Plant Defense Responses Induced by Two Herbivores and Consequences for Whitefly Bemisia tabaci

Better Together: Volatile-Mediated Intraguild Effects on the Preference of Tuta absoluta and Trialeurodes vaporariorum for Tomato Plants

Plant-herbivore interactions have been extensively studied in tomato plants and their most common pests. Tomato plant chemical defenses, both constitutive and inducible, play a role in mediating these interactions. Damaged tomato plants alter their volatile profiles, affecting herbivore preferences between undamaged and damaged plants. However, previous studies on tomato volatiles and herbivore preferences have yielded conflicting results, both in the volatile chemistry itself as well as in the attraction/repellent herbivore response. This study revisits the volatile-mediated interactions between tomato plants and two of their main herbivores: the leafminer Tuta absoluta and the whitefly Trialeurodes vaporariorum. Tomato plant volatiles were analyzed before and after damage by each of these herbivores, and the preference for oviposition (T. absoluta) and settling (T. vaporariorum) on undamaged and damaged plants was assessed both after conspecific and heterospecific damage. We found that both insects consistently preferred damaged plants over undamaged plants. The emission of herbivore-induced plant volatiles (HIPVs) increased after T. absoluta damage but decreased after T. vaporariorum damage. While some of our findings are in line with previous reports, T. absoluta preferred to oviposit on plants damaged by conspecifics, which differs from earlier studies. A comparison of HIPVs emitted after damage by T. absoluta and T. vaporariorum revealed differences in up- or down-regulation, as well as significant variations in specific compounds (12 for T. absoluta and 26 for T. vaporariorum damaged-plants). Only two compounds, β-caryophyllene and tetradecane, significantly varied because of damage by either herbivore, in line with the overall variation of the HIPV blend. Differences in HIPVs and herbivore preferences may be attributed to the distinct feeding habits of both herbivores, which activate different defensive pathways in plants. The plant's challenge in simultaneously activating both defensive pathways may explain the preference for heterospecific damaged plants found in this study, which are also in line with our own observations in greenhouses.

Laboratory evaluation of the life history of hibiscus mealybug, Nipaecocus viridis (Hemiptera: Pseudococcidae), on selected citrus and potential non-citrus hosts in Florida

Nipaecoccus viridis (Newstead) (Hemiptera: Pseudococcidae) is an emerging invasive pest of citrus. There is no information on the life history of N. viridis in relation to citrus cultivars produced in Florida. Here, we quantified the life-history parameters of N. viridis on 6 citrus and 3 non-citrus host plant species under laboratory conditions. We also investigated the effect of mode of reproduction (sexual vs. parthenogenic) on the fecundity of N. viridis. Nipaecoccus viridis was able to develop to maturity and reproduce on all host plant taxa tested. The individual host plants affected the developmental and survival rate, adult longevity, and offspring production of N. viridis. All citrus genotypes commercially grown in Florida were susceptible to N. viridis, with the exception of Citrus aurantium L., which suggests that there is need for management of this invasive pest in Florida citrus groves. Murraya paniculata L., Hibiscus rosa-sinensis L., and Punica granatum L. were also acceptable hosts for N. viridis. Sexually reproducing female N. viridis produced significantly more eggs than those reproducing asexually. Based on our results, propagation of M. paniculata and other citrus relatives should be discouraged around citrus groves in Florida. Given the fitness benefit of sexual reproduction in this pest, pheromone-based tools like mating disruption and mass trapping of adult males could be considered as part of an integrated pest management program with insecticides.

