Keywords
Plant defense, Insect herbivory, Jasmonate (JA), Elicitor, Effector
Plant defense, Insect herbivory, Jasmonate (JA), Elicitor, Effector
There are about 1 million insects and over 300,000 plants on our planet, and plant–insect interactions are the driving force of biodiversity. With long-term co-evolution, plants and insects have developed sophisticated mechanisms for adaptation1. In general, plants can recognize herbivore-/damage-/microbe-associated molecular patterns (HAMPs/DAMPs/MAMPs) and make the right defense. The early defense responses contain depolarization of the plasma transmembrane potential, changes of cytosolic Ca2+, reactive oxygen species (ROS) burst, and mitogen-activated protein kinase (MAPK)2,3. Most of these reactions are able to activate jasmonate (JA)-mediated plant defense4,5. JA is a main regulator of plant defense and its synthesis and regulation have been extensively studied6–9. Recent studies reveal new insights in JA oxidative metabolism and their negative regulation in the JA pathway10,11. In most plants, JA-Ile is the active signal recognized by the COI1 and promotes JAZ–COI1 interaction leading to JAZ degradation. This relieves the JAZ-interacting transcription factors to activate downstream defense gene expressions12–16. However, in Marchantia polymorpha, MpCOI1 recognized OPDA-Ile instead of JA-Ile. That work revealed the ligand-receptor co-evolution of the JA signaling pathway in land plants17. MYC2 is a well-studied transcription factor in JA signaling and can interact with both JAZ and MED25, the subunit of the mediator complex. The JAZ proteins recruit TOPLESS scaffold protein to inhibit gene transcription, whereas MED25 brings COI1 to MYC2 targeting promoters18. In this model, COI1 is thought to be the nuclear receptor. JAT1, which localizes at the nuclear envelope and plasma membrane, is the transporter responsible for the influx of JA-Ile into nucleus19. To balance the tradeoff between growth and defense, plants strictly regulate JA signaling to avoid a hypersensitive defense response20,21. Some development regulators, including SPLs and DELLAs, target JAZ or MYC transcription factors to modulate JA signaling output22–26. Interestingly, some insects use similar strategies to attenuate plant defense for fitness.
Herbivorous insects have different mouthparts and feeding habits. Active molecules from insects’ oral secretion (OS) (which contains regurgitant and saliva), frass, ovipositional fluids, and the endosymbionts of insects have a large impact on plant defense. Some of these molecules used by plants to trigger specialized defense are called elicitors, and those to weaken the plant defense response are defined as effectors. Plant–insect recognition is the first and also the key step of an effective defense in plants27,28. In this review, we discuss recent research advances in insect elicitors and effectors and their roles in plant–insect interactions.
Plant perception of an insect attack is the first step of defense. Insect herbivory raised diverse active molecules such as damage-associated molecules, insect-derived elicitors, and the plant endogenous molecules activated by insect digestive enzymes (Figure 1). The specific and efficient recognition of these active molecules guarantees the timely priming of plant defense29,30.
Wounding damage caused by insect herbivory will quickly trigger plant defense signaling. The first reported damage-related peptide signal was systemin, an 18–amino acid polypeptide cleaved from prosystemin (inactive form) in tomato upon wounding stimulus31. Systemin promotes JA accumulation and activates the expression of genes encoding proteinase inhibitors which have insecticide activity32. Whereas systemin had been reported long before, its receptor SYR1, a leucine-rich repeat receptor kinase (LRR-RK), was identified recently. The introgression line, which lacks SYR1 expression, is highly sensitive to Spodoptera littoralis33. Besides systemin, other wound-induced peptides had been identified in plants, including Arabidopsis, maize, and rice. The application of synthetic 23–amino acid maize Peps could mimic the Spodoptera exigua attack, and similar Peps were found in rice recently34,35. In Arabidopsis, AtPeps, which is generated from PROPEPs under the catalyzation of the cysteine protease METACASPASE4 (MC4), acts as signals to trigger both JA and SA signaling pathways36,37. Like the systemin-SYR1 module, the reported receptors of AtPeps—AtPEPR1 and AtPEPR2—are also classified in the LRR-RK family38,39.
From Spodoptera frugiperda larval OSs, researchers isolated a disulfide-bridged peptide (+ICDINGVCVDA−), termed inceptin, that can induce the accumulation of defense hormones such as ethylene, JA, and SA in cowpea plants. Inceptin is the proteolytic fragment of chloroplastic ATP synthase γ-subunit of cowpea plants digested by S. frugiperda larvae40,41. Recently, on BioRxiv, it was reported that the receptor of inceptin in plants was a leucine-rich repeat receptor-like protein, INR, which is distinguished from LRR-RKs by lacking an intracellular kinase domain42. These findings expand the paradigm of plant surface recognition of insect herbivory.
