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Younkin GC, Alani ML, Capador AP, Fischer HD, Mirzaei M, Hastings AP, Agrawal AA, Jander G. Cardiac glycosides protect wormseed wallflower (Erysimum cheiranthoides) against some, but not all, glucosinolate-adapted herbivores. THE NEW PHYTOLOGIST 2024; 242:2719-2733. [PMID: 38229566 PMCID: PMC11116068 DOI: 10.1111/nph.19534] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Received: 11/21/2023] [Accepted: 12/22/2023] [Indexed: 01/18/2024]
Abstract
The chemical arms race between plants and insects is foundational to the generation and maintenance of biological diversity. We asked how the evolution of a novel defensive compound in an already well-defended plant lineage impacts interactions with diverse herbivores. Erysimum cheiranthoides (Brassicaceae), which produces both ancestral glucosinolates and novel cardiac glycosides, served as a model. We analyzed gene expression to identify cardiac glycoside biosynthetic enzymes in E. cheiranthoides and characterized these enzymes via heterologous expression and CRISPR/Cas9 knockout. Using E. cheiranthoides cardiac glycoside-deficient lines, we conducted insect experiments in both the laboratory and field. EcCYP87A126 initiates cardiac glycoside biosynthesis via sterol side-chain cleavage, and EcCYP716A418 has a role in cardiac glycoside hydroxylation. In EcCYP87A126 knockout lines, cardiac glycoside production was eliminated. Laboratory experiments with these lines revealed that cardiac glycosides were highly effective defenses against two species of glucosinolate-tolerant specialist herbivores, but did not protect against all crucifer-feeding specialist herbivores in the field. Cardiac glycosides had lesser to no effect on two broad generalist herbivores. These results begin elucidation of the E. cheiranthoides cardiac glycoside biosynthetic pathway and demonstrate in vivo that cardiac glycoside production allows Erysimum to escape from some, but not all, specialist herbivores.
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Affiliation(s)
- Gordon C. Younkin
- Boyce Thompson Institute, 533 Tower Rd, Ithaca, NY 14853, USA
- Plant Biology Section, School of Integrative Plant Science, Cornell University, Ithaca, New York 14853
| | - Martin L. Alani
- Boyce Thompson Institute, 533 Tower Rd, Ithaca, NY 14853, USA
| | | | | | - Mahdieh Mirzaei
- Boyce Thompson Institute, 533 Tower Rd, Ithaca, NY 14853, USA
| | - Amy P. Hastings
- Department of Ecology and Evolutionary Biology, Cornell University, Ithaca, NY 14853, USA
| | - Anurag A. Agrawal
- Department of Ecology and Evolutionary Biology, Cornell University, Ithaca, NY 14853, USA
| | - Georg Jander
- Boyce Thompson Institute, 533 Tower Rd, Ithaca, NY 14853, USA
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Agrawal AA, Hastings AP, Duplais C. Testing the selective sequestration hypothesis: Monarch butterflies preferentially sequester plant defences that are less toxic to themselves while maintaining potency to others. Ecol Lett 2024; 27:e14340. [PMID: 38017619 DOI: 10.1111/ele.14340] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/28/2023] [Revised: 10/08/2023] [Accepted: 10/23/2023] [Indexed: 11/30/2023]
Abstract
Herbivores that sequester toxins are thought to have cracked the code of plant defences. Nonetheless, coevolutionary theory predicts that plants should evolve toxic variants that also negatively impact specialists. We propose and test the selective sequestration hypothesis, that specialists preferentially sequester compounds that are less toxic to themselves while maintaining toxicity to enemies. Using chemically distinct plants, we show that monarch butterflies sequester only a subset of cardenolides from milkweed leaves that are less potent against their target enzyme (Na+ /K+ -ATPase) compared to several dominant cardenolides from leaves. However, sequestered compounds remain highly potent against sensitive Na+ /K+ -ATPases found in most predators. We confirmed this differential toxicity with mixtures of purified cardenolides from leaves and butterflies. The genetic basis of monarch adaptation to sequestered cardenolides was also confirmed with transgenic Drosophila that were CRISPR-edited with the monarch's Na+ /K+ -ATPase. Thus, the monarch's selective sequestration appears to reduce self-harm while maintaining protection from enemies.
