Heterologous expression of the Mi-1.2 gene from tomato confers resistance against nematodes but not aphids in eggplant

Combating Root-Knot Nematodes (Meloidogyne spp.): From Molecular Mechanisms to Resistant Crops

Root-knot nematodes (RKNs; Meloidogyne spp.) are significant plant-parasitic nematodes that cause major yield losses worldwide. With growing awareness of the harmful effects of chemical pesticides on human health and the environment, there is an urgent need to develop alternative strategies for controlling RKN in agricultural fields. In recent years, implementing multiple approaches based on transcriptomics, genomics, and genome engineering, including modern platforms like CRISPR/Cas9, along with traditional genetic mapping, has led to great advances in understanding the plant-RKN interactions and the underlying molecular mechanisms of plant RKN resistance. In this literature review, we synthesize the contributions of relevant studies in this field and discuss key findings. This includes, for instance, transcriptomics studies that helped expand our understanding of plant RKN-resistance mechanisms, the overexpression of plant hormone-related genes, and the silencing of susceptibility genes that lead to plant RKN resistance. This review was conducted by searching scientific sources, including PubMed and Google Scholar, for relevant publications and filtering them using keywords such as RKN-plant defense mechanisms, host-plant resistance against RKN, and genetic mapping for RKN. This knowledge can be leveraged to accelerate the development of RKN-resistant plants and substantially improve RKN management in economically important crops.

All Roads Lead to Rome: Pathways to Engineering Disease Resistance in Plants

Unlike animals, plants are unable to move and lack specialized immune cells and circulating antibodies. As a result, they are always threatened by a large number of microbial pathogens and harmful pests that can significantly reduce crop yield worldwide. Therefore, the development of new strategies to control them is essential to mitigate the increasing risk of crops lost to plant diseases. Recent developments in genetic engineering, including efficient gene manipulation and transformation methods, gene editing and synthetic biology, coupled with the understanding of microbial pathogenicity and plant immunity, both at molecular and genomic levels, have enhanced the capabilities to develop disease resistance in plants. This review comprehensively explains the fundamental mechanisms underlying the tug-of-war between pathogens and hosts, and provides a detailed overview of different strategies for developing disease resistance in plants. Additionally, it provides a summary of the potential genes that can be employed in resistance breeding for key crops to combat a wide range of potential pathogens and pests, including fungi, oomycetes, bacteria, viruses, nematodes, and insects. Furthermore, this review addresses the limitations associated with these strategies and their possible solutions. Finally, it discusses the future perspectives for producing plants with durable and broad-spectrum disease resistance.

Barley MLA3 recognizes the host-specificity effector Pwl2 from Magnaporthe oryzae

Plant nucleotide-binding leucine-rich repeat (NLRs) immune receptors directly or indirectly recognize pathogen-secreted effector molecules to initiate plant defense. Recognition of multiple pathogens by a single NLR is rare and usually occurs via monitoring for changes to host proteins; few characterized NLRs have been shown to recognize multiple effectors. The barley (Hordeum vulgare) NLR gene Mildew locus a (Mla) has undergone functional diversification, and the proteins encoded by different Mla alleles recognize host-adapted isolates of barley powdery mildew (Blumeria graminis f. sp. hordei [Bgh]). Here, we show that Mla3 also confers resistance to the rice blast fungus Magnaporthe oryzae in a dosage-dependent manner. Using a forward genetic screen, we discovered that the recognized effector from M. oryzae is Pathogenicity toward Weeping Lovegrass 2 (Pwl2), a host range determinant factor that prevents M. oryzae from infecting weeping lovegrass (Eragrostis curvula). Mla3 has therefore convergently evolved the capacity to recognize effectors from diverse pathogens.

Marker assisted early generation identification of root knot disease resistant orange tomato segregants with multiple desirable alleles

Enhanced bioavailability of cis-isomers of lycopene, accumulated in orange-fruited tangerine mutant has broadened the scope of nutritional enrichment in tomato. At the same time, advancements in the field of marker assisted selection (MAS) have made the stacking of multiple desirable alleles through molecular breeding to develop superior tomato genotypes possible. Here we report seedling stage MAS from 146 F2 plants, to identify 3 superior performing, root knot disease resistant orange-fruited segregants. In the selected segregants, fruit weight ranged from 39.2 to 54.6 g, pericarp thickness ranged from 4.56 to 6.05 mm and total soluble solid content ranged from 3.65 to 4.87° Brix. Presence of parental diversity allowed identification of the other desirable alleles of the genes governing late blight and mosaic disease resistance, growth habit (determinate and indeterminate) as well as fruit elongation and firmness. Resistance to root knot disease of the selected 3 segregants was also validated through a unique method employing in vitro rooted stem cuttings subjected to artificial inoculation, where the resistant parent and the selected segregants developed no galls in comparison to ~ 24 galls developed in the susceptible parent. The selected segregants form the base for development of multiple disease resistant, nutritionally enriched orange-fruited determinate/indeterminate tomato lines with superior fruit quality. The study also highlights the utility of early generation MAS for detailed characterization of segregants, through which multiple desirable alleles can be precisely targeted and fixed to develop superior tomato genotypes.