Greenbug feeding-induced resistance to sugarcane aphids in sorghum

Plants are attacked by multiple insect pest species and insect herbivory can alter plant defense mechanisms. The plant defense responses to a specific herbivore may also contribute to the herbivore growth/survival on plants. Feeding by one insect species can modulate the plant defenses, which can either facilitate or hamper the colonization of subsequent incoming insects. However, little is known about the effect of sequential herbivory on sorghum plants. In this study, we demonstrate that a specialist aphid, sugarcane aphid (SCA; Melanaphis sacchari) grows faster on sorghum than a generalist aphid species, greenbug (GB; Schizaphis graminum). We also determined how the pre-infestation of SCA on sorghum affected the invasion of GB and vice-versa. Our sequential herbivory experiments revealed that SCA reproduction was lower on GB-primed sorghum plants, however, the reverse was not true. To assess the differences in plant defenses induced by specialist vs. generalist aphids, we monitored the expression of salicylic acid (SA) and jasmonic acid (JA) marker genes, and flavonoid biosynthetic pathway genes after 48 h of aphid infestation. The results indicated that GB infestation induced higher expression of SA and JA-related genes, and flavonoid pathway genes (DFR, FNR, and FNSII) compared to SCA infestation. Overall, our results suggested that GB-infested plants activate the plant defenses via phytohormones and flavonoids at early time points and hampers the colonization of incoming SCA, as well as explain the reproductive success of SCA compared to GB.

Plant jasmonic acid mediated contrasting effects of two citrus aphid species on Diaphorina citri Kuwayama

BACKGROUND

Herbivores may influence each other directly and through plant mediated inter-specific interactions. The Asian citrus psyllid (Diaphorina citri Kuwayama) and citrus aphids are key pests that can co-exist on citrus, but their plant-mediated interaction between them is unknown. Here we investigated plant-mediated effect of two citrus aphid species, the polyphagous Aphis spiraecola Patch and the oligophagous Aphis (Toxoptera) citricidus (Kirkaldy) on the feeding behavior and reproduction of Diaphorina citri, and explored the underlying mechanisms.

RESULTS

In comparison with those on aphid free plants, Diaphorina citri had decreased reproduction and reduced phloem feeding on Aphis spiraecola pre-infested plants, while the reproduction and feeding efficiency were increased on Aphis citricidus pre-infested plants. Jasmonic acid (JA) dependent defense was significantly activated by Diaphorina citri feeding on Aphis spiraecola pre-infested plants, but was suppressed by Diaphorina citri feeding on Aphis citricidus pre-infested plants compared with that on aphid free plant. By contrast, only one tested marker gene in salicylic acid (SA) signaling was activated by Diaphorina citri feeding on Aphis spiraecola pre-infested plants. Furthermore, exogenous application of methyl jasmonate, but not SA, conferred resistance against Diaphorina citri in our citrus trials.

CONCLUSION

Our results indicate that pre-infestation by two citrus aphid species differentially altered Diaphorina citri induced citrus JA dependent defense, which resulted in different effect on subsequent Diaphorina citri performance. © 2022 Society of Chemical Industry.

Oviposition by a Specialist Herbivore Increases Susceptibility of Canola to Herbivory by a Generalist Herbivore

Oviposition by specialist herbivores can alter the suitability of the host plant to subsequent infestation by other herbivores. In this study, we tested the effect of previous oviposition on canola, Brassica napus L., by a Brassica specialist, the diamondback moth (DBM), Plutella xylostella (L.) (Lepidoptera: Plutellidae), on subsequent herbivory by the generalist feeder, the bertha armyworm (BAW), Mamestra configurata Walker (Lepidoptera: Noctuidae). The effect of DBM oviposition on subsequent BAW oviposition and larval feeding was tested in no-choice and choice experiments. Oviposition of BAW was not altered by DBM eggs on canola plants, however, BAW had increased larval feeding on plants with DBM eggs. These results suggest that oviposition by a specialist herbivore increased the susceptibility of the host plant to generalist herbivory. In a preliminary experiment, salicylic acid, jasmonic acid, and its conjugates were not altered by DBM oviposition on canola, however, further experimentation is needed to determine if oviposition affects expression of plant defense pathways and other plant traits.

Bio-Efficacy of Chrysoeriol7, a Natural Chemical and Repellent, against Brown Planthopper in Rice

