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)

The plant specialized metabolite epicatechin- 3-gallate (EC3G) perturbs lipid metabolism and attenuates fat accumulation in pigeonpea pod borer, Helicoverpa armigera

Control of pod borer Helicoverpa armigera, a notorious polyphagous pest requires paramount attention with focus on environment-friendly management approaches. Overproduction of catechins (epigallocatechin-EGC and epicatechin-3-gallate-EC3G) in the pod borer-resistant pigeonpea wild relative, Cajanus platycarpus during continued herbivory prodded us to assess their underlying molecular effect on H. armigera. Significant reduction in larval and pupal growth parameters was observed when reared on artificial diet incorporated with 100 ppm EC3G vis a vis 100 ppm EGC and EGC + EC3G. Comparative RNAseq analyses of larvae that fed on normal and EC3G-incorporated diet revealed 62 differentially expressed genes dominated by detoxification and lipid metabolism. While lipase and fatty acid-binding protein 2-like were up-regulated, delta9-FADS-like involved in fatty acid synthesis was downregulated, indicating effect of EC3G on fat metabolism. Validation of RNAseq data by qPCR; midgut glutathione-S-transferase and esterase assays depicted increased lipolysis and reduced lipogenesis in EC3G-fed larvae. Additionally, differential accumulation of stearic acid and oleic acid in EC3G-fed and control larvae/adults ascertained perturbation in lipogenesis. Supported by modelling, molecular docking and simulations, we demonstrate the possible involvement of the insect adipokinetic hormone receptor (AKHR) in the EC3G-mediated response. The study demonstrates plant specialized metabolite EC3G as a potential candidate for H. armigera control.

RNAi-based knockdown of candidate gut receptor genes altered the susceptibility of Spodoptera frugiperda and S. litura larvae to a chimeric toxin Cry1AcF

Background

A multitude of Cry toxins (secreted by Bacillus thuringiensis or Bt) has been deployed globally either via transgenic mean or bio-pesticidal formulations in order to manage insect pests. However, Bt resistance development in insects is emerging as a major concern. To avoid this problem, multiple gene pyramiding or protein-engineered chimeric toxin-based strategy has been analyzed.

Methods

In the present study, one such chimeric toxin Cry1AcF (contain the swapped domains of Cry1Ac and Cry1F) was used to investigate its in vivo pathogenesis process in lepidopteran pests Spodoptera frugiperda and S. litura. A number of biochemical and molecular analysis were performed.

Results

Oral ingestion of Cry1AcF caused greater toxicity in S. frugiperda than S. litura with larvae displaying increased hemolymph melanization. Histopathology of the midgut transverse sections exhibited Cry1AcF-induced extensive gut damage in both the test insects followed by cytotoxicity in terms of reduced hemocyte numbers and viability. Elevated hemolymph phenoloxidase activity indicated the immune-stimulatory nature of Cry1AcF. In order to analyze the role of gut receptor proteins in Cry1AcF intoxication in test insects, we performed RNAi-mediated silencing using bacterially-expressed dsRNAs of individual receptor-encoding genes including CAD, ABCC2, ALP1 and APN. Target-specific induced downregulation of receptor mRNAs differentially altered the insect susceptibility to Cry1AcF toxin in our study. The susceptibility of ALP1 and APN dsRNA pre-treated S. frugiperda was considerably decreased when treated with Cry1AcF in LD₅₀ and LD₉₀ doses, whereas susceptibility of CAD and ABCC2 dsRNA pre-treated S. litura was significantly reduced when ingested with Cry1AcF in different doses. CAD/ABCC2-silenced S. frugiperda and ALP1/APN-silenced S. litura were vulnerable to Cry1AcF alike of control larvae. In conclusion, our results indicate ALP1/APN and CAD/ABCC2 as the functional receptor for Cry1AcF toxicity in S. frugiperda and S. litura, respectively.

