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Arthropod-Plant Interactions

, Volume 13, Issue 2, pp 289–297 | Cite as

Tripartite interactions between jasmonic/salicylic acid pathways, western flower thrips, and thrips-transmitted tomato zonate spot virus infection in Capsicuum annuum

  • Xue Zheng
  • Yong Chen
  • Lihua Zhao
  • Yongdui Chen
  • Limin Zheng
  • Kuanyu Zheng
  • Ye Mu
  • Xingyue Zhao
  • Yulin GaoEmail author
  • Jie ZhangEmail author
Original Paper

Abstract

The disease caused by Tomato zonate spot virus (TZSV), transmitted by Frankliniella occidentalis in a circulative-propagative manner, results in significant loss in production and quality in ornamentals and vegetable crops. Related to this recently found tospovirus (TZSV), knowledge involving the interaction between virus, vector, and host plants remains unknown. In this study, we investigated the effects of TZSV infection on the changes in concentration of the plant hormones jasmonic acid (JA) and salicylic acid (SA), and the transcriptional regulation of JA- and SA-associated genes. Additionally, we verified that JA and SA inhibit the fitness of the vector F. occidentalis. This work brings to a new perspective on plant—virus–vector interactions is proposed, which could be applied to control the TZSV infection.

Keywords

Antiviral defense Host plant resistance Plant hormones Tospovirus Vector fitness 

Introduction

The plant viruses belonging to the genus Orthotospovirus (Bunyavirales: Tospoviridae) are considered to be some of the most devastating pathogen agents on ornamentals and vegetable crops worldwide (Pappu et al. 2009; Scholthof et al. 2011). To date, many species of Orthotospoviruses have been reported in China (Yin et al. 2014; Zheng et al. 2016). By the application of electron microscopy and serological methods, Tomato zonate spot virus (TZSV), a new representative of the genus Orthotospovirus, was first isolated in the Yunnan Province in 2008 (Dong et al. 2008). As other Orthotospovirus species, the TZSV has been shown to have a wide range of hosts across seven families, including agricultural crops and ornamental plant species which are confirmed by field investigations and laboratory inoculations (Cai et al. 2011; Dong et al. 2008; Liu et al. 2015). Leaves of most TZSV-infected host plants usually turn yellow, withered, and necrotic, which significantly affects the growth of host plants (Fig. 1). That can result in severe yield losses (Dong et al. 2008, 2010). Since 2010, the disease induced by TZSV has been widely spread in many areas in Southwest China and poses a serious threat to the production of tomato, tobacco, pepper, and other economic crops by local farmers (Dong et al. 2010; Zheng et al. 2015; Niu et al. 2018).

Fig. 1

Typical disease symptoms at the 6 leaf stage induced by Tomato zonate spot virus infection at 7th and 14th day after inoculation (DAI)

Plant-mediated interactions between pathogens and herbivores are known to exert important influences on the epidemiology of plant diseases (Stout et al. 2006; Luan et al. 2014). In recent decades, attentions have been paid on the tripartite interaction between virus, plant, and insect vector. Plant viruses can directly and indirectly (via the host plant) modulate the biology and behavior of their vectors (Hogenhout et al. 2008; Mauck et al. 2012; Wei and Li 2016). At the same time, feeding by viruliferous vectors may induce plant defense responses and affects viruliferous vectors as well. These changes from plants may benefit (Stumpf and Kennedy 2007; Ogada et al. 2013), neutral (Thompson 2002) to negative (Chen et al. 2016) to insect vector. To date, molecular biologists and ecologists have joined forces to reveal the tripartite interaction between virus, plant, and insect vector. However, the interaction between host plants, TZSV, and the thrips vector is poorly understood.

