[Research progress in chemical interactions between plants and phytophagous insects]

There are complex chemical interactions between plants and phytophagous insects. On the one hand, when infested by phytophagous insects, plants can recognize herbivore-associated molecular patterns and trigger early signaling events and phytohormone-mediated signaling pathways. The activated signaling pathways thus result in the reconfiguration of transcriptomes and metabolomes as well as the increases in direct and indirect defensive compounds in plants, which in turn enhance the resistance of plants to phytophagous insects. On the other hand, phytophagous insects can recognize defense responses in plants and then inhibit or adapt to plant chemical defenses by secreting effector, sequestrating and detoxifying defensive compounds, and/or reducing sensitivity to defensive compounds. The deep analysis of chemical interactions between plant and phytophagous insects could improve the understanding of the relationship between insects and plants in theory and also provide important theoretical and technical guidance for the development of new technologies for crop pest control in practice.

Host plants alter their volatiles to help a solitary egg parasitoid distinguish habitats with parasitized hosts from those without

When attacked by herbivores, plants emit volatiles to attract parasitoids and predators of herbivores. However, our understanding of the effect of plant volatiles on the subsequent behaviour of conspecific parasitoids when herbivores on plants are parasitized is limited. In this study, rice plants were infested with gravid females of the brown planthopper (BPH) Nilaparvata lugens for 24 hr followed by another 24 hr in which the BPH eggs on plants were permitted to be parasitized by their egg parasitoid, Anagrus nilaparvatae; volatiles from rice plants that underwent such treatment were less attractive to subsequent conspecific parasitoids compared to the volatiles from plants infested with gravid BPH females alone. Chemical analysis revealed that levels of JA and JA-Ile as well as of four volatile compounds-linalool, MeSA, α-zingiberene and an unknown compound-from plants infested with BPH and parasitized by wasps were significantly higher than levels of these compounds from BPH-infested plants. Laboratory and field bioassays revealed that one of the four increased chemicals-α-zingiberene-reduced the plant's attractiveness to the parasitoid. These results suggest that host plants can fine-tune their volatiles to help egg parasitoids distinguish host habitats with parasitized hosts from those without.

Resistance of rice to insect pests mediated by suppression of serotonin biosynthesis

Rice is one of the world's most important foods, but its production suffers from insect pests, causing losses of billions of dollars, and extensive use of environmentally damaging pesticides for their control^1,2. However, the molecular mechanisms of insect resistance remain elusive. Although a few resistance genes for planthopper have been cloned, no rice germplasm is resistant to stem borers. Here, we report that biosynthesis of serotonin, a neurotransmitter in mammals³, is induced by insect infestation in rice, and its suppression confers resistance to planthoppers and stem borers, the two most destructive pests of rice². Serotonin and salicylic acid derive from chorismate⁴. In rice, the cytochrome P450 gene CYP71A1 encodes tryptamine 5-hydroxylase, which catalyses conversion of tryptamine to serotonin⁵. In susceptible wild-type rice, planthopper feeding induces biosynthesis of serotonin and salicylic acid, whereas in mutants with an inactivated CYP71A1 gene, no serotonin is produced, salicylic acid levels are higher and plants are more insect resistant. The addition of serotonin to the resistant rice mutant and other brown planthopper-resistant genotypes results in a loss of insect resistance. Similarly, serotonin supplementation in artificial diet enhances the performance of both insects. These insights demonstrate that regulation of serotonin biosynthesis plays an important role in defence, and may prove valuable for breeding insect-resistant cultivars of rice and other cereal crops.

First Report of Pyrimethanil Resistance in Botrytis cinerea from Stored Apples in Pennsylvania