Supplementary Information

The online version contains supplementary material available at 10.1007/s12298-023-01361-1.

Cloning and functional analysis of the root-knot nematode resistance gene NtRk1 in tobacco

Tobacco (Nicotiana tabacum L.) is an economically important crop worldwide. Root-knot nematodes (RKNs) are responsible for yield losses in tobacco and other crops, such as tomato, potato, peanut, and soybean. Therefore, screening for resistance genes that can prevent RKN infestation and the associated damage is crucial. However, there is no report of cloning tobacco RKN resistance genes to date. Here, we cloned the tobacco RKN resistance gene NtRk1 from the resistant variety TI706, using rapid amplification of cDNA ends. NtRk1 has high homology with other RKN resistance genes (CaMi in pepper, Mi-1.1 and Mi-1.2 in tomato). Under normal conditions, NtRk1 was barely expressed in the roots; however, following RKN infection, its expression level rapidly increased. Overexpression of NtRk1 in the susceptible cultivar "Changbohuang" enhanced its resistance to Meloidogyne incognita, while RNA interference of NtRk1 in the resistant cultivar K326 resulted in its susceptibility to M. incognita. Moreover, compared with resistant variety K326, we found the salicylic acid and jasmonic acid contents of RNAi plants decreased after inoculation with M. incognita, and confirmed that the function of NtRk1 is related to these phytohormones. These findings indicate that NtRk1 is an RKN resistance gene, which is abundantly expressed in response to RKN infection and may enhance host defense responses by elevating salicylic acid and jasmonic acid levels.

Engineering Resistance against Sclerotinia sclerotiorum Using a Truncated NLR (TNx) and a Defense-Priming Gene

The association of both cell-surface PRRs (Pattern Recognition Receptors) and intracellular receptor NLRs (Nucleotide-Binding Leucine-Rich Repeat) in engineered plants have the potential to activate strong defenses against a broad range of pathogens. Here, we describe the identification, characterization, and in planta functional analysis of a novel truncated NLR (TNx) gene from the wild species Arachis stenosperma (AsTIR19), with a protein structure lacking the C-terminal LRR (Leucine Rich Repeat) domain involved in pathogen perception. Overexpression of AsTIR19 in tobacco plants led to a significant reduction in infection caused by Sclerotinia sclerotiorum, with a further reduction in pyramid lines containing an expansin-like B gene (AdEXLB8) potentially involved in defense priming. Transcription analysis of tobacco transgenic lines revealed induction of hormone defense pathways (SA; JA-ET) and PRs (Pathogenesis-Related proteins) production. The strong upregulation of the respiratory burst oxidase homolog D (RbohD) gene in the pyramid lines suggests its central role in mediating immune responses in plants co-expressing the two transgenes, with reactive oxygen species (ROS) production enhanced by AdEXLB8 cues leading to stronger defense response. Here, we demonstrate that the association of potential priming elicitors and truncated NLRs can produce a synergistic effect on fungal resistance, constituting a promising strategy for improved, non-specific resistance to plant pathogens.

Exploiting Plant-Phytonematode Interactions to Upgrade Safe and Effective Nematode Control

Plant-parasitic nematodes (PPNs) bring about substantial losses of economic crops globally. With the environmental and health issues facing the use of chemical nematicides, research efforts should focus on providing economically effective and safe control methods. The sound exploitation of plant-PPN interactions is fundamental to such efforts. Initially, proper sampling and extraction techniques should be followed to avoid misleading nematode data. Recent evolutions in plant-PPN interactions can make use of diverse non-molecular and molecular approaches to boost plant defenses. Therefore, PPN control and increasing crop yields through single, sequential, dual-purpose, and simultaneous applications of agricultural inputs, including biocontrol agents, should be seriously attempted, especially within IPM schemes. The use of biologicals would ideally be facilitated by production practices to solve related issues. The full investment of such interactions should employ new views of interdisciplinary specialties in the relevant modern disciplines to optimize the PPN management. Having an accurate grasp of the related molecular events will help in developing tools for PPN control. Nonetheless, the currently investigated molecular plant-PPN interactions favoring plant responses, e.g., resistance genes, RNA interference, marker-assisted selection, proteinase inhibitors, chemo-disruptive peptides, and plant-incorporated protectants, are key factors to expanding reliable management. They may be applied on broader scales for a substantial improvement in crop yields.