To maximize transmission efficiency, viruses utilize a variety of strategies to disrupt phytohormone accumulation of their hosts, thus promoting pathogen spread (Robert-Seilaniantz et al. 2011; Collum and Culver 2016). Plant hormones play important roles in regulating all aspects of plant physiology including growth, development and reproduction (Bari and Jones 2009; Fahad et al. 2015; Stahl et al. 2018). Salicylic acid (SA), jasmonic acid (JA) and ethylene (Et) are primarily components of signal transduction cascades regulating plant defense responses against biotic stresses (Collum and Culver 2016; Stahl et al. 2018). Auxin (Aux), gibberellins (GA) cytokinins (CK), brassi-nosteroids (BR) and abscisic acid (ABA) also contribute to defense but play key roles in plant development and physiological processes (Collum and Culver 2016; Stahl et al. 2018). JA- and SA-mediated signal pathways play important roles in defending against infection by a broad spectrum of viruses in plants (Delaney et al. 1994; Shang et al. 2011; Erb et al. 2012; Zhu et al. 2014; Martini et al. 2014). In general, JA plays an important role in plants against phloem-feeding insects (Erb et al. 2012). However, the defense provided by the JA signaling pathway can be inhibited by insect-vectored viruses (Erb et al. 2012; Collum and Culver 2016). For instance, tomato yellow leaf curl virus (TYLCV), tomato yellow leaf curl China virus (TYLCCNV), rice ragged stunt virus (RRSV), cucumber mosaic virus (CMV) and TSWV infection decreased the level of JA or regulated gene expression (Abe et al. 2012; Zhang et al. 2012, 2018a; Shi et al. 2013, 2014; Sun et al. 2017). On the other hand, SA mediates the plant resistance to biotrophic pathogens. For example, TSWV and TYLCV infection elevated SA contents and induced SA-regulated gene expression in the plants (Abe et al. 2012; Shi et al. 2013). Makandar et al. (2012) demonstrated that SA accumulation increased in fungus-infected spikes of wheat, which was correlated with elevated expression of the SA-inducible pathogenesis-related 1 (PR1) gene, as well as Fusarium head blight resistance. Malamy et al. (1990) demonstrated that SA is involved in the resistance to TMV infection in tobacco plants. Recently, Zhang et al. (2018a) found that herbivore feeding can induce the SA signal to augment rice plants defense to rice blast disease. Conversely, virus infection has been shown to play a role in partially suppressing plant defenses induced by the viruliferous vectors (Li et al. 2018). These findings suggested that dynamic balance of JA and SA is involved in the interaction between host plant, virus, and vectors, influencing host resistance to the pathogen and/or vector.

To elucidate the regulation of the JA and SA pathway upon TZSV infection and the corresponding response of host plants and insect vectors, we measured the JA and SA content in TZSV-inoculated pepper plants by the LC–MS/MS method. We also analyzed the effect of altered JA and SA content on the behaviors and life history of the thrips vectors. Our findings involving the JA and SA signal pathway-mediated regulation of TZSV-pepper plants–thrips interactions provide fundamental insights into the TZSV pathogenesis and antiviral responses, which can direct the control and prevention of TZSV-mediated viral diseases.

Materials and methods

Virus isolation, plant material and insect vectors

The TZSV YN-Chili isolate (txid460926), originating from chili pepper (Capsicum annuum) in Yunnan, was used in this study (Dong et al. 2008). The isolate was maintained and cultured in Nicotiana benthamiana ‘Domin’ (Solanaceae). N. benthamiana seeds were kindly provided by the Yunnan Academy of Tobacco Agricultural Science. Hot pepper plants (Capsicum annum L. ‘Zunla-1’, Solanaceae) at six-leaf stage, grown in pots containing a mixture of vermiculite and organic fertilizer, were used for virus inoculation.

A non-viruliferous population of Frankliniella occidentalis Pergande (Thysanoptera, Thripidae) collected from tobacco plants (Nicotiana tabacum cv. NC89) in the Honghe Autonomous Prefecture (23°46′N, 113°38′E), Yunnan Province, China, were reared on green bean pods (Phaseolus vulgaris L., ‘Double blue 12’; Fabales: Fabaceae) in disposable plastic box (the length, width and height of the boxes were 15 × 21 × 7 cm, respectively) in the same substrate mentioned above a greenhouse at a temperature of 25 ± 1 °C, relative humidity of 70%, and a photoperiod of 14:10 h (light: dark) (Zheng et al. 2014).