Botrytis cinerea Pers.: Fr. (teleomorph Botryotinia fuckeliana [de Bary] Whetzel) causes gray mold on apple fruit. A survey of commercial packinghouses in Washington State revealed that it accounted for 28% of the decay in storage (1). Fungicide application coupled with cultural practices are the primary method of control as all commercial apple cultivars are susceptible to gray mold. In February 2013, gray mold was observed at ~5% incidence for commercially packed 'Gala' apple fruit that had been treated with Penbotec (active ingredient: pyrimethanil, Shield-Bright, Pace International) prior to controlled atmosphere storage in Pennsylvania. Eight infected apple fruit were collected, placed in 80 count boxes on cardboard trays, and stored at 4°C. One isolate was obtained from each decayed apple, placed on potato dextrose agar (PDA) petri plates, and incubated at 20°C with natural light. Eight single-spore isolates were identified as B. cinerea based on cultural characteristics. Species level identification was executed by obtaining mycelial genomic DNA, amplifying the ITS rDNA, and sequencing the ~550-bp amplicon directly (2). MegaBLAST analysis of the 2X consensus for the 8 isolates revealed 100% identity to B. cinerea ITS sequences in GenBank (KF156296.1 and JX867227.1) with E values of 0.0, thus confirming the morphological identification. Minimum inhibitory concentration (MIC) was determined using conidial suspensions obtained from ~14-day-old plates (10⁴ spores/ml) and a range (0 to 500 μg/ml) of technical grade pyrimethanil on three replicated 96-well microtiter plates containing a defined medium for each experiment. Conidial proliferation was inhibited at 250 μg/ml for all eight isolates and the experiment was conducted four times. To further define the resistance levels between the isolates, mycelial growth analysis using a plug of actively growing mycelium from the margin of ~3-day-old plates was conducted with a defined medium three times with technical grade pyrimethanil with three plates per experiment. Five isolates grew at 250 μg/ml (highly resistant), while three did not (moderately resistant). To assess resistance in vivo, organic 'Gala' apples were rinsed with soap and water, sprayed with 70% ethanol, placed on trays, and allowed to air dry. Apples were wounded with a sterile finishing nail, inoculated with 20 μl of a conidial suspension (10⁴ spores/ml) of either a moderately or a highly resistant isolate, and dipped in the labeled application rate of Penbotec at 500 μg/ml or sterile water for 30 s. Fruit were stored in 100 count boxes at 22°C for 5 days and decay incidence and severity were recorded. Ten fruit composed a replicate per treatment and the experiment was repeated. Water inoculated controls were symptomless and water-dipped inoculated fruit had 100% decay. Penbotec-treated fruit had 100% decay incidence and mean lesion diameters of 37.6 (±13.1 mm) for the highly, and 35.7 (±9.0 mm) for the moderately resistant isolate. This is the first report of pyrimethanil resistance in B. cinerea from decayed apples collected from a commercial packinghouse in Pennsylvania. The results indicate that pyrimethanil resistance has developed in B. cinerea, which can result in control failures on Penbotec-treated fruit during storage. Furthermore, it emphasizes the need for additional tools to manage gray mold on apple fruit and may pose issues for export concerning the spread of fungicide-resistant inoculum. References: (1) Y.-K. Kim and C. L. Xiao. Plant Dis. 92:940, 2008. (2) T. J. White et al. Page 315 in: PCR Protocols: A Guide to Methods and Applications. Academic Press, San Diego, CA, 1990.

Feeding-induced interactions between two rice planthoppers, Nilaparvata lugens and Sogatella furcifera (Hemiptera: Delphacidae): effects on feeding and honeydew excretion

A series of laboratory experiments was conducted using electrical penetration graph, salivary flange, and honeydew measurement to study the effects of feeding-induced intra- and interspecific interactions on feeding behavior and honeydew excretion between planthoppers Nilaparvata lugens (Stål) and Sogatela furcifera (Horváth). Feeding-induced intra- and interspecific interactions affected many measures of feeding behavior. The number of salivary flanges, mean duration of pathway activities per insect, and mean duration from first probe to first sustained phloem ingestion for both N. lugens and S. furcifera were significantly shorter on rice plants with feeding-induced conspecific and heterospecific effects than those for planthoppers fed on control plants. Feeding-induced intra- and interspecific interactions also affected the duration per insect of phloem ingestion for both N. lugens and S. furcifera. The durations per insect of phloem ingestion on host plants with feeding-induced conspecific and heterospecific effects were significantly longer than those on control plants. An asymmetric facilitative effect of induced interspecific interactions on the weight of honeydew excreted was detected, because only the honeydew weights of S. furcifera were significantly increased by the induced heterospecific effect on both varieties. The results demonstrated that the facilitative effects on honeydew excretion were consistent with previously documented effects on performance. Both facilitative effects on honeydew excretion and performance were asymmetrical, with more benefits to S. furcifera from N. lugens. Such facilitative effects might be mainly related to altered nutrient status and induced allelochemistry in rice.

