Junxin Yan · Yi Tan · Yaru Lv · Fei Wang ·Yongqiang Zhang · Defu Chi

Abstract To study the Effect of jasmonates (JAs) on the resistance of economic forest plants to insects, Rosa rugosa‘Plena’ leaves were treated with 1 mmol/L jasmonic acid(JA), methyl jasmonate (MeJA) and Z-jasmone, then the content of tannin and total phenol in leaves and the feeding area of Monolepta hieroglyphica adults on leaves were measured. Changes in the activities of detoxif ication enzymes in adult M. hieroglyphica that had fed on leaves treated with JAs were also studied. Tannin and total phenol levels in leaves increased signif icantly after treatment with JAs, and tannin level was 1.36-1.55-fold higher than in the control after treatment with 1 mmol/L MeJA. The total content of phenol in leaves treated with 1.0 mmol/L Z-jasmone increased by 1.33-2.20-fold compared with those of the control. The activities of detoxif ication enzymes in adults were inhibited to diff ering degrees: activity of alkaline phosphatase (AKP) f irst increased, then decreased; the activities of acid phosphatase (ACP), glutathione S-transferases(GSTs) and carboxylesterase (CarE) following treatment with 1 mmol/L MeJA were signif icantly reduced and were 22%-31%, 11%-26%, and 11%-31% lower than those of the control, respectively. Moreover, the feeding area of adult M. hieroglyphica on the leaves treated with JAs was signif icantly reduced ( P < 0.05). The feeding area of economic forest R. rugosa ‘Plena’ leaves treated with 1 mmol/L MeJA decreased by 17%-43% compared with that of the control.Moreover, the decrease in the adult M. hieroglyphica feeding area was highly positively correlated with the content of tannin and positively correlated with the contents of total phenol of economic forest R. rugosa ‘Plena’ leaves. The reduced feeding area of adult M. hieroglyphica was highly negatively correlated with the activities of AKP and ACP and negatively correlated with those of the GSTs. In conclusion, the use of 1 mmol/L MeJA can noticeably decrease the deleterious Effects of adult M. hieroglyphica .

Keywords Jasmonates · Economic forest Rosa rugosa‘Plena’ · Monolepta hieroglyphica · Detoxif ication enzyme

Introduction

A common pest of economic forests in recent years,Monolepta hieroglyphica(Coleoptera; Chrysomelidae), chews notches and holes in plant leaves as it feeds on them, hindering photosynthesis and seriously aff ecting plant growth(Tian et al. 2014). Its hosts include a variety of trees, such as members of the Leguminosae, Aceraceae, Caprifoliaceae,Ulmaceae, Juglandaceae and Rosaceae families, but it prefers those of Rosaceae and causes substantial economic losses (Chen et al. 2007; Guo et al. 2018).Rosa rugosa‘Plena’, a deciduous rosaceous shrub, is one of the most important economic forest tree species in the world (Li et al.2018; Yan et al. 2018) because it contains a large amount of commercially valuable essential oils, which serve as the main raw material for various perfumes, soaps and edible fragrances. In addition, its petals and fruits are used in the food and pharmaceutical industries, and the seeds contain approximately 14% oil, which can be extracted for industrial use (Maciag and Kalemba 2015; Sheng et al. 2018; Chang et al. 2019; Rusanov et al. 2019). However, these plants are vulnerable to diseases and insect pests; adults ofM. hieroglyphicaare one of the main pests (Jafari et al. 2017; Yan et al. 2017). Heavy use of pesticides to control the insect can easily devalueR. rugosa‘Plena’, threaten the environment and human health and greatly increase the rate of insecticide resistance in insects, (Gracen and Guthrie 1986; Firake et al. 2017; Bass and Jones 2018; Yan et al. 2018; Macel et al. 2019). Therefore, an environmentally friendly method is needed against adultM. hieroglyphica.