Understanding Molecular Plant–Nematode Interactions to Develop Alternative Approaches for Nematode Control

Developing control measures of plant-parasitic nematodes (PPNs) rank high as they cause big crop losses globally. The growing awareness of numerous unsafe chemical nematicides and the defects found in their alternatives are calling for rational molecular control of the nematodes. This control focuses on using genetically based plant resistance and exploiting molecular mechanisms underlying plant–nematode interactions. Rapid and significant advances in molecular techniques such as high-quality genome sequencing, interfering RNA (RNAi) and gene editing can offer a better grasp of these interactions. Efficient tools and resources emanating from such interactions are highlighted herein while issues in using them are summarized. Their revision clearly indicates the dire need to further upgrade knowledge about the mechanisms involved in host-specific susceptibility/resistance mediated by PPN effectors, resistance genes, or quantitative trait loci to boost their effective and sustainable use in economically important plant species. Therefore, it is suggested herein to employ the impacts of these techniques on a case-by-case basis. This will allow us to track and optimize PPN control according to the actual variables. It would enable us to precisely fix the factors governing the gene functions and expressions and combine them with other PPN control tactics into integrated management.

Insect pest management in the age of synthetic biology

Arthropod crop pests are responsible for 20% of global annual crop losses, a figure predicted to increase in a changing climate where the ranges of numerous species are projected to expand. At the same time, many insect species are beneficial, acting as pollinators and predators of pest species. For thousands of years, humans have used increasingly sophisticated chemical formulations to control insect pests but, as the scale of agriculture expanded to meet the needs of the global population, concerns about the negative impacts of agricultural practices on biodiversity have grown. While biological solutions, such as biological control agents and pheromones, have previously had relatively minor roles in pest management, biotechnology has opened the door to numerous new approaches for controlling insect pests. In this review, we look at how advances in synthetic biology and biotechnology are providing new options for pest control. We discuss emerging technologies for engineering resistant crops and insect populations and examine advances in biomanufacturing that are enabling the production of new products for pest control.

Biological control of nematodes infecting eggplant in Egypt

Background

Eggplant is of great renown in Egypt and worldwide as it is favorable and high value vegetable crop. Yet, many plant-parasitic nematodes (PPNs) can penetrate the roots of this plant and feed on it, causing heavy losses in its yield. Meloidogyne spp. (root-knot nematodes, RKNs) and Rotylenchulus reniformis rank high among these parasites in Egypt.

Main body of the abstract

This review compiles PPN ecology, biology and economic value from the pest management point of view. Applying production practices and other preventive methods against PPNs should be a priority. Accelerating and unprecedented public concern about excessive use of chemical nematicides leading to health problems, development of pest resistance, and environmental contamination, has boosted interest in developing eco-friendly biologicals as safe substitutional materials. Hence, safe biological control agents (BCAs) are reviewed herein to prove that they are quintessential to nematode control on eggplant in Egypt. Thus, BCAs should reliably operate with other agricultural inputs in integrated pest management programs against eggplant nematodes.

Short conclusion

A few paradigms are shown herein which foster BCA practical use via additive or synergistic relation with compatible inputs such as compost, chemicals, and manure. Upgrading their delivery techniques, beneficial interaction and persistence in fields with nematode-infected eggplants should be broadly tested and greatly disseminated.

Engineering plant disease resistance against biotrophic pathogens

Breeding for disease resistance against microbial pathogens is essential for food security in modern agriculture. Conventional breeding, although widely accepted, is time consuming. An alternative approach is generating crop plants with desirable traits through genetic engineering. The collective efforts of many labs in the past 30 years have led to a comprehensive understanding of how plant immunity is achieved, enabling the application of genetic engineering to enhance disease resistance in crop plants. Here, we briefly review the engineering of disease resistance against biotrophic pathogens using various components of the plant immune system. Recent breakthroughs in immune receptors signaling and systemic acquired resistance (SAR), along with innovations in precise gene editing methods, provide exciting new opportunities for the development of improved environmentally friendly crop varieties that are disease resistant and high-yield.

