The first line of defence is mounted by pattern recognition receptors, or PRRs, which sit at the cell surface and detect conserved microbial molecules known as microbe-associated molecular patterns. These include bacterial flagellin, the translation factor EF-Tu, fungal chitin and bacterial peptidoglycan. When a PRR such as the flagellin receptor FLS2 binds its ligand, it recruits a co-receptor kinase called BAK1, forming an active receptor complex that triggers a cascade of phosphorylation events. Structural studies over the past decade have revealed the atomic details of these interactions, showing how ligand binding induces dimerization and allosteric activation of the kinase domains. The review emphasizes that receptor-like proteins, which lack an intracellular kinase domain, rely on partner kinases such as SOBIR1 and BAK1 to transduce signals, and that recent work has clarified how these complexes assemble and activate downstream pathways.

Once a PRR is activated, a receptor-like cytoplasmic kinase called BIK1 is released from the receptor complex and phosphorylates a remarkable range of targets. BIK1 directly activates the NADPH oxidase RbohD, driving a burst of reactive oxygen species that reinforces cell walls and signals to neighbouring cells. It also regulates calcium channels, including CNGCs and OSCA1.3, allowing cytosolic calcium concentrations to rise rapidly. This calcium influx is not merely a byproduct; it is a central hub of immune signalling. Calcium-dependent protein kinases decode the calcium signature and phosphorylate transcription factors and metabolic enzymes, while calcium-activated metacaspases release immunomodulatory peptides from damaged cells. The review highlights how this spatiotemporally controlled calcium symphony, as one group of researchers has described it, links pattern recognition to gene expression, stomatal closure and the production of antimicrobial compounds.

The second branch of plant immunity operates inside the cell. Intracellular nucleotide-binding leucine-rich repeat receptors, or NLRs, detect pathogen effectors, the virulence proteins that pathogens inject into host cells to suppress pattern-triggered immunity. The gene-for-gene concept, articulated by Harold Flor in the 1970s, predicted that for every resistance gene in the plant there is a corresponding avirulence gene in the pathogen. Decades of work have vindicated this idea at the molecular level. Some NLRs directly bind effectors, while others act as guards or decoys, monitoring host proteins that effectors modify. Integrated domains, which are fusions of host target mimics into the NLR architecture, allow single receptors to sense effectors that manipulate specific host processes. The review traces this conceptual evolution from the cloning of the first NLR genes in the 1990s to the recent explosion of structural biology that has revealed how these receptors work.

The most striking recent discovery in this field concerns resistosomes. When certain NLRs recognize their cognate effectors, they do not simply change shape; they oligomerize into large, ordered complexes that function as channels or enzymes. The ZAR1 resistosome, for example, forms a pentameric calcium-permeable channel in the plasma membrane, flooding the cytosol with calcium and triggering cell death at the infection site. Helper NLRs of the NRC and ADR1 families form hexameric resistosomes with similar channel activity, while TIR-domain-containing NLRs act as NAD-cleaving enzymes that produce small signalling molecules. Recent work has shown that helper NLR resistosome clusters assemble upon activation of sensor NLRs, and that a wheat CCG10-NLR receptor forms an octameric resistosome. These structures represent a remarkable convergence with animal immunity, where inflammasomes perform analogous functions, and they explain how a single molecular recognition event can be amplified into a full-scale defensive response.

Perhaps the most important conceptual shift in recent years is the recognition that pattern-triggered immunity and effector-triggered immunity are not separate pathways but mutually reinforcing branches of a single system. Studies published in 2021 demonstrated that PRR signalling is required for full NLR-mediated resistance, and that activation of TIR signalling boosts pattern-triggered responses. The EDS1-PAD4-ADR1 node, a complex of lipase-like proteins and helper NLRs, mediates both branches, providing a molecular bridge between cell surface and intracellular perception. This mutual potentiation means that a plant that has already mounted a pattern-triggered response is primed to respond more vigorously to effector recognition, and vice versa. The review argues that this integration explains why plants with compromised PRR function are often more susceptible to disease even when they possess functional NLR genes.