Induced resistance in plants is a defense response induced by external injury or an exogenous substance (Chang et al.2016; Vishwanathan et al. 2020). Jasmonates (JAs), a general term for jasmonic acid (JA) and its derivatives, are natural plant hormones that share a common cyclopentanone structure. They play a role in signal transmission and hormone regulation in plants by inducing resistance to adverse conditions, growth and development (Wang 2009;Sripontan and Hwang 2016). JAs, such as JA and methyl jasmonate (MeJA), can help plants produce an induced defense response similar to those produced in response to insect predation and to accumulate anti-insect substances, which can disturb the behavior or physiology of pests, thus aff ecting their growth and reducing their deleterious Effects (Pickett et al. 2007; Delaney et al. 2013; Falk et al. 2014; Zas et al.2014). Dam et al. ( 2000) found that the use of exogenous JA on tobacco (Nicotiana attenuata) slowed the development and increased mortality ofManduca sextaowing to changes in the rate of its feeding.

Tannins are important insect-resistance compounds in plants; a high concentration of tannin even leads to the death ofHelicoverpa armigera(Sharma et al. 2009). Phenolic substances are a large family of plant secondary metabolites,which are produced in plants after an exogenous injury. The accumulation of phenolic substances can inhibit insects, bacteria and fungi (Matsuki et al. 2004). After the addition of high concentrations of phenolic compounds to cotton (Gossypium hirsutum) leaves, the larval mass ofH. armigeraandSpodoptera lituradecreased signif icantly, and the rate of mortality had increased signif icantly on the 10 th day (Dixit et al. 2017). Moreover, studies have shown that the detoxif ication enzymes inH. armigeradecreased following feeding with forage that contained high quantities of tannin (Chen et al. 2003).

Insects, however, have detoxif ication enzymes such as phosphatase, glutathioneS-transferases (GSTs) and carboxylesterase (CarE) that are involved in the oxidation, reduction,hydrolysis, conjugation and other metabolic reactions of various exogenous toxins (Yang et al. 2001; Prapanthadara et al. 2000; Feng et al. 2001). These detoxif ication enzymes in insects have attracted increasing amounts of attention(Despres et al. 2007; Boulogne et al. 2012).

Here we measured changes in the activities of alkaline phosphatase (AKP), acid phosphatase (ACP), GSTs and CarE in adultM. hieroglyphicaafter the insects had fed onR. rugosa‘Plena’ leaves treated with JAs. We also studied the correlation between tannins, total phenols, leaf area fed on by adultM. hieroglyphica, and the activities of detoxif ication enzymes in adultM. hieroglyphica. This goal of this study was to shed light on the potential use of JAs to control adultM. hieroglyphicain economic forests.

Material and methods

Field experiments were started in May in the rose plantation at Yulin Village (45.89°N, 126.59°E), Harbin City,Heilongjiang Province, China using 1-year-old cuttings ofR. rugosa‘Plena’ that had been propagated for 3 months.Healthy plants with similar amounts of growth were sprayed separately with 20 mL of 1 mmol/L JA, MeJA,Z-jasmone or distilled water (control) per plant, with 40 seedlings per treatment. Plants for each treatment were covered with plastic f ilm to prevent any interaction of the treatments. The leaves were collected on days 1, 3 and 5 after spraying and placed in plastic bags that sealed. Leaf samples were immediately frozen in liquid nitrogen and stored at − 40 °C for quantifying tannins and total phenols.

Adult specimens ofM. hieroglyphicawere collected from an economic rose forest in Yulin Village, Harbin City, Heilongjiang Province, China. The insects used in these experiments were fed with fresh ‘Plena’ leaves in rearing cages and then raised in a light incubator (16 h light /8 h dark) at 25 ± 0.5 °C and relative humidity of 70 ± 5%. Adults of the same size that were clearly healthy were selected for the feeding test after 24 h of starvation. The feeding experiment was started immediately after the plants were sprayed with the JAs. Five adults were covered with a 15 × 20 cm gauze net on the middle leaf of a ‘Plena’ plant, and the net was tightened with a thin rope. After adults had fed for 1, 3, and 5 days, the leaves were collected to measure the feeding area.There were 60 adultM. hieroglyphicaper treatment group,and each plant was covered with a 45 × 30 cm gauze net.The living adults were collected 1, 3 and 5 days later and preserved at − 40 °C until the respective compounds were extracted. Each experiment was conducted in triplicate.

