Silencing a cotton bollworm P450 monooxygenase gene by plant-mediated RNAi impairs larval tolerance of gossypol

Genetically engineered plants and foods: a scientist's analysis of the issues (part II)

Genetic engineering provides a means to introduce genes into plants via mechanisms that are different in some respects from classical breeding. A number of commercialized, genetically engineered (GE) varieties, most notably canola, cotton, maize and soybean, were created using this technology, and at present the traits introduced are herbicide and/or pest tolerance. In 2007 these GE crops were planted in developed and developing countries on more than 280 million acres (113 million hectares) worldwide, representing nearly 10% of rainfed cropland. Although the United States leads the world in acres planted with GE crops, the majority of this planting is on large acreage farms. In developing countries, adopters are mostly small and resource-poor farmers. For farmers and many consumers worldwide, planting and eating GE crops and products made from them are acceptable and even welcomed; for others GE crops raise food and environmental safety questions, as well as economic and social issues. In Part I of this review, some general and food issues related to GE crops and foods were discussed. In Part II, issues related to certain environmental and socioeconomic aspects of GE crops and foods are addressed, with responses linked to the scientific literature.

New insights into peritrophic matrix synthesis, architecture, and function

The peritrophic matrix (PM) is a chitin and glycoprotein layer that lines the invertebrate midgut. Although structurally different, it is functionally similar to the mucous secretions of the vertebrate digestive tract. The PM is a physical barrier, protecting the midgut epithelium from abrasive food particles, digestive enzymes, and pathogens infectious per os. It is also a biochemical barrier, sequestering and, in some cases, inactivating ingested toxins. Finally, the PM compartmentalizes digestive processes, allowing for efficient nutrient acquisition and reuse of hydrolytic enzymes. The PM consists of an organized lattice of chitin fibrils held together by chitin binding proteins. Glycans fill the interstitial spaces, creating a molecular sieve, the properties of which are dependent on the immediate ion content and pH. In this review, we have integrated recent structural and functional information to create a holistic model for the PM. We also show how this information may generate novel technologies for use in insect pest management.

RNA interference-mediated knockdown of a cytochrome P450, CYP6BG1, from the diamondback moth, Plutella xylostella, reduces larval resistance to permethrin

We have previously reported that a cytochrome P450, CYP6BG1, from Plutella xylostella was found to be overexpressed in 4th instars of a permethrin resistant strain and inducible in the susceptible counterpart. The findings suggested potential involvement of CYP6BG1 in permethrin resistance, hence warranted a functional analysis. To assess the functional link of the gene to permethrin resistance, we adopted RNA interference-mediated gene silencing (RNAi) by dsRNA droplet feeding. Here, real time PCR analyses show that oral delivery of dsRNA can efficiently reduce the expression of CYP6BG1. Knockdown of CYP6BG1 transcript was evident in midgut and larval tissues enclosed in carcass. As a consequence of knockdown, a significant reduction in resistance of larvae fed CYP6BG1 dsRNA was observed after 24 and 48h of exposure to permethrin. In addition, CYP6BG1 dsRNA feeding to larvae led to reduced total P450 activities of microsomal preparations toward model substrates p-nitroanisole and benzyloxyresorufin. These results indicate that the overexpressed CYP6BG1 participate in enhanced metabolism of permethrin, thereby, resistance. The knockdown of a non-overexpressed P450, CYP6BF1v4, from the same resistant P. xylostella strain did not lead to changes in the level of resistance to permethrin, supporting further the specific involvement of CYP6BG1 in the resistance.

Disruption of Spodoptera exigua larval development by silencing chitin synthase gene A with RNA interference

RNA interference (RNAi) is a powerful tool for rapidly analyzing gene functions. However, little is known about the possible use of dsRNA/siRNA as a pest control method. Here, we demonstrate that dsRNA/siRNA can induce the silence of chitin synthase gene A (CHSA), which is an important gene for the growth and development of cuticles and trachea in beet armyworm, Spodoptera exigua. Based on the in vitro RNAi experiments in an insect cell line (Trichoplusia ni High 5), in vivo RNAi was performed by injecting synthesized dsRNA/siRNA into the 4th instar larvae of S. exigua. Significantly lower levels of CHSA transcripts were detected. In addition, the cuticle of these insects was disordered and the epithelial walls of larval trachea did not expand uniformly in injected individuals. Moreover, Injections significantly increased abnormalities relative to control larvae. These results highlighted the possibility of dsRNA/siRNA for gene function studies in lepidopteran insects and future pest control.

