Exploiting natural variation to identify insect-resistance genes

Melanaphis sacchari/sorghi complex: current status, challenges and integrated strategies for managing the invasive sap-feeding insect pest of sorghum

Melanaphis sacchari (Zehntner;Hemiptera: Aphididae), sugarcane aphid (SCA), is an invasive phloem-feeder found worldwide with a wide host range of economically important plants including sorghum and sugarcane. Given its high reproductive capacity and ability to rapidly spread over long distances, SCA presents challenges for effective control, leading to substantial economic losses. Recent studies have identified two multiloci SCA genotypes specialized in feeding on sugarcane (MLL-D) and sorghum (MLL-F) in the USA, which raises concerns as the USA is the second largest sorghum-producing country. This has encouraged research towards identifying these two biotypes where some research has stated them as two species; MLL-D clade to be M. sacchari and MLL-F clade to be M. sorghi Theobald (Hemiptera: Aphididae), sorghum aphid (SA). This review aims at compiling research progress that has been made on understanding the SCA/SA species complex. Furthermore, this review also highlights a wide range of management strategies against SCA/SA that includes both biological and chemical methods. In addition, the review emphasizes studies examining host plant resistance to understand and evaluate the role of R-genes and phytohormones such as jasmonic acid, salicylic acid and ethylene against SCA. Beside this, plant volatiles and other secondary metabolites such as flavonoids, terpenes and phytanes are also explored as potential control agents. Being an invasive pest, a single management tactic is inadequate to control SCA population and hence, integrated pest management practices incorporating physical, cultural and biological control methods should be implemented with exclusive chemical control as a last resort, which this review examines in detail. © 2024 The Author(s). Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

Assessment of Different Conventional and Biofortified Wheat Genotypes Based on Biology and Damage Pattern of Rhyzopertha dominica and Trogoderma granarium

The lesser grain borer, Rhyzopertha dominica (F.) (Coleoptera: Bostrichidae) and khapra beetle, Trogoderma granarium E. (Coleoptera: Dermestidae) are primary stored-grain insect pests. Differences in certain biological and physical parameters of both pest species and wheat genotypes were investigated under laboratory conditions. Zinc (Zn)-biofortified (Zincol-2016 and Akbar-2019) and conventional (Arooj-2022, Nawab-2021, Dilkash-2021, Bhakkar Star-2019) wheat genotypes were used in this study. Zn-biofortified genotypes outperformed the conventional ones, with significant differences observed in fecundity, percent adult emergence, total developmental duration, percent grain damage, and weight loss of both insect species. The results further revealed that the fecundity of R. dominica and T. granarium were lowest on Akbar-2019 in both the free-choice test (42.50 and 33.17) and no-choice test (35.50 and 32.50), respectively. Similarly, percent adult emergence of both insect species was also lowest on Akbar-2019 in both the free-choice test (69.78 and 70.28%) and no-choice test (67.38 and 70.71%). The total developmental period also showed significant variation among the tested genotypes. The longest developmental period was recorded in Akbar-2019, i.e., 44.33 and 58.83 days, for R. dominica and T. granarium, respectively. Similarly, percent grain damage (13.23 and 10.33%) and weight loss (3.62 and 2.12%) were found to be minimum in Akbar-2019 for both pest species, respectively. Additionally, a positive correlation was observed between grain moisture content and damage parameters, suggesting that the higher moisture content may aggravate the percent grain damage and weight loss. These findings indicate that the nutritional qualities of Zn-biofortified wheat genotypes negatively affected the development of both insect species; thus, it can be an efficacious approach not only for ensuring food security but also for protecting grains against storage pests.

Rice varietal resistance to the vector Sogatella furcifera hinders transmission of Southern rice black-streaked dwarf virus

BACKGROUND

The Southern rice black-streaked dwarf virus (SRBSDV) transmitted by Sogatella furcifera constitutes a threat to sustainable rice production. However, most rice varieties are highly vulnerable to SRBSDV, whereas the occurrence of the viral disease varies significantly under field conditions. This study aimed to evaluate the potential of rice varietal resistance to S. furcifera in reducing SRBSDV transmission.

RESULTS

Among the five rice varieties, Zhongzheyou8 and Deyou108 exhibited high resistance to S. furcifera, Baixiangnuo33 was susceptible, and TN1 and Diantun502 were highly susceptible. The S. furcifera generally showed non-preference for and low feeding on the Zhongzheyou8 and Deyou108 plants, which may explain the resistance of these varieties to S. furcifera. Transmission of SRBSDV by S. furcifera was significantly impaired on the resistant varieties, both inoculation and acquisition rates were much lower on Zhongzheyou8 than on TN1. The short durations of S. furcifera salivation and phloem-related activities and the low S. furcifera feeding amount may explain the reduced SRBSDV inoculation and acquisition rates associated with Zhongzheyou8. Spearman's rank correlation revealed a significant negative correlation between S. furcifera resistance and SRBSDV transmission among the tested varieties.

CONCLUSION

The results indicate that rice varietal resistance to the vector S. furcifera hinders SRBSDV transmission, which is largely associated with the host plant selection and feeding behaviors of the vector. The current findings shed light on the management of the SRBSDV viral disease through incorporation of S. furcifera resistant rice varieties in the management protocol. © 2024 Society of Chemical Industry.

Volatiles of different resistant cotton varieties mediate the host preference of Mirid bug Apolygus lucorum

Cotton, a crucial economic crop, is also the preferred host plant of the mirid bug Apolygus lucorum. In our previous field experiments, we found that cotton cultivars Kelin 08-15 and BR-S-10 (healthy and herbivore-damaged plants) exhibit distinct attraction and repellence to A. lucorum, respectively. However, the key plant volatiles determining attraction or repulsion effects remain unknown. Here, we investigated the volatiles emitted by these two cotton cultivars before and after herbivore infestation. We found that susceptible Kelin 08-15 emitted a greater diversity and quantity of volatiles than those of BR-S-10, with herbivore-damaged cottons releasing more volatile substances. Electroantennogram (EAG) recordings further revealed that 15 representative volatiles identified above could elicited electrophysiological responses in female and male A. lucorum antennae. Among them, behavioral assays showed that two compounds, 1,3-Diethylbenzene and 4-Ethylbenzaldehyde, exhibited attractive properties, whereas six volatiles including Hexyl Acrylate, Cumene, 2,4-Dimethylstyrene, Eucalyptol, Linalool and Butyl Acrylate demonstrated repellent effects on A. lucorum. Taken together, our findings suggest the critical role of volatile compounds in mediating bug-plant interactions and provide a foundation for the development of strategies to prevent and control of A. lucorum in cotton fields.

Aphid-Induced Volatiles and Subsequent Attraction of Natural Enemies Varies among Sorghum Cultivars

The production of herbivore-induced plant volatiles (HIPVs) is a type of indirect defense used by plants to attract natural enemies and reduce herbivory by insect pests. In many crops little is known about genotypic variation in HIPV production or how this may affect natural enemy attraction. In this study, we identified and quantified HIPVs produced by 10 sorghum (Sorghum bicolor) cultivars infested with a prominent aphid pest, the sorghum aphid (Melanaphis sorghi Theobald). Volatiles were collected using dynamic headspace sampling techniques and identified and quantified using GC-MS. The total amounts of volatiles induced by the aphids did not differ among the 10 cultivars, but overall blends of volatiles differed significantly in composition. Most notably, aphid herbivory induced higher levels of methyl salicylate (MeSA) emission in two cultivars, whereas in four cultivars, the volatile emissions did not change in response to aphid infestation. Dual-choice olfactometer assays were used to determine preference of the aphid parasitoid, Aphelinus nigritus, and predator, Chrysoperla rufilabris, between plants of the same cultivar that were un-infested or infested with aphids. Two aphid-infested cultivars were preferred by natural enemies, while four other cultivars were more attractive to natural enemies when they were free of aphids. The remaining four cultivars elicited no response from parasitoids. Our work suggests that genetic variation in HIPV emissions greatly affects parasitoid and predator attraction to aphid-infested sorghum and that screening crop cultivars for specific predator and parasitoid attractants has the potential to improve the efficacy of biological control.