Freshly collected ‘Plena’ leaves were freeze-dried at − 57 °C for 24 h and ground to a powder in a frozen mortar. The powder was sifted through a 60-mesh sieve and stored at − 20 °C. The sample was placed in a tube, 10 mL of 70% methanol was added, the tube sealed, shaken well,and incubated at room temperature for 24 h. The solution was centrifuged at 5000 rpm for 10 min at 4 °C using a high speed tabletop refrigerated centrifuge (Heraeus Multifuge X1R, ThermoFisher Scientif ic, Waltham, MA, USA), and the supernatant was used as the source of tannins. Tannins were quantif ied using the vanillin-HCl method as described by Robert ( 1971) with minor modif ications. A volume of 3 mL of 4% vanillin in methanol, 1.5 mL of concentrated HCl and 0.5 mL of supernatant was added to a test tube covered with aluminum foil, which was stoppered and shaken well, then incubated in a 20 °C water bath for 20 min. A solution of 3 mL of 4% vanillin in methanol, 1.5 mL of concentrated HCl and 0.5 mL of 70% methanol was used as the blank control. The absorbance was measured three times at 510 nm using a spectrophotometer (Cary 60 UCVIS UV, Agilent Technologies, Carpinteria, CA, USA).Catechin (Sigma-Aldrich, Shanghai, China) was measured at 510 nm and used as the standard to calculate the content of condensed tannins in the extract.

Total phenol content was quantif ied as described by Yan et al ( 2018); 0.2 g leaves was added to 5 mL of precooled 1%v/v HCl-methanol solution, fully ground, extracted, centrifuged at 6000 rpm at 4 °C for 20 min and incubated at 4 °C for 24 h. Absorbance of the supernatant was measured three times at 280 nm. The total phenol content was calculated as micrograms of phenolic compounds per gram of fresh mass using a standard curve based on gallic acid (Sigma-Aldrich,Shanghai, China).

The feeding area onR. rugosa‘Plena’ was calculated using the square grid method (Yan et al. 2017).

ACP and AKP activities were quantif ied as described by Bessey et al. ( 1946) using disodiump-nitrophenyl phosphate as the substrate. The reaction mixture for ACP was 2.3 mL 0.1 mol/L (pH 4.6) acetic acid buff er, 0.5 mL substrate and 0.2 mL enzyme solution. The reaction mixture of AKP was 2.3 mL 0.05 mol/L (pH 10.0) carbonic acid buff er, 0.5 mL substrate and 0.2 mL enzyme solution. After incubation at 37 °C for 30 min, the reaction was terminated by the addition of 2 mL 0.1 mol/L NaOH, and absorbance was measured at 400 nm. For the control, the enzyme solution was replaced with the same amount of buff er. Each assay was done in triplicate.

The activities of GSTs were measured as described by Franciosa and Bergé ( 1995). The enzyme was extracted in the same manner as that of CarE. The reaction mixture consisted of 0.1 mL 1 × 10 -4 mol/L substrate 1-chloro-2,4-dinitrobenzene (CDNB), 0.4 mL 1 × 10 -3 mol/L GSH, 3.4 mL 0.1 mol/L phosphate-buff ered saline (PBS) (pH 7.0) and 0.1 mL enzyme solution, which was mixed and incubated in a 25 °C water bath for 5 min. The optical density was measured at 340 nm with 0.1 mol/L PBS (pH 7.0) as the control.