But immunity is expensive. Uncontrolled activation of PRRs or NLRs causes stunted growth, autoimmunity and reduced seed yield, a phenomenon known as the growth-defence trade-off. Plants have evolved multiple mechanisms to keep immune signalling in check. Negative regulators such as the protein phosphatase PP2C38 dephosphorylate BIK1, while ubiquitin ligases control BIK1 protein homeostasis. The BAK1 co-receptor is shared between immune receptors and the brassinosteroid growth receptor BRI1, creating a molecular competition between defence and growth signalling. BIK1 inversely modulates these two pathways, promoting immunity while suppressing brassinosteroid responses. Membrane nanodomains physically separate immune and growth receptor complexes, even when they share signalling components. The review also describes how phytosulfokine signalling, mediated by the receptor PSKR1, balances growth and defence in the rhizosphere, and how transcription factors such as BZR1 mediate trade-offs at the gene expression level.

Environmental conditions add another layer of complexity. Temperature, light and humidity all modulate immune responses. Elevated temperatures can suppress NLR-mediated resistance, but recent work has identified CBP60g family transcription factors as key determinants of temperature sensitivity, offering a route to climate-resilient immunity. Light quality matters too: the blue-light receptor CRY1 acts as a switch that balances photosynthesis and defence, while phytochromes regulate salicylic acid and jasmonic acid pathways in rice. High humidity dampens salicylic acid signalling and NPR1 function, promoting disease, and daily humidity oscillations entrain the circadian clock to influence plant physiology. The microbiome also plays a role, with rhizosphere bacteria suppressing local root immune responses through pH modulation, and leaf microbiota influencing autoimmunity in Arabidopsis. The review presents a schematic model of these interplays, emphasizing that immunity cannot be understood in isolation from the environment.

The practical implications are profound. Understanding the mechanisms of immune receptor activation and regulation opens the door to engineering disease-resistant crops with optimized fitness. Researchers have transferred PRRs between plant families to confer broad-spectrum bacterial resistance, stacked multiple PRRs in potato against Phytophthora infestans, and used reverse engineering of FLS2 to expand recognition of evasive flagellin epitopes. NLR engineering has progressed from single amino acid mutations that expand effector recognition to designer receptors with novel specificities, nanobody fusions that redirect NLRs to new targets, and interfamily co-transfer of sensor and helper NLR pairs. Genome editing of susceptibility genes has produced multipathogen resistance without agronomic penalty, and inducible overexpression of transcription factors such as Ideal Plant Architecture1 improves both yield and disease resistance in rice. Machine learning and AlphaFold-guided design are now being applied to predict effector targets and redesign immune proteins. The review concludes that the next frontier lies in combining these engineering strategies with a deep understanding of growth-defence balance, so that crops can be made both resistant and productive in a changing climate.

Subject of Research: Mechanisms of pathogen perception, immune signalling and growth-defence balance in plant immunity

Article Title: Mechanisms and balanced regulation of plant immunity

Article References: Chai, J., Wang, Y., & Wang, Y. (2026). Mechanisms and balanced regulation of plant immunity. Nature Reviews Molecular Cell Biology. https://doi.org/10.1038/s41580-026-01009-4

Image Credits: AI Generated

DOI: 10.1038/s41580-026-01009-4

Keywords: plant immunity, pattern recognition receptors, NLR receptors, resistosomes, calcium signalling, growth-defence trade-off, effector recognition, immune receptor engineering, BIK1, BAK1, PTI, ETI

The review, authored by Vishal Patil, Rizwana Rehsawla, and Apurba K. Barman, synthesizes the molecular and transcriptional regulation of plant defense responses to aphid infestation. Its central message is that plant immunity against aphids is not a single reaction but a multilayered, highly coordinated network involving phytohormones, calcium signaling, mitogen-activated protein kinases, reactive oxygen species, and a vast cast of transcription factors that translate these signals into gene expression programs. Understanding this architecture, the authors argue, provides a framework for breeding, genome editing, and elicitor-based strategies that could yield durable, sustainable aphid resistance in crops.

At the first line of defense stand physical barriers: trichomes, glandular hairs, and the cuticular wax layer, all of which reduce aphid movement, penetration efficiency, and settling. Once stylets successfully penetrate plant tissue, the plant initiates pattern-triggered immunity, or PTI. Cell membrane-localized pattern-recognition receptors detect herbivore-associated molecular patterns and damage-associated molecular patterns generated during probing. Although aphids lack the classical elicitors found in chewing insects, such as beta-glucosidase and fatty acid amides, several aphid-associated molecules do trigger PTI-like responses. The green peach aphid, Myzus persicae, carries a salivary elicitor protein of 3 to 10 kilodaltons that activates defense genes in Arabidopsis, and its whole-body extract activates BAK1-mediated PTI, including upregulation of the camalexin biosynthetic gene PAD3. Remarkably, GroEL, a chaperonin derived from the aphid’s endosymbiotic bacterium Buchnera aphidicola, is delivered into host plants during feeding and acts as a molecular pattern that induces BAK1-dependent signaling and reduces aphid fecundity.

