Unconventional Routes To Developing Insect-resistant Crops
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Host plant susceptibility
A condition where some or all genetic variants of an insect species are able to
successfully colonize and reproduce on some or all genetic variants of a plant
species. [Compatible relationship].
Tolerance
A form of defense against herbivores where a plant species or genotype is
able to withstand or recover from damage or injury, such that significant loss
of fitness (i.e., survival, reproduction) is avoided
Insect virulence
The ability of an insect to overcome host defenses, causing alterations at the
physiological, molecular, and biochemical level of the host that ultimately
result in some form of damage or loss of fitness.
Insect non-virulence/avirulence
The inability of a genetic variant of a herbivorous insect species to overcome
host defenses, resulting in reduced insect population growth and reduced
damage to the host plant
Plant-Insect hologenomics
Plant and insect genomes co-evolve with a vast number of microbial
symbionts co-occurring in communities (i.e., microbiomes) that vary in
complexity, interrelatedness, and degree of intimacy with the host organism.
- The conglomerate genome (host + microbiome) is thus considered the
hologenome, the study of which is hologenomics.](https://image.slidesharecdn.com/pbt602sureshj-250725043300-370f41ed/85/Unconventional-Routes-To-Developing-Insect-resistant-Crops-7-320.jpg)
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D. Reconfiguration of metabolism
Metabolic habitat modification of host plant by insect,
reallocation of resources: growth versus defense
E. Production of key molecules mediating interactions
small RNAs, insect effector proteins, elicitor proteins
[insect, plant, or microbe derived; plant volatile emissions;
plant defensive chemicals] antifeedants, growth inhibitors
F. Late-stage or long-term responses
Repair mechanisms, detoxification, defense priming for
future interactions
G. Epigenetic or trans-generational responses
Maternal effects on gene expression, methylation patterns,
histone modifications](https://image.slidesharecdn.com/pbt602sureshj-250725043300-370f41ed/85/Unconventional-Routes-To-Developing-Insect-resistant-Crops-11-320.jpg)
Unconventional Routes To Developing Insect-resistant Crops
- 1. Unconventional Routes To Developing Insect- resistant Crops Dr. Suresh R. Jambagi Research Associate Division of Genomic Resources ICAR- National Bureau of Agricultural Insect Resources, Bengaluru jambagisuru@gmail.com
- 2. CONTENTS 1 • Introduction 2 • Key Concepts 3 • Unconventional approaches 4 • I. Bridging Plant-Insect divide 5 • II. Uncharted approaches to sustainable insect pest management 6 • Conclusion 2
- 3. 3 INTRODUCTION Much concerns over widespread use of insecticides and enhanced insect pest virulence under climate change. This fuels 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 much needed. Field appears to be static, urgently needing shifts in approaches to identify novel mechanisms by which insects colonize plants and plants avoid insect pressure.
- 4. 4 Necessary steps for advancing holistic methodologies that capture complex plant-insect molecular interactions. So, 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
- 5. 5 Studies bridging the plant-insect divide that no longer treat insects and plants as separate entities are needed to - (1) Develop durable tactics for managing insect virulence to plant defenses, (2) Identify novel sources of HPR to combat agricultural pests, and (3) Rely on predominately proactive rather than reactive management of insect virulence.
- 6. 6 KEY CONCEPTS Compatible interaction A condition when an insect is able to overcome molecular and physical plant responses, allowing them to colonize, feed, and reproduce. When plants do not have the molecular capability to trigger an effective response to prevent colonization, it is considered to be susceptible Incompatible interaction A condition where an organism is not able to feed, colonize, and reproduce on a plant. In such cases, plants might display an array of responses to prevent the attack. When those responses are effective, the organisms that display them are considered resistant Host plant resistance (HPR) Immunity of a plant species or genotype to colonization and feeding by an insect herbivore. Non-host plant resistance Immunity of an entire plant species to all genetic variants of a non-adapted pest or pathogen species. - For plant-insect interactions, this means a particular herbivorous insect species is unable to colonize (i.e., feed, reproduce, live or survive on) a non-host plant species
- 7. 7 Host plant susceptibility A condition where some or all genetic variants of an insect species are able to successfully colonize and reproduce on some or all genetic variants of a plant species. [Compatible relationship]. Tolerance A form of defense against herbivores where a plant species or genotype is able to withstand or recover from damage or injury, such that significant loss of fitness (i.e., survival, reproduction) is avoided Insect virulence The ability of an insect to overcome host defenses, causing alterations at the physiological, molecular, and biochemical level of the host that ultimately result in some form of damage or loss of fitness. Insect non-virulence/avirulence The inability of a genetic variant of a herbivorous insect species to overcome host defenses, resulting in reduced insect population growth and reduced damage to the host plant Plant-Insect hologenomics Plant and insect genomes co-evolve with a vast number of microbial symbionts co-occurring in communities (i.e., microbiomes) that vary in complexity, interrelatedness, and degree of intimacy with the host organism. - The conglomerate genome (host + microbiome) is thus considered the hologenome, the study of which is hologenomics.