Besides plant signal molecules activated by insect feeding, a number of reported elicitors are derived from insects themselves and most of them belong to HAMPs43. It had been reported that the OS, the oviposition and the honeydew of insects could induce a plant defense response, including the accumulation of JA and secondary metabolism44. These insect-derived elicitors can be classified as fatty acid derivatives, enzymes, and some other proteins43.
The first identified fatty acid–amino acid conjugate (FAC) elicitor was volicitin, which was isolated from S. exigua larval OSs. Volicitin can induce the emission of volatiles in maize to attract predators45. After volicitin, other forms of FACs from various insect OSs had been found in succession46,47. In Nicotiana attenuate, FACs from Manduca sexta activate the MAPK pathway48. Besides FACs, califerins, the sulfooxy fatty acids that exist in OSs of grasshopper (Schistocerca americana) larvae, also have elicitor activity49. Glucose oxidases (GOXs) and β-glucosidase are enzyme-like elicitors. GOX is identified from Helicoverpa zea and specifically activates defense response in tomato50,51. The β-glucosidase in Pieris brassicae larval OSs triggers the emission of volatiles from wounded cabbage leaves and this attracts predators such as parasitic wasp52,53. Lipase and phospholipase C are other types of salivary enzyme-like elicitors. Lipase of Schistocerca gregaria OS elevates oxylipin accumulation and defense response in Arabidopsis54. Phospholipase C of Spodoptera frugiperda induces the accumulation of proteinase inhibitors in corn55.
The above-mentioned elicitors are from chewing insects. The elicitors from the piercing-sucking insects are isolated largely from salivary glands. The mucin-like salivary protein (NlMLP) of planthopper (Nilaparvata lugens) is a double-edged sword. On one hand, it contributes to the formation of salivary sheaths for successful feeding; on the other hand, it was used by plants to trigger a defense response, like Ca2+ mobilization, the MEK2 MAPK cascades, and JA signaling transduction, thereby reducing the performance of N. lugens56. Tetranins is another characterized elicitor identified from Tetranychus urticae. Tetranins increases the expression of defense genes and activates JA, salicylic acid (SA), and abscisic acid biosynthesis in plant. It also promotes volatile emission to attract predatory mites57.
Some elicitors are from endosymbionts. MAMPs could be released through herbivory OSs and recognized by plants to induce pattern-triggered immunity (PTI)58,59. The chaperon GroEL from the endosymbionts Buchnera of potato aphids (Macrosiphum euphorbiae) induces oxidative bursts and PTI in Arabidopsis60. From the S. littoralis larval OSs, the porin-like proteins most likely of bacterial origin can induce the early response of plant defense61. A recent report reveals that some elicitors are from honeydew-associated microbes in sucking arthropods62.
To adapt to their host plants, insects have developed multilayered means for fitness. Besides releasing elicitors, the insect releases effectors that disturb host plant defense response for successful feeding63. The reported insect effectors are identified from both the herbivory itself and insect-related microbiomes (Figure 1).
The first reported insect effector was GOX from the chewing insect, H. zea, which inhibits nicotine accumulation and elevates the SA-mediated PR-1a protein level in tobacco64,65. Notably, the same GOX protein induces plant response in tomato50,51, which we discussed in the ‘Elicitors secreted by insects’ section. This suggests that the same protein acts as the effector or as the elicitor depending on their interacted host plant. Another piece of evidence in support of insect effectors is that the S. littoralis larvae that fed on OS pretreated plants had a greater weight increase66.
The direct interaction with JA signaling-related components is an efficient way for herbivory effectors to inhibit plant defense. In our recent work, we isolated a venom-like protein termed HARP1, which is identified from the OS of Helicoverpa armigera. HARP1 can interact with multiple JAZ proteins of Arabidopsis and cotton plants to prevent COI1-mediated JAZ degradation, thereby blocking the JA signaling output67. SSGP-71 is an E3 ubiquitin ligase–mimicking protein in Hessian fly (Mayetiola destructor). It allows the insect to hijack the plant proteasome and block the basal immunity68. These studies fill in the gap of the working mechanism about how insects manipulate effectors to block plant defense for better adaptation.
Some insect effectors inhibit plant defense by interfering with the crosstalk between SA and JA. For example, Bt56 from the whitefly (Bemisia tabaci) enhanced the performance of the whitefly on tobacco by decreasing JA signaling through the antagonism between JA and SA. Bt56 could directly interact with KNOTTED 1-like homeobox transcription factor NTH202 and eliminate the negative modulation of NTH202 on SA accumulation69. Armet, the effector of pea aphid (Acyrthosiphon pisum) protein, induced SA accumulation by blocking SA methylation and enhanced the pathogen resistance in plants, reflecting a novel tripartite interaction of insect–plant–pathogen70,71.
