Diane E. Ullman

Cross-talk between jasmonate and salicylate plant defense pathways: effects on several plant parasites

Plants are often attacked by many herbivorous insects and pathogens at the same time. Two important suites of responses to attack are mediated by plant hormones, jasmonate and salicylate, which independently provide resistance to herbivorous insects and pathogens, respectively. Several lines of evidence suggest that there is negative cross-talk between the jasmonate and salicylate response pathways. This biochemical link between general plant defense strategies means that deploying defenses against one attacker can positively or negatively affect other attackers. In this study, we tested for cross-talk in the jasmonate and salicylate signaling pathways in a wild tomato and examined the effects of cross-talk on an array of herbivores of cultivated tomato plants. In the wild cultivar, induction of defenses signaled by salicylate reduced biochemical expression of the jasmonate pathway but did not influence performance of S. exigua caterpillars. This indicates that the signal interaction is not a result of agricultural selection. In cultivated tomato, biochemical attenuation of the activity of a defense protein (polyphenol oxidase) in dual-elicited plants resulted in increased of performance of cabbage looper caterpillars, but not thrips, spider mites, hornworm caterpillars or the bacteria Pseudomonas syringae pv. tomato. In addition, we tested the effects of jasmonate-induced resistance on the ability of thrips to vector tomato spotted wilt virus. Although thrips fed less on induced plants, this did not affect the level of disease. Thus, the negative interaction between jasmonate and salicylate signaling had biological consequences for two lepidopteran larvae but not for several other herbivores tested or on the spread of a disease.

Infection with a plant virus modifies vector feeding behavior

Vector infection by some animal-infecting parasites results in altered feeding that enhances transmission. Modification of vector behavior is of broad adaptive significance, as parasite fitness relies on passage to a new host, and vector feeding is nearly always essential for transmission. Although several plant viruses infect their insect vectors, we have shown that vector infection by a plant virus alters feeding behavior. Here we show that infection with Tomato spotted wilt virus (TSWV), type member of the only plant-infecting genus in the Bunyaviridae, alters the feeding behavior of its thrips vector, Frankliniella occidentalis (Pergande). Male thrips infected with TSWV fed more than uninfected males, with the frequency of all feeding behaviors increasing by up to threefold, thus increasing the probability of virus inoculation. Importantly, infected males made almost three times more noningestion probes (probes in which they salivate, but leave cells largely undamaged) compared with uninfected males. A functional cell is requisite for TSWV infection and cell-to-cell movement; thus, this behavior is most likely to establish virus infection. Some animal-infecting members of the Bunyaviridae (La Crosse virus and Rift Valley fever virus) also cause increased biting rates in infected vectors. Concomitantly, these data support the hypothesis that capacity to modify vector feeding behavior is a conserved trait among plant- and animal-infecting members of the Bunyaviridae that evolved as a mechanism to enhance virus transmission. Our results underscore the evolutionary importance of vector behavioral modification to diverse parasites with host ranges spanning both plant and animal kingdoms.

Variability in the Response of Macrosiphum euphorbiae and Myzus persicae (Hemiptera: Aphididae) to the Tomato Resistance Gene Mi

Abstract Mi is a dominant locus in tomato that confers resistance to both root knot nematodes, Meloidogyne spp., and potato aphids, Macrosiphum euphorbiae (Thomas). The goal of this study was to investigate the breadth of Mi-mediated aphid resistance. To examine variability in potato aphid response, the effect of Mi on seven isolates of the potato aphid from California, New Jersey, North Carolina, France, and The Netherlands was measured in greenhouse bioassays. Assays were also performed to determine if Mi impacted another aphid species that attacks tomato, Myzus persicae (Sulzer). Tomato varieties carrying Mi had a profound negative impact on the potato aphid isolates from France and The Netherlands. In contrast, other potato aphid isolates from New Jersey, North Carolina, and California proved to be virulent, or capable of overcoming Mi. Mi-mediated resistance also did not extend to the green peach aphid isolates tested. These results demonstrate that Mi-mediated aphid resistance is isolate-specific, and it may not extend to all aphid species. Our data also show that virulent potato aphids are geographically widespread, and, unlike virulent nematode isolates, are relatively common. Virulence is persistent and is maintained even in the absence of selection pressure by Mi. The response of a particular aphid isolate to Mi could not be predicted on the basis of aphid color, because both avirulent and virulent isolates were found within the pink and green biotypes of the potato aphid.