Feeding-induced interactions between Nilaparvata lugens and Laodelphax striatellus (Hemiptera: Delphacidae): effects on feeding behavior and honeydew excretion

Using electrical penetration graph, salivary flange, and honeydew measurement, this study investigated the effects of feeding-induced intra- and interspecific interactions on feeding behavior and honeydew excretion of brown planthopper (Nilaparvata lugens) compared with small brown planthopper (Laodelphax striatellus). Results showed that many measures of feeding behavior were affected by feeding-induced intra- and interspecific interactions on two different rice varieties. There were significantly fewer salivary flanges for both brown planthopper and small brown planthopper on rice plants with feeding-induced conspecific or heterospecific effects than on relevant control plants. In contrast, only small brown planthopper on rice plants with feeding-induced heterospecific effects had significantly fewer salivary flanges than those with feeding-induced conspecific effects. The mean durations of pathway activities per insect and mean durations from first probe to first sustained phloem ingestion for small brown planthopper were significantly shorter, whereas the mean duration per insect of phloem ingestion was significantly longer, on rice plants with feeding-induced heterospecific effects than those on relevant control plants, as well as rice plants with feeding-induced conspecific effects. Honeydew weights of small brown planthopper were significantly increased by the induced heterospecific effect. Thus, all results indicated indirect, asymmetrical, facilitative effects of induced interspecific interactions on the feeding behavior and honeydew weight for small brown planthopper on both varieties. These findings are consistent with the previously documented asymmetrical effects on performance, with more benefits to small brown planthopper from brown planthopper indirectly. The change of nutrient and induced allelochemistry in host plant probably underlies these facilitative effects.

Altered disease development in the eui mutants and Eui overexpressors indicates that gibberellins negatively regulate rice basal disease resistance

Gibberellins (GAs) form a group of important plant tetracyclic diterpenoid hormones that are involved in many aspects of plant growth and development. Emerging evidence implicates that GAs also play roles in stress responses. However, the role of GAs in biotic stress is largely unknown. Here, we report that knockout or overexpression of the Elongated uppermost internode (Eui) gene encoding a GA deactivating enzyme compromises or increases, respectively, disease resistance to bacterial blight (Xanthomonas oryzae pv. oyrzae) and rice blast (Magnaporthe oryzae). Exogenous application of GA(3) and the inhibitor of GA synthesis (uniconazol) could increase disease susceptibility and resistance, respectively, to bacterial blight. Similarly, uniconazol restored disease resistance of the eui mutant and GA(3) decreased disease resistance of the Eui overexpressors to bacterial blight. Therefore, the change of resistance attributes to GA levels. In consistency with this, the GA metabolism genes OsGA20ox2 and OsGA2ox1 were down-regulated during pathogen challenge. We also found that PR1a induction was enhanced but the SA level was decreased in the Eui overexpressor, while the JA level was reduced in the eui mutant. Together, our current study indicates that GAs play a negative role in rice basal disease resistance, with EUI as a positive modulator through regulating the level of bioactive GAs.

[Ovicidal activity of nine essential oils against Chrysomya megacephara in bacon and kipper]

To exploit safe ovicides against the flies that attack bacon and kipper, the ovicidal activity of 9 plant essential oils was investigated by a modified egg-dipping method. The results indicated that the essential oils from Cinnamomun cassia, Eugenia caryophyllata and Illicum verum had higher ovicidal activity, with their LC50 values being 0.428, 1.605 and 2.489 mg x ml(-1), respectively. Through GC/MS analysis, a total of 22 components were identified from C. cassia oil, among which, cinamaldehyde was the most predominant one, accounting for 92.33% of the total. The LC50 of synthesized cinamaldehyde (97.33%) was 0.281 mg x ml(-1), being a little higher than the LC50 of C. cassia oil, suggesting that cinamaldehyde was the main ovicidal component of C. cassia oil and could replace the latter as an ovicide against harmful flies.

The rice (E)-beta-caryophyllene synthase (OsTPS3) accounts for the major inducible volatile sesquiterpenes

Terpenoids serve as both constitutive and inducible defense chemicals in many plant species, and volatile terpenes participate in plant a indirect defense by attracting natural enemies of the herbivores. The rice (Oryza sativa L.) genome contains about 50 genes encoding putative terpene synthases (TPSs). Here we report that two of the rice sesquiterpene synthase genes, OsTPS3 and OsTPS13, encode (E)-beta-caryophyllene synthase and (E,E)-farnesol synthase, respectively. In vitro, the recombinant protein of OsTPS3 catalyzed formation of (E)-beta-caryophyllene and several other sesquiterpenes, including beta-elemene and alpha-humulene, all being components of inducible volatiles of rice plants. The transcript levels of OsTPS3 exhibit a circadian rhythm of fluctuation, and its expression was also greatly induced by methyl jasmonate (MeJA). In addition, expression of OsTPS3 in transgenic plants of Arabidopsis thaliana resulted in emitting high quantities of OsTPS3 products. We also overexpressed OsTPS3 in rice plants which then produced more (E)-beta-caryophyllene after MeJA treatment. Finally, we found that the MeJA-treated transgenic rice plants attracted more parasitoid wasps of Anagrus nilaparvatae than the wild-type. These results demonstrate that OsTPS3, an enzyme catalyzing the formation of volatile sesquiterpenes, plays a role in indirect defense of rice plants.