Virus inoculation

We used a frictional inoculation method to artificially infect pepper plants at the 6 leaf stage (Dong et al. 2008). Leaves of TZSV-infected N. benthamiana plants were homogenated in PBS buffer. The homogenate was then applied uniformly to 2 adaxial leaves (1st and 2nd euphylla) per pepper plant. Ten minutes after virus inoculation, the inoculated pepper leaves were washed with deionized water. Pepper plants inoculated with PBS buffer were used as controls. Virus-inoculated pepper plants were cultured in a greenhouse at 25 ± 1 °C, relative humidity of 70%, and a photoperiod of 14:10 h (light/dark). The pepper sap concentration was 100 mg ml−1, which equaled the diseased fresh leaf weight (mg) divided by the volume of inoculation homogenate. Eighty pepper plants were used for virus inoculation, and 80 plants were used as controls.

Phytohormone measurement and treatment

Eight treatments in each containing three plants that had been either inoculated with the virus (treatment 1–4) or with PBS (control, treatment 5–8) were used to measure the phytohormone. At the 7th (treatment 1 and 5) and 14th (treatment 2 and 6) day post-inoculation, five pepper leaves (3rd to 7th euphylla) in each plant were collected for phytohormone measurements. At the 14th day post-inoculation, 20 thrips larvae were grouped and allowed to feed on each whole plant in a cage (an acryl cylinder chamber with air ventilation windows covered with a fine mesh). After infestation times of 2 days (treatment 3 and 7) and 3 days (treatment 4 and 8), the cages and thrips within were removed, 5 pepper leaves (3rd to 7th euphylla) in each plant were collected to measure the change of JA and SA content compared with those in pepper leaves without thrips feeding. The experiment was repeated three times.

Phytohormone analysis

JA and SA were extracted and quantified by LC–MS/MS as described previously (Segarra et al. 2006; Wu et al. 2007). The leaf samples of 150 mg were frozen in liquid nitrogen and ground to a fine powder. Then, 1 ml ethyl acetate, spiked with labeled internal standards (13C2-JA, 13C6-JA-Ile, D4-SA, and D6-ABA, each at 100 ng), was added to each sample. After centrifugation at 13,000×g for 10 min at 4 °C, supernatants were transferred to clean tubes (Eppendorf, Hauppauge, NY, USA) and the liquids were evaporated to dryness in a vacuum concentrator (Eppendorf AG, Hamburg, Germany). Each residue was resuspended in 0.5 Ml 70% (v/v) methanol and centrifuged (15 min, 13,000×g, 4 °C) to remove particles. The supernatants were analyzed on an HPLC-tandem mass spectrometry device (1200L LC/MS system, Varian, Inc., Walnut Creek, CA, USA). Three replicated leaf samples were tested for virus-infected and control plants.

RNA extraction and quantitative real-time PCR (qRT-PCR)

Pepper leaves at the 6 leaf stage were inoculated with TZSV mechanically as described above. Eight treatments in each containing three plants that had been either inoculated with the virus (treatment 1–4) or with PBS (control, treatment 5–8) were used to measure the expression of SA and JA biosynthesis and defense marker genes in pepper. At 7th (treatment 1 and 5) and 14th (treatment 2 and 6) day after inoculation, 3 leaves (3rd to 5th euphylla) in each plant were cut off and frozen in liquid nitrogen. Leaves were also collected and frozen in liquid nitrogen from plants that had been infested with thrips for 2 (treatment 3 and 7) and 3 (treatment 4 and 8) days, 14 days after inoculation with the virus or PBS. Total RNA was extracted from the samples using InvitrogenTM TRIzolTM Reagent (Life TechnologiesTM, Carlsbad, CA, USA), while 1 µg of RNA was reverse transcribed in a 10-µl reaction system using the AMV RNA PCR Kit (TaKaRa, Bio Inc., Japan). According to the manufacturer’s protocol, each qPCR reaction included 2 µl cDNA template (tenfold dilution), 0.4 µl of each primer (10 mM), 0.4 µl Rox Reference Dye, and 10 µl SYBR Green PCR Master Mix kit (Promega Corp, USA) in a total volume of 20 µl. The PCR procedure consisted of one cycle at 95 °C for 30 s; 40 cycles of 95 °C for 5 s and 60 °C for 30 s. All primers used in this study (Supplemental Table 1) were designed using Primer 5.0 based on the sequences deposited in the National Center for Biotechnology Information database and Sol Genomics Network for Capsicum annuum Genome Data. Three replicate assays were performed with independently isolated RNA samples. The expression of the β-tubulin gene was used as an internal standard to verify the absence of any significant variation in overall cDNA levels. The experiment was repeated three times.