A protocol for increased throughput phenotyping of plant resistance to the pollen beetle

BACKGROUND

Improving crop resistance to insect herbivores is a major research objective in breeding programs. Although genomic technologies have increased the speed at which large populations can be genotyped, breeding programs still suffer from phenotyping constraints. The pollen beetle (Brassicogethes aeneus) is a major pest of oilseed rape for which no resistant cultivar is available to date, but previous studies have highlighted the potential of white mustard as a source of resistance and introgression of this resistance appears to be a promising strategy. Here we present a phenotyping protocol allowing mid-throughput (i.e., increased throughput compared to current methods) acquisition of resistance data, which could then be used for genetic mapping of QTLs.

RESULTS

Contrasted white mustard genotypes were selected from an initial field screening and then evaluated for their resistance under controlled conditions using a standard phenotyping method on entire plants. We then upgraded this protocol for mid-throughput phenotyping, by testing two alternative methods. We found that phenotyping on detached buds did not provide the same resistance contrasts as observed with the standard protocol, in contrast to the phenotyping protocol with miniaturized plants. This protocol was then tested on a large panel composed of hundreds of plants. A significant variation in resistance among genotypes was observed, which validates the large-scale application of this new phenotyping protocol.

CONCLUSION

The combination of this mid-throughput phenotyping protocol and white mustard as a source of resistance against the pollen beetle offers a promising avenue for breeding programs aiming to improve oilseed rape resistance. © 2022 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

Plant resistance against whitefly and its engineering

Plants face constant threats from insect herbivores, which limit plant distribution and abundance in nature and crop productivity in agricultural ecosystems. In recent decades, the whitefly Bemisia tabaci, a group of phloem-feeding insects, has emerged as pests of global significance. In this article, we summarize current knowledge on plant defenses against whitefly and approaches to engineer plant resistance to whitefly. Physically, plants deploy trichome and acylsugar-based strategies to restrain nutrient extraction by whitefly. Chemically, toxic secondary metabolites such as terpenoids confer resistance against whitefly in plants. Moreover, the jasmonate (JA) signaling pathway seems to be the major regulator of whitefly resistance in many plants. We next review advances in interfering with whitefly-plant interface by engineering of plant resistance using conventional and biotechnology-based breeding. These breeding programs have yielded many plant lines with high resistance against whitefly, which hold promises for whitefly control in the field. Finally, we conclude with an outlook on several issues of particular relevance to the nature and engineering of plant resistance against whitefly.

Comparative Transcriptome Analysis Reveals the Molecular Basis of Brassica napus in Response to Aphid Stress

Rapeseed is a globally important economic crop that can be severely impacted by aphids. However, our understanding of rapeseed resistance to aphid stress is very limited. In this study, we analyzed the resistance characteristics of the low aphid-susceptible variety APL01 and the highly aphid-susceptible variety Holly in response to aphid stress. APL01 had a more significant inhibitory effect on aphid proliferation compared with Holly during the early stage of inoculation, whereas Holly showed stronger tolerance to aphid stress compared with APL01 during the later stage of inoculation. Through transcriptome, physiological, and gene expression analyses, it was revealed that chitinase activity, catalase activity, calcium signal transduction, and activation of systemic acquired resistance might be involved in aphid resistance in B. napus. The degree of inhibition of photosynthesis in plants under aphid stress directly determines the tolerance of B. napus to aphid stress. Furthermore, four promising candidate genes were screened from eight genes related to rapeseed response to biotic stress through RT-qPCR analysis of gene expression levels. These research findings represent an important step forward in understanding the resistance of rapeseed to aphid stress and provide a solid foundation for the cloning of genes responsible for this resistance.

Genome-wide association study reveals novel SNPs and genes in Gossypium hirsutum underlying Aphis gossypii resistance

A. gossypii resistance showed great variability in G. hirsutum varieties. One hundred and seventy-six SNPs associated with A. gossypii resistance were identified using GWAS. Four candidate resistance genes were functionally validated. Aphis gossypii is an economically important sap-feeding pest and is widely distributed in the world's cotton-producing regions. Identification of cotton genotypes and developing cultivars with improved A. gossypii resistance (AGR) is essential and desirable for sustainable agriculture. In the present study, A. gossypii was offered no choice but to propagate on 200 Gossypium hirsutum accessions. A relative aphid reproduction index (RARI) was used to evaluate the AGR, which showed large variability in cotton accessions and was classified into 6 grades. A significantly positive correlation was found between AGR and Verticillium wilt resistance. A total of 176 SNPs significantly associated with the RARI were identified using GWAS. Of these, 21 SNPs could be repeatedly detected in three replicates. Cleaved amplified polymorphic sequence, a restriction digestion-based genotyping assay, was developed using SNP1 with the highest observed -log10(P-value). Four genes within the 650 kb region of SNP1 were further identified, including GhRem (remorin-like), GhLAF1 (long after far-red light 1), GhCFIm25 (pre-mRNA cleavage factor Im 25 kDa subunit) and GhPMEI (plant invertase/pectin methylesterase inhibitor superfamily protein). The aphid infection could induce their expression and showed a significant difference between resistant and susceptible cotton varieties. Silencing of GhRem, GhLAF1 or GhCFIm25 could significantly increase aphid reproduction on cotton seedlings. Silencing of GhRem significantly reduced callose deposition, which is reasonably believed to be the cause for the higher AGR. Our results provide insights into understanding the genetic regulation of AGR in cotton and suggest candidate germplasms, SNPs and genes for developing cultivars with improved AGR.

Allelic variation of terpene synthases drives terpene diversity in the wild species of the Freesia genus

Terpene synthases (TPSs) play pivotal roles in conferring the structural diversity of terpenoids, which are mainly emitted from flowers, whereas the genetic basis of the release of floral volatile terpenes remains largely elusive. Though quite similar in sequence, TPS allelic variants still function divergently, and how they drive floral terpene diversity in closely related species remains unknown. Here, TPSs responsible for the floral scent of wild Freesia species were characterized, and the functions of their natural allelic variants, as well as the causal amino acid residues, were investigated in depth. Besides the 8 TPSs previously reported in modern cultivars, 7 additional TPSs were functionally evaluated to contribute to the major volatiles emitted from wild Freesia species. Functional characterization of allelic natural variants demonstrated that allelic TPS2 and TPS10 variants changed the enzymatic capacity while allelic TPS6 variants drove the diversity of floral terpene products. Further residue substitution analysis revealed the minor residues determining the enzyme catalytic activity and product specificity. The clarification of TPSs in wild Freesia species reveals that allelic TPS variants evolved differently to determine the interspecific floral volatile terpenes in the genus and might be used for modern cultivar improvement.

Transcriptomic profiling of near-isogenic lines reveals candidate genes for a significant locus conferring metribuzin resistance in wheat

BACKGROUND

Weeds reduce wheat yields in dryland farming systems. Herbicides such as metribuzin are commonly used to control weeds. However, wheat has a narrow safety margin against metribuzin. Standing crops such as wheat with weeds in the same field can also be killed by the same dose of metribuzin. Therefore, it is important to identify metribuzin resistance genes and understand the resistance mechanism in wheat for sustainable crop production. A previous study identified a significant metribuzin resistance wheat QTL, Qsns.uwa.4 A.2, explaining 69% of the phenotypic variance for metribuzin resistance.

RESULTS

Two NIL pairs with the most contrasting performance in the metribuzin treatment and different in genetic backgrounds were compared using RNA sequence analysis, identifying nine candidate genes underlying Qsns.uwa.4 A.2 responsible for metribuzin resistance. Quantitative RT-qPCR further validated the candidate genes, with TraesCS4A03G1099000 (nitrate excretion transporter), TraesCS4A03G1181300 (aspartyl protease), and TraesCS4A03G0741300 (glycine-rich proteins) identified as key factors for metribuzin resistance.

CONCLUSION

Identified markers and key candidate genes can be used for selecting metribuzin resistance in wheat.