The activity of CarE was measured as described by Van Asperen ( 1962) with minor modif ications. The insect sample was placed in a glass homogenizer with precooled 0.1 mol/L PBS, pH 7.0, was added with 10 mL/head and homogenized on ice. The homogenate was centrifuged at 12,000 rpm for 20 min at 4 °C, and the supernatant was used as the enzyme solution to be tested. The reaction solution contained 0.01 mL enzyme solution, 3.49 mL 0.04 mol/L PBS (containing 10 -7 mol/L physostigmine) and 0.5 mL 4 × 10 -4 mol/L T-naphthyl acetate (T-NA) solution and was incubated in a 37 °C water bath for 30 min. A 0.5 mL of chromogenic solution (2:5 v/v of 1% fast blue B salt with a 5% solution SDS) was added and the solution incubated at room temperature for 10 min. PBS (0.04 mol/L containing 10 -7 mol/L physostigmine) was used as the control. The optical density at 600 nm was measured. T-naphthol was used as the standard, and the amount of α-naphthol generated by the enzymatic reaction was obtained from the standard curve.Each experiment was conducted in triplicate.

Bovine serum albumin was used as the standard protein as described by Bradford ( 1976), and the content of soluble protein in the enzyme solution was determined using Coomassie brilliant blue G-250.

A one-way analysis of variance (ANOVA) and Dunnett test were used to test for signif icant diff erences among treatment means for a specif ic sampling time (P< 0.05). Pearson’s correlation was used to test for a correlation in pairwise comparisons of all variables (feeding area, enzymatic activities, tannins and phenols). SPSS v. 22 (IBM, Armonk,NY, USA) was used for all analyses.

Results

Effects of JAs on tannins of leaves

JAs signif icantly increased the content of tannin in leaves during the entire experimental period (P< 0.05) (Fig. 1 a).JA increased the tannin content by 1.28-1.48-fold compared with the control. Tannin content was 1.42-1.58-fold higher than the control after treatment with MeJA and 1.15-1.20-fold higher than the control after treatment withZ-jasmone.

Effects of JAs on total phenols in leaves

JAs signif icantly increased the total phenol content during the experiment (P< 0.05) (Fig. 1 b). On days 1, 3 and 5, the total phenols in leaves treated with JA increased by 1.54-,1.67-, and 1.71-fold compared with that of the control group,respectively; MeJA increased total phenols by 1.34-, 1.62-,and 1.56-fold compared with the control, respectively,Z-jasmone increased total phenols by 1.33-, 1.53-, and 2.20-fold compared with the control, respectively.

Fig. 1 Effects of treatment with various jasmonates on tannin a and total phenol b levels in leaves of Rosa rugosa ‘Plena’ over time. Different letters indicate signif icant diff erences among treatments at a specif ic time ( P < 0.05). JA, jasmonic acid; MeJA, methyl jasmonate

Change in feeding area of adult M. hieroglyphica on leaves treated with various JAs

Application of the three types of exogenous JAs signif icantly decreased the feeding area of adults compared with that of the control group (P< 0.05) (Fig. 2). On days 1, 3 and 5, the feeding area on the plants treated with JA was reduced by 15%, 38%, and 40%, respectively, compared with the control.On days 1, 3 and 5, the feeding area of the plants treated with MeJA decreased by 17%, 40%, and 43% compared with that of the control group, respectively. On days 1, 3 and 5, the feeding area of the plants treated withZ-jasmone was lower than that of the controls by 11%, 25%, and 27%, respectively.Thus, the application of three types of JAs on leaves reduced the feeding area of adults to diff ering extents.

Fig. 2 Effect of treatments with various jasmonates on feeding area of adult Monolepta hieroglyphica on Rosa rugosa ‘Plena’ over time.Diff erent letters indicate signif icant diff erences among treatments at a specif ic time ( P < 0.05). JA, jasmonic acid; MeJA, methyl jasmonate

Activity of AKP from adult M. hieroglyphica that fed on leaves treated with JAs

AKP increased at f irst and then decreased after the insects fed on leaves treated with JAs (Fig. 3 a). On the f irst day, the AKP of adultM. hieroglyphicafed onR. rugosa‘Plena’leaves treated with JA, MeJA andZ-jasmone increased by 31%, 35%, and 19% compared with the control, respectively.AKP on the third day was 25%-35% lower than that of the control and reduced by 10%-20% on the f ifth day compared with that of the control.