Soybean Resistance Locus Rhg1 Confers Resistance to Multiple Cyst Nematodes in Diverse Plant Species

Cyst nematodes consistently threaten agricultural production, causing billions of dollars in losses globally. The Rhg1 (resistance to Heterodera glycines 1) locus of soybean (Glycine max) is the most popular resistance source used against soybean cyst nematodes (H. glycines). Rhg1 is a complex locus that has multiple repeats of an ≈30-kilobase segment carrying three genes that contribute to resistance. We investigated whether soybean Rhg1 could function in different plant families, conferring resistance to their respective cyst nematode parasites. Transgenic Arabidopsis thaliana and potato (Solanum tuberosum) plants expressing the three soybean Rhg1 genes were generated. The recipient Brassicaceae and Solanaceae plant species exhibited elevated resistance to H. schachtii and Globodera rostochiensis and to G. pallida, respectively. However, some negative consequences including reduced root growth and tuber biomass were observed upon Rhg1 expression in heterologous species. One of the genes at Rhg1 encodes a toxic version of an alpha-SNAP protein that has been demonstrated to interfere with vesicle trafficking. Using a transient expression assay for Nicotiana benthamiana, native Arabidopsis and potato alpha-SNAPs (soluble NSF [N-ethylamine sensitive factor] attachment protein) were found to compensate for the toxicity of soybean Rhg1 alpha-SNAP proteins. Hence, future manipulation of the balance between Rhg1 alpha-SNAP and the endogenous wild-type alpha-SNAPs (as well as the recently discovered soybean NSF-RAN07) may mitigate impacts of Rhg1 on plant productivity. The multispecies efficacy of soybean Rhg1 demonstrates that the encoded mechanisms can function across plant and cyst nematode species and offers a possible avenue for engineered resistance in diverse crop species.

Tomato Natural Resistance Genes in Controlling the Root-Knot Nematode

The root-knot nematode (RKN) is one of the most dangerous and widespread types of nematodes affecting tomatoes. There are few methods for controlling nematodes in tomatoes. Nature resistance genes (R-genes) are important in conferring resistance against nematodes. These genes that confer resistance to the RKN have already been identified as Mi-1, Mi-2, Mi-3, Mi-4, Mi-5, Mi-6, Mi-7, Mi-8, Mi-9, and Mi-HT. Only five of these genes have been mapped. The major problem is that their resistance breaks down at high temperatures. Some of these genes still work at high temperatures. In this paper, the mechanism and characteristics of these natural resistance genes are summarized. Other difficulties in using these genes in the resistance and how to improve them are also mentioned.

Overexpression of the Prunus sogdiana NBS-LRR Subgroup Gene PsoRPM2 Promotes Resistance to the Root-Knot Nematode Meloidogyne incognita in Tobacco

Root-knot nematodes (RKNs), particularly Meloidogyne incognita, are the most devastating soil-borne pathogens that significantly affect the production of Prunus spp. fruit. RKN infection is difficult to control and consequently causes massive yield losses each year. However, several germplasms of wild Prunus spp. have been shown to display resistance to M. incognita. Consequently, both the isolation of novel plant resistance (R) genes and the characterization of their resistance mechanisms are important strategies for future disease control. R proteins require the co-chaperone protein HSP90-SGT1-RAR1 to achieve correct folding, maturation, and stabilization. Here, we used homologous cloning to isolate the R gene PsoRPM2 from the RKN-resistant species Prunus sogdiana. PsoRPM2 was found to encode a TIR-NB-LRR-type protein and react with significantly elevated PsoRPM2 expression levels in response to RKN infection. Transient expression assays indicated PsoRPM2 to be located in both the cytoplasm and the nucleus. Four transgenic tobacco lines that heterologously expressed PsoRPM2 showed enhanced resistance to M. incognita. Yeast two-hybrid analysis and bimolecular fluorescence complementation analysis demonstrated that both PsoRAR1 and PsoRPM2 interacted with PsoHSP90-1 and PsoSGT1, but not with one another. These results indicate that the observed PsoRPM2-mediated RKN resistance requires both PsoHSP90-1 and PsoSGT1, further suggesting that PsoRAR1 plays a functionally redundant role in the HSP90-SGT1-RAR1 co-chaperone.

Engineering resistance to virus transmission

Engineering plants for resistance to virus transmission by invertebrate vectors has lagged behind other forms of plant protection. Vectors typically transmit more than one virus. Thus, vector resistance could provide a wider range of protection than defenses directed solely against one virus or virus group. We discuss current knowledge of vector-host-virus interactions, the roles of viral gene products in host and vector manipulation, and the effects of semiochemicals on host-vector interactions, and how this knowledge could be employed to disrupt transmission dynamics. We also discuss how resistance to vectors could be generated through genetic engineering or gene editing or indirectly through use of biocontrol using plant-resident viruses that infect vectors.