Reactive oxygen species in plants: an invincible fulcrum for biotic stress mitigation

Climate change-associated environmental vagaries have amplified the incidence of pests and pathogens on plants, thus imparting the increased quest for management strategies. Plants respond to stresses through intricate signaling networks that regulate diverse cellular mechanisms. Reactive oxygen species (ROS) are cardinal towards the maintenance of normal plant activities as well as improving stress management. Plants that exhibit a fine balance between ROS levels and its management apparently mitigate stresses better. There have been very many compendiums on signaling and management of ROS during several abiotic stresses. However, expansion of knowledge related to ROS induction and homeostasis during biotic stresses is pertinent. Hence, considering its importance, we provide insights in this review on how plants signal and manage ROS upon an oxidative burst during their interaction with pathogens and herbivores. Substantial degree of molecular changes and pivotal roles of ROS have been detected during phyto-pathogen/herbivore interactions, opening novel platforms to understand signaling/management of events under varied biotic stresses. It is interesting to know that, though plants react to biotic stresses through oxidative burst, receptors and elicitors involved in the signal transduction differ across stresses. The review provides explicit details about the specific signaling of ROS production in plants under pathogen and herbivore attack. Furthermore, we also provide an update about tackling the accumulated ROS under biotic stresses as another pivotal step. ROS signaling and homeostasis can be exploited as critical players and a fulcrum to tackle biotic stresses, thus paving the way for futuristic combinatorial stress management strategies. KEY POINTS: • The review is a comprehension of redox signaling and management in plants during herbivory and pathogen infection • Reactive oxygen species (ROS) is an important factor during normal plant activities as well as in their response to stresses. Diverse modes of ROS signaling and management have been observed during both biotic stresses independently • Exploration of plant biology in multi-stress resistant plants like the crop wild relatives could pave the way for combinatorial management of stress for a better tomorrow.

Deciphering of Pod Borer [Helicoverpa armigera (Hübner)] Resistance in Cajanus platycarpus (Benth.) Offers Novel Insights on the Reprogramming and Role of Flavonoid Biosynthesis Pathway

Management of pod borer, Helicoverpa armigera in pigeonpea (Cajanus cajan L.), an important legume crop, has been a pertinent endeavor globally. As with other crops, wild relatives of pigeonpea are bestowed with various resistance traits that include the ability to deter the H. armigera. Understanding the molecular basis of pod borer resistance could provide useful leads for the management of this notorious herbivore. Earlier studies by our group in deciphering the resistance response to herbivory through multiomics approaches in the pigeonpea wild relative, Cajanus platycarpus, divulged the involvement of the flavonoid biosynthesis pathway, speculating an active chemical response of the wild relative to herbivory. The present study is a deeper understanding of the chemical basis of pod borer (H. armigera) resistance in, C. platycarpus, with focus on the flavonoid biosynthesis pathway. To substantiate, quantification of transcripts in H. armigera-challenged C. platycarpus (8 h, 24 h, 48 h, 96 h) showed dynamic upregulation (up to 11-fold) of pivotal pathway genes such as chalcone synthase, dihydroflavonol-4-reductase, flavonoid-3'5'-hydroxylase, flavonol synthase, leucoanthocyanidin reductase, and anthocyanidin synthase. Targeted LC-MS analyses demonstrated a concomitant increase (up to 4-fold) in naringenin, kaempferol, quercetin, delphinidin, cyanidin, epigallocatechin, and epicatechin-3-gallate. Interestingly, H. armigera diet overlaid with the over-produced flavonoids (100 ppm) showed deleterious effects on growth leading to a prolonged larval period demonstrating noteworthy coherence between over-accumulation of pathway transcripts/metabolites. The study depicts novel evidence for the directed metabolic reprogramming of the flavonoid biosynthesis pathway in the wild relative to pod borer; plant metabolic potential is worth exploiting for pest management.

Relevance of methionine sulfoxide reductase(s) (MSR) as candidate proteins in redox homeostasis-mediated resistance response to Helicoverpa armigera (Hübner) in the pigeonpea wild relative Cajanus platycarpus (Benth.) Maesen

Pod borer, Helicoverpa armigera, a polyphagus herbivore causes extensive economic losses to crops, including pigeonpea. Exploitation of pod borer resistance in wild relatives is pertinent due to the absence of resistance sources in cultivated pigeonpea and crossing-incompatibility with the resistant wild relatives. We present leads obtained in deeper understanding of pod borer resistance mechanism in Cajanus platycarpus, a pigeonpea wild relative. Surge in cellular ROS during herbivory leads to redox-PTMs (post translational modifications) of methionine-rich proteins including antioxidant enzymes, causing oxidative damage. Plants then officiate methionine sulfoxide reductases (MSRs), that maintain the redox status of methionine and hence homeostasis. We demonstrate functionality of MSRs (MSRA and MSRB) in the resistance response of the wild relative to pod borer. Among 5 MSRA and 3 MSRB genes, CpMSRA2 and CpMSRB1 were herbivore-responsive based on expression during herbivory. Clues about the stress-responsiveness were obtained upon analyses of cis-elements and co-expressing genes. Apparently, the wild relative followed a non-canonical mode of redox management, as divulged by antioxidant enzymes and the scavenging capacity. Differential lipid peroxidation as an early response provided evidences for an effective redox management in the wild relative. This is the first report signifying redox homeostasis in the resistance response towards herbivory.