Activity of ACP from adult M. hieroglyphica that fed on leaves treated with JAs

ACP was signif icantly inhibited after adults had fed on theR. rugosa‘Plena’ leaves treated with JAs (P< 0.05)(Fig. 3 b). On days 1, 3 and 5, ACP activity from insects that fed on leaves treated with JA was 11%, 26%, and 24%lower, respectively, than in the controls. On days 1, 3, and 5, ACP activity in insects that fed on leaves of MeJA-treated reduced 22%, 31%, and 25% compared with the controls,respectively, and that ofZ-jasmone-treated was 19%, 24%and 18% lower than in the controls, respectively.

Activity of GSTs from adult M. hieroglyphica that fed on leaves treated with JAs

Activities of GSTs were signif icantly inhibited after adults fed on leaves treated with JA (10%-13% lower than controls) and MeJA (11%-26% lower than controls) (Fig. 3 c).Z-jasmone had no signif icant Effect.

Fig. 3 Activity of AKP a, ACP b, GSTs c and CarE d from adult Monolepta hieroglyphica that fed on Rosa rugosa ‘Plena’ treated with JAs. Diff erent letters indicate signif icant diff erences among treatments at a specif ic time ( P < 0.05). JA, jasmonic acid; MeJA, methyl jasmonate

Activity of CarE fron adult M. hieroglyphica that fed on leaves treated with JAs

CarE activity decreased by 8%-27% in leaves treated with JA compared with the controls, and there were signif icant diff erences between the two groups (JA-treated and the control) on days 1 and 5 (Fig. 3 d). Activity was 11%-31% lower than that of the control after the insects had fed on MeJAtreated leaves. InZ-jasmone-treated leaves, CarE activity in the insect was signif icantly lower than in the control group only on day 1; the levels did not diff er signif icantly on days 3 and 5.

Pearson’s correlation analysis of pairwise comparisons of all variables

The decrease in adultM. hieroglyphicafeeding area was highly positively correlated with tannin content (p< 0.01)and positively correlated with the content of total phenol inR. rugosa‘Plena’ leaves (Table 1). The reduced feeding area of adultM. hieroglyphicawas highly negatively correlated with activities of AKP and ACP (p< 0.01) and negatively correlated with activity of GSTs.

Discussion

Phenolic compounds are secondary metabolites that serve as important defensive compounds in plants (Rani and Jyothsna 2010; War et al. 2011; Sharma et al. 2009; Kaur et al.2017). Tannins, key phenolic compounds that provide multiresistance, can specif ically combine with proteins to aff ect the feeding, growth and digestion of insects (Barbehenn and Constabel 2011; War et al. 2012). JAs can transmit resistance information within plants and induce plants to produce certain defense compounds (Wasternack et al. 2015; Tang et al. 2020). Ibrahim et al. ( 2018) demonstrated that the tannin and total phenol levels in kale (Brassica oleraceavar.acephala‘Khanyari’) treated with 1 mmol/L JA increased by 1.79- and 2.38-fold compared with those of the control,respectively. Ketabchi et al. ( 2014) showed that a foliar spray treatment of 0.1 mmol/L MeJA increased the total phenols in wheat (Triticum aestivum‘Falat’) plants compared with those of untreated plants. In addition, tannins in American chestnut (Castanea dentata) foliage increased after the application of 1.5 mmol/L JA (Cooper and Rieske 2008). In the present study, the contents of tannins and total phenols inR.rugosa‘Plena’ leaves treated with 1 mmol/L JA, MeJA andZ-jasmone were higher than in the control at all sampling times, consistent with the results of Ibrahim et al. ( 2018).Furthermore, tannin levels after treatment with JA or MeJA were signif icantly higher than after treatment withZ-jasmone. However, the optimal concentration of exogenous JAsfor increasing the defense compounds diff ered from those in other studies, which may have been caused by diff ering defense responses to induction by JAs. Although the precursor substances of tannins are phenolic compounds such as gallic acid and catechin, the trend in the variation in the tannin contents in our study diff ered somewhat from that obtained for the total phenol content after treatment with JAs. The reason may be that the biochemical reactions and metabolic pathways in plants are highly interconnected and complex. In addition, the induction of diff erent secondary metabolic pathways mediated by JAs is regulated diff erently.Moreover, the diff erent metabolic pathways and tannin and total phenol levels in plant tissues aff ect the rate of synthesis and the amount of tannins and total phenols, thus, leading to diff erences in the resistance reactions produced by identical inducing factors in the same tree species. In addition,when more than one pathway is induced, the sensitivity and lag range of each pathway may diff er (Pauwels et al. 2009;Florian et al. 2005).