Expression of a Cystatin Transgene in Eggplant Provides Resistance to Root-knot Nematode, Meloidogyne incognita

Root-knot nematodes (RKN) cause substantial yield decline in eggplant and sustainable management options to minimize crop damage due to nematodes are still limited. A number of genetic engineering strategies have been developed to disrupt the successful plant-nematode interactions. Among them, delivery of proteinase inhibitors from the plant to perturb nematode development and reproduction is arguably the most effective strategy. In the present study, transgenic eggplant expressing a modified rice cystatin (OC-IΔD86) gene under the control of the root-specific promoter, TUB-1, was generated to evaluate the genetically modified nematode resistance. Five putative transformants were selected through PCR and genomic Southern blot analysis. Expression of the cystatin transgene was confirmed in all the events using western blotting, ELISA and qPCR assay. Upon challenge inoculation, all the transgenic events exhibited a detrimental effect on RKN development and reproduction. The best transgenic line (a single copy event) showed 78.3% inhibition in reproductive success of RKN. Our results suggest that cystatins can play an important role for improving nematode resistance in eggplant and their deployment in gene pyramiding strategies with other proteinase inhibitors could ultimately enhance crop yield.

Transient Expression of Candidatus Liberibacter Asiaticus Effector Induces Cell Death in Nicotiana benthamiana

Candidatus Liberibacter asiaticus "Las" is a phloem-limited bacterial plant pathogen, and the most prevalent species of Liberibacter associated with citrus huanglongbing (HLB), a devastating disease of citrus worldwide. Although, the complete sequence of the Las genome provides the basis for studying functional genomics of Las and molecular mechanisms of Las-plant interactions, the functional characterization of Las effectors remains a slow process since remains to be cultured. Like other plant pathogens, Las may deliver effector proteins into host cells and modulate a variety of host cellular functions for their infection progression. In this study, we identified 16 putative Las effectors via bioinformatics, and transiently expressed them in Nicotiana benthamiana. Diverse subcellular localization with different shapes and aggregation patterns of the effector candidates were revealed by UV- microscopy after transient expression in leaf tissue. Intriguingly, one of the 16 candidates, Las5315mp (mature protein), was localized in the chloroplast and induced cell death at 3 days post inoculation (dpi) in N. benthamiana. Moreover, Las5315mp induced strong callose deposition in plant cells. This study provides new insights into the localizations and potential roles of these Las effectors in planta.

Host genetic resistance to root-knot nematodes, Meloidogyne spp., in Solanaceae: from genes to the field

Root-knot nematodes (RKNs) heavily damage most solanaceous crops worldwide. Fortunately, major resistance genes are available in a number of plant species, and their use provides a safe and economically relevant strategy for RKN control. From a structural point of view, these genes often harbour NBS-LRR motifs (i.e. a nucleotide binding site and a leucine rich repeat region near the carboxy terminus) and are organised in syntenic clusters in solanaceous genomes. Their introgression from wild to cultivated plants remains a challenge for breeders, although facilitated by marker-assisted selection. As shown with other pathosystems, the genetic background into which the resistance genes are introgressed is of prime importance to both the expression of the resistance and its durability, as exemplified by the recent discovery of quantitative trait loci conferring quantitative resistance to RKNs in pepper. The deployment of resistance genes at a large scale may result in the emergence and spread of virulent nematode populations able to overcome them, as already reported in tomato and pepper. Therefore, careful management of the resistance genes available in solanaceous crops is crucial to avoid significant reduction in the duration of RKN genetic control in the field. From that perspective, only rational management combining breeding and cultivation practices will allow the design and implementation of innovative, sustainable crop production systems that protect the resistance genes and maintain their durability.

Bean Common Mosaic Virus and Bean Common Mosaic Necrosis Virus: Relationships, Biology, and Prospects for Control

The closely related potyviruses Bean common mosaic virus (BCMV) and Bean common mosaic necrosis virus (BCMNV) are major constraints on common bean (Phaseolus vulgaris) production. Crop losses caused by BCMV and BCMNV impact severely not only on commercial scale cultivation of this high-value crop but also on production by smallholder farmers in the developing world, where bean serves as a key source of dietary protein and mineral nutrition. In many parts of the world, progress has been made in combating BCMV through breeding bean varieties possessing the I gene, a dominant gene conferring resistance to most BCMV strains. However, in Africa, and in particular in Central and East Africa, BCMNV is endemic and this presents a serious problem for deployment of the I gene because this virus triggers systemic necrosis (black root disease) in plants possessing this resistance gene. Information on these two important viruses is scattered throughout the literature from 1917 onward, and although reviews on resistance to BCMV and BCMNV exist, there is currently no comprehensive review on the biology and taxonomy of BCMV and BCMNV. In this chapter, we discuss the current state of our knowledge of these two potyviruses including fundamental aspects of classification and phylogeny, molecular biology, host interactions, transmission through seed and by aphid vectors, geographic distribution, as well as current and future prospects for the control of these important viruses.