The extracellular DNA and hydrogen peroxide that are released by damaged cells can trigger plant defense30. Therefore, some insects secrete effectors to eliminate the production of these DAMPs. The planthopper (Laodelphax striatellus) secretes salivary DNase II, which acts as an effector by erasing extracellular DNA, and the Trichoplusia ni salivary catalase functions as an ROS scavenger to reduce hydrogen peroxide, thus inhibiting ROS burst and other plant defense responses72,73.
Moreover, some effectors were reported to target other defense-related proteins in plants. A set of saliva proteins in aphids were proven to have effector activity through proteomic combined RNA sequencing (RNA-seq) analysis63,74–79. A macrophage migration inhibitory factor (MIF) from pea aphid saliva inhibits immune response in N. benthamiana and improves aphid performance. Interestingly, the MIFs in vertebrates are also involved in the immune pathway, suggesting the highly conserved function of MIF80,81. Vacuolar protein sorting-associated protein 52 (VPS52) in potato (Solanum tuberosum) has negative impacts on green peach aphid (Myzus persicae) infection. M. persicae saliva-secreted protein Mp1 targets the VPS52 as an effective virulence strategy77,82. Me10 from M. euphorbiae interacts with tomato TFT7, a 14-3-3 isoform involved in aphid resistance, and enhances aphid longevity and fecundity83. Some effectors can target the host cell wall. Expansin-like protein (HaEXPB2) from the nematode (Heterodera avenae) binds to cellulose of tobacco, thereby increasing nematode infectivity84.
The effectors mentioned above are generated from the insect itself. Other effectors are also derived from insect-borne microbe. Although the exact effector components need to be explored, it was found that Colorado potato beetle (Leptinotarsa decemlineata) larvae suppress tomato defense response by exploiting bacteria in their OSs and gut85,86. Besides bacteria, some active molecules from vector-borne pathogens are reported to interfere with plant defense and are of benefit for their insect vectors living on host plants87. The phytoplasm protein SAP11 and SAP54 of aster yellows phytoplasma strain witches’ broom was proposed to promote aphid colonization and also interfere with plant development88–90. The βC1 of tomato yellow leaf curl China virus directly interacts with MYC2 protein to decrease the MYC2-regulated terpene synthase, thereby reducing plant resistance to the whitefly91. The 2b protein of the aphid-borne cucumber mosaic virus (CMV) stabilizes JAZ proteins by direct interaction, thus blocking JA signaling output, and this benefits aphid (M. persicae) performance on the host plant92. The C2 protein of tomato yellow leaf curl virus can also compromise JA signaling in tobacco by interacting with plant ubiquitin to block JAZ1 protein degradation, thereby reducing plant resistance to the insect vector whitefly93. These studies reveal the intricate interaction of plant–virus–insect vector. In Table 1, we summarize the reported insect-associated elicitors and effectors from different species and their probable roles.
Name | Origin | Biofunction | References | ||
---|---|---|---|---|---|
Elicitors | Plant-derived | Systemin | Wounded tomato plants | Perceived by SYR1, induce accumulation of proteinase inhibitor and ethylene, and induce oxidative bursts | 31,33 |
PEPs | Wounded plants (Arabidopsis, maize, rice) | Induce defensin and burst of hydrogen peroxide (H2O2) after perceiving by PEPRs | 36,38,39 | ||
Inceptin | Degradation of cowpea ATP synthase by Spodoptera frugiperda during herbivory | Increase the concentration of JA and SA by interacting with INR | 40,42 | ||
Derived from insect | Volicitin | Spodoptera exigua | Induce volatiles emission in corn | 45 | |
Caeliferins | Schistocerca americana | 49 | |||
GOX | Helicoverpa zea, Ostrinia nubilalis | Specifically promote defense response in tomato | 50,51 | ||
β-glucosidase | Pieris brassicae | Increase volatile emission in cabbage | 52 | ||
Lipase | Schistocerca gregaria | Elevate the oxylipins accumulation in Arabidopsis | 54 | ||
Phospholipase C | S. frugiperda | Trigger proteinase inhibitors accumulation in corn | 55 | ||
Bruchins | Bruchus pisorum | Induce neoplasms formation beneath the insect egg in pea | 94,95 | ||
NlMLP | Nilaparvata lugens | Induce plant defense response in rice | 56 | ||
Tetranins | Tetranychus urticae | Cytosolic calcium influx and membrane depolarization induce biosynthesis of JA. SA and ABA in kidney bean | 57 | ||
GroEL |
Buchnera in Macrosiphum euphorbiae | Induce PTI and ROS accumulation in Arabidopsis | 60 | ||
Porin-like proteins | Bacteria in Spodoptera littoralis | Trigger membrane potential changes and cytosolic Ca2+ elevations in Arabidopsis and Vicia faba | 61 | ||
