Comparative Proteomic and Nutritional Composition Analysis of Independent Transgenic Pigeon Pea Seeds Harboring cry1AcF and cry2Aa Genes and Their Nontransgenic Counterparts

Unraveling the proteomic changes involved in the resistance response of Cajanus platycarpus to herbivory by Helicoverpa armigera

The pigeonpea wild relative Cajanus platycarpus is resistant to Helicoverpa armigera, one of the major pests responsible for yield losses in Cajanus cajan. Deciphering the molecular mechanism underlying host plant resistance is pertinent to identify proteins that aid in the mitigation of the insect pest. The present study adopted comparative proteomics as a tool to interpret the resistance mechanism(s) in C. platycarpus vis-à-vis C. cajan during continued herbivory (up to 96 h). Over-representation analysis of the differentially expressed proteins implicated a multi-dimensional resistance response accomplished by both physical and chemical barriers in C. platycarpus. While the chemical basis for resistance was depicted by the upregulation of proteins playing a rate limiting role in the phenylpropanoid pathway, the physical basis was provided by the regulation of proteins involved in microtubule assembly and synthesis of lignins. Upregulation of proteins in the polyamine pathway indicated the role of metabolite conjugates to be negatively affecting herbivore growth. Reallocation of resources and diversion of metabolic flux to support the production of secondary metabolites could be the probable approach in the wild relative against herbivory. Our study provided deeper insights into the pod borer resistance mechanism in C. platycarpus for utility in crop improvement. KEY POINTS: • Pod borer resistance in Cajanus platycarpus is multi-dimensional. • Pod borer resistance has been arbitrated to cell wall rigidity and secondary metabolites. • Phenylpropanoid pathway derivatives apparently shaped the plant chemical defense against pod borer.

Assessment of Pigeonpea (Cajanus cajan L.) transgenics expressing Bt ICPs, Cry2Aa and Cry1AcF under nethouse containment implicated an effective control against herbivory by Helicoverpa armigera (Hübner)

BACKGROUND

Pigeonpea is a source of quality proteins and the main constituent of a well-balanced diet for majority of Indian population. One of the major constraints in the production of pigeonpea is a polyphagous insect pest, Helicoverpa armigera. Non-availability of resistant sources in the germplasm and limitations in conventional breeding have been key factors for continued yield losses. Additionally, hazards of chemical fertilizers on the environment have prompted the scientific community to develop alternative strategies. Bacillus thuringiensis (Bt) insecticidal proteins (ICPs) have emerged as the most reliable source for the control of insect pests through transgenics.

RESULTS

Transgenic pigeonpea plants harboring validated Bt ICPs, Cry2Aa and Cry1AcF were developed by a non-tissue culture based in planta transformation strategy and assessed for integration of Transfer-DNA (T-DNA) and efficacy against pod borer under in vitro conditions. For the first time this study demonstrates the successful evaluation of 19 transgenic pigeonpea events (11 with cry2Aa and 8 with cry1AcF) under soil and pot conditions in a nethouse containment. The stability in the performance was assessed stringently by deliberate H. armigera larval challenging. The trial identified ten promising events of both the genes that portrayed reduced damage to the herbivore.

CONCLUSION

We present the first ever successful evaluation of pigeonpea transgenics with the ability to mitigate pod borer under nethouse conditions. The transgenics depicted molecular evidence for the stability of T-DNA integration, consistency in the expression of Cry proteins and resistance against H. armigera. These events can form a pool of useful transgenics to manage the devastating pod borer. © 2019 Society of Chemical Industry.

Comparative transcriptome analyses provide novel insights into the differential response of Pigeonpea (Cajanus cajan L.) and its wild relative (Cajanus platycarpus (Benth.) Maesen) to herbivory by Helicoverpa armigera (Hübner)

Deeper insights into the resistance response of Cajanus platycarpus were obtained based on comparative transcriptomics under Helicoverpa armigera infestation. Devastation by pod borer, Helicoverpa armigera is one of the major factors for stagnated productivity in Pigeonpea. Despite possessing a multitude of desirable traits including pod borer resistance, wild relatives of Cajanus spp. have remained under-utilized due to linkage drag and cross-incompatibility. Discovery and deployment of genes from them can provide means to tackle key pests like H. armigera. Transcriptomic differences between Cajanus platycarpus and Cajanus cajan during different time points (0, 18, 38, 96 h) of pod borer infestation were elucidated in this study. For the first ever time, we demonstrated captivating variations in their response; C. platycarpus apparently being reasonably agile with effectual transcriptomic reprogramming to deter the insect. Deeper insights into the differential response were obtained by identification of significant GO-terms related to herbivory followed by combined KEGG and ontology analyses. C. platycarpus portrayed a multilevel response with cardinal involvement of SAR, redox homeostasis and reconfiguration of primary metabolites leading to a comprehensive defense response. The credibility of RNA-seq analyses was ascertained by transient expression of selected putative insect resistance genes from C. platycarpus viz., chitinase (CHI4), Alpha-amylase/subtilisin inhibitor (IAAS) and Flavonoid 3_5 hydroxylase (C75A1) in Nicotiana benthamiana followed by efficacy analysis against H. armigera. qPCR validated results of the study provided innovative insights and useful leads for development of durable pod borer resistance.