Interaction of Tomato Spotted Wilt Tospovirus (TSWV) Glycoproteins with a Thrips Midgut Protein, a Potential Cellular Receptor for TSWV

ABSTRACT Interactions between viral and cellular membrane fusion proteins mediate virus penetration of cells for many arthropod-borne viruses. Electron microscope observations and circumstantial evidence indicate insect acquisition of tomato spotted wilt virus (TSWV) (genus Tospovirus, family Bunyaviridae) is receptor mediated, and TSWV membrane glycoproteins (GP1 and GP2) serve as virus attachment proteins. The tospoviruses are plant-infecting members of the family Bunyaviridae and are transmitted by several thrips species, including Frankliniella occidentalis. Gel overlay assays and immunolabeling were used to investigate the putative role of TSWV GPs as viral attachment proteins and deter mine whether a corresponding cellular receptor may be present in F. occidentalis. A single band in the 50-kDa region was detected with murine monoclonal antibodies (MAbs) to the TSWV-GPs when isolated TSWV or TSWV-GPs were used to overlay separated thrips proteins. This band was not detected when blots were probed with antibody to the non-structural protein encoded by the small RNA of TSWV or the TSWV nucleocapsid protein, nor were proteins from nonvector insects labeled. Anti-idiotype antibodies prepared to murine MAbs against GP1 or GP2 specifically labeled a single band at 50 kDa in Western blots and the plasmalemma of larval thrips midguts. These results support the putative role of the TSWV GPs as viral attachment proteins and identified potential cellular receptor(s) in thrips.

Morphological variation in Bemisia endosymbionts.

SummaryThe ultrastructure of the endosymbionts of several populations of whitefly (Homoptera: Aleyrodidae) was examined using transmission electron microscopy. Consistent differences in morphology and relative number of endosymbionts were observed between species and biotypes of whitefly within the Bemisia taxon.Bemisia argentifolii (=B. tabaci B biotype) individuals from Hawaii, Florida, and Arizona contained two morphological types of microorganisms housed within the mycetocyte cells of immature whiteflies. In contrast, individuals from populations ofB. tabaci A biotype from Arizona and Mexico, andB. tabaci Jatropha biotype from Puerto Rico, consistently contained three distinct morphological types of microorganisms within their mycetocytes. Organisms fromB. tabaci A and Jatropha biotypes differed from each other in the relative frequency of each type of microorganism. These observations suggest that different whitefly biotypes may have variable combinations of micro-fauna, with some possibly unique to each group, and furthers the hypothesis that variation in whitefly endosymbionts may be associated with the development of biotypes.

Thrips as vectors of tospoviruses

Publisher Summary This chapter reviews the thrips– tospovirus pathosystem and the cellular and molecular determinants of thrips acquisition of tospoviruses. Viruses in the genus Tospovirus (family Bunyaviridae) are transmitted by thrips and have become an ever increasing problem for the producers of agricultural and horticultural crops worldwide. The genus Tospovirus is the genus within the Bunyaviridae containing plant-infecting viruses. Tomato spotted wilt virus ( TSWV) is the type species of this genus. Thrips cause significant direct damage to plants, but it is their transmission of tospoviruses that is most difficult to control and frequently causes the most severe damage to crops. At least ten species of thrips transmit tospoviruses, all of which are in the Thysanopteran family Thripidae. Most thrips vector species deposit their eggs into plant tissue and the eggs hatch after 2–3 days, depending on temperature and plant host. For tospoviruses to be transmitted by thrips, they must be acquired by the larvae. Thus, only immature thrips that acquire tospoviruses or adults arising from such immatures are important to the transmission of the virus. This concept is extremely important in managing tospoviruses, because only the plants that serve as hosts for both the insect and the virus are important in epidemics.

Mi-Mediated Resistance Against the Potato Aphid Macrosiphum euphorbiae (Hemiptera: Aphididae) Limits Sieve Element Ingestion

Abstract The effect of the tomato (Lycopersicon esculentum Miller) resistance gene, Mi, on the feeding behavior of the potato aphid, Macrosiphum euphorbiae (Thomas), was studied using an AC electronic monitoring system. Aphids were monitored for at least 16 h on nearly isogenic tomato lines, and we observed waveforms corresponding to the stylet pathway phase, sieve element and nonphloem phase. The time to the first probe was not significantly different between resistant and susceptible tomato lines, indicating that leaf surface components were not involved in the resistance. Also, time to the first sieve element contact, as indicated by the sieve element phase waveform, was not significantly different, indicating that aphids were able to locate the sieve element equally well on resistant and susceptible lines. However, the duration of the sieve element phase was 7- to 10-fold longer on susceptible plants than on resistant plants. The insect also made more frequent and briefer probes on resistant tomato than on susceptible tomato. These results indicate that the mechanism of resistance mediated by Mi involves limitation of the duration of the sieve element phase, and salivation and ingestion of phloem fluids associated with it.