Green revolution to grain revolution: Florigen in the frontiers

Burgeoning human population dents, globally, the brimming buffer stock as well as gain in food grain production. However, an imminent global starvation was averted through precise scientific intervention and pragmatic policy changes in the 1960s and was eulogized as the "Green Revolution". Miracle rice and wheat obtained through morphometric changes in the ideotype of these two crops yielded bumper harvest that nucleated in Asia and translated into Latin America. The altered agronomic traits in these two crops were the result of tinkering with the phyto-hormone "Gibberellin'. Recently, another plant hormone 'Cytokinin' has gained prominence for its involvement in the grain revolution in rice and other field crops. Suo moto homeostasis of CK by the cytokinin oxidase enzyme governs the cardinal shoot apical meristem that produces new flowering primordia thereby enhancing grain number. Similarly, the flowering hormone 'Florigen' impacts sympodia formation, flowering, and fruit production in tomato. The role of heterozygosity induced heterosis by florigen in revolutionizing tomato production and cellular homeostasis of CK by CK oxidising enzyme (CKX) in enhancing rice production has been path-breaking. This review highlights role of phytohormones in grain revolution and crop specific fine-tuning of gibberellins, cytokinins and florigen to accomplish maximum yield potential in field crops.

Success of microbial genes based transgenic crops: Bt and beyond Bt

Transgenic technology could hold the key to help farmers to fulfill the ever increasing fast-paced global demand for food. Microbes have always wondered us by their potentials and thriving abilities in the extreme conditions. The use of microorganisms as a gene source in transgenic development is a promising option for crop improvement. The aforesaid approach has already for improving the characteristics of food, industrial, horticulture, and floriculture crops. Many transgenic crops containing microbial genes have been accepted by the farmers and consumers worldwide over the last few decades. The acceptance has brought remarkable changes in the status of society by providing food safety, economic, and health benefits. Among transgenic plants harboring microbial genes, Bacillus thuringiensis (Bt) based transgenic were more focused and documented owing to its significant performance in controlling insects. However, other microbial gene-based transgenic plants have also reserved their places in the farmer's field globally. Therefore, in this review, we have thrown some light on successful transgenic plants harboring microbial genes other than Bt, having application in agriculture. Also, we presented the role of microbial genetic element and product thereof in the inception of biotechnology and discussed the potential of microbial genes in crop improvement.

Exploitation of Novel Bt ICPs for the Management of Helicoverpa armigera (Hübner) in Cotton (Gossypium hirsutum L.): A Transgenic Approach

Cotton is a commercial crop of global importance. The major threat challenging the productivity in cotton has been the lepidopteron insect pest Helicoverpa armigera or cotton bollworm which voraciously feeds on various plant parts. Biotechnological interventions to manage this herbivore have been a universally inevitable option. The advent of plant genetic engineering and exploitation of Bacillus thuringiensis (Bt) insecticidal crystal proteins (ICPs) marked the beginning of plant protection in cotton through transgenic technology. Despite phenomenal success and widespread acceptance, the fear of resistance development in insects has been a perennial concern. To address this issue, alternate strategies like introgression of a combination of cry protein genes and protein-engineered chimeric toxin genes came into practice. The utility of chimeric toxins produced by domain swapping, rearrangement of domains, and other strategies aid in toxins emerging with broad spectrum efficacy that facilitate the avoidance of resistance in insects toward cry toxins. The present study demonstrates the utility of two Bt ICPs, cry1AcF (produced by domain swapping) and cry2Aa (produced by codon modification) in transgenic cotton for the mitigation of H. armigera. Transgenics were developed in cotton cv. Pusa 8–6 by the exploitation of an apical meristem-targeted in planta transformation protocol. Stringent trait efficacy-based selective screening of T₁ and T₂ generation transgenic plants enabled the identification of plants resistant to H. armigera upon deliberate challenging. Evaluation of shortlisted events in T₃ generation identified a total of nine superior transgenic events with both the genes (six with cry1AcF and three with cry2Aa). The transgenic plants depicted 80–100% larval mortality of H. armigera and 10–30% leaf damage. Molecular characterization of the shortlisted transgenics demonstrated stable integration, inheritance and expression of transgenes. The study is the first of its kind to utilise a non-tissue culture-based transformation strategy for the development of stable transgenics in cotton harbouring two novel genes, cry1AcF and cry2Aa for insect resistance. The identified transgenic events can be potential options toward the exploitation of unique cry genes for the management of the polyphagous insect pest H. armigera.

