actively growing tissues, which instead need to use resources for biomass production. It is intriguing to consider whether the epichloae obey the predictions of ODT and GDBH, considering that many epichloae protect their hosts by synthesizing insecticidal alkaloids, but they are also evolutionarily derived from plant-pathogenic fungi (Moon et al., 2004) and do not always enhance host fitness (Faeth et al., 2004). In order to address these questions, it is necessary to understand how secondary metabolism of the epichloae is regulated in symbio. The production of lolines in MF-N. uncinatum and MF-N. siegelii is an ideal test case because the lolines accumulate to very high levels—up to 1.9% dry weight—in regrowth of clipped plants (Craven et al., 2001). Here, we test the hypotheses that LOL gene expression and substrate availability correlate with LA levels in younger versus older leaf tissues and in response to clipping in MF-N. uncinatum and MF-N. siegelii symbiota.     

Focus on Metabolism: Changes in Whole-Plant Metabolism during the Grain-Filling Stage in Sorghum Grown under Elevated CO2 and Drought

Projections indicate an elevation of the atmospheric CO₂ concentration ([CO₂]) concomitant with an intensification of drought for this century, increasing the challenges to food security. On the one hand, drought is a main environmental factor responsible for decreasing crop productivity and grain quality, especially when occurring during the grain-filling stage. On the other hand, elevated [CO₂] is predicted to mitigate some of the negative effects of drought. Sorghum (Sorghum bicolor) is a C₄ grass that has important economical and nutritional values in many parts of the world. Although the impact of elevated [CO₂] and drought in photosynthesis and growth has been well documented for sorghum, the effects of the combination of these two environmental factors on plant metabolism have yet to be determined. To address this question, sorghum plants (cv BRS 330) were grown and monitored at ambient (400 µmol mol⁻¹) or elevated (800 µmol mol⁻¹) [CO₂] for 120 d and subjected to drought during the grain-filling stage. Leaf photosynthesis, respiration, and stomatal conductance were measured at 90 and 120 d after planting, and plant organs (leaves, culm, roots, prop roots, and grains) were harvested. Finally, biochemical composition and intracellular metabolites were assessed for each organ. As expected, elevated [CO₂] reduced the stomatal conductance, which preserved soil moisture and plant fitness under drought. Interestingly, the whole-plant metabolism was adjusted and protein content in grains was improved by 60% in sorghum grown under elevated [CO₂].Global food demand is projected to increase up to 110% by the middle of this century (Tilman et al., 2011; Alexandratos and Bruinsma, 2012), particularly due to a rise in world population that is likely to plateau at about 9 billion people (Godfray et al., 2010). Additionally, the average concentration of atmospheric CO₂ ([CO₂]) has increased 1.75 µmol mol⁻¹ per year between 1975 and today, reaching 400 µmol mol⁻¹ in April 2015 (NOAA, 2015). According to the A2 emission scenario from the U.S. Environmental Protection Agency, in the absence of explicit climate change policy, atmospheric CO₂ concentrations will reach 800 µmol mol⁻¹ by the end of this century. The increasing atmospheric [CO₂] is resulting in global climate changes, such as reduction in water availability and elevation in temperature. These factors are expected to heavily influence food production in the next years (Godfray and Garnett, 2014; Magrin et al., 2014).Sorghum (Sorghum bicolor) is a C₄ grass, considered a staple food grain for millions of the poorest and most food-insecure people in the semiarid tropics of Africa, Asia, and Central America, serving as an important source of energy, proteins, vitamins, and minerals (Taylor et al., 2006). Moreover, this crop is used for animal feed and as industrial raw material in developed countries such as the United States, which is the main world producer (FAO, 2015). With a fully sequenced genome (Paterson et al., 2009) and