Development of F. occidentalis fed on pepper plants treated with MeSA and MeJA

Phytohormone treatment

At the 7th day post-inoculation, MeJA (100 µM) and MeSA (10 µM) were applied to pepper plants by continuous spraying the plants for 3 days with an ethanol solution (10 ml per plant) containing the phytohormones, and 99.99% ethanol was applied as control. At the 10th day post-inoculation, we used the method of Zheng et al. (2014) to assess the effects of MeSA and MeJA on the development F. occidentalis. Briefly, 20 newly emerged adult females were collected, divided into groups of 5 and each group was placed in a Petri dish (12 cm diameter) containing 25 mm-diameter leaf disks, with or without MeSA or MeJA, over moistened filter papers. The adults were removed after they had oviposited on the leaf disks during 12 h. After the eggs hatched, 40 newly larvae were transferred individually into clean, ventilated plastic Petri dishes (7 cm diameter) containing MeSA-, MeJA- or ethanol-treated pepper leaf discs placed on moist filter paper. Each Petri dish was sealed with parafilm to prevent F. occidentalis from escaping. The leaf disks were changed daily by freshly cut ones onto which thrips larvae were transferred. Growth of F. occidentalis in each Petri dish was monitored daily until the emergence of adults. The duration of each developmental stage of F. occidentalis was recorded for all differently treated plants. All dead individuals at any developmental stage were excluded when calculating the average developmental time for a specific stage. Each of the experiments was repeated five times. The Petri dishes, lined with moist filter paper, were placed in climate chambers and maintained at 25 ± 1 °C, relative humidity of 70% and a photoperiod of 16: 8 h (light: dark).

Life-table parameters of F. occidentalis fed on pepper plants treated with MeSA and MeJA

One hundred individuals of first larval instar on day after hatching were transferred individually into clean, ventilated plastic petri dishes (7 cm diameter) lined with moistened filter paper, containing a leaf disc cut from the adaxial leaves from a pepper plant treated with MeSA, MeJA or ethanol. The number of first-instar larvae that developed into the 2nd instar larvae (Su1) or the prepupal-to-pupal stage (Su2) was recorded. The number of male and female adults that emerged was counted daily to determine the emergence rate (Er) and sex ratio (Sr). The leaf discs were inspected thoroughly under a stereomicroscope and the number of un-hatched eggs was recorded. The female fecundity (Fy) was evaluated based on the average number of eggs produced per female until the end of adult life. Rate of hatched eggs (Hr) was calculated as the ratio of the total number of first-instar larvae plus the number of non-hatched eggs divided by the total number of first-instar larvae recorded.

The MeSA-, MeJA- or ethanol-treated leaf disc were replaced every 2 days in each Petri dish, and removed leaf discs were kept in separate petri dishes in order to continue the observation of egg hatching using a stereomicroscope. The experiment was repeated three times. The population growth index (I) was calculated as:
$${N_{\text{t}}}={N_{\text{O}}} \times {{\text{S}}_{\text{u}}}1 \times {{\text{S}}_{\text{u}}}2 \times {E_{\text{r}}} \times {S_{\text{r}}} \times {F_{\text{y}}} \times {H_{\text{r}}}$$
$$I=\frac{{{N_{\text{t}}}}}{{{N_{\text{O}}}}}$$
where NO was the number of first-instar larvae initially added to each Petri dish and Nt was the number of first-instar larvae recorded in the next generation.

Data analysis

All data were analyzed using the SPSS (origin), version 11.5. One-way analysis of variance (ANOVA) or Student’s t test was used to determine the significance of difference between the control and treatment per each variant. Percentage data were arcsine square-root transformed before the analysis.