CRISPR/Cas9 for Insect Pests Management: A Comprehensive Review of Advances and Applications

Insect pests impose a serious threat to agricultural productivity. Initially, for pest management, several breeding approaches were applied which have now been gradually replaced by genome editing (GE) strategies as they are more efficient and less laborious. CRISPR/Cas9 (Clustered Regularly Interspaced Short Palindromic Repeat/CRISPR-associated system) was discovered as an adaptive immune system of bacteria and with the scientific advancements, it has been improvised into a revolutionary genome editing technique. Due to its specificity and easy handling, CRISPR/Cas9-based genome editing has been applied to a wide range of organisms for various research purposes. For pest control, diverse approaches have been applied utilizing CRISPR/Cas9-like systems, thereby making the pests susceptible to various insecticides, compromising the reproductive fitness of the pest, hindering the metamorphosis of the pest, and there have been many other benefits. This article reviews the efficiency of CRISPR/Cas9 and proposes potential research ideas for CRISPR/Cas9-based integrated pest management. CRISPR/Cas9 technology has been successfully applied to several insect pest species. However, there is no review available which thoroughly summarizes the application of the technique in insect genome editing for pest control. Further, authors have highlighted the advancements in CRISPR/Cas9 research and have discussed its future possibilities in pest management.

(Z)-3-hexenol primes callose deposition against whitefly-mediated begomovirus infection in tomato

Rapid callose accumulation has been shown to mediate defense in certain plant-virus interactions. Exposure to the green leaf volatile (Z)-3-hexenol (Z-3-HOL) can prime tomato (Solanum lycopersicum) for an enhanced defense against subsequent infection by whitefly-transmitted Tomato yellow leaf curl virus (TYLCV). However, the molecular mechanisms affecting Z-3-HOL-induced resistance are poorly understood. Here, we explored the mechanisms underlying Z-3-HOL-induced resistance against whitefly-transmitted TYLCV infection and the role of callose accumulation during this process. Tomato plants pre-treated with Z-3-HOL displayed callose priming upon whitefly infestation. The callose inhibitor 2-deoxy-d-glucose abolished Z-3-HOL-induced resistance, confirming the importance of callose in this induced resistance. We also found that Z-3-HOL pre-treatment enhanced salicylic acid levels and activated sugar signaling in tomato upon whitefly infestation, which increased the expression of the cell wall invertase gene Lin6 to trigger augmented callose deposition against TYLCV infection resulting from whitefly transmission. Using virus-induced gene silencing, we demonstrated the Lin6 expression is relevant for sugar accumulation mediated callose priming in restricting whitefly-transmitted TYLCV infection in plants that have been pre-treated with Z-3-HOL. Moreover, Lin6 induced the expression of the callose synthase gene Cals12, which is also required for Z-3-HOL-induced resistance of tomato against whitefly-transmitted TYLCV infection. These findings highlight the importance of sugar signaling in the priming of callose as a defense mechanism in Z-3-HOL-induced resistance of tomato against whitefly-transmitted TYLCV infection. The results will also increase our understanding of defense priming can be useful for the biological control of viral diseases.

Common resistance mechanisms are deployed by plants against sap-feeding herbivorous insects: insights from a meta-analysis and systematic review

Despite their abundance and economic importance, the mechanism of plant resistance to sap-feeding insects remains poorly understood. Here we deploy meta-analysis and data synthesis methods to evaluate the results from electrophysiological studies describing feeding behaviour experiments where resistance mechanisms were identified, focussing on studies describing host-plant resistance and non-host resistance mechanisms. Data were extracted from 108 studies, comprising 41 insect species across eight insect taxa and 12 host-plant families representing over 30 species. Results demonstrate that mechanisms deployed by resistant plants have common consequences on the feeding behaviour of diverse insect groups. We show that insects feeding on resistant plants take longer to establish a feeding site and have their feeding duration suppressed two-fold compared with insects feeding on susceptible plants. Our results reveal that traits contributing towards resistant phenotypes are conserved across plant families, deployed against taxonomically diverse insect groups, and that the underlying resistance mechanisms are conserved. These findings provide a new insight into plant-insect interaction and highlight the need for further mechanistic studies across diverse taxa.

Estimating the Susceptibility of Four Wheat Cultivars to the Saw-Toothed Grain Beetle Oryzaephilus surinamensis (L.) (Coleoptera: Silvanidae)

The quality and quantity of wheat are severely affected by insect infestation during storage. Excessive use of chemical pesticides has resulted in several environmental issues, in addition to the emergence of pesticide-resistant strains. Extensive research has been conducted to determine sustainable alternatives, such as the selection and cultivation of insect-resistant crop varieties. The aim of this study was to determine the degree of susceptibility of four wheat cultivars, namely, Albelad, Aldwasair, Australia, and Najran (NJ), to Oryzaephilus surinamensis (L.) using four parameters: F1 progeny, length of developmental period, Dobie's index of susceptibility (DI), and percentage weight loss (PWL). Albelad was highly susceptible (DI =17.44) to O. surinamensis, followed by Najran (DI =8.42) and Australia (DI =6.18) (susceptible and moderately susceptible, respectively), whereas Aldwasair was moderately resistant (DI =4.67). Furthermore, there were significant positive correlations between DI and the mean number of F1 progeny (R² = 0.98) and between DI and grain weight loss (R² = 0.95). However, a significantly negative correlation (R² = −0.86) was observed between DI and beetle developmental period. These findings will aid the development of breeding programs to maintain the quality and quantity of wheat grains during storage.

Resistance Management through Brassica Crop–TuMV–Aphid Interactions: Retrospect and Prospects

Turnip mosaic virus (TuMV) is an important threat to the yield and quality of brassica crops in China, and has brought serious losses to brassica crops in the Far East, including China and the north. Aphids (Hemiptera, Aphidoidea) are the main mediators of TuMV transmission in field production, and not only have strong virus transmission ability (small individuals, strong concealment, and strong fecundity), but are also influenced by the environment, making them difficult to control. Till now, there have been few studies on the resistance to aphids in brassica crops, which depended mainly on pesticide control in agriculture production. However, the control effect was temporarily effective, which also brought environmental pollution, pesticide residues in food products, and destroyed the ecological balance. This study reviews the relationship among brassica crop–TuMV, TuMV–aphid, and brassica crop–aphid interactions, and reveals the influence factors (light, temperature, and CO2 concentration) on brassica crop–TuMV–aphid interactions, summarizing the current research status and main scientific problems about brassica crop–TuMV–aphid interactions. It may provide theoretical guidance for opening up new ways of aphid and TuMV management in brassica crops.

Nematode RALF-Like 1 Targets Soybean Malectin-Like Receptor Kinase to Facilitate Parasitism

Soybean [Glycine max (L.) Merr. ] is one of the most strategical oilseed crops that provides sustainable source of protein and oil worldwide. Cultivation of soybean is severely affected by root-knot nematode (RKN). However, the mechanism of RKN parasitism to soybeans is largely unknown. In this study, we identify GmLMM1, which encodes a homolog of FERONIA-like receptor kinase in soybean, as a susceptible gene toward nematode. Mutations of GmLMM1 exhibit enhanced resistance against the RKN Meloidogyne incognita. RNA-sequencing (RNA-seq) analysis reveals a similar differential expression pattern for genes regulated by GmLMM1 (Gmlmm1 vs. wild-type) and M. incognita (M. incognita vs. mock), supporting the role of GmLMM1 in M. incognita infection. Unlike FERONIA in Arabidopsis, GmLMM1 specifically binds to MiRALF1 and AtRALF23 that suppress plant immunity, but not MiRALF3 and AtRALF1. Moreover, we found that the single-nucleotide polymorphism (SNP) in GmLMM1 leads to the natural resistance against RKNs in soybeans. Collectively, these findings uncover GmLMM1 as a susceptible target of nematode RALF-like 1 and provide new genetic resource for nematode resistant breeding.

Transcriptomic and metabolomic changes triggered by Macrosiphum rosivorum in rose (Rosa longicuspis)

BACKGROUND

Rose is one of the most popular flowers in the wold. Its field growth and quality are negatively affected by aphids. However, the defence mechanisms used by rose plants against aphids are unclear. Therefore, to understand the defence mechanism of rose under aphid stress, transcriptome and metabolome techniques were used to investigate the regulation mechanism in R. longicuspis infected with M. rosivorum.