Expression of v-cath gene from HearNPV in tobacco confers an antifeedant effect against Helicoverpa armigera

Biotechnology solutions for insect control on crops largely depend on the expression of Bacillus thuringiensis insecticidal proteins to kill pests. V-CATH, a cathepsin L-like cysteine protease from baculoviruses, has been shown to play an essential role in host insect liquefaction. In this study, the v-cath gene from Helicoverpa armigera single nucleopolyhedrovirus (HearNPV) was cloned into the pBI121 binary vector under the control of CaMV35S promoter, and was expressed in tobacco via Agrobacterium-mediated transformation. PCR and RT-PCR analyses of T(1) kanamycin-resistant tobacco progeny plants confirmed the integration and transcription of the v-cath gene. Using a leaf-disk bioassay, antifeedant activity toward H. armigera was tested. Our result showed that, when feeding the first-instar larvae of H. armigera with leaves of transgenic plants, the v-cath transgene expression has a profound antifeedant effect. Most importantly, the growth and development of the insect were inhibited when transferred from leaf-feeding to artificial diet. Our result demonstrated that v-cath gene from baculovirus induced antifeedant effect against H. armigera, resulted in larval stunting and retarded insect development, and has the potential to be used as an alternative way to generate transgenic plants for insect pest control.

RNA interference in the termite Reticulitermes flavipes through ingestion of double-stranded RNA

RNA interference (RNAi) represents a breakthrough technology for conducting functional genomics research in non-model organisms and for the highly targeted control of insect pests. This study investigated RNAi via voluntary feeding in the economically important pest termite, Reticulitermes flavipes. We used a high-dose double-stranded (ds) RNA feeding approach to silence two termite genes: one encoding an endogenous digestive cellulase enzyme and the other a caste-regulatory hexamerin storage protein. Contrary to results from previous low-dose studies that examined injection-based RNAi, high-dose silencing of either gene through dsRNA feeding led to significantly reduced group fitness and mortality. Hexamerin silencing in combination with ectopic juvenile hormone treatments additionally led to lethal molting impacts and increased differentiation of presoldier caste phenotypes (a phenotype that is not capable of feeding). These results provide the first examples of insecticidal effects from dsRNA feeding in a termite. Additionally, these results validate a high-throughput bioassay approach for use in (i) termite functional genomics research, and (ii) characterizing target sites of conventional and novel RNAi-based termiticides.

RNAi-mediated crop protection against insects

Downregulation of the expression of specific genes through RNA interference (RNAi), has been widely used for genetic research in insects. The method has relied on the injection of double-stranded RNA (dsRNA), which is not possible for practical applications in crop protection. By contrast, specific suppression of gene expression in nematodes is possible through feeding with dsRNA. This approach was thought to be unfeasible in insects, but recent results have shown that dsRNA fed as a diet component can be effective in downregulating targeted genes. More significantly, expression of dsRNA directed against suitable insect target genes in transgenic plants has been shown to give protection against pests, opening the way for a new generation of insect-resistant crops.

Environmental RNA interference

The discovery of RNA interference (RNAi), the process of sequence-specific gene silencing initiated by double-stranded RNA (dsRNA), has broadened our understanding of gene regulation and has revolutionized methods for genetic analysis. A remarkable property of RNAi in the nematode Caenorhabditis elegans and in some other multicellular organisms is its systemic nature: silencing signals can cross cellular boundaries and spread between cells and tissues. Furthermore, C. elegans and some other organisms can also perform environmental RNAi: sequence-specific gene silencing in response to environmentally encountered dsRNA. This phenomenon has facilitated significant technological advances in diverse fields including functional genomics and agricultural pest control. Here, we describe the characterization and current understanding of environmental RNAi and discuss its potential applications.

Cloning and RNAi-mediated functional characterization of MaLac2 of the pine sawyer, Monochamus alternatus

Laccase, a member of a group of proteins collectively known as multicopper oxidases, is hypothesized to play an important role in insect cuticle sclerotization by oxidizing catechols in the cuticle to their corresponding quinones, which then catalyze protein cross-linking reactions. Laccase 2 has been proved as the gene required for beetle cuticle tanning through RNA interference (RNAi) experiments on red flour beetle Tribolium castaneum. The pine sawyer beetle, Monochamus alternatus (Coleoptero: Cerambycidae) is the insect serving as a major vector of the pinewood nematode, Bursaphelenchus xylophilus, which is the causative agent for pine wilt disease. The cDNA of MaLac2 was cloned from the insect in this study. The conceptual amino-acid sequence deduced was much conserved with other known insect laccases, particularly with the enzyme of Tribolium castaneum. Injection in hemolymph of pine sawyer larva of dsRNA targeting the laccase 2 mRNA leads to important alterations of the tanning, hardening and sclerotization of the pupal and adult cuticles. Defaults appear in a dose-dependent manner and high loads of dsRNA are lethal. The decrease of the endogenous laccase 2 mRNA affects the procuticle which is thinner and without the characteristic piling up of successive layers. The observations reinforce the role of laccase 2 as an essential phenoloxidase for making cuticle.