RNA-Seq reveals a xenobiotic stress response in the soybean aphid, Aphis glycines, when fed aphid-resistant soybean

BACKGROUND

While much recent research has expanded our understanding of the molecular interactions between aphids and their host plants, it is lacking for the soybean aphid, Aphis glycines. Since its North American invasion, A. glycines has become one of the most damaging insect pests on this important crop. Five soybean genes for host plant resistance to A. glycines have been identified, but populations of A. glycines have already adapted to overcome these resistance genes. Understanding the molecular interactions between resistant soybean and A. glycines can provide clues to its adaptation mechanisms. Here, we used RNA-Sequencing to compare and contrast A. glycines gene expression when fed resistant (Rag1) and susceptible soybean.

RESULTS

Combining results from a previous A. glycines transcriptome, we generated 64,860 high quality transcripts, totaling 41,151,086 bases. Statistical analysis revealed 914 genes with significant differential expression. Most genes with higher expression in A. glycines on resistant plants (N = 352) were related to stress and detoxification such as cytochrome P450s, glutathione-S-transferases, carboxyesterases, and ABC transporters. A total of 562 genes showed lower transcript abundance in A. glycines on resistant plants. From our extensive transcriptome data, we also identified genes encoding for putative salivary effector proteins (N = 73). Among these, 6 effector genes have lower transcript abundance in A. glycines feeding on resistant soybean.

CONCLUSIONS

Overall, A. glycines exhibited a pattern typical of xenobiotic challenge, thereby validating antibiosis in Rag1, presumably mediated through toxic secondary metabolites. Additionally, this study identified many A. glycines genes and gene families at the forefront of its molecular interaction with soybean. Further investigation of these genes in other biotypes may reveal adaptation mechanisms to resistant plants.

Engineering plants for aphid resistance: current status and future perspectives

The current status of development of transgenic plants for improved aphid resistance, and the pros and cons of different strategies are reviewed and future perspectives are proposed. Aphids are major agricultural pests that cause significant yield losses of crop plants each year. Excessive dependence on insecticides for aphid control is undesirable because of the development of insecticide resistance, the potential negative effects on non-target organisms and environmental pollution. Transgenic plants engineered for resistance to aphids via a non-toxic mode of action could be an efficient alternative strategy. In this review, the distribution of major aphid species and their damages on crop plants, the so far isolated aphid-resistance genes and their applications in developments of transgenic plants for improved aphid resistance, and the pros and cons of these strategies are reviewed and future perspectives are proposed. Although the transgenic plants developed through expressing aphid-resistant genes, manipulating plant secondary metabolism and plant-mediated RNAi strategy have been demonstrated to confer improved aphid resistance to some degree. So far, no aphid-resistant transgenic crop plants have ever been commercialized. This commentary is intended to be a helpful insight into the generation and future commercialization of aphid-resistant transgenic crops in a global context.

Arabidopsis dual resistance proteins, both RPS4 and RRS1, are required for resistance to bacterial wilt in transgenic Brassica crops

Bacterial wilt phytopathogen Ralstonia solanacearum is a serious soil-borne disease that attacks several economically important plants worldwide, including Brassicaceae. Previous studies indicate that recognition of avirulence (Avr)-effector PopP2 by resistance (R) protein, RRS1-R, and physical interaction between RRS1-R and PopP2 in the nucleus are required for resistance. Of late, we showed that a pair of Arabidopsis thaliana TIR-NLR proteins, RRS1 and RPS4, function together in disease resistance against multiple pathogen isolates. Here, we report that dual R proteins, RRS1 and RPS4, from A. thaliana ecotype Wassilewskija confer resistance to bacterial wilt in transgenic Brassica crops. For practical applications, this finding may provide a new strategy for developing disease resistant plants that express R genes from other plants.