Results

Effect of TZSV infection on the SA pathway but not JA pathway in pepper plants

The TZSV-induced disease symptoms usually appeared 7 days after inoculation with TZSV (Fig. 1d). As shown in Table 1, the SA content was significantly down-regulated in TZSV-infected pepper plants compared with those in healthy pepper plants. At 7th day after inoculation (DAI), the amount of SA in healthy and TZSV-inoculated pepper plants differed by nearly 20 ng g−1, while at 14th DAI the difference was less marked (7 ng g−1). TZSV infection had neither significantly effect on JA accumulation at 7th DAI nor at 14th DAI, as there was no significant difference between healthy and TZSV-infected pepper plants at 7th or 14th DAI. We also examined the expression of SA and JA biosynthesis and defense marker genes in pepper using qRT-PCR, which are used to indicate regulation by these two phytohormone signaling pathways. The transcript levels of the SA defense pathway genes: non-expressor of pathogenesis-related genes 1 (NPR1), PR1, isochorismate synthase 1 (ICS1) and phenylalanine ammonia-lyase (PAL) were down-regulated in the TZSV-inoculated pepper plants when compared with those in the healthy plants both at 7th DAI and 14th DAI (Fig. 2), indicating that TZSV suppresses the SA responses in pepper plants. In contrast, the transcripts for JA-associated genes, including jasmonoyl-l-isoleucine (JA-Ile), OPR3 (12-oxo-phytodienoic acid reductase 3), LOX1 (lipoxygenase 1), LOX2, LOX3, LOX4, LOX5, LOX6 and LOX7 remained unchanged in the TZSV-inoculated pepper plants when compared with those in the healthy pepper plants both at 7th DAI and 14th DAI (Fig. 2). These results indicated that SA-mediated but not JA-mediated signaling pathway was probably involved in the response to TZSV infection in pepper plants.

Table 1

Concentration of JA and SA in leaves of Capsicum annuum ‘Zunla-1’ after inoculated Tomato zonate spot virus

Phytohormone treatment

Samples date after treatment (d)

Virus treatment

PBS treatment (control)

SA concentration (ng g−1 FW)

7

16.01 ± 4.11aA

35.76 ± 1.92bA

14

18.25 ± 2.20aA

25.44 ± 3.25bB

JA concentration (ng g−1 FW)

7

10.99 ± 6.38aA

11.03 ± 1.91aA

14

8.64 ± 1.35aA

12.05 ± 2.24aA

Mean ± SE is the mean of data obtained from three replicates with standard error. Different lowercase letters of the same row and different uppercase letters of the same column indicate significant differences between treatment and control based on Student’s t test (at the level P = 0.05)

Fig. 2

Expression of different genes related to the JA- and SA-regulated marker genes after inoculated Tomato zonate spot virus onto the 6 leaf stage Capsicum annuum ‘Zunla-1’ plants. The expression level of JA- and SA-regulated marker genes was normalized relative to β-tubulin transcript. a 7 days after inoculation. b 14 days after inoculation. Each histogram bar represents the mean (± SE), including three replicates; means were compared between control and treatment using Student’s t test (at the level P = 0.05). Statistical differences are indicated by asterisks

Induction of signaling in SA but not in JA in pepper plants after thrips feeding

As shown in Table 2, thrips feeding resulted in a significant higher level of SA in the leaves of TZSV-inoculated and healthy pepper plants when compared with those in the control plants (Table 2). In addition, SA levels were higher after 3 days of thrips feeding compared to after 2 days, although a slightly lower level of SA in the leaves of TZSV-inoculated and healthy pepper plants after thrips feeding (Table 2), indicating that thrips feeding can induce the SA pathway on both kinds of TZSV-inoculated and healthy pepper plants. However, the concentration of JA was slightly lower in virus-infected plants than uninfected plants, but this difference was not substantial (Table 2). These results showed that thrips feeding induced SA accumulation in the leaves of pepper plants, so that the down-regulation of SA level by TZSV infection was counteracted 2 days after thrips feeding. Thrips feeding had no effect on JA accumulation in the leaves of pepper plants. We also checked the effect of thrips feeding on the expression of SA- and JA-associated genes. As shown in Fig. 3, the expression of NPR1, PR1, ICS1 and PAL was significantly up-regulated by thrips feeding for 2 days and 3 days, which is in line with the increase in SA levels in the leaves of pepper plants after 2 days or 3 days of thrips feeding. However, the expression of genes responsible for JA biogenesis such as JA-Ile, OPR3, LOX1, LOX2, LOX3, LOX4, LOX5, LOX6 and LOX7 was not affected by thrips feeding for 2 days and 3 days. Together, these results indicated that induction of signaling in SA but not in JA in pepper plants after thrips feeding.