Herbivory-induced changes in the small-RNA transcriptome and phytohormone signaling in Nicotiana attenuata

Phytohormones mediate the perception of insect-specific signals and the elicitation of defenses during insect attack. Large-scale changes in a plant's transcriptome ensue, but how these changes are regulated remains unknown. Silencing of RNA-directed RNA polymerase 1 (RdR1) makes Nicotiana attenuata highly susceptible to insect herbivores, suggesting that defense elicitation is under the direct control of small-RNAs (smRNAs). Using 454-sequencing, we characterized N. attenuata's smRNA transcriptome before and after insect-specific elicitation in wild-type (WT) and RdR1-silenced (irRdR1) plants. We predicted the targets of N. attenuata smRNAs in the genes related to phytohormone signaling (jasmonic acid, JA-Ile, and ethylene) known to mediate resistance responses, and we measured the elicited dynamics of phytohormone biosynthetic transcripts and phytohormone levels in time-course experiments with field- and glasshouse-grown plants. RdR1 silencing severely altered the induced transcript accumulation of 8 of the 10 genes, reduced JA, and enhanced ethylene levels after elicitation. Adding JA completely restored the insect resistance of irRdR1 plants. irRdR1 plants had photosynthetic rates, growth, and reproductive output indistinguishable from that of WT plants, suggesting unaltered primary metabolism. We conclude that the susceptibility of irRdR1 plants to herbivores is due to altered phytohormone signaling and that smRNAs play a central role in coordinating the large-scale transcriptional changes that occur after herbivore attack. Given the diversity of smRNAs that are elicited after insect attack and the recent demonstration of the ability of ingested smRNAs to silence transcript accumulation in lepidopteran larvae midguts, the smRNA responses of plants may also function as direct defenses.

Highly specific gene silencing by artificial miRNAs in rice

Background

Endogenous microRNAs (miRNAs) are potent negative regulators of gene expression in plants and animals. Artificial miRNAs (amiRNAs)–designed to target one or several genes of interest–provide a new and highly specific approach for effective post-transcriptional gene silencing (PTGS) in plants.

Methodology

We devised an amiRNA-based strategy for both japonica and indica type strains of cultivated rice, Oryza sativa. Using an endogenous rice miRNA precursor and customized 21mers, we designed amiRNA constructs targeting three different genes (Pds, Spl11, and Eui1/CYP714D1). Upon constitutive expression of these amiRNAs in the varieties Nipponbare (japonica) and IR64 (indica), the targeted genes are down-regulated by amiRNA-guided cleavage of the transcripts, resulting in the expected mutant phenotypes. The effects are highly specific to the target gene, the transgenes are stably inherited and they remain effective in the progeny.

Conclusion/Significance

Our results not only show that amiRNAs can efficiently trigger gene silencing in a monocot crop, but also that amiRNAs can effectively modulate agronomically important traits in varieties used in modern breeding programs. We provide all software tools and a protocol for the design of rice amiRNA constructs, which can be easily adapted to other crops. The approach is suited for candidate gene validation, comparative functional genomics between different varieties, and for improvement of agronomic performance and nutritional value.

Nematode resistance

Plant-parasitic nematodes are major pests of both temperate and tropical agriculture. Many of the most damaging species employ an advanced parasitic strategy in which they induce redifferentiation of root cells to form specialized feeding structures able to support nematode growth and reproduction over several weeks. Current control measures, particularly in intensive agriculture systems, rely heavily on nematicides but alternative strategies are required as effective chemicals are withdrawn from use. Here, we review the different approaches that are being developed to provide resistance to a range of nematode species. Natural, R gene-based resistance is currently exploited in traditional breeding programmes and research is ongoing to characterize the molecular basis for the observed resistant phenotypes. A number of transgenic approaches hold promise, the best described being the expression of proteinase inhibitors to disrupt nematode digestion. The application of plant-delivered RNA interference (RNAi) to silence essential nematode genes has recently emerged as a potentially valuable resistance strategy.

Insecticide resistance and implications for future aphid management in Australian grains and pastures: a review

Aphids can cause substantial damage to cereals, oilseeds and legumes through direct feeding and through the transmission of plant pathogenic viruses. Aphid-resistant varieties are only available for a limited number of crops. In Australia, growers often use prophylactic sprays to control aphids, but this strategy can lead to non-target effects and the development of insecticide resistance. Insecticide resistance is a problem in one aphid pest of Australian grains in Australia, the green peach aphid (Myzus persicae). Molecular analyses of field-collected samples demonstrate that amplified E4 esterase resistance to organophosphate insecticides is widespread in Australian grains across Australia. Knockdown resistance to pyrethroids is less abundant, but has an increased frequency in areas with known frequent use of these insecticides. Modified acetylcholinesterase resistance to dimethyl carbamates, such as pirimicarb, has not been found in Australia, nor has resistance to imidacloprid. Australian grain growers should consider control options that are less likely to promote insecticide resistance, and have reduced impacts on natural enemies. Research is ongoing in Australia and overseas to provide new strategies for aphid management in the future.