Aphids counter with an arsenal of salivary effectors delivered through watery saliva, while gelling saliva forms a protective stylet sheath. Some effectors suppress plant immunity and produce effector-triggered susceptibility: MpC002, PIntO1 (Mp1), and PIntO2 enhance M. persicae colonization on specific hosts, while Mp55 and GroEL suppress glucosinolate, callose, and reactive oxygen species responses to promote aphid reproduction. The macrophage migration inhibitory factor MIF1 and the effector Sm9723 from the Indian grain aphid inhibit defense gene expression, callose deposition, and hypersensitive cell death, making them potential RNAi targets. Yet the arms race cuts both ways. Cathepsin B3 from M. persicae triggers ROS accumulation via an EDR1-like kinase, limiting phloem feeding, and the effector Mp10 induces defenses and reduces aphid fecundity. When intracellular nucleotide-binding leucine-rich repeat receptors, or NLRs, detect these effectors, a stronger effector-triggered immunity ensues. Classic examples include the tomato Mi gene conferring resistance against the potato aphid, the melon VAT gene encoding a CC-NBS-LRR protein with dual resistance to Aphis gossypii and aphid-transmitted viruses, the wheat NLR gene Adnr1 with its integrated WRKY domain, and the recently discovered noncanonical sorghum resistance proteins RMES1A and RMES1B, which interact with the aphid effector MsEF1 to trigger ROS bursts.

Beyond local responses, plants mount systemic defenses. Systemic acquired resistance, or SAR, involves mobile signals including salicylic acid, methyl salicylate, jasmonic acid, pipecolic acid, N-hydroxy pipecolic acid, azelaic acid, and reactive oxygen waves that prime distal tissues for faster, stronger responses to subsequent attacks. Exogenous salicylic acid application strengthens defense enzymes and phenolic compounds in wheat, reducing grain aphid fecundity and survival, while methyl salicylate exposure in barley reduces Rhopalosiphum padi settling and feeding. In parallel, induced systemic resistance is triggered by plant growth-promoting microbes. Bacillus amyloliquefaciens primes broad beans against the pea aphid, Bacillus subtilis 26D protects wheat against greenbug and bird cherry-oat aphid, Bacillus velezensis YC7010 activates PAD4-mediated defense in Arabidopsis, and Beauveria bassiana elicits resistance in tomato. Microbial protein elicitors such as PeaT1, PeBL1, Hrip1, and PeBA1 suppress cabbage aphids by activating jasmonic acid, salicylic acid, and ethylene signaling. Intriguingly, not all microbial effects are beneficial: Pseudomonas fluorescens can enhance green peach aphid performance in Arabidopsis by suppressing abscisic acid signaling, illustrating induced systemic susceptibility.

Phytohormones orchestrate the entire defensive response. Salicylic acid is a central regulator of aphid resistance, orchestrating pathogenesis-related gene expression, reactive oxygen species, secondary metabolite production, and volatile emission. Russian wheat aphid infestation selectively elevates salicylic acid and peroxidase activity in resistant wheat; Mi-1-mediated defense in tomato depends on salicylic acid and MAPK cascades; and sorghum PAL genes are induced by sugarcane aphid infestation to enhance salicylic acid-dependent resistance. Jasmonic acid, traditionally associated with chewing herbivores, also contributes significantly to aphid resistance by regulating secondary metabolites, proteinase inhibitors, and antioxidant defenses. In resistant soybean, jasmonic acid-isoleucine accumulates in response to avirulent aphids but is suppressed by virulent ones, while in sorghum, jasmonic acid plays a dichotomous role, initially deterring sugarcane aphid settling but later promoting feeding and proliferation. Ethylene often acts in conjunction with jasmonic acid to fine-tune antixenotic responses, though it can be manipulated: cucumber mosaic virus infection in pepper increases ethylene production, which actually attracts aphid vectors. Abscisic acid emerges as a double-edged sword, supporting tolerance through water relations modulation but also exploited by soybean aphids to suppress salicylic and jasmonic acid defenses.