Table 2

Concentration of JA and SA in leaves of Capsicum annuum ‘Zunla-1’ after Thrips feeding

Phytohormone treatment

Sampling date after treatment (d)

Virus + thrips treatment

PBS + thrips treatment

PBS treatment (control)

SA concentration (ng g−1 FW)

2

34.16 ± 4.15aA

36.85 ± 2.40aA

26.62 ± 2.37bA

3

45.28 ± 4.65aB

48.20 ± 3.44aB

26.89 ± 3.14 bA

JA concentration (ng g−1 FW)

2

8.95 ± 1.44aA

13.12 ± 1.65aA

13.45 ± 2.56aA

3

9.12 ± 2.05aA

13.65 ± 2.18aA

12.96 ± 3.19aA

Mean ± SE is the mean of data obtained from three replicates with standard error. Different lowercase letters of the same row and different uppercase letters of the same column indicate significant differences between treatment and control based on Tukey’s honestly significant difference test (at the level P = 0.05)

Fig. 3

Expression of different JA- and SA-regulated marker genes after thrips feeding on the 6 leaf stage Capsicum annuum ‘Zunla-1’ plants. The expression level of JA- and SA-regulated marker genes was normalized relative to β-tubulin transcript. a 2 days after feeding. b 3 days after feeding. Each histogram bar represents the mean (± SE), including three replicates; means were compared between control and treatment using Tukey’s honestly significant difference test (at the level P = 0.05). Statistical differences are indicated by different letters above bars

JA and SA treatments alter the TZSV pathogenesis and thrips life table

To further confirm the role of SA- and JA-regulated defenses in thrips resistance, pepper plants were treated with MeJA or MeSA, and the performance of thrips was examined. We found that when pepper leaves were treated with either MeJA or MeSA, the duration of the developmental stages of thrips was significantly increased in the populations kept in petri dishes (Table 3). The developmental duration for thrips fed on MeJA-treated pepper plants took longer in eggs (1.62 times), 1st instar (1.41 times), 2nd instar (1.26 times), prepupa (1.46 times) and pupa (1.50 times) compared to that of thrips fed on water-treatment pepper plants. Likewise, thrips fed on pepper plants treated with MeSA also developed slower in eggs (1.39), 1st instar (1.21), 2nd instar (1.19), prepupa (1.39) and pupa (1.40) times than those of controls. Overall, development of thrips from egg to adult took 3.5 days longer on MeSA-treated leaves, and 5 days on MeJA-treated leaves compared to untreated leaves.

Table 3

Effect of application of exogenous phytohormone on the developmental duration of Frankliniella occidentalis on leaves of Capsicum annuum ‘Zunla-1’ (number of days)

Stadium

Control

MeSA

MeJA

Egg

2.65 ± 0.22a

3.68 ± 0.15b

4.28 ± 0.08c

1st instar

2.02 ± 0.10a

2.44 ± 0.14b

2.85 ± 0.14c

2nd instar

4.06 ± 0.15a

4.85 ± 0.16b

5.12 ± 0.22b

Prepupa

1.15 ± 0.06a

1.60 ± 0.12b

1.68 ± 0.10b

Pupa

2.08 ± 0.14a

2.94 ± 0.18b

3.12 ± 0.22b

Mean ± SE is the mean of data obtained for three replicates with standard error. Different lowercase letters of the same row indicate significant differences between treatment and control based on Tukey’s honestly significant difference test (at the level P = 0.05)

The survival rate of thrips from 1st instar to 2nd instar and from prepupa to pupa was lower for thrips fed on MeSA- and MeJA-treated leaf discs (Table 4). Additionally, the adult emergence, female oviposition, egg hatchability, the population quantity in next generation and population index of thrips population were lower for thrips fed on MeSA- and MeJA-treated leaf discs compared to untreated ones. The sex ratio was not influenced by the application of either MeSA or MeJA (Table 4). These findings suggested that JA and SA had a negative impact on the thrips vectors.