- 8. 8 UNCONVENTIONAL APPROACHES I. Bridging Plant-Insect divide II. Uncharted approaches to sustainable insect pest management a) Embracing microbial partnerships: IPM in the Hologenomics Era b) Tolerating damage to survive: rethinking pest resistance traits c) Breeding for incompatibility: learning from interactions with non-host plants d) Adapting to crop domestication: counteracting tradeoffs a) Expanding the multidimensional framework for molecular plant-insect interactions b) A systems biology approach to plant- insect interactions
- 9. 9 I. Bridging Plant-Insect divide II. Uncharted approaches to sustainable insect pest management a) Expanding the multidimensional framework for molecular plant-insect interactions Plant responses to insect feeding and insect counterresponses are dynamic processes. Many levels of organizational and functional complexity, executed at different time scales ranging from fractions of seconds to days or even months. The well-known layers of defense concept in plants can therefore be expanded to include both plants and insects. Understanding of the molecular physiology of plant-insect interactions could lead to advances in crop protection.
- 10. 10 Layers Of Molecular Interaction Between Plant And Insects A. Plant-insect recognition/perception plant detection of herbivore feeding, insect choice of host plant and detection of defensives B. Early signaling responses Calcium waves and reactive oxygen species production, wounding responses, other early-stage/immediate responses C. Hormone-mediated signalling Plant phytohormones pathways, peptide hormones regulating herbivore digestive processes
- 11. 11 D. Reconfiguration of metabolism Metabolic habitat modification of host plant by insect, reallocation of resources: growth versus defense E. Production of key molecules mediating interactions small RNAs, insect effector proteins, elicitor proteins [insect, plant, or microbe derived; plant volatile emissions; plant defensive chemicals] antifeedants, growth inhibitors F. Late-stage or long-term responses Repair mechanisms, detoxification, defense priming for future interactions G. Epigenetic or trans-generational responses Maternal effects on gene expression, methylation patterns, histone modifications
- 12. 12
- 13. 13 Recent developments Sl. No. Approaches Reference 1 Plant perception of insect eggs Hilker and Fatouros, 2016; Bittner et al., 2017 2 Induction of defenses in response to deposition of feces (e.g., caterpillar frass) on plant tissues Ray et al., 2016 3 Long-term associations between insects and host plants Hohenstein et al., 2019 4 Epigenetic mechanisms and trans-generational or multiannual effects (e.g., wheat aphids, bilberry folivores,) Hu et al., 2018; Benevenuto et al., 2018 Studies unraveling the time series of molecular events that take place during plant-insect interactions, using both controlled laboratory experiments and broader field-based assessments, are therefore critical to advancing our knowledge of HPR and insect virulence.
- 14. 14 b) A systems biology approach to plant-insect interactions Molecular profiling or multi-species omics approaches are increasingly being used to characterize and predict plant-microbe and insect microbe molecular interactions. But have yet to be embraced by the research community Currently, the parallel responses of plants and insects during colonization or feeding are unexplored. First step toward developing a systems biology approach: Conduct research from a unified multi-species perspective rather than focusing on insect or plant responses alone. I. Bridging Plant-Insect divide II. Uncharted approaches to sustainable insect pest management
- 15. 15 Emerging areas of research: 1. Role of small regulatory RNAs play in mediating plant-insect interactions (Wang et al., 2017) Ex: microRNAs, plant or insect-derived 2. Microbial dimension to plant-insect interactions It make us to understand tri-trophic molecular interactions important for crop health and protection from pests in diverse agroecosystems. Ex: plant production of volatile chemicals not only mediates insect attraction to and identification of suitable hosts but can alert neighboring plants to the presence of herbivores and preemptively trigger defenses that disarm insect attackers (Dicke and Baldwin, 2010). Note: Induced susceptibility and obviation of resistance are understudied factors that could contribute to improved pest management in agroecosystems.