Characterization of Cucurbita maxima phloem serpin-1 (CmPS-1): a developmentally regulated elastase inhibitor.

We report on the molecular, biochemical, and functional characterization of Cucurbita maxima phloem serpin-1 (CmPS-1), a novel 42-kDa serine proteinase inhibitor that is developmentally regulated and has anti-elastase properties. CmPS-1 was purified to near homogeneity from C. maxima (pumpkin) phloem exudate and, based on microsequence analysis, the cDNA encoding CmPS-1 was cloned. The association rate constant (k a ) of phloem-purified and recombinant His6-tagged CmPS-1 for elastase was 3.5 ± 1.6 × 105 and 2.7 ± 0.4 × 105 m − 1 s− 1, respectively. The fraction of complex-forming CmPS-1, Xinh, was estimated at 79%. CmPS-1 displayed no detectable inhibitory properties against chymotrypsin, trypsin, or thrombin. The elastase cleavage sites within the reactive center loop of CmPS-1 were determined to be Val347-Gly348 and Val350-Ser351 with a 3:2 molar ratio. In vivo feeding assays conducted with the piercing-sucking aphid,Myzus persicae, established a close correlation between the developmentally regulated increase in CmPS-1 within the phloem sap and the reduced ability of these insects to survive and reproduce onC. maxima. However, in vitro feeding experiments, using purified phloem CmPS-1, failed to demonstrate a direct effect on aphid survival. Likely roles of this novel phloem serpin in defense against insects/pathogens are discussed.

Heterologous Expression of the Mi-1.2 Gene from Tomato Confers Resistance Against Nematodes but Not Aphids in Eggplant

The Mi-1.2 gene in tomato (Solanum lycopersicum) is a member of the nucleotide-binding leucine-rich repeat (NBLRR) class of plant resistance genes, and confers resistance against root-knot nematodes (Meloidogyne spp.), the potato aphid (Macrosiphum euphorbiae), and the sweet potato whitefly (Bemisia tabaci). Mi-1.2 mediates a rapid local defensive response at the site of infection, although the signaling and defensive pathways required for resistance are largely unknown. In this study, eggplant (S. melongena) was transformed with Mi-1.2 to determine whether this gene can function in a genetic background other than tomato. Eggplants that carried Mi-1.2 displayed resistance to the root-knot nematode Meloidogyne javanica but were fully susceptible to the potato aphid, whereas a susceptible tomato line transformed with the same transgene was resistant to nematodes and aphids. This study shows that Mi-1.2 can confer nematode resistance in another Solanaceous species. It also indicates that the requirements for Mi-mediated aphid and nematode resistance differ. Potentially, aphid resistance requires additional genes that are not conserved between tomato and eggplant.

Variation in Tomato spotted wilt virus titer in Frankliniella occidentalis and its association with frequency of transmission.

Tomato spotted wilt virus (TSWV) is transmitted in a persistent propagative manner by Frankliniella occidentalis, the western flower thrips. While it is well established that vector competence depends on TSWV acquisition by young larvae and virus replication within the insect, the biological factors associated with frequency of transmission have not been well characterized. We hypothesized that the number of transmission events by a single adult thrips is determined, in part, by the amount of virus harbored (titer) by the insect. Transmission time-course experiments were conducted using a leaf disk assay to determine the efficiency and frequency of TSWV transmission following 2-day inoculation access periods (IAPs). Virus titer in individual adult thrips was determined by real-time quantitative reverse transcriptase-PCR (qRT-PCR) at the end of the experiments. On average, 59% of adults transmitted the virus during the first IAP (2 to 3 days post adult-eclosion). Male thrips were more efficient at transmitting TSWV multiple times compared with female thrips of the same cohort. However, females harbored two to three times more copies of TSWV-N RNA per insect, indicating that factors other than absolute virus titer in the insect contribute to a successful transmission event. Examination of virus titer in individual insects at the end of the third IAP (7 days post adult-eclosion) revealed significant and consistent positive associations between frequency of transmission and virus titer. Our data support the hypothesis that a viruliferous thrips is more likely to transmit multiple times if it harbors a high titer of virus. This quantitative relationship provides new insights into the biological parameters that may influence the spread of TSWV by thrips.