RESULT

In our study, after inoculation with M. rosivorum, M. rosivorum quickly colonized R. longicuspis. A total of 34,202 genes and 758 metabolites were detected in all samples. Under M. rosivorum stress, R. longicuspis responded by MAPK cascades, plant hormone signal transduction pathway activation, RlMYBs and RlERFs transcription factors expression and ROS production. Interestingly, the 'brassinosteroid biosynthesis' pathway was significantly enriched in A3 d-vs.-A5 d. Further analysis showed that M. rosivorum induced the biosynthesis of secondary metabolites such as terpenoids, tannins and phenolic acids, among others. Importantly, the 'glutathione metabolic' and 'glucosinolate biosynthesis' pathways were significantly enriched, which involved in the rose against aphids.

CONCLUSION

Our study provides candidate genes and metabolites for Rosa defence against aphids. This study provides a theoretical basis for further exploring the molecular regulation mechanism of rose aphid resistance and aphid resistance breeding in the future.

Unconventional routes to developing insect-resistant crops

Concerns over widespread use of insecticides and heightened insect pest virulence under climate change continue to fuel the need for environmentally safe and sustainable control strategies. However, to develop such strategies, a better understanding of the molecular basis of plant-pest interactions is still needed. Despite decades of research investigating plant-insect interactions, few examples exist where underlying molecular mechanisms are well characterized, and even rarer are cases where this knowledge has been successfully applied to manage harmful agricultural pests. Consequently, the field appears to be static, urgently needing shifts in approaches to identify novel mechanisms by which insects colonize plants and plants avoid insect pressure. In this perspective, we outline necessary steps for advancing holistic methodologies that capture complex plant-insect molecular interactions. We highlight novel and underexploited approaches in plant-insect interaction research as essential routes to translate knowledge of underlying molecular mechanisms into durable pest control strategies, including embracing microbial partnerships, identifying what makes a plant an unsuitable host, capitalizing on tolerance of insect damage, and learning from cases where crop domestication and agronomic practices enhance pest virulence.

Potato Zebra Chip: An Overview of the Disease, Control Strategies, and Prospects

Potato (Solanum tuberosum L.) is an important food crop worldwide. As the demand for fresh and processed potato products is increasing globally, there is a need to manage and control devastating diseases such as zebra chip (ZC). ZC disease causes major yield losses in many potato-growing regions and is associated with the fastidious, phloem-limited bacterium Candidatus Liberibacter solanacearum (CLso) that is vectored by the potato-tomato psyllid (Bactericera cockerelli Šulc). Current management measures for ZC disease mainly focus on chemical control and integrated pest management strategies of the psyllid vector to limit the spread of CLso, however, they add to the costs of potato production. Identification and deployment of CLso and/or the psyllid resistant cultivars, in combination with integrated pest management, may provide a sustainable long-term strategy to control ZC. In this review, we provide a brief overview of the ZC disease, epidemiology, current management strategies, and potential new approaches to manage ZC disease in the future.

In-field screening for host plant resistance to Delia radicum and Brevicoryne brassicae within selected rapeseed cultivars and new interspecific hybrids

Rapeseed (Brassica napus) can be attacked by a wide range of pests, for example, cabbage root fly (Delia radicum) and cabbage aphid (Brevicoryne brassicae). One of the best methods of pest management is breeding for insect resistance in rapeseed. Wild genotypes of Brassicaceae and rapeseed cultivars can be used as a source of resistance. In 2017, 2018, and 2019, field trials were performed to assess the level of resistance to D. radicum and B. brassicae within 53 registered rapeseed cultivars and 31 interspecific hybrid combinations originating from the resources of the Department of Genetics and Plant Breeding of Poznań University of Life Sciences (PULS). The level of resistance varied among genotypes and years. Only one hybrid combination and two B. napus cultivars maintained high level of resistance in all tested years, i.e., B. napus cv. Jet Neuf × B. carinata – PI 649096, Galileus, and Markolo. The results of this research indicate that resistance to insects is present in Brassicaceae family and can be transferred to rapeseed cultivars. The importance of continuous improvement of rapeseed pest resistance and the search for new sources of resistance is discussed; furthermore, plans for future investigations are presented.

Glucosinolate Biosynthesis and the Glucosinolate–Myrosinase System in Plant Defense

Insect pests represent a major global challenge to important agricultural crops. Insecticides are often applied to combat such pests, but their use has caused additional challenges such as environmental contamination and human health issues. Over millions of years, plants have evolved natural defense mechanisms to overcome insect pests and pathogens. One such mechanism is the production of natural repellents or specialized metabolites like glucosinolates. There are three types of glucosinolates produced in the order Brassicales: aliphatic, indole, and benzenic glucosinolates. Upon insect herbivory, a “mustard oil bomb” consisting of glucosinolates and their hydrolyzing enzymes (myrosinases) is triggered to release toxic degradation products that act as insect deterrents. This review aims to provide a comprehensive summary of glucosinolate biosynthesis, the “mustard oil bomb”, and how these metabolites function in plant defense against pathogens and insects. Understanding these defense mechanisms will not only allow us to harness the benefits of this group of natural metabolites for enhancing pest control in Brassicales crops but also to transfer the “mustard oil bomb” to non-glucosinolate producing crops to boost their defense and thereby reduce the use of chemical pesticides.

An Acylsucrose-Producing Tomato Line Derived from the Wild Species Solanum pimpinellifolium Decreases Fitness of the Whitefly Trialeurodes vaporariorum

Simple Summary

The greenhouse whitefly, Trialeurodes vaporariorum is an insect pest of many plant crops including tomato and is especially harmful because it is a vector for a number of plant viral diseases. In this paper, an improved tomato line bred to produce glandular trichomes that exudate the deterrent compounds acylsucroses, which was introgressed from a wild tomato species, was demonstrated to decrease fitness of the insect and showed as a means for controlling the pests and, indirectly, could be an aid to reduce virus transmission to tomato plants.

Abstract

A combination of biological control and host plant resistance would be desirable for optimally controlling the greenhouse whitefly, Trialeurodes vaporariorum in tomato crops. Whitefly settlement preference, oviposition, and survivorship were evaluated on ABL 10-4 and ‘Moneymaker’, two nearly-isogenic tomato lines with, and without, whitefly-resistance traits based on type IV leaf glandular trichomes derived from the tomato wild species Solanum pimpinellifolium, respectively. Significantly reduced preference of T. vaporariorum adult whiteflies for ABL 10-4 leaves was observed. Moreover, T. vaporariorum altered its abaxial–adaxial settling performance on leaves of ABL 10-4 plants. A significantly lower tendency to settle on abaxial leaf surface was observed in ABL 10-4 compared to Moneymaker plants. Furthermore, T. vaporariorum deposited fewer eggs and exhibited a significantly reduced egg to adult survivorship in ABL 10-4 than in Moneymaker plants. Therefore, reduced fitness and distorted performance were observed for T. vaporariorum on ABL 10-4 tomato plants supporting that type IV leaf glandular trichomes might protect them from this pest and, indirectly, from the viruses it transmits.

Measurement of biorational effect of imidacloprid on some aphids spp. as well as on wheat (Triticum aestivum L.) using biochemical parameters and ISSR-PCR

Aphid species are the most common insect pest infesting wheat plants. The LC values of imidacloprid against aphid spp., ISSR-PCR molecular markers and biochemical parameters in wheat plant in the present study were carried out. The LC₅₀ of imidacloprid on Rhopalosiphum maidis, Rhopalosiphum padi, Sitobion avenae, and Schzaphis graminum were 71.626, 85.853, 91.05, and 115.806, respectively. While, the biochemical parameters in treated wheat plants including; total protein, RNA, and DNA contents increased in all experimental groups. Moreover, total carbohydrates and soluble sugar also increased in treated wheat seedlings. Also, the genetic diversity studies revealed that among six ISSR primers produced 36 amplified DNA fragments in a molecular weight range of (480-1545 bp). About 28 fragments were monomorphic, and 8 fragments were polymorphic. It can be concluded that the effectiveness of imidacloprid against sucking insects in treated wheat plants may due to few genetic variations and some metabolic changes in the Triticum. aestivum. PRACTICAL APPLICATIONS: The current study indicated that the potent efficacy of imidacloprid against different aphid spp. which were the most insect pest in wheat plants. As well as the application of imidacloprid as seed dressing insecticide, significantly increase carbohydrate, protein, amino acids, and RNA content in T. aestivum plants. While there was no significant increase in DNA in treated one. The recorded increase in these biochemical parameters may due to the increase in the gene expressions as a result of the application of such pesticides. These results explain the extra mechanism of the effect of imidacloprid against sucking insects. Also, the conducted results explain the effect of imidacloprid as a seed dressing insecticide in wheat crop protection from sucking insects and increasing seed germination, increasing seed wheat and number of seeds/spike.