- 16. 16 a) Embracing microbial partnerships: IPM in the Hologenomics Era Microbes influence plant-insect interactions through both direct and indirect mechanisms (Giron et al., 2017). Microbial communities can augment the intrinsic defensive capabilities of their host (plant or insect) or provide novel functions that influence the coevolutionary arms race between plants and insects. Ex: Microbes enable insects to contend with plant chemical defences in a multitude of ways, including counteracting harmful effects through direct detoxification or metabolism, reducing oxidative stress, and by synthesizing phytohormones to manipulate plant physiology (Mason, 2020). I. Bridging Plant-Insect divide II. Uncharted approaches to sustainable insect pest management
- 17. 17 Do you know…? Virulence-enhancing bacteria occurring in insect saliva have been found in Colorado potato beetle larva (Chung et al., 2013) and the Russian wheat aphid (Luna et al., 2018) Known fact Insect-vectored plant pathogens (e.g., viruses, phytoplasma) can manipulate host plant development and defense responses (Carr et al., 2018). Concept-1: Microbial communities are not only responsible for enhancing insect pest virulence, they also contribute to plant anti-herbivore defenses. Ex: Some plant- and soil-associated microbes (e.g., Pseudomonas spp.) are multi-talented colonizers that suppress plant pathogens and stimulate plant defenses, essentially acting as plant beneficial and insect pathogens (Vacheron et al., 2019). Concept-2: Microbes play a role in plant detection of chewing insects Ex: Symbiotic relationships with AM fungi are one of the best example of microbial-mediated defense priming against chewing herbivores in tomato (Rivero et al., 2021).
- 18. 18 Microbial manipulation strategies for pest management 1. Genetic modification of insect microbiomes To reduce virulence by disrupting fundamental symbiotic relationships or turning resident microbiota against their insect host (Qadri et al., 2020). 2. Use of beneficial plant associated microbes It has generated considerable interest as a means to enhance natural defense against insect pests (Kaur et al., 2020). 3. plant or soil microbes could be used to attract insects to unsuitable hosts or to trap crops. 4. Plant-associated entomopathogens targeting insect pests are also particularly promising candidates for biocontrol applications in agroecosystems (Wei et al., 2020).
- 19. 19 5. Use of microbes on leaves to deter aphid colonization by altering visual cues used in host plant detection (Hendry et al., 2018). 6. Modification of plant volatile profiles, beneficial root microbes can also enhance natural biocontrol of pests (Pangesti et al., 2015). 7. The development of insect pest suppressive soils Harnesses the benefits of soil-borne insect pathogens through agricultural practices that promote ecological diversity (reduced tillage and crop rotations) (Hokkanen and MenzlerHokkanen, 2018)
- 20. 20 b) Tolerating damage to survive: rethinking pest resistance traits Tolerance arises as a form of compatible relationship where no loss of fitness occurs in either the plant or insect. Measurements of tolerance traits markedly differ between chewing and sap-feeding insects. Chewing insects: focuses on thresholds of plant damage (defoliation) and regrowth rates (Welter, 1991). Sap-feeding insects: combination of relative damage ratings and insect population size (e.g., Pierson et al., 2010). I. Bridging Plant-Insect divide II. Uncharted approaches to sustainable insect pest management
- 21. 21 Allowing insect injury to occur seems counterintuitive, but research shows a number of compensatory mechanisms can enable long-term survival and reproduction in the face of seemingly high insect pressure. Ex: Best-studied examples of tolerance occurs in soybean, where a suite of mechanisms (upregulation of detoxification pathways, increased photosynthetic rates) enable plants to withstand high levels of aphid pressure while maintaining yield (Chapman et al., 2018). Research on tolerance mechanisms has primarily involved sap-feeding pests (aphids).
- 22. 22 DEMERITS: Grower and consumer perception is another challenge to overcome when incorporating tolerance into IPM strategies because ‘‘clean plants’’ are often viewed as necessary for high yield and quality products. Acceptance of tolerant plant products will likely depend on what tissues (e.g., fruits versus leaves) show damage or presence of insects and how crops are used or sold (e.g., fresh market, wholesale, processed) DO YOU KNOW? Microbes found in herbivore saliva and honeydew excretions are also known to trigger plant defence responses (Wari et al., 2019). Mining herbivore microbiomes for candidate microbes that mediate plant defence responses (i.e., trigger tolerance mechanisms) could be a valuable resource for developing novel microbial bio stimulants with dual functions that promote plant vigor while also counteracting pest injury.