Table 1 Pearson’s correlation analysis

In a study on the Effect of MeJA-induced defenses in rice (Oryza sativa) againstCnaphalocrocis medinalis, 2.5 and 5 mmol/L of MeJA Effectively reduce feeding (Senthil-Nathan 2019). The application of 0.05 and 0.1 mmol/L JA on sweet pepper leaves (Capsicum annuum) signif icantly reduced the food intake of adultLiriomyza trifollito 39% and 19% of the control, respectively (Tebayashi et al. 2007). Feeding ofSpodoptera exiguaon f ield-grown tomato plants treated with 1 mmol JA was reduced, as mass and survival of the pest (Thaler et al. 1999). The present study also showed that the feeding area of adultM. hieroglyphicaon leaves ofR. rugosa‘Plena’ treated with JAs was reduced compared with the controls (P< 0.05), and feeding area decreased by 17%-43% after treatment with 1 mmol/L MeJA compared with the controls. Moreover, compared withZ-jasmone, JA and MeJA was more Effective at inducing resistance to feeding, similar to the results of Thaler et al.( 1999). Treatment with exogenous JAs can activate the JA signaling pathway, an important component of induced systemic resistance (ISR) in plants (Hu et al. 2018). After the plant is sprayed with JAs, corresponding induction signals are generated in the local tissue; this treatment stimulates the synthesis of endogenous JAs, which are quickly transmitted to the whole plant (Koo and Howe 2009; War et al. 2012).Therefore, the exogenous JAs might activate the expression of JA signaling pathway to induce an ISR response.

Phosphatase, which is involved in transphosphorylation,is an important metabolic enzyme in many biochemical reactions in insects and plays a vital role in the detoxif ication,metabolism and growth of insects (Senthil-Nathan et al.2005). When phosphatase activity is inhibited, the release of phosphorus required for energy metabolism decreases,aff ecting the normal physiological activities in insects (Senthil-Nathan et al. 2006). A decrease in phosphatase activity will weaken the resistance of insects to pesticides and other exogenous substances, which will aff ect their feeding, growth, development and survival (Bilal et al. 2018).Our results showed that AKP activity increased at f irst,then decreased after adultM. hieroglyphicafed on leaves ofR. rugosa‘Plena’ treated with diff erent types of JAs; the activity of ACP was inhibited to varying degrees depending on the treatment. These results are similar to those of Bodnaryk and Rymerson ( 1994). However, our results diff er from those reported by Jiang ( 2017), in which treatment of leaves ofLarix olgensiswith MeJA induced higher levels of phosphatase activity in 5th instarsDendrolimus superanscompared with the control. This diff erence may be due to diff erences in the activity of the detoxif ication enzyme induced by JAs.

GSTs, important detoxif ication enzymes that are widely distributed in organisms, can catalyze the degradation of harmful substances for removal from the body (Ku et al.1994; Francis et al. 2005). The activities of GSTs inSitobion avenaeincreased in response to application of alkaloids, suggesting that GSTs are involved in the detoxif ication of plant defense substances (Cai et al. 2009). Activities of GSTs inH. armigerawere signif icantly reduced after the pests fed on cotton leaves treated with 0.4 mmol/L MeJA(Yang et al. 2013). In the present study, the activities of GSTs in adultM. hieroglyphicawere signif icantly inhibited when the insects fed on leaves treated with JA and MeJA(P< 0.05), butZ-jasmone did not induce a signif icant diff erence compared to the control plants, likely because JA derivatives trigger diff erent parts of the JA signaling pathway.