Distinct defensive activity of phenolics and phenylpropanoid pathway genes in different cotton varieties toward chewing pests

Identifying the maximum level of inherent defense against harmful insects in natural variation among wild lineages of crop plants may result in high yield tolerant varieties and reducing use of chemical insecticides. However, knowledge of natural cotton genotypes with high insect-resistance is still indistinguishable at the biochemical or molecular level. In the present study, different cultivated Gossypium hirsutum varieties were evaluated for their inherent insect-tolerance against two major cottons chewing pests. The insect bio-assay identified two tolerant and one susceptible cotton varieties. The study demonstrates difference in phenolic acids, proanthocyanidin and tannin accumulation in tolerant and susceptible varieties. The post-infestation of chewing pests increases transcript level of the phenylpropanoid pathway genes were detected in tolerant varieties as compared to the susceptible varieties. Altogether, chewing pest-tolerance level in cotton varieties is the cumulative effect of enhanced phenylpropanoid pathway genes expression and secondary metabolite leading to defense responses to conventional host plant.

Aphid populations showing differential levels of virulence on Capsicum accessions

The green peach aphid, Myzus persicae, is one of the most threatening pests in pepper cultivation and growers would benefit from resistant varieties. Previously, we identified two Capsicum accessions as susceptible and three as resistant to M. persicae using an aphid population originating from the Netherlands (NL). Later on we identified an aphid population originating from a different geographical region (Switserland, SW) that was virulent on all tested Capsicum accessions. The objective of the current work is to describe in detail different aspects of the interaction between two aphid populations and two selected Capsicum accessions (one that was susceptible [PB2013046] and one that was resistant [PB2013071] to population NL), including biochemical processes involved. Electrical penetration graph (EPG) recordings showed similar feeding activities for both aphid populations on PB2013046. On accession PB2013071 the aphid population SW was able to devote significantly more time to phloem ingestion than population NL. We also studied plant defense response and found that plants of accession PB2013046 could not induce an accumulation of reactive oxygen species and callose formation after infestation with either aphid population. However, plants of PB2013071 induced a stronger defense response after infestation by population NL than after infestation by population SW. Based on these results, population SW of M. persicae seems to have overcome the resistance of PB2013071 that prevented feeding of aphids from NL population. The potential mechanism by which SW population overcomes the resistance is discussed.

Aphid resistance in Capsicum maps to a locus containing LRR-RLK gene analogues

A QTL for aphid resistance on pepper chromosome 2 was identified and validated. This QTL affects aphid survival and reproduction, and was fine mapped to a locus containing LRR-RLK analogues. Myzus persicae is one of the most threatening insect pests that adversely affects pepper (Capsicum) cultivation. Resistance to aphids was previously identified in Capsicum baccatum. This study aimed at elucidating the genetics of aphid resistance in C. baccatum. A QTL analysis was carried out for M. persicae resistance in an F₂ population derived from an intraspecific cross between a highly resistant plant and a susceptible plant. Survival and reproduction were used as resistance parameters. Interval mapping detected two QTLs affecting aphid survival (Rmpas-1) and reproduction (Rmprp-1), respectively, both localized in the same area and sharing the same top marker on chromosome 2. Use of this marker as co-factor in multiple-QTL mapping analysis revealed a second, minor QTL (Rmprp-2) only affecting aphid reproduction, on chromosome 4. Fine mapping confirmed the effects of Rmpas-1 and Rmprp-1 and narrowed the major QTL Rmprp-1 down to a genomic region of 96 kb which is predicted to encode four analogues of resistance genes of the receptor-like kinase family containing a leucine-rich repeat domain (LRR-RLKs). This work provides not only initial information for breeding aphid-resistant pepper varieties, but also forms the basis for future molecular analysis of gene(s) involved in aphid resistance.

Defence gene expression and phloem quality contribute to mesophyll and phloem resistance to aphids in wild barley

Aphids, including the bird cherry-oat aphid (Rhopalosiphum padi), are significant agricultural pests. The wild relative of barley, Hordeum spontaneum 5 (Hsp5), has been described to be partially resistant to R. padi, with this resistance proposed to involve higher thionin and lipoxygenase gene expression. However, the specificity of this resistance to aphids and its underlying mechanistic processes are unknown. In this study, we assessed the specificity of Hsp5 resistance to aphids and analysed differences in aphid probing and feeding behaviour on Hsp5 and a susceptible barley cultivar (Concerto). We found that partial resistance in Hsp5 to R. padi extends to two other aphid pests of grasses. Using the electrical penetration graph technique, we show that partial resistance is mediated by phloem- and mesophyll-based resistance factors that limit aphid phloem ingestion. To gain insight into plant traits responsible for partial resistance, we compared non-glandular trichome density, defence gene expression, and phloem composition of Hsp5 with those of the susceptible barley cultivar Concerto. We show that Hsp5 partial resistance involves elevated basal expression of thionin and phytohormone signalling genes, and a reduction in phloem quality. This study highlights plant traits that may contribute to broad-spectrum partial resistance to aphids in barley.

Proteomic analysis by iTRAQ-PRM provides integrated insight into mechanisms of resistance in pepper to Bemisia tabaci (Gennadius)

BACKGROUND

The Bemisia tabaci is a major leaf feeding insect pest to pepper (Capsicum annuum), causing serious damage to pepper growth and yield. It is particularly important to study the mechanism of pepper resistance to B. tabaci, and to breed and promote the varieties of pepper resistant to B. tabaci. However, very limited molecular mechanism is available about how plants perceive and defend themselves from the destructive pest. Proteome technologies have provided an idea method for studying plant physiological processes in response to B. tabaci.

RESULTS

Here, a highly resistant genotype and a highly susceptible genotype were exposed to B. tabaci feeding for 48 h to explore the defense mechanisms of pepper resistance to B. tabaci. The proteomic differences between both genotypes were compared using isobaric tag for relative and absolute quantification (iTRAQ). The quantitative data were validated by parallel reaction monitoring (PRM). The results showed that 37 differential abundance proteins (DAPs) were identified in the RG (resistant genotype), while 17 DAPs were identified in the SG (susceptible genotype) at 48 h after B. tabaci feeding. 77 DAPs were identified when comparing RG with SG without feeding. The DAP functions were determined for the classification of the pathways, mainly involved in redox regulation, stress response, protein metabolism, lipid metabolism and carbon metabolism. Some candidate DAPs are closely related to B. tabaci resistance such as annexin D4-like (ANN4), calreticulin-3 (CRT3), heme-binding protein 2-like (HBP1), acidic endochitinase pcht28-like (PR3) and lipoxygenase 2 (LOX2).

CONCLUSIONS

Taken together, this study indicates complex resistance-related events in B. tabaci interaction, provides novel insights into the molecular mechanism underlying the response of plant to B. tabaci, and identifies some candidate proteins against B. tabaci attack.

Improving plant-resistance to insect-pests and pathogens: The new opportunities through targeted genome editing

The advantages of high input agriculture are fading away due to degenerating soil health and adverse effects of climate change. Safeguarding crop yields in the changing environment and dynamics of pest and pathogens, has posed new challenges to global agriculture. Thus, integration of new technologies in crop improvement has been imperative for achieving the breeding objectives in faster ways. Recently, enormous potential of genome editing through engineered nucleases has been demonstrated in plants. Continuous refinements of the genome editing tools have increased depth and breadth of their applications. So far, genome editing has been demonstrated in more than fifty plant species. These include model species like Arabidopsis, as well as important crops like rice, wheat, maize etc. Particularly, CRISPR/Cas9 based two component genome editing system has been facile with wider applicability. Potential of genome editing has unfurled enormous possibilities for engineering diverse agronomic traits including durable resistance against insect-pests and pathogens. Novel propositions of developing insect and pathogen resistant crops by genome editing include altering the effector-target interaction, knocking out of host-susceptibility genes, engineering synthetic immune receptor eliciting broad spectrum resistance, uncoupling of antagonistic action of defense hormones etc. Alternatively, modification of insect genomes has been used either to create gene drive or to counteract resistance to various insecticides. The distinct advantage of genome editing system is that it can knock out specific target region in the genome without leaving the unwanted vector backbone. In this article, we have reviewed the novel opportunities offered by the genome editing technologies for developing insect and pathogen resistant crop-types, their future prospects and anticipated challenges.