over 45,000 accessions representing a large geographic and genetic diversity, sorghum is a good model system in which to study the impact of global climate changes in C₄ grasses.The increase in [CO₂] in the atmosphere, which is the main driver of global climate changes (Meehl et al., 2007), is predicted to boost photosynthesis rates and productivity in a series of C₃ legumes and cereals, mainly due to a decrease in the photorespiration process (Grashoff et al., 1995; Long et al., 2006). On the contrary, due to their capacity to concentrate CO₂ in bundle sheath cells and reduce photorespiration to virtually zero, C₄ plants are unlikely to respond to the elevation of atmospheric [CO₂] (Leakey, 2009). However, even for C₄ plants, elevated [CO₂] can ameliorate the effects caused by drought, maintaining higher photosynthetic rates. This is due to an improvement in the efficiency of water use that is achieved by the reduction in stomatal conductance (Leakey et al., 2004; Markelz et al., 2011).The rate of photosynthesis as well as the redistribution of photoassimilates accumulated in different plant tissues during the day and/or during vegetative growth are crucial to grain development, and later, to its filling (Schnyder, 1993). Due to this relationship, any environmental stress such as drought occurring during the reproductive phase has the potential to result in poor grain filling and losses in yield (Blum et al., 1997). For instance, postanthesis drought can cause up to 30% decrease in yield (Borrell et al., 2000). It is also known that elevated [CO₂], drought, high temperature, and any combinations of these stresses can lead to significant changes in grain composition (Taub et al., 2008; Da Matta et al., 2010; Uprety et al., 2010; Madan et al., 2012), suggesting diverse metabolic alterations and/or adaptations that occur in the plant when it is cultivated in such conditions.Although the impacts of elevated [CO₂] and drought on photosynthesis and the growth of sorghum have been well documented (Conley et al., 2001; Ottman et al., 2001; Wall et al., 2001), no attention has been given to the impact of the combination of these two environmental changes on plant metabolism and composition. Regarding physiology, studies on the growth of sorghum under elevated [CO₂] and drought showed an increase of the net assimilation rate of 23% due to a decrease of 32% in stomatal conductance (Wall et al., 2001). This resulted in sorghum’s ability to use water 17% more efficiently (Conley et al., 2001). An improvement in the final overall biomass under elevated [CO₂] and drought has also been described (Ottman et al., 2001), but without a significant effect in grain yield (Wall et al., 2001).Few studies have been monitoring metabolic pathways in plants under elevated [CO₂] (Li et al., 2008; Aranjuelo et al., 2013) and drought (Silvente et al., 2012; Nam et al., 2015; Wenzel et al., 2015). Furthermore, to our knowledge, there are only two reports in which metabolite profiles or metabolic pathways were investigated under the combination of these two environmental conditions (Sicher and Barnaby, 2012; Zinta et al., 2014). Although it is widely accepted that whole-plant metabolism and composition can impact grain filling and yield, metabolic studies conducted so far have focused on a specific plant organ. For instance, Sicher and Barnaby (2012) analyzed the metabolite profile of leaves from maize (Zea mays) plants that were grown under elevated [CO₂] and drought, but they did not show how those environmental changes could have affected the metabolism of other tissues (e.g. culm and roots) or how they might have influenced the biomass or grain composition.In order to address how the combination of elevated [CO₂] and drought can modify whole-plant metabolism as well as biomass composition in sorghum, this study aimed to (1) evaluate photosynthesis, growth, and yield; (2) underline the differences in biomass composition and primary metabolite profiles among leaves, culm, roots, prop roots, and grains; and (3) determine the effect of elevated [CO₂] and drought on the primary metabolism of each organ.     

