Identification of genes involved in wild crucifer Rorippa indica resistance response on mustard aphid Lipaphis erysimi challenge

Enhancing aphid resistance in horticultural crops: a breeding prospective

Increasing agricultural losses caused by insect infestations are a significant problem, so it is important to generate pest-resistant crop varieties to address this issue. Several reviews have examined aphid-plant interactions from an entomological perspective. However, few have specifically focused on plant resistance mechanisms to aphids and their applications in breeding for aphid resistance. In this review, we first outline the types of resistance to aphids in plants, namely antixenosis, tolerance (cell wall lignification, resistance proteins), and antibiosis, and we discuss strategies based on each of these resistance mechanisms to generate plant varieties with improved resistance. We then outline research on the complex interactions amongst plants, viruses, and aphids, and discuss how aspects of these interactions can be exploited to improve aphid resistance. A deeper understanding of the epigenetic mechanisms related to induced resistance, i.e. the phenomenon where plants become more resistant to a stress they have encountered previously, may allow for its exploitation in breeding for aphid resistance. Wild relatives of crop plants serve as important sources of resistance traits. Genes related to these traits can be introduced into cultivated crop varieties by breeding or genetic modification, and de novo domestication of wild varieties can be used to exploit multiple excellent characteristics, including aphid resistance. Finally, we discuss the use of molecular design breeding, genomic data, and gene editing to generate new aphid-resistant, high-quality crop varieties.

De novo transcriptome assembly and global analysis of differential gene expression of aphid tolerant wild mustard Rorippa indica (L.) Hiern infested by mustard aphid Lipaphis Erysimi (L.) Kaltenbach

Rapeseed-mustard, the oleiferous Brassica species are important oilseed crops cultivated all over the globe. Mustard aphid Lipaphis erysimi (L.) Kaltenbach is a major threat to the cultivation of rapeseed-mustard. Wild mustard Rorippa indica (L.) Hiern shows tolerance to mustard aphids as a nonhost and hence is an important source for the bioprospecting of potential resistance genes and defense measures to manage mustard aphids sustainably. We performed mRNA sequencing of the R. indica plant uninfested and infested by the mustard aphids, harvested at 24 hours post-infestation. Following quality control, the high-quality reads were subjected to de novo assembly of the transcriptome. As there is no genomic information available for this potential wild plant, the raw reads will be useful for further bioinformatics analysis and the sequence information of the assembled transcripts will be helpful to design primers for the characterization of specific gene sequences. In this study, we also used the generated resource to comprehensively analyse the global profile of differential gene expression in R. indica in response to infestation by mustard aphids. The functional enrichment analysis of the differentially expressed genes reveals a significant immune response and suggests the possibility of chitin-induced defense signaling.

Characterization of STP4 promoter in Indian mustard Brassica juncea for use as an aphid responsive promoter

OBJECTIVE

Brassica juncea, a major oilseed crop, suffers substantial yield losses due to infestation by mustard aphids (Lipaphis erysimi). Unavailability of resistance genes within the accessible gene pool underpins significance of the transgenic strategy in developing aphid resistance. In this study, we aimed for the identification of an aphid-responsive promoter from B. juncea, based on the available genomic resources.

RESULTS

A monosaccharide transporter gene, STP4 in B. juncea was activated by aphids and sustained increased expression as the aphids colonized the plants. We cloned the upstream intergenic region of STP4 and validated its stand-alone aphid-responsive promoter activity. Further, deletion analysis identified the putative cis-elements important for the aphid responsive promoter activity.

CONCLUSION

The identified STP4 promoter can potentially be used for driving high level aphid-inducible expression of transgenes in plants. Use of aphid-responsive promoter instead of constitutive promoters can potentially reduce the metabolic burden of transgene-expression on the host plant.