QTL mapping of insect resistance components of Solanum galapagense

QTLs for insect resistance parameters, trichome type IV development, and more than 200 non-volatile metabolites, including 76 acyl sugars, all co-locate at the end of Chromosome 2 of Solanum galapagense. Host plant resistance is gaining importance as more and more insecticides are being banned due to environmental concerns. In tomato, resistance towards insects is found in wild relatives and has been attributed to the presence of glandular trichomes and their specific phytochemical composition. In this paper, we describe the results from a large-scale QTL mapping of data from whitefly resistance tests, trichome phenotyping and a comprehensive metabolomics analysis in a recombinant inbred line population derived from a cross between the cultivated Solanum lycopersicum and the wild relative S. galapagense, which is resistant to a range of pest insects. One major QTL (Wf-1) was found to govern the resistance against two different whitefly species. This QTL co-localizes with QTLs for the presence of trichomes type IV and V, as well as all 76 acyl sugars detected and about 150 other non-volatile phytochemicals, including methyl esters of the flavonols myricetin and quercetin. Based on these results, we hypothesize that Wf-1 is regulating the formation of glandular trichome type IV on the leaf epidermis, enabling the production and accumulation of bioactive metabolites in this type of trichomes.

Combining QTL mapping with transcriptome and metabolome profiling reveals a possible role for ABA signaling in resistance against the cabbage whitefly in cabbage

Whiteflies are among the world's most significant agricultural pests and chemical insecticides are extensively used to reduce crop damage to acceptable levels. However, nearly all insecticides pose a threat to the environment and alternative control methods, such as breeding of crop varieties that are inherently insect-resistant, are needed. Previously, a strong source of plant-age dependent resistance to the cabbage whitefly (Aleyrodes proletella) has been identified in the modern white cabbage (Brassica oleracea var. capitata) variety Rivera. However, nothing is known about the molecular mechanisms or the genes involved in this resistance. In the present study, a multidisciplinary approach combining transcriptome and metabolome profiling with genetic mapping was used to identify the molecular players of whitefly resistance in cabbage. Transcriptome profiles of young (susceptible) and older (resistant) Rivera plants were analyzed using RNA sequencing. While many genes involved in general processes were differentially expressed between both ages, several defense-related processes were overrepresented in the transcriptome profile of older plants. Hormone measurements revealed that jasmonic acid (JA) levels decreased upon whitefly infestation at both plant ages. Interestingly, abscisic acid (ABA) levels showed contrasting effects in response to whitefly infestation: ABA levels were reduced in young plants but induced in older plants upon whitefly feeding. Auxin levels were significantly lower in older plants compared with young plants, independent of whitefly presence, while glucosinolate levels were higher. Additionally, whitefly performance was monitored in an F2 population derived from a cross between Rivera and the susceptible white cabbage variety Christmas Drumhead. Significant QTL intervals were mapped on chromosome 2 and 9 for oviposition rate and whitefly adult survival, respectively. Several genes that were higher expressed in older plants and located in the identified QTL intervals were orthologous to Arabidopsis genes that have been related to ABA signaling, suggesting a role for ABA in the regulation of resistance towards whiteflies. Our results show that combining different omics approaches is a useful strategy to identify candidate genes underlying insect resistance.

Broad spectrum insect resistance and metabolites in close relatives of the cultivated tomato

Wild relatives of tomato possess effective means to deal with several pests, among which are a variety of insects. Here we studied the presence of resistance components against Trialeurodes vaporariorum, Myzus persicae, Frankliniella occidentalis, and Spodoptera exigua in the Lycopersicon group of Solanum section Lycopersicon by means of bioassays and comprehensive metabolite profiling. Broad spectrum resistance was found in Solanum galapagense and a few accessions of S. pimpinellifolium. Resistance to the sap sucking insects may be based on the same mechanism, but different from the caterpillar resistance. Large and highly significant differences in the leaf metabolomes were found between S. galapagense, containing type IV trichomes, and its closest relative S. cheesmaniae, which lacks type IV trichomes. The most evident differences were the relatively high levels of different methylated forms of the flavonoid myricetin and many acyl sucrose structures in S. galapagense. Possible candidate genes regulating the production of these compounds were identified in the Wf-1 QTL region of S. galapagense, which was previously shown to confer resistance to the whitefly B. tabaci. The broad spectrum insect resistance identified in S. galapagense will be very useful to increase resistance in cultivated tomato.

Prospects of herbivore egg-killing plant defenses for sustainable crop protection

Due to a growing demand of food production worldwide, new strategies are suggested to allow for sustainable production of food with minimal effects on natural resources. A promising alternative to the application of chemical pesticides is the implementation of crops resistant to insect pests. Plants produce compounds that are harmful to a wide range of attackers, including insect pests; thus, exploitation of their natural defense system can be the key for the development of pest-resistant crops. Interestingly, some plants possess a unique first line of defense that eliminates the enemy before it becomes destructive: egg-killing. Insect eggs can trigger (1) direct defenses, mostly including plant cell tissue growth or cell death that lead to eggs desiccating, being crushed or falling off the plant or (2) indirect defenses, plant chemical cues recruiting natural enemies that kill the egg or hatching larvae (parasitoids). The consequences of plant responses to eggs are that insect larvae do not hatch or that they are impeded in development, and damage to the plant is reduced. Here, we provide an overview on the ubiquity and evolutionary history of egg-killing traits within the plant kingdom including crops. Up to now, little is known on the mechanisms and on the genetic basis of egg-killing traits. Making use of egg-killing defense traits in crops is a promising new way to sustainably reduce losses of crop yield. We provide suggestions for new breeding strategies to grow egg-killing crops and improve biological control.

SNP-markers in Allium species to facilitate introgression breeding in onion

BACKGROUND

Within onion, Allium cepa L., the availability of disease resistance is limited. The identification of sources of resistance in related species, such as Allium roylei and Allium fistulosum, was a first step towards the improvement of onion cultivars by breeding. SNP markers linked to resistance and polymorphic between these related species and onion cultivars are a valuable tool to efficiently introgress disease resistance genes. In this paper we describe the identification and validation of SNP markers valuable for onion breeding.

RESULTS

Transcriptome sequencing resulted in 192 million RNA seq reads from the interspecific F1 hybrid between A. roylei and A. fistulosum (RF) and nine onion cultivars. After assembly, reliable SNPs were discovered in about 36 % of the contigs. For genotyping of the interspecific three-way cross population, derived from a cross between an onion cultivar and the RF (CCxRF), 1100 SNPs that are polymorphic in RF and monomorphic in the onion cultivars (RF SNPs) were selected for the development of KASP assays. A molecular linkage map based on 667 RF-SNP markers was constructed for CCxRF. In addition, KASP assays were developed for 1600 onion-SNPs (SNPs polymorphic among onion cultivars). A second linkage map was constructed for an F2 of onion x A. roylei (F2(CxR)) that consisted of 182 onion-SNPs and 119 RF-SNPs, and 76 previously mapped markers. Markers co-segregating in both the F2(CxR) and the CCxRF population were used to assign the linkage groups of RF to onion chromosomes. To validate usefulness of these SNP markers, QTL mapping was applied in the CCxRF population that segregates for resistance to Botrytis squamosa and resulted in a QTL for resistance on chromosome 6 of A. roylei.

CONCLUSIONS

Our research has more than doubled the publicly available marker sequences of expressed onion genes and two onion-related species. It resulted in a detailed genetic map for the interspecific CCxRF population. This is the first paper that reports the detection of a QTL for resistance to B. squamosa in A. roylei.