Unidentified | Gut-associated bacteria in H. zea | Increase salivary GOX to induce defense in tomato | 96,97 | ||
Unidentified | Honeydew-associated microbes N. lugens | Induce accumulation of phytoalexins and volatile emission in rice | 62,98 | ||
Effectors | Insect-derived | GOX | H. zea | Decrease nicotine accumulation in tobacco | 64 |
HARP1 | Helicoverpa armigera | Interact with and stabilize JAZs, depress JA signaling in Arabidopsis | 67 | ||
SSGP-71 | Mayetiola destructor | Interact with Skp, decrease plant proteasome activity, thus block hormone signaling in wheat | 68 | ||
Bt56 | Bemisia tabaci | Interact with NTH202 to increase SA biosynthesis, thus decrease JA response in tobacco | 69 | ||
Armet | Acyrthosiphon pisum | Help feeding of insect, induce SA accumulation and pathogen response in N. benthamiana and Medicago truncatula | 70,71 | ||
DNase II | Laodelphax striatellus | Erase extracellular DNA released by damaged cell in rice | 72 | ||
Catalase | Trichoplusia ni | Reduce H2O2 in tomato | 73 | ||
C002 | A. pisum, Myzus persicae | ApC002 and MpC002 help insect foraging and feeding on fava bean and N. benthamiana, respectively | 63,74 | ||
MIF | A. pisum, M. persicae | Improve aphid performance, inhibit immune response in N. benthamiana | 80 | ||
Mp1 | M. persicae | Interact with VPS52 to relocalize to vesicle-like structures and enhance insect virulence in Arabidopsis and potato | 82 | ||
Me10 | M. euphorbiae | Interact with TFT7, enhance the longevity and fecundity on tomato | 83 | ||
Mp42, Mp55 Me23 | M. persicae, M. euphorbiae | Increase aphid reproduction, suppress N. benthamiana defenses | 99,100 | ||
HaEXPB2 | Heterodera avenae | Bind to cellulous and target cell wall when parasitizing N. benthamiana | 84 | ||
Phosphatase 2C | M. destructor | Interfere with the wheat signal transduction pathway possibly by phosphatase ability | 101 | ||
Endo-beta-1,4- Glucanase (NIEG1) | N. lugens | Degrade celluloses in plant cell wall, enable insect stylet to reach the rice phloem | 102 | ||
NcSP75 | Nephotettix cincticeps | Help successful ingestion from sieve elements of rice | 103 | ||
NcSP84 | N. cincticeps | Suppress accumulation of Ca2+ and H2O2 and sieve element clogging in rice | 104 | ||
NlSEF1 | N. lugens | Help successful ingestion from sieve elements of rice | 105 | ||
Derived from insect-borne microbe | Unidentified | Gut and oral secretion–associated bacteria in Colorado potato beetle | Suppress tomato defense response Bind and destabilize TCPs, reduce plant defense in Arabidopsis | 85,86 | |
SAP11 | Aster yellows witches’ broom in Macrosteles quadrilineatus | 106 | |||
SAP54 | Aster yellows witches’ broom in M. quadrilineatus | Degrade MTFs through interacting with RAD23, influence floral development in Arabidopsis | 90 | ||
βC1 | Tomato yellow leaf curl China virus in B. tabaci | Interact with MYC2 and suppress MYC2-regulated terpene synthesis in Arabidopsis | 91 | ||
2b | Cucumber mosaic virus (CMV) in M. persicae | Interact with and stable JAZ protein, blocking JA signaling in Arabidopsis | 92 | ||
C2 | Tomato yellow leaf curl virus in B. tabaci | Interact with plant ubiquitin, blocking JA signaling in tobacco | 93 |
JA is a conserved defense regulator in the plant kingdom. On one hand, various elicitors can be recognized by plants to trigger JA signaling. On the other hand, the JA pathway tends to be targeted by a diverse range of attackers for fitness (Figure 1). Some insect effectors have a mechanism similar to that of the virus proteins in blocking JA signaling67,91–93. It would be interesting to study whether there are relationships between the phylogeny of insect effectors and viral proteins. Although numerous elicitors and effectors were identified, their target proteins, the underlying mechanisms, and the transportation mechanisms of the effectors entering plant cells are largely unknown and deserve further investigation. In plants, JA is integrated with multiple signaling to form a complex and flexible defense network. Recent research has revealed the intricate defense network shaped by insect herbivory69,107–109. Studies have also shown that insects can use plant defense metabolites to find their host plants and to fend off predators110,111; this gives new insight into plant–insect interactions. Further investigations will greatly enrich our knowledge of the complex and flexible interactions between plants and insects and will also be helpful for breeding insect-proof crops112,113.
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Competing Interests: No competing interests were disclosed.
Competing Interests: No competing interests were disclosed.
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