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Affiliation(s)
- Anurag A Agrawal
- Department of Ecology and Evolutionary Biology, Cornell University, Ithaca, New York, USA
- Department of Entomology, Cornell University, Ithaca, New York, USA
| | - Amy P Hastings
- Department of Ecology and Evolutionary Biology, Cornell University, Ithaca, New York, USA
| | - Christophe Duplais
- Department of Entomology, Cornell AgriTech, Cornell University, Geneva, New York, USA
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Younkin GC, Alani ML, Capador AP, Fischer HD, Mirzaei M, Hastings AP, Agrawal AA, Jander G. Cardiac glycosides protect wormseed wallflower ( Erysimum cheiranthoides) against some, but not all, glucosinolate-adapted herbivores. BIORXIV : THE PREPRINT SERVER FOR BIOLOGY 2023:2023.09.19.558517. [PMID: 37790475 PMCID: PMC10542140 DOI: 10.1101/2023.09.19.558517] [Citation(s) in RCA: 1] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 10/05/2023]
Abstract
The chemical arms race between plants and insects is foundational to the generation and maintenance of biological diversity. We asked how the evolution of a novel defensive compound in an already well-defended plant lineage impacts interactions with diverse herbivores. Erysimum cheiranthoides (Brassicaceae), which produces both ancestral glucosinolates and novel cardiac glycosides, served as a model.We analyzed gene expression to identify cardiac glycoside biosynthetic enzymes in E. cheiranthoides and characterized these enzymes via heterologous expression and CRISPR/Cas9 knockout. Using E. cheiranthoides cardiac glycoside-deficient lines, we conducted insect experiments in both the laboratory and field.EcCYP87A126 initiates cardiac glycoside biosynthesis via sterol side chain cleavage, and EcCYP716A418 has a role in cardiac glycoside hydroxylation. In EcCYP87A126 knockout lines, cardiac glycoside production was eliminated. Laboratory experiments with these lines revealed that cardiac glycosides were highly effective defenses against two species of glucosinolate-tolerant specialist herbivores but did not protect against all crucifer-feeding specialist herbivores in the field. Cardiac glycosides had lesser to no effect on two broad generalist herbivores.These results begin elucidation of the E. cheiranthoides cardiac glycoside biosynthetic pathway and demonstrate in vivo that cardiac glycoside production allows Erysimum to escape from some, but not all, specialist herbivores.
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Affiliation(s)
- Gordon C. Younkin
- Boyce Thompson Institute, 533 Tower Rd, Ithaca, NY 14853, USA
- Plant Biology Section, School of Integrative Plant Science, Cornell University, Ithaca, New York 14853
| | - Martin L. Alani
- Boyce Thompson Institute, 533 Tower Rd, Ithaca, NY 14853, USA
| | | | | | - Mahdieh Mirzaei
- Boyce Thompson Institute, 533 Tower Rd, Ithaca, NY 14853, USA
| | - Amy P. Hastings
- Department of Ecology and Evolutionary Biology, Cornell University, Ithaca, NY 14853, USA
| | - Anurag A. Agrawal
- Department of Ecology and Evolutionary Biology, Cornell University, Ithaca, NY 14853, USA
| | - Georg Jander
- Boyce Thompson Institute, 533 Tower Rd, Ithaca, NY 14853, USA
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Agrawal AA, Hastings AP. Tissue-specific plant toxins and adaptation in a specialist root herbivore. Proc Natl Acad Sci U S A 2023; 120:e2302251120. [PMID: 37216531 PMCID: PMC10235950 DOI: 10.1073/pnas.2302251120] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/09/2023] [Accepted: 04/17/2023] [Indexed: 05/24/2023] Open
Abstract
In coevolution between plants and insects, reciprocal selection often leads to phenotype matching between chemical defense and herbivore offense. Nonetheless, it is not well understood whether distinct plant parts are differentially defended and how herbivores adapted to those parts cope with tissue-specific defense. Milkweed plants produce a diversity of cardenolide toxins and specialist herbivores have substitutions in their target enzyme (Na+/K+-ATPase), each playing a central role in milkweed-insect coevolution. The four-eyed milkweed beetle (Tetraopes tetrophthalmus) is an abundant toxin-sequestering herbivore that feeds exclusively on milkweed roots as larvae and less so on milkweed leaves as adults. Accordingly, we tested the tolerance of this beetle's Na+/K+-ATPase to cardenolide extracts from roots versus leaves of its main host (Asclepias syriaca), along with sequestered cardenolides from beetle tissues. We additionally purified and tested the inhibitory activity of dominant cardenolides from roots (syrioside) and leaves (glycosylated aspecioside). Tetraopes' enzyme was threefold more tolerant of root extracts and syrioside than leaf cardenolides. Nonetheless, beetle-sequestered cardenolides were more potent than those in roots, suggesting selective uptake or dependence on compartmentalization of toxins away from the beetle's enzymatic target. Because Tetraopes has two functionally validated amino acid substitutions in its Na+/K+-ATPase compared to the ancestral form in other insects, we compared its cardenolide tolerance to that of wild-type Drosophila and CRISPR-edited Drosophila with Tetraopes' Na+/K+-ATPase genotype. Those two amino acid substitutions accounted for >50% of Tetraopes' enhanced enzymatic tolerance of cardenolides. Thus, milkweed's tissue-specific expression of root toxins is matched by physiological adaptations in its specialist root herbivore.
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Affiliation(s)
- Anurag A. Agrawal
- Department of Ecology and Evolutionary Biology, Cornell University, Ithaca, NY14853
- Department of Entomology, Cornell University, Ithaca, NY14853
| | - Amy P. Hastings
- Department of Ecology and Evolutionary Biology, Cornell University, Ithaca, NY14853
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