Table 4

Effect of application of exogenous phytohormone on the life table parameters of Frankliniella occidentalis on leaves of Capsicum annuum ‘Zunla-1’ (number of days)

Parameters

Control

MeSA

MeJA

N 0

100

100

100

Su1

0.92 ± 0.04a

0.85 ± 0.04a

0.82 ± 0.05a

Su2

0.94 ± 0.06a

0.86 ± 0.24b

0.80 ± 0.12b

Er (%)

95.42 ± 0.12a

94.86 ± 0.14b

94.15 ± 0.10b

S r

0.54 ± 0.16a

0.53 ± 0.16a

0.52 ± 0.20a

F y

21.86 ± 2.45a

18.25 ± 1.22ab

15.12 ± 1.74b

Hr (%)

84.85 ± 2.55a

78.05 ± 1.85b

72.60 ± 2.62c

N t

826.52a

523.49b

352.64c

I

8.27a

5.23b

3.53c

N0, number of individuals of the initial population; Su1, survival rate of thrips from first instar to second instar; Su2, survival rate of thrips from prepupa to pupa; Er, number of emerged adults; Sr, sex ratio; Fy, female fecundity (number of eggs per female lifetime); Hr, rate of hatched eggs; Nt, number of first-instar larvae in the next generation; I, population index; Mean ± SE is the mean of data obtained for three replicates with standard error. Different lowercase letters of the same row indicate significant differences between based on Tukey’s honestly significant difference test (at the level P = 0.05)

Discussion

Vector-borne pathogens often manipulate the traits of their hosts in ways for their own benefits (Belliure et al. 2005; Luan et al. 2014). Rice ragged stunt virus (RRSV) infection decreased the JA content in rice plants and suppressed the JA-mediated antiviral defense, promoting virus infection (Zhang et al. 2018a). Our present results demonstrated that TZSV infection can repress the SA accumulation as well as genes functioning in SA pathway to some extent in pepper plants against thrips (Fig. 2; Table 1). Additional feeding by F. occidentalis, the thrips vector of TZSV can induce of signaling in SA in both TZSV infection and healthy pepper plants (Fig. 3; Table 2), whereas TZSV infection could slightly inhibit the level of SA and JA in the leaves of pepper plants when compared with those in the healthy plants after thrips feeding (Fig. 3; Table 2). Thus, the SA pathway was inhibited by TZSV infection. We further showed that the exogenous application of MeJA and MeSA onto pepper plants significantly decreased important life history parameters of thrips (Tables 3, 4). These findings suggest that TZSV is responsible for the suppression of the plant defences and modulates the mutualistic effects during infection.

In the tritrophic interaction, herbivores may benefit from being viruliferous because of the response of the host plant defense system to pathogens infection (Belliure et al. 2005; Mauck et al. 2012; Martini et al. 2014). Many studies have shown that the SA and JA pathways are universal in plant defense systems to pathogens infections and insect herbivores (Huang et al. 2018). In general, SA pathway regulates a set of basal immune system in the plant’s response to attack by pathogens and insect herbivores (Pieterse et al. 2006), whereas JA pathway regulates the generation of plant secondary metabolites and defensive proteins that are harmful to herbivores (Pieterse and Dicke 2007). Previous studies have demonstrated that TSWV infection increased SA contents and induced SA-regulated gene expression in Arabidopsis plants (Abe et al. 2012). TYLCV and Bemisia tabaci (Hemiptera: Aleyrodidae) infection simultaneously increased the endogenous SA levels and induced the SA-regulated defense system in tomato plants (Shi et al. 2013). In addition, SA-mediated plant defense responses can have negative effects on the insect herbivores (Rodriguez-Saona et al. 2010; Zhang et al. 2012; Jaouannet et al. 2014). In the present study, TZSV infection indirectly enhances the performance of its vector, thrips, by means of the antagonistic SA-regulated host plant defense systems. Furthermore, thrips feeding can induce of signaling in SA in the leaves of pepper plants, which could counteract the down-regulation of SA level by TZSV infection. These results, together with the increased level of SA in a plant increased its attraction to thrips (Abe et al. 2012), we speculate that TZSV-induced modulation of host innate defense responses to herbivory have been implicated in enhanced performance of F. occidentalis and their preference for such plants and thus assist in the spread of TZSV.