Natural factors regulating mustard aphid dynamics in cabbage

Lipaphis erysimi (L.) Kaltenbach (Hemiptera: Aphididae) is one of the most important pests of brassica crops, mainly causing losses due to sap sucking, toxin injection and viral transmission. Knowledge about the main natural factors that regulate populations of this pest, as well as its critical mortality stage, is crucial for the development of integrated pest management of L. erysimi. Here, we determined the critical stage and key mortality factors for L. erysimi in cabbage using an ecological life table. Causes of mortality at each stage of L. erysimi development were monitored daily in the field for seven seasons. From the experimental data, we determined the key factor and critical stage of mortality through correlation and regression analyses. The nymphal stage, especially first instar nymphs, was critical for L. erysimi mortality. The key mortality factors were, in descending order of importance, physiological disturbances and predation by Syrphidae, Coccinellidae and Solenopsis ants. Therefore, control measures should target early stages of L. erysimi and the use of cabbage cultivars that have negative effects against L. erysimi may be a promising strategy for its management. Our results may be useful for plant geneticists who could develop new cabbage cultivars based on these findings. In addition, conservation measures of the main predators of L. erysimi may contribute to the natural control of this pest.

Overexpression of biologically safe Rorippa indica defensin enhances aphid tolerance in Brassica juncea

Transgenic mustard plants ( Brassica juncea ) expressing non-allergenic and biologically safe RiD peptide show higher tolerance against Lipaphis erysimi. Rorippa indica defensin (RiD) has previously been reported as a novel insecticidal protein derived from a wild crucifer Rorippa indica. RiD was found to have an effective insecticidal property against mustard aphid, Lipaphis erysimi. In the present study, RiD was highly upregulated in R. indica during aphid infestation initiating a defense system mediated by jasmonic acid (JA), but not by salicylic acid (SA)/abscisic acid (ABA). RiD has also been assessed for biosafety according to the FAO/WHO guideline (allergenicity of genetically modified foods; Food And Agriculture Organisation of the United Nations, Rome, Italy, 2001) and Codex Alimentarius Guideline (Guidelines for the design and implementation of national regulatory food safety assurance programme associated with the use of veterinary drugs in food producing animals. Codex Alimentarius Commission. GL, pp 71-2009, 2009). The purified protein was used to sensitize BALB/c mice and they showed normal histopathology of lung and no elevated IgE level in their sera. As the protein was found to be biologically safe and non-allergenic, it was used to develop transgenic Brassica juncea plants with enhanced aphid tolerance, which is one of the most important oilseed crops and is mostly affected by the devastating pest-L. erysimi. The transgene integration was monitored by Southern hybridization, and the positive B. juncea lines were further analyzed by Western blot, ELISA, immunohistolocalization assays and in planta insect bioassay. Transgenic plants expressing RiD conferred a higher level of tolerance against L. erysimi. All these results demonstrated that RiD is a novel, biologically safe, effective insecticidal agent and B. juncea plants expressing RiD are important components of integrated pest management.

Identification and comprehensive evaluation of reference genes for RT-qPCR analysis of host gene-expression in Brassica juncea-aphid interaction using microarray data

Brassica juncea is a chief oil yielding crop in many parts of the world including India. With advancement of molecular techniques, RT-qPCR based study of gene-expression has become an integral part of experimentations in crop breeding. In RT-qPCR, use of appropriate reference gene(s) is pivotal. The virtue of the reference genes, being constant in expression throughout the experimental treatments, needs to be validated case by case. Appropriate reference gene(s) for normalization of gene-expression data in B. juncea during the biotic stress of aphid infestation is not known. In the present investigation, 11 reference genes identified from microarray database of Arabidopsis-aphid interaction at a cut off FDR ≤0.1, along with two known reference genes of B. juncea, were analyzed for their expression stability upon aphid infestation. These included 6 frequently used and 5 newly identified reference genes. Ranking orders of the reference genes in terms of expression stability were calculated using advanced statistical approaches such as geNorm, NormFinder, delta Ct and BestKeeper. The analysis suggested CAC, TUA and DUF179 as the most suitable reference genes. Further, normalization of the gene-expression data of STP4 and PR1 by the most and the least stable reference gene, respectively has demonstrated importance and applicability of the recommended reference genes in aphid infested samples of B. juncea.

Development of PR genes panel for screening aphid-tolerant cultivars in Brassica juncea

The exorbitant yield loss incurred by Indian farmers every year (10-90%) in rapeseed-mustard (Brassica juncea) is chiefly attributed to the progressive infestation of mustard fields by Lipaphis erysimi (Kalt.), a major insect pest belonging to the family of Homoptera. Currently there are no successful tolerant cultivars developed by conventional means in Brassica juncea with systemic plant responses in the form of direct or indirect defenses against aphid attack. Lack of specific methods for screening large numbers of genotypes required in breeding for selection of tolerant cultivars in mustard is one of the main causes of slow progress in developing resistant varieties of Brassica juncea. Traditional phenotype-based breeding has to be augmented with recent molecular approaches for potential genotype selection and cultivar development in Brassica juncea. In current study a pathogen-responsive gene panel was developed which could be used for expression-assisted breeding program in mustard for selection of tolerant types against aphid infestation, minimizing the huge crop losses suffered by farmers every year.