Non-host plant extracts reduce oviposition of Plutella xylostella (Lepidoptera: Plutellidae) and enhance parasitism by its parasitoid Cotesia plutellae (Hymenoptera: Braconidae)

Botanical preparations, usually from non-host plants, can be used to manipulate the behaviour of insect pests and their natural enemies. In this study, the effects of extracts of Chrysanthemum morifolium, a non-host plant of the diamondback moth, Plutella xylostella (Linnaeus), on the olfactory and oviposition responses of this phytophagous insect and on levels of parasitism by its specialist parasitoid Cotesia plutellae (Kurdjumov) were examined, using Chinese cabbage Brassica campestris L. ssp. pekinensis as the test host plant. Olfactometer tests showed that volatiles of chrysanthemum extract-treated host plants were less attractive to P. xylostella females than those from untreated host plants; and in contrast, volatiles of the chrysanthemum extract-treated host plants were more attractive to females of its parasitoid C. plutellae than those from untreated host plants. Oviposition preference tests showed that P. xylostella females laid only a small proportion of their eggs on chrysanthemum extract-treated host plants, while ovipositing parasitoid females parasitized a much higher proportion of host larvae feeding on the treated host plants than on untreated host plants. These results suggest that certain non-host plant compounds, when applied onto a host plant, may render the plant less attractive to a phytophagous insect but more attractive to its parasitoids. Application of such non-host plant compounds can be explored to develop push-pull systems to reduce oviposition by a pest insect and at the same time enhance parasitism by its parasitoids in crops.

Attraction of the parasitoid Anagrus nilaparvatae to rice volatiles induced by the rice brown planthopper Nilaparvata lugens

Anagrus nilaparvatae, an egg parasitoid of the rice brown planthopper Nilaparvata lugens, was attracted to volatiles released from N. lugens-infested plants, whereas there was no attraction to volatiles from undamaged plants, artificially damaged plants, or volatiles from N. lugens nymphs, female adults, eggs, honeydew, and exuvia. There was no difference in attractiveness between plants infested by N. lugens nymphs or those infested by gravid females. Attraction was correlated with time after infestation and host density; attraction was only evident between 6 and 24 hr after infestation by 10 adult females per plant, but not before or after. Similarly, after 24 hr of infestation, wasps were attracted to plants with 10 to 20 female planthoppers, but not to plants with lower or higher numbers of female planthoppers. The attractive time periods and densities may be correlated with the survival chances of the wasps' offspring, which do not survive if the plants die before the wasps emerge. Wasps were also attracted to undamaged mature leaves of a rice plant when one of the other mature leaves had been infested by 10 N. lugens for 1 d, implying that the volatile cues involved in host location by the parasitoid are systemically released. Collection and analyses of volatiles revealed that 1 d of N. lugens infestation did not result in the emission of new compounds or an increase in the total amount of volatiles, but rather the proportions among the compounds in the blend were altered. The total amounts and proportions of the chemicals were also affected by infestation duration. These changes in volatile profiles might provide the wasps with specific information on host habitat quality and thus could explain the observed behavioral responses of the parasitoid.

Exogenous application of jasmonic acid induces volatile emissions in rice and enhances parasitism of Nilaparvata lugens eggs by the parasitoid Anagrus nilaparvatae

Jasmonate signaling pathway plays an important role in induced plant defense against herbivores and pathogens, including the emission of volatiles that serve as attractants for natural enemies of herbivores. We studied the volatiles emitted from rice plants that were wounded and treated with jasmonic acid (JA) and their effects on the host-searching behavior of the rice brown planthopper, Nilaparvata lugens (Stål), and its mymarid egg parasitoid Anagrus nilaparvatae Pang et Wang. Female adults of N. lugens significantly preferred to settle on JA-treated rice plants immediately after release. The parasitoid A. nilaparvatae showed a similar preference and was more attracted to the volatiles emitted from JA-treated rice plants than to volatiles from control plants. This was also evident from greenhouse and field experiments in which parasitism of N. lugens eggs by A. nilaparvatae on plants that were surrounded by JA-treated plants was more than twofold higher than on control plants. Analyses of volatiles collected from rice plants showed that JA treatment dramatically increased the release of volatiles, which included aliphatic aldehydes and alcohols, monoterpenes, sesquiterpenes, methyl salicylate, n-heptadecane, and several as yet unidentified compounds. These results confirm an involvement of the JA pathway in induced defense in rice plants and demonstrate that the egg parasitoid A. nilaparvatae exploits plant-provided cues to locate hosts. We explain the use of induced plant volatiles by the egg parasitoid by a reliable association between planthopper feeding damage and egg presence.