Transgenic cowpea plants expressing Bacillus thuringiensis Cry2Aa insecticidal protein imparts resistance to Maruca vitrata legume pod borer

Fertile independent transgenic cowpea lines expressing the BtCry2Aa toxin with increased resistance to the most devastating lepidopteran insect pest, Maruca pod borer has been developed for the first time. Cowpea is a staple legume important for food and nutritional security in sub-Saharan Africa and Asia, where its production is limited by the key pest, legume pod borer (Maruca vitrata). Cowpea varieties resistant to M. vitrata are not known, hence, development of Maruca pod borer resistance cowpea through genetic engineering is a promising approach to improve its production. In the present study, transgenic cowpea plants expressing Bacillus thuringiensis Cry2Aa insecticidal protein were developed for the first time using Agrobacterium tumefaciens-mediated transformation of cotyledonary explants. T₀ plants recovered from Agrobacterium cocultured explants on medium containing 120 mgl^-1 of kanamycin were identified on the basis of the presence of transgenes by PCR, their integration into genome by Southern hybridization and expression of their transcripts by semi quantitative PCR (sqRT-PCR) and quantitative Real-time-PCR (qRT-PCR) and protein by Western blot analysis. The transformation efficiency obtained was 3.47% with 11 independent T₀ transgenic lines. The bioefficacy of Cry2Aa protein expressed in randomly selected four T₀ plant's leaves and pods was evaluated by feeding Maruca pod borer demonstrated a significant lower damage and a high level of Maruca mortality (more than 90%) for all these Bt lines. The inheritance of transgenes from T₀ to T₁ progeny plants was demonstrated by PCR analysis. The transgenic plants generated in this study can be used in cowpea breeding program for durable and sustainable legume pod borer resistance.

Agrobacterium tumefaciens-mediated in planta transformation strategy for development of transgenics in cotton (Gossypium hirsutum L.) with GFP as a visual marker

Cotton (Gossypium hirsutum L.), a mercantile crop plant, is grown worldwide for fiber and seed oil. As with other economically important crops, cotton is bogged down with many biotic and abiotic stress factors. Towards this, genetic engineering offers numerous protocols to engineer plants for better resilience. However, recalcitrance of cotton to plant tissue culture has been the major constraint for successful in vitro regeneration. Hence, alternate methods that evade tissue culture regeneration have been envisaged. Non tissue culture-based in planta transformation strategies are in vogue due to amenability and ease in the generation of transgenic plants. In the present study, we demonstrate the utility of an in planta transformation protocol and establishment of a stringent selection agent-based screening for the identification of transgenics. The genotype independent nature of the protocol was validated in cotton cv. Pusa 8-6 using GFP. Preliminary transformation efficiency of 28% was achieved with a screening efficiency of 20% in the presence of hygromycin. The proof of T-DNA integration by various molecular and expression analysis in T1 and T2 generations proved that this technique can be employed to generate transgenic cotton.

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.

An Apical Meristem-Targeted in planta Transformation Method for the Development of Transgenics in Flax (Linum usitatissimum): Optimization and Validation

Efficient regeneration of explants devoid of intrinsic somaclonal variations is a cardinal step in plant tissue culture, thus, a vital component of transgenic technology. However, recalcitrance of economically important crops to tissue culture-based organogenesis ensues a setback in the use of transgenesis in the genetic engineering of crop plants. The present study developed an optimized, genotype-independent, nonconventional tissue culture-independent in planta strategy for the genetic transformation of flax/linseed. This apical meristem-targeted in planta transformation protocol will accelerate value addition in the dual purpose industrially important but recalcitrant fiber crop flax/linseed. The study delineated optimization of Agrobacterium tumefaciens-mediated transformation and stable T-DNA (pCambia2301:GUS:nptII) integration in flax. It established successful use of a stringent soilrite-based screening in the presence of 30 mg/L kanamycin for the identification of putative transformants. The amenability, authenticity, and reproducibility of soilrite-based kanamycin screening were further verified at the molecular level by GUS histochemical analysis of T₀ seedlings, GUS and nptII gene-specific PCR, genomic Southern hybridization for stable integration of T-DNA, and expression analysis of transgenes by sqRT-PCR. This method resulted in a screening efficiency of 6.05% in the presence of kanamycin, indicating amenability of in planta flax transformation. The strategy can be a promising tool for the successful development of transgenics in flax.