Functional characterization of Rorippa indica defensin and its efficacy against Lipaphis erysimi

Rorippa indica, a wild crucifer, has been previously reported as the first identified plant in the germplasm of Brassicaceae known to be tolerant towards the mustard aphid Lipaphis erysimi Kaltenbach. We herein report the full-length cloning, expression, purification and characterization of a novel R. indica defensin (RiD) and its efficacy against L. erysimi. Structural analysis through homology modeling of RiD showed longer α-helix and 3rd β-sheet as compared to Brassica juncea defensin (BjD). Recombinant RiD and BjD was purified for studying its efficacy against L. erysimi. In the artificial diet based insect bioassay, the LC50 value of RiD against L. erysimi was found to be 9.099 ± 0.621 µg/mL which is far lower than that of BjD (43.51 ± 0.526 µg/mL). This indicates the possibility of RiD having different interacting partner and having better efficacy against L. erysimi over BjD. In the transient localization studies, RiD signal peptide directed the RiD: yellow fluorescent protein (YFP) fusion protein to the apoplastic regions which indicates that it might play a very important role in inhibiting nutrient uptake by aphids which follow mainly extracellular route to pierce through the cells. Hence, the present study has a significant implication for the future pest management program of B. juncea through the development of aphid tolerant transgenic plants.

Rorippa indica Regeneration via Somatic Embryogenesis Involving Frog Egg-like Bodies Efficiently Induced by the Synergy of Salt and Drought Stresses

Frog egg-like bodies (FELBs), novel somatic embryogenesis (SE) structures first observed in Solanum nigrum, were induced in Rorippa indica. NaCl-mediated salt and mannitol-mimicked drought stresses induced FELBs in R. indica, which is very different from the induction by plant growth regulators (PGRs) under low light condition that was used in S. nigrum FELB induction. It demonstrated that NaCl or mannitol supplements alone could induce FELBs in R. indica, but with low induction rates, while the synergy of NaCl and mannitol significantly increased the FELB induction rates. For the combination of 5.0 g/L mannitol and 10.0 g/L NaCl the highest FELB induction rate (100%) was achieved. It suggests that the synergy of drought and salt stresses can replace PGRs to induce FELBs in R. indica. On medium supplemented with 1.0 mg/L gibberellic acid all the inoculated in vitro FELBs developed into multiple plantlets. Morphological and histological analyses confirmed the identity of FELBs induced in R. indica and revealed that FELBs originate from root cortex cells.

Expression profiling of selected glutathione transferase genes in Zea mays (L.) seedlings infested with cereal aphids

The purpose of this report was to evaluate the expression patterns of selected glutathione transferase genes (gst1, gst18, gst23 and gst24) in the tissues of two maize (Zea mays L.) varieties (relatively resistant Ambrozja and susceptible Tasty Sweet) that were colonized with oligophagous bird cherry-oat aphid (Rhopalosiphum padi L.) or monophagous grain aphid (Sitobion avenae L.). Simultaneously, insect-triggered generation of superoxide anion radicals (O₂^•−) in infested Z. mays plants was monitored. Quantified parameters were measured at 1, 2, 4, 8, 24, 48 and 72 h post-initial aphid infestation (hpi) in relation to the non-infested control seedlings. Significant increases in gst transcript amounts were recorded in aphid-stressed plants in comparison to the control seedlings. Maximal enhancement in the expression of the gst genes in aphid-attacked maize plants was found at 8 hpi (gst23) or 24 hpi (gst1, gst18 and gst24) compared to the control. Investigated Z. mays cultivars formed excessive superoxide anion radicals in response to insect treatments, and the highest overproduction of O₂^•− was noted 4 or 8 h after infestation, depending on the aphid treatment and maize genotype. Importantly, the Ambrozja variety could be characterized as having more profound increments in the levels of gst transcript abundance and O₂^•− generation in comparison with the Tasty Sweet genotype.