Brown planthopper (BPH, Nilaparvata lugens Stal.) is the most damaging rice pest affecting stable rice yields worldwide. Currently, methods for controlling BPH include breeding a BPH-resistant cultivar and using synthetic pesticides. Nevertheless, the continuous cultivation of resistant cultivars allows for the emergence of various resistant races, and the use of synthetic pesticides can induce environmental pollution as well as the emergence of unpredictable new pest species. As plants cannot migrate to other locations on their own to combat various stresses, the production of secondary metabolites allows plants to protect themselves from stress and tolerate their reproduction. Pesticides using natural products are currently being developed to prevent environmental pollution and ecosystem disturbance caused by synthetic pesticides. In this study, after BPH infection in rice, chrysoeriol7 (C7), a secondary metabolite that induces resistance against BPH, was assessed. After C7 treatment and BPH infection, relative expression levels of the flavonoid-related genes were elevated, suggesting that in plants subjected to BPH, compounds related to flavonoids, among the secondary metabolites, play an important role in inducing resistance. The plant-derived natural compound chrysoeriol7 can potentially thus be used to develop environmentally friendly pesticides. The suggested control of BPH can be effectively used to alleviate concerns regarding environmental pollution and to construct a relatively safe rice breeding environment.

Improvement of Bemisia tabaci (Hemiptera: Aleyrodidae) Fitness on Chinese Kale upon Simultaneous Herbivory by Plutella xylostella (Lepidoptera: Plutellidae)

Simple Summary

Different herbivores feeding on the same plant can interact through plant-mediated effects. Cotton whitefly and diamondback moth are two of the most destructive pests in the world, and they often occur together in cruciferous plants. However, how the performance and fitness of them are affected when co-occurring in the same host plant remains unclear. The present study demonstrates that cotton whitefly has become a dominant competitor by gaining increased fitness benefits when it is mixed with DBM on the same host plant irrespective of sequences of their arrival, which may be one of the reasons for the rapid expansion and outbreak of the whitefly population worldwide.

Abstract

Bemisia tabaci and the diamondback moth (DBM), Plutella xylostella, are two major cosmopolitan pests that often occur together and cause severe economic losses to cruciferous crops. However, little is known about how they interact with each other. To determine the effects of defense responses induced by the two pests on the biology and population dynamics of the herbivores, we studied the performance and fitness of B. tabaci and DBM when they damaged Chinese kale simultaneously and in different orders. The results showed that DBM pre-infestation shortened the developmental duration, increased longevity, oviposition days, and fecundity of B. tabaci. Meanwhile, the intrinsic rate of increase (r), net reproductive rate (R₀) and finite rate of increase (λ) of B. tabaci increased significantly with dual infection as compared with only B. tabaci infestation. In contrast, B. tabaci pre-infestation reduced the longevity and oviposition days of DBM, but the population parameters r, R₀, and λ did not vary significantly compared with only DBM infestation. Thus, co-infestation of B. tabaci and DBM was beneficial to the performance of the B. tabaci population. The present findings highlight that B. tabaci has become a dominant competitor when mixing with DBM on the same host plant.

Responses of pea plants to multiple antagonists are mediated by order of attack and phytohormone crosstalk

Plants are often attacked by multiple antagonists and traits of the attacking organisms and their order of arrival onto hosts may affect plant defences. However, few studies have assessed how multiple antagonists, and varying attack order, affect plant defence or nutrition. To address this, we assessed defensive and nutritional responses of Pisum sativum plants after attack by a vector herbivore (Acrythosiphon pisum), a nonvector herbivore (Sitona lineatus), and a pathogen (Pea enation mosaic virus, PEMV). We show viruliferous A. pisum induced several antipathogen plant defence signals, but these defences were inhibited by S. lineatus feeding on peas infected with PEMV. In contrast, S. lineatus feeding induced antiherbivore defence signals, and these plant defences were enhanced by PEMV. Sitona lineatus also increased abundance of plant amino acids, but only when they attacked after viruliferous A. pisum. Our results suggest that diverse communities of biotic antagonists alter defence and nutritional traits of plants through complex pathways that depend on the identity of attackers and their order of arrival onto hosts. Moreover, we show interactions among a group of biotic stressors can vary along a spectrum from antagonism to enhancement/synergism based on the identity and order of attackers, and these interactions are mediated by a multitude of phytohormone pathways.

Changes in plant responses induced by an arthropod influence the colonization behavior of a subsequent herbivore

BACKGROUND

Plants in nature can be sequentially attacked by different arthropod herbivores. Feeding by one arthropod species may induce plant-defense responses that might affect the performance of a later-arriving herbivorous species. Understanding these interactions can help in developing pest-management strategies. In tomato, the sweet-potato whitefly Bemisia tabaci and the two-spotted spider mite Tetranychus urticae are key pests that frequently cohabit on the same plant. We studied whether colonization by one species can either facilitate or impede later colonization of tomato plants by conspecific or heterospecific individuals.