A Physiological and Behavioral Mechanism for Leaf Herbivore-Induced Systemic Root Resistance

Indirect plant-mediated interactions between herbivores are important drivers of community composition in terrestrial ecosystems. Among the most striking examples are the strong indirect interactions between spatially separated leaf- and root-feeding insects sharing a host plant. Although leaf feeders generally reduce the performance of root herbivores, little is known about the underlying systemic changes in root physiology and the associated behavioral responses of the root feeders. We investigated the consequences of maize (Zea mays) leaf infestation by Spodoptera littoralis caterpillars for the root-feeding larvae of the beetle Diabrotica virgifera virgifera, a major pest of maize. D. virgifera strongly avoided leaf-infested plants by recognizing systemic changes in soluble root components. The avoidance response occurred within 12 h and was induced by real and mimicked herbivory, but not wounding alone. Roots of leaf-infested plants showed altered patterns in soluble free and soluble conjugated phenolic acids. Biochemical inhibition and genetic manipulation of phenolic acid biosynthesis led to a complete disappearance of the avoidance response of D. virgifera. Furthermore, bioactivity-guided fractionation revealed a direct link between the avoidance response of D. virgifera and changes in soluble conjugated phenolic acids in the roots of leaf-attacked plants. Our study provides a physiological mechanism for a behavioral pattern that explains the negative effect of leaf attack on a root-feeding insect. Furthermore, it opens up the possibility to control D. virgifera in the field by genetically mimicking leaf herbivore-induced changes in root phenylpropanoid patterns.Insect herbivores constantly compete for plants as a primary terrestrial source of organic carbon and nitrogen (Denno et al., 1995). Consequently, resource competition is thought to be a major determinant of the distribution and abundance of insects in natural and agricultural systems (Begon et al., 2006). Recent evidence suggests, however, that in many cases, insect herbivore competition may not follow the traditional theoretical assumptions of direct interference and/or resource exploitation, but may be determined by indirect plant-mediated effects (Kaplan and Denno, 2007; Poelman et al., 2008). Among the most striking examples of indirect plant-mediated interactions is the interplay between root- and leaf-feeding insects (Blossey and Hunt-Joshi, 2003). Despite their nonoverlapping feeding niches, leaf and root herbivores determine each other’s performance through shared host plants (Bezemer and van Dam, 2005). Although root feeders can have positive or negative effects on leaf feeders (van Dam and Heil, 2011), the effect of leaf herbivores on root consumers is predominantly negative (Johnson et al., 2012; Huang et al., 2014).Despite the increasing number of examples demonstrating negative effects of leaf attack on root herbivores (Tindall and Stout, 2001; Blossey and Hunt-Joshi, 2003; Soler et al., 2007; Gill et al., 2011), the mechanisms underlying this form of systemic induced resistance remain poorly understood (Erb et al., 2008; Rasmann and Agrawal, 2008). Pieris brassicae, for instance, was found to increase glucosinolate levels in the roots, which correlated with a reduced survival of the root feeder Delia radicum (Soler et al., 2007). Understanding why root feeders perform worse on leaf-infested plants would allow for more detailed investigations regarding the adaptive and evolutionary context of the phenomenon, and may allow for its exploitation in agriculture (for instance, by triggering root resistance through targeted leaf treatments).A promising system to study the mechanisms and agroecological consequences of plant-mediated interactions between herbivores is maize (Zea mays) and its associated pests. In the field, maize is attacked by a suite of herbivores, including leaf feeders, stem borers, and root feeders. The highly specialized root-feeding larvae of the western corn rootworm Diabrotica virgifera virgifera cause significant plant damage and yield loss in the United States and Eastern Europe. Earlier studies demonstrated that D. virgifera attack increases leaf resistance against Spodoptera spp. by triggering drought stress responses (Erb et al., 2009, 2011b). In the opposite direction, leaf feeding by Spodoptera spp. caterpillars reduces D. virgifera growth and development in a sequence-specific manner in the laboratory and the field (Erb et al., 2011c; Gill et al., 2011). D. virgifera was subsequently demonstrated to avoid leaf-infested plants by detecting and responding to a reduction in root ethylene emissions (Robert et al., 2012). However, it remains unclear whether nonvolatile chemical changes in the roots of leaf-infested maize plants affect D. virgifera foraging and performance. In this study, we explored the hypothesis that leaf infestation by Spodoptera spp. caterpillars triggers a short-range avoidance response in D. virgifera. Through a combination of bioactivity-guided fractionation of root extracts and biochemical and molecular manipulation, we show that systemic changes in soluble phenylpropanoid derivatives trigger a strong avoidance response in D. virgifera. We furthermore demonstrate that this avoidance response is mediated by systemic internal signals and is triggered specifically by herbivory, suggesting that D. virgifera actively and specifically recognizes and avoids leaf-infested plants.     