CarE primarily degrades exogenous and some endogenous harmful substances in the organism for excretion from the body (Cui et al. 2011). Our study demonstrated that CarE activity of adultM. hieroglyphicatreated with JA, MeJA andZ-jasmone was signif icantly inhibited on the f irst day, and JA and MeJA were more Effective thanZ-jasmone. Moreover, the MeJA treatment signif icantly inhibited CarE activity during the entire experiment, indicating that the inhibition of CarE was relatively long-lasting. When plants are treated with exogenous JAs, their cell membrane receptors are stimulated, mediating a series of reactions to activate plant defense genes. The activation of defense genes leads to the reconf iguration of metabolic pathways that may produce different defense Effects (Kessler and Baldwin 2001). Moreover, JA is the substrate for a reaction catalyzed by jasmonic acid carboxyl methyltransferase (JMT) to produce MeJA,and JA can be oxidized to dihydrojasmonic acid, which is then decarboxylated to formZ-jasmone (Florian et al. 2005).Since the basic structure of MeJA and JA is a cyclopentane,MeJA likely can be easily converted to JA, which directly enters the plant defense system from an intermediate step to activate the defense system, omitting the multi-step signalstarting process (Seo et al. 2001). However, the conversion ofZ-jasmone to JA is more complex and takes longer; thus,compared with JA and MeJA, treatment withZ-jasmone has a slightly weaker inhibitory Effect on the activity of CarE of the adultM. hieroglyphica, but the inhibitory Effect ofZ-jasmone treatment was still signif icantly stronger than that of the control group. Our results were consistent with those of Yang et al. ( 2013), who demonstrated that CarE activity ofH. armigerawas reduced when it fed on cotton seedlings fumigated with MeJA. Exogenous JAs might activate the JA signaling pathway and induce the defense reaction inR.rugosa‘Plena’, thus producing a variety of toxic substances to inhibit the activities of detoxif ication enzymes in insects,thus, reducing the deleterious Effects ofM. hieroglyphica(Kessler and Baldwin 2002).

The feeding area of adultM. hieroglyphicawas highly negatively correlated with the content of tannin and negatively correlated with the content of total phenol in ‘Plena’leaves. Moreover, the reduction in feeding area was highly negatively correlated with the activities of AKP and ACP and negatively correlated with that of the GSTs. The tannins and total phenols in leaves increased after treatment with JAs; thus, the detoxif ication activity in the adultM. hieroglyphicadecreased, and the feeding area also decreased.Our results were similar to those of Gui et al. ( 2005) and Yang et al. ( 2013) who reported the application of MeJA on tea (Camellia sinensis) and cotton (G. hirsutum) leaves signif icantly inhibited the detoxif ication enzymes ofEctropis obliquaandH. armigera, respectively, thus, reducing the deleterious Effects of pests. Using exogenous hormones to improve plant resistance and reduce pest damage is a relatively new concept of environmentally friendly integrated management of insect pests. Detailed research on the physiological, biochemical responses and molecular mechanisms of anti-insect metabolites induced in plants is now needed.

Conclusions

Treatment with 1.0 mmol/L JA, MeJA andZ-jasmone can activate defense reactions inR. rugosa‘Plena’, increase the levels of tannins and total phenols in the plant, and decrease the activities of AKP, ACP, GSTs, and CarE in adultM.hieroglyphica. The treatments also reduced the feeding area on leaves. Treatment with 1.0 mmol/L MeJA was the most Effective at inducing a defense response. The decrease in the feeding area of adultM. hieroglyphicawas highly positively correlated with the tannin content and positively correlated with the level of total phenols in the treated leaves. The reduced feeding area of adultM. hieroglyphicawas highly negatively correlated with the activities of AKP and ACP and negatively correlated with those of GSTs.

Compliance with ethical standards

Conf lict of interestThe authors declare that they have no conf lict of interest.

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