- 23. 23 c) Breeding for incompatibility: Learning from interactions with non-host plants Mechanisms of non-host insect resistance are rarely investigated. Insect feeding is perceived differently and triggers distinct responses in preferred versus non-preferred host plants (Wang et al., 2016). While, insect rejection of non-hosts is uniquely influenced by a number of factors, including volatile chemical profiles, nutrient composition, and levels of defensive chemicals (Powell et al., 2006). A non-host plant is one where incompatible interactions involving mismatched cues for host recognition, production of toxic defensive chemicals, and/or low nutritional quality prevent sustained insect colonization and reproduction. I. Bridging Plant-Insect divide II. Uncharted approaches to sustainable insect pest management
- 24. 24 Plant-insect incompatibility resulting from non-host resistance could therefore provide a novel route to achieve efficient, broad- spectrum, and durable resistance against a vast number of potential plant foes. Characterization of beneficial or defensive mutualisms between microbes and non-host plants is another route to uncovering mechanisms governing resistance to insects that could provide hologenetic targets for breeding
- 25. 25 Strategies To Attain The Goal: 1. Cross-species hybridization between susceptible crops and non-host plants. 2. Breeding for non-host traits that boost insect resistance (e.g., highly abundant or insect-induced volatiles, interactions with microbial symbionts that prime immune defenses). 3. Lack of natural variation for resistance in host and nonhost plant species can also be overcome by using genome editing technologies such as CRISPR/Cas and/or through chemical or radiation mutagenesis, or by augmenting the plant microbiome with beneficial microbial inoculants that function in herbivore resistance.
- 26. 26 Examples: 1. Aphid probing behaviour can occur regardless of host suitability, but individuals may be unable to successfully feed from non-host plants or phloem feeding may occur but non-host-plant defensive chemicals or low nutritional quality causes mortality (Dastranj et al., 2018). 2. Ladybird beetle, Epilachna vigintioctopunctata, host recognition combined with layers of constitutive and induced defenses are proposed to determine acceptance and feeding on compatible versus non-host plants (Shinogi et al., 2005).
- 27. 27 d) Adapting to crop domestication: counteracting tradeoffs Crop domestication is among the most important events in human history but has resulted in a number of unintentional consequences that pose a challenge for agricultural sustainability. Approaches: 1. Back-To-The-Wild Concept In order to counteract the negative consequences of domestication for plant-insect interactions, several ‘‘back-to-the- wild’’ approaches could be used to identify the basis of domestication syndromes and provide candidates for improved breeding efforts. Ex: Wheat: a transcriptomic and metabolic comparison between wild and domesticated genotypes identified genomic differences in both plant groups that could influence interactions with insect herbivores. While wild wheat genotypes have high physical defenses (higher trichome density), their domesticated counterparts have high chemical defenses against aphids (Batyrshina et al., 2020). I. Bridging Plant-Insect divide II. Uncharted approaches to sustainable insect pest management
- 28. 28 2. Screening of wild crop relatives For identifying candidates for hybridization in breeding programs aiming to develop new pest-resistant varieties. Ex: Enhanced aphid resistance was identified in a screen of wild Solanum accessions challenged with two common species which could be useful for improving domesticated tomato varieties (Frechette et al., 2010). NOTE: Wild crop relatives are generally less productive from a yield perspective, they display a larger degree of improved resistance to stresses, including resistance to aphids. Thus, approaches combining wild species with elite varieties could help to develop high-yielding and pest-resistant crops.
- 29. 29 3. Alleviation of selective pressures Implementation of management practices that alleviate selective pressures associated with agricultural environments might be another strategy to sustainably counteract negative effects of domestication on insect resistance in crop plants. Research shows increased pest adaptation and evolution when repetitive and homogeneous management strategies (irrigation, fertilization, and tillage regimes) are used in agricultural systems (Jin et al., 2015). Ex: Incorporation of crop rotation and cover cropping strategies could lead to decreased pesticide application. Must to be consider: Management should also consider how best to alleviate pest pressure by maintaining beneficial microbes that help plants defend themselves against insect damage.
- 30. 30 The main issue is not a general lack of information on the intricate molecular mechanisms employed by plants and insects in their ever-evolving interactions. Instead, the problem lies in our limited ability thus far to translate knowledge into broader biological principles and solutions to challenges faced in modern agroecosystems. CONCLUSION