iTRAQ-based quantitative tissue proteomic analysis of differentially expressed proteins (DEPs) in non-transgenic and transgenic soybean seeds

The unintended effects of transgenesis have increased food safety concerns, meriting comprehensive evaluation. Proteomic profiling provides an approach to directly assess the unintended effects. Herein, the isobaric tags for relative and absolute quantitation (iTRAQ) comparative proteomic approach was employed to evaluate proteomic profile differences in seed cotyledons from 4 genetically modified (GM) and 3 natural genotypic soybean lines. Compared with their non-GM parents, there were 67, 61, 13 and 22 differentially expressed proteins (DEPs) in MON87705, MON87701 × MON89788, MON87708, and FG72. Overall, 170 DEPs were identified in the 3 GM soybean lines with the same parents, but 232 DEPs were identified in the 3 natural soybean lines. Thus, the differences in protein expression among the genotypic varieties were greater than those caused by GM. When considering ≥2 replicates, 4 common DEPs (cDEPs) were identified in the 3 different GM soybean lines with the same parents and 6 cDEPs were identified in the 3 natural varieties. However, when considering 3 replicates, no cDEPs were identified. Regardless of whether ≥2 or 3 replicates were considered, no cDEPs were identified among the 4 GM soybean lines. Therefore, no feedback due to GM was observed at the common protein level in this study.

Expression of Cry2Aa, a Bacillus thuringiensis insecticidal protein in transgenic pigeon pea confers resistance to gram pod borer, Helicoverpa armigera

Pigeon pea is an important legume infested by a plethora of insect pests amongst which gram pod borer Helicoverpa armigera is very prominent. Imparting resistance to this insect herbivore is of global importance in attaining food security. Expression of insecticidal crystal proteins (ICP) in diverse crops has led to increased resistance to several pests. We report in this paper, expression of Cry2Aa in transgenic pigeon pea and its effectiveness towards H. armigera by employing Agrobacterium-mediated in planta transformation approach. Approximately 0.8% of T₁ generation plants were identified as putative transformants based on screening in the presence of 70 ppm kanamycin as the selection agent. Promising events were further recognized in advanced generations based on integration, expression and bioefficacy of the transgenes. Seven T₃ lines (11.8% of the selected T1 events) were categorized as superior as these events demonstrated 80–100% mortality of the challenged larvae and improved ability to prevent damage caused by the larvae. The selected transgenic plants accumulated Cry2Aa in the range of 25–80 µg/g FW. The transgenic events developed in the study can be used in pigeon pea improvement programmes for pod borer resistance.

Proteomic Analysis of Pigeonpea (Cajanus cajan) Seeds Reveals the Accumulation of Numerous Stress-Related Proteins

Pigeonpea is one of the major sources of dietary protein for more than a billion people living in South Asia. This hardy legume is often grown in low-input and risk-prone marginal environments. Considerable research effort has been devoted by a global research consortium to develop genomic resources for the improvement of this legume crop. These efforts have resulted in the elucidation of the complete genome sequence of pigeonpea. Despite these developments, little is known about the seed proteome of this important crop. Here, we report the proteome of pigeonpea seed. To enable the isolation of maximum number of seed proteins, including those that are present in very low amounts, three different protein fractions were obtained by employing different extraction media. High-resolution two-dimensional (2-D) electrophoresis followed by MALDI-TOF-TOF-MS/MS analysis of these protein fractions resulted in the identification of 373 pigeonpea seed proteins. Consistent with the reported high degree of synteny between the pigeonpea and soybean genomes, a large number of pigeonpea seed proteins exhibited significant amino acid homology with soybean seed proteins. Our proteomic analysis identified a large number of stress-related proteins, presumably due to its adaptation to drought-prone environments. The availability of a pigeonpea seed proteome reference map should shed light on the roles of these identified proteins in various biological processes and facilitate the improvement of seed composition.