The controlled application of SA or JA to plants could offer a viable approach to inducing host plant resistance to against pathogens and insect herbivores. For instance, applying 0.06 mM JA and then 0.1 mM SA 24 h later, enhanced resistance to Cucumber mosaic virus (CMV), Tobacco mosaic virus (TMV) and Turnip crinkle virus (TCV) in arabidopsis, tobacco, tomato and hot pepper, respectively, as indicated by reduction of virus accumulation (Shang et al. 2011). In addition, altered JA or SA accumulation mediated by such stimulus as fungi, bacteria and viruliferous vectors has also been shown to regulate host resistance to virus infection. Beris et al. (2018) demonstrated that induction of the SA signaling pathway in tomato occurred after treatment with MBI600, a strain of Bacillus amyloliquefaciens Priest U. A. (Bacillales, Bacillaceae), resulting in an enhanced plant defense response to TSWV and Potato virusY (PVY) infection. Alternatively, induced plant defenses against herbivores have been shown to be modulated by JA and SA signaling pathways. For instance, exogenous JA, but not SA, significantly decreased mealybug feeding time and reduced nymphal performance on rice (Zhang et al. 2018b). Meanwhile, the same study also found that mealybugs benefit from suppression of JA-regulated defenses by exhibiting enhanced nymphal performance. Sun et al. (2017) showed that the performance of whiteflies was elevated on tomato plants deficient in JA-related resistance, but reduced on plants with a high level of JA-related resistance, when compared to the performance of whiteflies on plants of the wild type, indicating that JA-related resistance plays an important role in the tripartite interactions between whitefly, begomovirus and tomato plants. In our research, we also found that applying 100 µM JA or 10 µM SA have a negative impact on the thrips vectors (Tables 3, 4). Thus, it provides a comprehensive insight to control insect vectors by using exogenous SA for blocking the horizontal transmission of viruses.

In conclusion, our findings reveal that a tospoviruses, TZSV, can suppress the plant defenses result in increased thrips vector multiplication. The indirect mutualism between a virus and its vectors may assist in the spread of TZSV. However, the biological function of elevated JA on TZSV infection is still unknown. In the future, efforts will be warranted to reveal the details of the relationships among TZSV, thrips and host plants.

Notes

Acknowledgements

The study was supported by the National Natural Science Foundation of China (31360430, 31460461, 31560499, 31660508, 31871936 and 31871935), the Science and Technology Program of Yunnan Province (2016FB063, 2018FA020), the Fund for Reserve Talents of Young and Middle-aged Academic and Technical Leaders of Yunnan Province (2015HB081), the Basic R & D Special Fund Business of Fujian Province (2017R1025-9), the Fund for Hundred Talent Program of Young Science Elite of the Fujian Academy of Agricultural Sciences (YC2016-5), the Program for Innovative Research Team of Fujian Academy of Agricultural Sciences (STIT2017-3-2) and Fujian Science and Technology Agency of China (2017NZ003-1-4).

Supplementary material

11829_2019_9683_MOESM1_ESM.docx (14 kb)
Supplementary material 1 (DOCX 14 KB)

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Copyright information

© Springer Nature B.V. 2019

Authors and Affiliations

  1. 1.Yunnan Province Key Laboratory of Agricultural Biotechnology, Biotechnology and Germplasm Resource InstituteYunnan Academy of Agricultural SciencesKunmingChina
  2. 2.Fujian Key Laboratory for Monitoring and Integrated Management of Crop Pests, Institute of Plant ProtectionFujian Academy of Agricultural SciencesFuzhouChina
  3. 3.College of Life SciencesSouthwest Forestry UniversityKunmingChina
  4. 4.State Key Laboratory for Biology of Plant Diseases and Insect Pests, Institute of Plant ProtectionChinese Academy of Agricultural SciencesBeijingChina
  5. 5.Institute of Plant ProtectionHunan Academy of Agricultural SciencesChangshaChina

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