Quantitative resistance against Bemisia tabaci in Solanum pennellii: Genetics and metabolomics

The whitefly Bemisia tabaci is a serious threat in tomato cultivation worldwide as all varieties grown today are highly susceptible to this devastating herbivorous insect. Many accessions of the tomato wild relative Solanum pennellii show a high resistance towards B. tabaci. A mapping approach was used to elucidate the genetic background of whitefly-resistance related traits and associated biochemical traits in this species. Minor quantitative trait loci (QTLs) for whitefly adult survival (AS) and oviposition rate (OR) were identified and some were confirmed in an F2 BC1 population, where they showed increased percentages of explained variance (more than 30%). Bulked segregant analyses on pools of whitefly-resistant and -susceptible F2 plants enabled the identification of metabolites that correlate either with resistance or susceptibility. Genetic mapping of these metabolites showed that a large number of them co-localize with whitefly-resistance QTLs. Some of these whitefly-resistance QTLs are hotspots for metabolite QTLs. Although a large number of metabolite QTLs correlated to whitefly resistance or susceptibility, most of them are yet unknown compounds and further studies are needed to identify the metabolic pathways and genes involved. The results indicate a direct genetic correlation between biochemical-based resistance characteristics and reduced whitefly incidence in S. pennellii.

Automated video tracking of thrips behavior to assess host-plant resistance in multiple parallel two-choice setups

BACKGROUND

Piercing-sucking insects cause severe damage in crops. Breeding for host-plant resistance can significantly reduce the yield losses caused by these insects, but host-plant resistance is a complex trait that is difficult to phenotype quickly and reliably. Current phenotyping methods mainly focus on labor-intensive and time-consuming end-point measurements of plant fitness. Characterizing insect behavior as a proxy for host-plant resistance could be a promising time-saving alternative to end-point measurements.

RESULTS

We present a phenotyping platform that allows screening for host-plant resistance against Western flower thrips (WFT, Frankliniella occidentalis (Pergande)) in a parallel two-choice setup using automated video tracking of thrips behavior. The platform was used to establish host-plant preference of WFT with a large plant population of 345 wild Arabidopsis accessions and the method was optimized with two extreme accessions from this population that differed in resistance towards WFT. To this end, the behavior of 88 WFT individuals was simultaneously tracked in 88 parallel two-choice arenas during 8 h. Host-plant preference of WFT was established both by the time thrips spent on either accession and various behavioral parameters related to movement (searching) and non-movement (feeding) events.

CONCLUSION

In comparison to 6-day end-point choice assays with whole plants or detached leaves, the automated video-tracking choice assay developed here delivered similar results, but with higher time- and resource efficiency. This method can therefore be a reliable and effective high throughput phenotyping tool to assess host-plant resistance to thrips in large plant populations.

A Jasmonate-Inducible Defense Trait Transferred from Wild into Cultivated Tomato Establishes Increased Whitefly Resistance and Reduced Viral Disease Incidence

Whiteflies damage tomatoes mostly via the viruses they transmit. Cultivated tomatoes lack many of the resistances of their wild relatives. In order to increase protection to its major pest, the whitefly Bemisia tabaci and its transmitted Tomato Yellow Leaf Curl Virus (TYLCV), we introgressed a trichome-based resistance trait from the wild tomato Solanum pimpinellifolium into cultivated tomato, Solanum lycopersicum. The tomato backcross line BC₅S₂ contains acylsucrose-producing type-IV trichomes, unlike cultivated tomatoes, and exhibits increased, yet limited protection to whiteflies at early development stages. Treatment of young plants with methyl jasmonate (MeJA) resulted in a 60% increase in type-IV trichome density, acylsucrose production, and enhanced resistance to whiteflies, leading to 50% decrease in the virus disease incidence compared to cultivated tomato. Using transcriptomics, metabolite analysis, and insect bioassays we established the basis of this inducible resistance. We found that MeJA activated the expression of the genes involved in the biosynthesis of the defensive acylsugars in young BC₅S₂ plants leading to enhanced chemical defenses in their acquired type-IV trichomes. Our results show that not only constitutive but also these inducible defenses can be transferred from wild into cultivated crops to aid sustainable protection, suggesting that conventional breeding strategies provide a feasible alternative to increase pest resistance in tomato.

Genetic variation in NIN1 and C/VIF1 genes is significantly associated with Populus angustifolia resistance to a galling herbivore, Pemphigus betae

The identification of genes associated with ecologically important traits provides information on the potential genetic mechanisms underlying the responses of an organism to its natural environment. In this study, we investigated the genetic basis of host plant resistance to the gall-inducing aphid, Pemphigus betae, in a natural population of 154 narrowleaf cottonwoods (Populus angustifolia). We surveyed genetic variation in two genes putatively involved in sink-source relations and a phenology gene that co-located in a previously identified quantitative trait locus for resistance to galling. Using a candidate gene approach, three major findings emerged. First, natural variation in tree resistance to galling was repeatable. Sampling of the same tree genotypes 20 years after the initial survey in 1986 show that 80% of the variation in resistance was due to genetic differences among individuals. Second, we identified significant associations at the single nucleotide polymorphism and haplotype levels between the plant neutral invertase gene NIN1 and tree resistance. Invertases are a class of sucrose hydrolyzing enzymes and play an important role in plant responses to biotic stress, including the establishment of nutrient sinks. These associations with NIN1 were driven by a single nucleotide polymorphism (NIN1_664) located in the second intron of the gene and in an orthologous sequence to two known regulatory elements. Third, haplotypes from an inhibitor of invertase (C/VIF1) were significantly associated with tree resistance. The identification of genetic variation in these two genes provides a starting point to understand the possible genetic mechanisms that contribute to tree resistance to gall formation. We also build on previous work demonstrating that genetic differences in sink-source relationships of the host influence the ability of P. betae to manipulate the flow of nutrients and induce a nutrient sink.

Novel Genes Affecting the Interaction between the Cabbage Whitefly and Arabidopsis Uncovered by Genome-Wide Association Mapping

Plants have evolved a variety of ways to defend themselves against biotic attackers. This has resulted in the presence of substantial variation in defense mechanisms among plants, even within a species. Genome-wide association (GWA) mapping is a useful tool to study the genetic architecture of traits, but has so far only had limited exploitation in studies of plant defense. Here, we study the genetic architecture of defense against the phloem-feeding insect cabbage whitefly (Aleyrodes proletella) in Arabidopsis thaliana. We determined whitefly performance, i.e. the survival and reproduction of whitefly females, on 360 worldwide selected natural accessions and subsequently performed GWA mapping using 214,051 SNPs. Substantial variation for whitefly adult survival and oviposition rate (number of eggs laid per female per day) was observed between the accessions. We identified 39 candidate SNPs for either whitefly adult survival or oviposition rate, all with relatively small effects, underpinning the complex architecture of defense traits. Among the corresponding candidate genes, i.e. genes in linkage disequilibrium (LD) with candidate SNPs, none have previously been identified as a gene playing a role in the interaction between plants and phloem-feeding insects. Whitefly performance on knock-out mutants of a number of candidate genes was significantly affected, validating the potential of GWA mapping for novel gene discovery in plant-insect interactions. Our results show that GWA analysis is a very useful tool to gain insight into the genetic architecture of plant defense against herbivorous insects, i.e. we identified and validated several genes affecting whitefly performance that have not previously been related to plant defense against herbivorous insects.

Green leaf volatiles: biosynthesis, biological functions and their applications in biotechnology

Plants have evolved numerous constitutive and inducible defence mechanisms to cope with biotic and abiotic stresses. These stresses induce the expression of various genes to activate defence-related pathways that result in the release of defence chemicals. One of these defence mechanisms is the oxylipin pathway, which produces jasmonates, divinylethers and green leaf volatiles (GLVs) through the peroxidation of polyunsaturated fatty acids (PUFAs). GLVs have recently emerged as key players in plant defence, plant-plant interactions and plant-insect interactions. Some GLVs inhibit the growth and propagation of plant pathogens, including bacteria, viruses and fungi. In certain cases, GLVs released from plants under herbivore attack can serve as aerial messengers to neighbouring plants and to attract parasitic or parasitoid enemies of the herbivores. The plants that perceive these volatile signals are primed and can then adapt in preparation for the upcoming challenges. Due to their 'green note' odour, GLVs impart aromas and flavours to many natural foods, such as vegetables and fruits, and therefore, they can be exploited in industrial biotechnology. The aim of this study was to review the progress and recent developments in research on the oxylipin pathway, with a specific focus on the biosynthesis and biological functions of GLVs and their applications in industrial biotechnology.