Molecular characterization of ZzR1 resistance gene from Zingiber zerumbet with potential for imparting Pythium aphanidermatum resistance in ginger

Soft rot disease caused by the oomycete Pythium aphanidermatum (Edson) Fitzp. is the most economically significant disease of ginger (Zingiber officinale Rosc.) in tropical countries. All available ginger cultivars are susceptible to this pathogen. However a wild ginger relative viz., Zingiber zerumbet L. Smith, was identified as a potential soft rot resistance donor. In the present study, a putative resistance (R) gene designated, ZzR1 was isolated and characterized from Z. zerumbet using sequence information from Zingiber RGCs identified in our earlier experiments. Analysis of the 2280 bp segment revealed a 2157 bp open reading frame (ORF) encoding a putative cytoplasmically localized protein. The deduced ZzR1 protein shared high homology with other known R-genes belonging to the CC-NBS-LRR (coiled coil-nucleotide binding site-leucine rich repeat) class and had a calculated molecular weight of 84.61kDa. Real-time PCR analysis of ZzR1 transcription in Z. zerumbet following pathogen infection demonstrated activation at 3 hpi thus suggesting an involvement of ZzR1 in Z. zerumbet defense mechanism. Although many R-genes have been characterized from different taxa, none of them will help in the development of resistant ginger cultivars owing to the phenomenon of "Restricted Taxonomic Functionality" (RTF). Thus ZzR1 gene characterized from the resistant wild Zingiber accession represents a valuable genomic resource for ginger improvement programs. This first report on R-gene isolation from the Zingiber secondary gene pool is pivotal in designing strategies for engineering resistance in ginger, which is otherwise not amenable to conventional improvement programs owing to sexual reproduction barriers.

Impacts of resistance gene genetics, function, and evolution on a durable future

Studies on resistance gene function and evolution lie at the confluence of structural and molecular biology, genetics, and plant breeding. However, knowledge from these disparate fields has yet to be extensively integrated. This review draws on ideas and information from these different fields to elucidate the influences driving the evolution of different types of resistance genes in plants and the concurrent evolution of virulence in pathogens. It provides an overview of the factors shaping the evolution of recognition, signaling, and response genes in the context of emerging functional information along with a consideration of the new opportunities for durable resistance enabled by high-throughput DNA sequencing technologies.

Aphid resistance in Brassica crops: challenges, biotechnological progress and emerging possibilities

Aphids, (Hemiptera: Aphidoidea) a nefarious insect pest of Brassicaceae members including major vegetable and oilseed crops have coevolved with their host plant and emerged as most economically important insect pest of crop Brassicas. Their atypical feeding mechanism and unusual reproductive biology made them intractable to control below economic threshold level of damage to the crops. To a large extent aphid infestation is controlled by spraying agrochemicals of systemic mode of action and rarely by biological control. Use of systemic insecticides is highly cost intensive as well poses bigger threat of their incorporation in dietary chain. Breeding for genetic resistance against aphids has not been possible owing to the non-availability of resistance source within the crossable germplasms and lack of knowledge of the genetics of the trait. Genetic engineering with insect resistant transgenes seems to be the only potential avenue to address this difficult-to-accomplish breeding objective. Some success had been achieved in terms of developing aphid resistant cultivars through genetic engineering however, commercial utilization of such crops are still awaited. Thus protection of crops against aphids necessarily requires more research to identify either more effective insecticidal transgenes or biological phenomenon that can usher to new mechanism of resistance. The present review is an attempt to highlight the current status and possible avenues to develop aphid resistance in Brassicaceae crops.

Tritrophic interactions among Macrosiphum euphorbiae aphids, their host plants and endosymbionts: investigation by a proteomic approach

The Mi-1.2 gene in tomato confers resistance against certain clones of the potato aphid (Macrosiphum euphorbiae). This study used 2D-DIGE coupled with protein identification by MALDI-TOF-MS to compare the proteome patterns of avirulent and semivirulent potato aphids and their bacterial endosymbionts on resistant (Mi-1.2+) and susceptible (Mi-1.2-) tomato lines. Avirulent aphids had low survival on resistant plants, whereas the semivirulent clone could colonize these plants. Eighty-two protein spots showed significant quantitative differences among the four treatment groups, and of these, 48 could be assigned putative identities. Numerous structural proteins and enzymes associated with primary metabolism were more abundant in the semivirulent than in the avirulent aphid clone. Several proteins were also up-regulated in semivirulent aphids when they were transferred from susceptible to resistant plants. Nearly 25% of the differentially regulated proteins originated from aphid endosymbionts and not the aphid itself. Six were assigned to the primary endosymbiont Buchnera aphidicola, and 5 appeared to be derived from a Rickettsia-like secondary symbiont. These results indicate that symbiont expression patterns differ between aphid clones with differing levels of virulence, and are influenced by the aphids' host plant. Potentially, symbionts may contribute to differential adaptation of aphids to host plant resistance.