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

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.Plants suffer from numerous pathogen and herbivore challenges in both natural and agricultural environments, often facing multiple simultaneous threats (Casteel and Hansen, 2014). For example, many plant pathogens depend on insect vectors for transmission, including over 75% of all described plant viruses (Nault, 1997). Thus, plants must recognize, prioritize, and mount the most appropriate response to both the insect that is feeding and the pathogen being transmitted. Despite constant attack, plants persist, largely due to a sophisticated surveillance system. Plants respond with an arsenal of defenses that may be morphological, biochemical, or molecular in nature (Jones and Dangl, 2006; Jander and Howe, 2008). Nevertheless, pathogens and insects successfully colonize plants by actively compromising plant perception and/or defense responses.Recent studies show that synergisms exist between challengers, where both parties benefit during dual attack. For example, some virus infections can decrease plant defenses against insects, increasing plant palatability and vector fitness. Consequently, improved insect performance will increase the number of viruliferous vectors, promoting virus transmission to new hosts (Mauck et al., 2010; Casteel and Jander, 2013; Casteel et al., 2014; Li et al., 2014). Thus, vector-plant interactions represent a critical and synergistic relationship, ultimately determining survival and host range. Although numerous studies have examined virus-plant interactions, few have examined the molecular and genetic mechanisms mediating plant-virus-vector interactions and alterations in plant defenses (Li et al., 2014; Mauck et al., 2014).While defenses vary widely across plant species, the phytohormones that regulate their production are somewhat conserved. Modulation of hormone composition, timing, and concentration specifies plant responses to an attack (Mur et al., 2006; Verhage et al., 2010) and represents an excellent target for compromising defenses. Numerous studies have demonstrated that at least three phytohormones, jasmonic acid (JA), salicylic acid (SA), and ethylene (ET), have major roles in orchestrating plant defense responses (Bari and Jones, 2009; Erb et al., 2012; Pieterse et al., 2012). In general, SA signaling is critical for defense responses against a wide range of pathogens, including viruses (Glazebrook, 2005; Carr et al., 2010). Production of JA and ET, meanwhile, are involved in regulation of plant response to herbivores, necrotrophic pathogens, and nonpathogenic microbes (Glazebrook, 2005; Howe and Jander, 2008; Van der Ent et al., 2009). Virus infection can also alter JA and ET signaling (Carr et al., 2010; Lewsey et al., 2010; Wei et al., 2010; Mauck et al., 2014).Together, Arabidopsis (Arabidopsis thaliana), the green peach aphid (Myzus persicae), and Turnip mosaic virus (TuMV) constitute an excellent model system for investigating the molecular and biochemical mechanisms that underlie plant-aphid-virus interactions. As a well-studied model plant, Arabidopsis provides numerous genetic resources that can be used to investigate responses to aphid feeding and virus infection. The green peach aphid is a broad-host-range aphid and the world’s most prolific plant virus vector, transmitting more than 100 different viral species (Kennedy et al., 1962). The green peach aphid is the most common aphid pest on Arabidopsis in greenhouses and growth chambers (Bush et al., 2006), and we also have observed it feeding from Arabidopsis growing in nature. Due to the agricultural relevance of the green peach aphid, there is a large body of literature about the biology of this insect and its interactions with host plants, going back more than 100 years. More recently, several research groups have initiated projects to study plant defense against aphids using Arabidopsis and the green peach aphid as a model system (de Vos et al., 2007; Louis and Shah, 2013). TuMV is a positive-strand RNA virus that infects not only Arabidopsis but also hundreds of other species in more than 40 plant families (Walsh and Jenner, 2002). It is considered to be one of the most damaging viruses for vegetable crops worldwide (Tomlinson, 1987; Nguyen et al., 2013; Yasaka et al., 2015) and is transmitted by the green peach aphid and many other aphid species in both natural and agricultural settings (Shattuck, 1992). Largely due to its ability to systemically infect Arabidopsis (Sánchez et al., 1998; Martín Martín et al., 1999), TuMV has become a model for potyvirus-host interactions (Walsh and Jenner, 2002).In this study, we investigate the role of phytohormone signals in TuMV’s ability to suppress plant defense and enhance aphid fecundity during infection of host plants. First, we show that TuMV infection induces SA and ET accumulation in Arabidopsis. Next, using genetic and pharmacological analyses, we demonstrate that ET signaling is necessary for TuMV-initiated suppression of plant defense responses and enhanced aphid reproduction in plants. Further, we show that expression of the viral protein Nuclear Inclusion a-Protease (NIa-Pro) alters ET responses and that ET is also required for NIa-Pro’s role in suppressing aphid-induced defense in virus-infected plants. This molecular, biochemical, and genetic evidence reveals that TuMV may modulate ET responses not only to increase plant susceptibility to infection but also to increase vector performance.