Normal adult survival but reduced Bemisia tabaci oviposition rate on tomato lines carrying an introgression from S. habrochaites

Background

Host plant resistance has been proposed as one of the most promising approaches in whitefly management. Already in 1995 two quantitative trait loci (Tv-1 and Tv-2) originating from S. habrochaites CGN1.1561 were identified that reduced the oviposition rate of the greenhouse whitefly (Trialeurodes vaporariorum). After this first study, several others identified QTLs affecting whitefly biology as well. Generally, the QTLs affecting oviposition were highly correlated with a reduction in whitefly survival and the presence of high densities of glandular trichomes type IV. The aim of our study was to further characterize Tv-1 and Tv-2, and to determine their role in resistance against Bemisia tabaci.

Results

We selected F₂ plants homozygous for the Tv-1 and Tv-2 QTL regions and did three successive backcrosses without phenotypic selection. Twenty-three F₂BC₃ plants were phenotyped for whitefly resistance and differences were found in oviposition rate of B. tabaci. The F₂BC₃ plants with the lowest oviposition rate had an introgression on Chromosome 5 in common. Further F₂BC_4, F₂BC₄S₁ and F₂BC₄S₂ families were developed, genotyped and phenotyped for adult survival, oviposition rate and trichome type and density. It was possible to confirm that an introgression on top of Chr. 5 (OR-5), between the markers rs-2009 and rs-7551, was responsible for reducing whitefly oviposition rate.

Conclusion

We found a region of 3.06 Mbp at the top of Chr. 5 (OR-5) associated with a reduction in the oviposition rate of B. tabaci. This reduction was independent of the presence of the QTLs Tv-1 and Tv-2 as well as of the presence of trichomes type IV. The OR-5 locus will provide new opportunities for resistance breeding against whiteflies, which is especially relevant in greenhouse cultivation.

Electronic supplementary material

The online version of this article (doi:10.1186/s12863-014-0142-3) contains supplementary material, which is available to authorized users.

Behavioral assays for studies of host plant choice and adaptation in herbivorous insects

The association of insect herbivores with their host plants is influenced by behaviors governing acceptance of those plants for feeding and oviposition. Behavioral changes accompany and may even precede host range expansion. Characterization and quantification of specific behaviors often form the basis of studies on host plant adaptation and chemical ecology. Behavioral assays of insects are usually designed to measure attraction for feeding or oviposition in relation to their host plants or specific chemistry. We review behavioral assays of insect herbivores with host plants or the volatiles they emit, with special consideration given to design, analysis, and interpretation to maximize ecological relevance. A toolkit of robust assays can help address fundamental issues at the intersection of ecology and evolution, such as the underpinnings of plant-insect interactions and the identification of genes involved in host race formation.

Overexpression of IRM1 enhances resistance to aphids in Arabidopsis thaliana

Aphids are insects that cause direct damage to crops by the removal of phloem sap, but more importantly they spread devastating viruses. Aphids use their sophisticated mouthpart (i.e. stylet) to feed from the phloem sieve elements of the host plant. To identify genes that affect host plant resistance to aphids, we previously screened an Arabidopsis thaliana activation tag mutant collection. In such mutants, tagged genes are overexpressed by a strong 35S enhancer adjacent to the natural promoter, resulting in a dominant gain-of-function phenotype. We previously identified several of these mutants on which the aphid Myzus persicae showed a reduced population development compared with wild type. In the present study we show that the gene responsible for the phenotype of one of the mutants is At5g65040 and named this gene Increased Resistance to Myzus persicae 1 (IRM1). Overexpression of the cloned IRM1 gene conferred a phenotype identical to that of the original mutant. Conversely, an IRM1 knockout mutant promoted aphid population development compared to the wild type. We performed Electrical Penetration Graph analysis to investigate how probing and feeding behaviour of aphids was affected on plants that either overexpressed IRM1 or contained a knockout mutation in this gene. The EPG results indicated that the aphids encounter resistance factors while reaching for the phloem on the overexpressing line. This resistance mechanism also affected other aphid species and is suggested to be of mechanical nature. Interestingly, genetic variation for IRM1 expression in response to aphid attack was observed. Upon aphid attack the expression of IRM1 was initially (after 6 hours) induced in ecotype Wassilewskija followed by suppression. In Columbia-0, IRM1 expression was already suppressed six hours after the start of the infestation. The resistance conferred by the overexpression of IRM1 in A. thaliana trades off with plant growth.

Identification and QTL mapping of whitefly resistance components in Solanum galapagense

Solanum galapagense is closely related to the cultivated tomato and can show a very good resistance towards whitefly. A segregating population resulting from a cross between the cultivated tomato and a whitefly resistant S. galapagense was created and used for mapping whitefly resistance and related traits, which made it possible to study the genetic basis of the resistance. Quantitative trait loci (QTL) for adult survival co-localized with type IV trichome characteristics (presence, density, gland longevity and gland size). A major QTL (Wf-1) was found for adult survival and trichome characters on Chromosome 2. This QTL explained 54.1 % of the variation in adult survival and 81.5 % of the occurrence of type IV trichomes. A minor QTL (Wf-2) for adult survival and trichome characters was identified on Chromosome 9. The major QTL was confirmed in F3 populations. Comprehensive metabolomics, based on GCMS profiling, revealed that 16 metabolites segregating in the F2 mapping population were associated with Wf-1 and/or Wf-2. Analysis of the 10 most resistant and susceptible F2 genotypes by LCMS showed that several acyl sugars were present in significantly higher concentration in the whitefly resistant genotypes, suggesting a role for these components in the resistance as well. Our results show that whitefly resistance in S. galapagense seems to inherit relatively simple compared to whitefly resistance from other sources and this offers great prospects for resistance breeding as well as elucidating the underlying molecular mechanism(s) of the resistance.

Resistance to sap-sucking insects in modern-day agriculture

Plants and herbivores have co-evolved in their natural habitats for about 350 million years, but since the domestication of crops, plant resistance against insects has taken a different turn. With the onset of monoculture-driven modern agriculture, selective pressure on insects to overcome resistances has dramatically increased. Therefore plant breeders have resorted to high-tech tools to continuously create new insect-resistant crops. Efforts in the past 30 years have resulted in elucidation of mechanisms of many effective plant defenses against insect herbivores. Here, we critically appraise these efforts and - with a focus on sap-sucking insects - discuss how these findings have contributed to herbivore-resistant crops. Moreover, in this review we try to assess where future challenges and opportunities lay ahead. Of particular importance will be a mandatory reduction in systemic pesticide usage and thus a greater reliance on alternative methods, such as improved plant genetics for plant resistance to insect herbivores.

Perception of insect feeding by plants

The recognition of phytophagous insects by plants induces a set of very specific responses aimed at deterring tissue consumption and reprogramming metabolism and development of the plant to tolerate the herbivore. The recognition of insects by plants requires the plant's ability to perceive chemical cues generated by the insects and to distinguish a particular pattern of tissue disruption. Relatively little is known about the molecular basis of insect perception by plants and the signalling mechanisms directly associated with this perception. Importantly, the insect feeding behaviour (piercing-sucking versus chewing) is a decisive determinant of the plant's defence response, and the mechanisms used to perceive insects from different feeding guilds may be distinct. During insect feeding, components of the saliva of chewing or piercing-sucking insects come into contact with plant cells, and elicitors or effectors present in this insect-derived fluid are perceived by plant cells to initiate the activation of specific signalling cascades. Although receptor-ligand interactions controlling insect perception have yet not been molecularly described, a significant number of regulatory components acting downstream of receptors and involved in the activation of defence responses against insects has been reported. Some of these regulators mediate changes in the phytohormone network, while others directly control gene expression or the redox state of the cell. These processes are central in the orchestration of plant defence responses against insects.