Resistance to aphid vectors of virus disease

The majority of plant viruses rely on vectors for their transmission and completion of their life cycle. These vectors comprise a diverse range of life forms including insects, nematodes, and fungi with the most common of these being insects. The geographic range of many of these vectors is continually expanding due to climate change. The viruses that they carry are therefore also expanding their range to exploit novel and naïve plant hosts. There are many forms of naturally occurring vector resistance ranging from broad nonhost resistance to more specific types of inducible resistance. Understanding and exploiting the many and varied forms of natural resistance to virus vectors is therefore extremely important for current and future agricultural production systems. To demonstrate the range and extent of these resistance mechanisms, this chapter will primarily focus on aphids to highlight key developments appropriate to plant-insect-virus interactions.

Mechanisms of recognition in dominant R gene mediated resistance

One branch of plant innate immunity is mediated through what is traditionally known as race-specific or gene-for-gene resistance wherein the outcome of an attempted infection is determined by the genotypes of both the host and the pathogen. Dominant plant disease resistance (R) genes confer resistance to a variety of biotrophic pathogens, including viruses, encoding corresponding dominant avirulence (Avr) genes. R genes are among the most highly variable plant genes known, both within and between populations. Plant genomes encode hundreds of R genes that code for NB-LRR proteins, so named because they posses nucleotide-binding (NB) and leucine-rich repeat (LRR) domains. Many matching pairs of NB-LRR and Avr proteins have been identified as well as cellular proteins that mediate R/Avr interactions, and the molecular analysis of these interactions have led to the formulation of models of how products of R genes recognize pathogens. Data from multiple NB-LRR systems indicate that the LRR domains of NB-LRR proteins determine recognition specificity. However, recent evidence suggests that NB-LRR proteins have co-opted cellular recognition co-factors that mediate interactions between Avr proteins and the N-terminal domains of NB-LRR proteins.

Vegetables

The conscious promotion of health by an appropriate, balanced diet has become an important social request. Vegetable thereby possesses a special importance due to its high vitamin, mineral and dietary fibre content. Major progress has been made over the past few years in the transformation of vegetables. The expression of several genes has been inhibited by sense gene suppression, and new traits caused by new gene constructs are stably inherited. This chapter reviews advances in various traits such as disease resistance, abiotic stress tolerance, quality improvement, pharmaceutical and industrial application. Results are presented from most important vegetable families, like Solanaceae, Brassicaceae, Fabaceae, Cucurbitaceae, Asteraceae, Apiaceae, Chenopodiaceae and Liliaceae. Although many research trends in this report are positive, only a few transgenic vegetables have been released from confined into precommercial testing or into use.

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.

Gene expression of different wheat genotypes during attack by virulent and avirulent Hessian fly (Mayetiola destructor) larvae

Wheat and its relatives possess a number of resistance (R) genes specific for the Hessian fly (HF) [Mayetiola destructor (Say)]. HF populations overcome R gene resistance by evolving virulence. Virulent HF larvae manipulate the plant to produce a nutritionally enhanced feeding tissue and, probably, also suppress plant defense responses. Using two wheat R genes, H9 and H13, and three HF strains (biotypes) differing in virulence for H9 and H13, we conducted a genome-wide transcriptional analysis of gene expression during compatible interactions with virulent larvae and incompatible interactions with avirulent larvae. During both types of interactions, a large number of genes (>1,000) showed alterations in gene expression. Analysis of genes with known functions revealed that major targets for differential regulation were genes that encoded defense proteins or enzymes involved in the phenylpropanoid, cell wall, and lipid metabolism pathways. A combination of the enhancement of antibiosis defense, the evasion of nutrient metabolism induction, and the fortification and expansion of the cell wall are likely the collective mechanism for host-plant resistance observed during incompatible interactions. To overcome this resistance, virulent larvae appeared to suppress antibiosis defense while inducing nutrient metabolism, weakening cell wall, and inhibiting plant growth.

Plant-aphid interactions: molecular and ecological perspectives

Many aphids are major agricultural pests because of their unparalleled reproductive capacity and their ability to manipulate host plant physiology. Aphid population growth and its impact on plant fitness are strongly influenced by interactions with other organisms, including plant pathogens, endophytes, aphid endosymbionts, predators, parasitoids, ants, and other herbivores. Numerous molecular and genomic resources have recently been developed to identify sources of aphid resistance in plants, as well as potentially novel targets for control in aphids. Moreover, the same model systems that are used to explore direct molecular interactions between plants and aphids can be utilized to study the ecological context in which they occur.

Sources of natural resistance to plant viruses: status and prospects

SUMMARY Globally, virus diseases are common in agricultural crops and have a major agronomic impact. They are countered through the deployment of genetic resistance against the virus, or through the use of a range of farming practices based upon the propagation of virus-free plant material and the exclusion of the virus vectors from the growing crop. We review here the current status of our knowledge of natural virus resistance genes, and consider the future prospects for the deployment of these genes against virus infection.