RESULTS

B. tabaci females showed a strong preference for and increased oviposition on plants previously colonized by conspecifics. In contrast, plants infested with T. urticae repelled B. tabaci females and reduced their oviposition rate by 86%. Although females of T. urticae showed no preference between conspecific-infested or uninfested plants, we observed a 50% reduction in the number of eggs laid on conspecific-infested plants. Both herbivorous arthropods up-regulated the expression of genes involving the jasmonic acid and abscisic acid pathways, increasing emissions of fatty-acid derivatives, but only B. tabaci increased the expression of genes related to the salicylic acid pathway and the total amount of phenylpropanoids released. Terpenoids were the most abundant compounds in the volatile blends; many terpenoids were emitted at different rates, which might have influenced the arthropods' host selection.

CONCLUSION

Our results indicate that B. tabaci infestation facilitated subsequent infestations by conspecifics and mites, while T. urticae infestation promoted herbivore-induced resistance. Based on both the molecular and behavioral findings, a novel sustainable pest-management strategy is discussed.

Response of Protective Enzymes in Western Flower Thrips (Thysanoptera: Thripidae) to Two Leguminous Plants

The western flower thrips, Frankliniella occidentalis, is a major invasive pest of commercially important crops worldwide. We compared the activities of superoxide dismutase (SOD), peroxidase (POD), and catalase (CAT) and the expressions of two putative SOD and two putative POD sequences in second instar larvae and adults after three generations of adaptation to kidney bean and broad bean plants. The results showed that the SOD, POD, and CAT activities in adults were significantly higher than those in the second instar larvae. The SOD activities were significantly higher in both the second instar larvae and the adults fed on kidney bean (Phaseolus vulgaris) plants versus broad bean (Vicia faba) plants, whereas the POD and CAT activities showed the opposite trend. The gene expression data showed that the FoPOD-2 expression levels were lower in the second instar larvae after three generations of feeding on broad bean plants versus kidney bean plants. The expression levels of FoSOD-1 and FoSOD-2, and FoPOD-1 under broad bean plant treatment were higher than those under kidney bean plant treatment. Additionally, gene expression fluctuated among the different generations. Our results indicated that western flower thrips demonstrated plasticity in gene expression and activity of protective enzymes, which is related to their adaptability to the host plants. Western flower thrips can change the expression of protective enzyme genes and enzyme activity in vivo to better adapt to kidney bean and broad bean plants.

Identification and evaluation of reliable reference genes for quantitative real-time PCR analysis in tea plants under differential biotic stresses

The selection of reliable reference genes (RGs) for normalization under given experimental conditions is necessary to develop an accurate qRT-PCR assay. To the best of our knowledge, only a small number of RGs have been rigorously identified and used in tea plants (Camellia sinensis (L.) O. Kuntze) under abiotic stresses, but no critical RG identification has been performed for tea plants under any biotic stresses till now. In the present study, we measured the mRNA transcriptional levels of ten candidate RGs under five experimental conditions; these genes have been identified as stable RGs in tea plants. By using the ΔCt method, geNorm, NormFinder and BestKeeper, CLATHRIN1 and UBC1, TUA1 and SAND1, or SAND1 and UBC1 were identified as the best combination for normalizing diurnal gene expression in leaves, stems and roots individually; CLATHRIN1 and GAPDH1 were identified as the best combination for jasmonic acid treatment; ACTIN1 and UBC1 were identified as the best combination for Toxoptera aurantii-infested leaves; UBC1 and GAPDH1 were identified as the best combination for Empoasca onukii-infested leaves; and SAND1 and TBP1 were identified as the best combination for Ectropis obliqua regurgitant-treated leaves. Furthermore, our results suggest that if the processing time of the treatment was long, the best RGs for normalization should be recommended according to the stability of the proposed RGs in different time intervals when intragroup differences were compared, which would strongly increase the accuracy and sensitivity of target gene expression in tea plants under biotic stresses. However, when the differences of intergroup were compared, the RGs for normalization should keep consistent across different time points. The results of this study provide a technical guidance for further study of the molecular mechanisms of tea plants under different biotic stresses.