The true integrators of this signaling web are transcription factors, which bind cis-regulatory DNA elements within complex gene regulatory networks. The WRKY family stands out as a major regulatory hub. In Arabidopsis, sixteen WRKY genes are rapidly induced by cabbage aphid feeding, with WRKY75 showing the strongest upregulation. WRKY70 and WRKY72 are essential for Mi-1-mediated defense in tomato and Arabidopsis, CmWRKY48 overexpression in chrysanthemum reduces aphid growth, and tobacco NtWRKY28 enhances resistance by activating phenylpropanoid and lignin biosynthesis. Genome-wide association studies in sorghum identified SbWRKY86 as a major locus for sugarcane aphid resistance. Aphids fight back: the wheat aphid salivary protein SmCSP4 interacts with TaWRKY76 to modulate salicylic acid accumulation, and Arabidopsis WRKY22 suppresses salicylic and jasmonic acid defenses to promote aphid performance, demonstrating that WRKYs can act as both positive and negative regulators.

MYB transcription factors, defined by their conserved helix-turn-helix DNA-binding domain, likewise integrate metabolic and defense pathways. In wheat, TaMYB19, TaMYB29, and TaMYB44 activate phloem-based defenses by inducing callose synthases and phloem lectins, while cotton GhMYB18 enhances resistance to Aphis gossypii by activating salicylic acid and flavonoid pathways. In chrysanthemum, CmMYB19 and CmMYB15 activate lignin-biosynthetic genes to restrict aphid multiplication, and CRISPR/Cas9 knockouts of the MYB genes BjA06.GL1 and BjB02.GL1 in Brassica juncea produce glabrous, aphid-susceptible leaves. NAC transcription factors integrate jasmonic acid, salicylic acid, ethylene, and abscisic acid signals with cell wall modification and reactive oxygen homeostasis, contributing to resistance in medicago, melon, soybean, sorghum, and maize. AP2/ERF factors link ethylene signaling to defense, with tomato Pti5 mediating ethylene-independent antibiotic defense against the potato aphid in synergy with Mi-1.2, and bHLH factors, including the cotton MYC2-like GhMYC1374, regulate flavonoid and gossypol biosynthesis to confer aphid resistance.

Underpinning all of this is an elaborate chemical arsenal of plant secondary metabolites. Alkaloids, terpenoids, saponins, pyrethrins, glucosinolates, phenols, flavonoids, lectins, lignin, and tannins act through direct toxicity, antifeedant activity, and repellence, while herbivore-induced plant volatiles such as (E)-beta-farnesene recruit predators and parasitoids, from ladybird beetles to the parasitoid Aphidius ervi. Specialized proteins, including phloem lectins and protease inhibitors, disrupt aphid digestive physiology. The wild peach relative Prunus davidiana even produces betulin via the cytochrome P450 gene PpCYP716A1, an aphid-specific toxin that spares beneficial insects. Translating this knowledge into practice, the review highlights modern breeding tools: wild germplasm introgression, QTL mapping and genome-wide association studies that have located resistance loci such as ApRVII in pea and SbWRKY86 in sorghum, CRISPR-mediated knockout of the sugar transporter VST1 in watermelon, and transgenic cotton expressing fungal lectins that reduced aphid populations by roughly sixty-nine percent. Epigenetic mechanisms, including DNA methylation changes induced by aphid feeding, may further contribute to defense priming and stress memory.

The authors caution that plant defense is inherently constrained by growth-defense trade-offs, mediated by shared regulatory hubs, and that constitutive transcription factor expression risks pleiotropic effects on growth and yield. Fine-tuning the spatial, temporal, or inducible expression of these regulators, through promoter engineering, genome editing, RNAi, virus-induced gene silencing, and marker-assisted selection, will be critical to decoupling defense activation from yield penalties. As multi-omics approaches continue to illuminate the molecular choreography of plant-aphid interactions, from signal perception at the stylet puncture to systemic transcriptional reprogramming, the prospect of crop varieties that combine robust aphid resistance with stable yields moves steadily closer, offering agriculture a path away from chemical insecticide dependence and toward genuinely sustainable pest management.

Subject of Research: Molecular and transcriptional regulation of plant defense responses to aphid infestation

Article Title: Molecular and transcriptional regulation of plant defense responses to aphid infestation

Article References: Patil, V., Rehsawla, R., & Barman, A. K. (2026). Molecular and transcriptional regulation of plant defense responses to aphid infestation. Stress Biology, 6(1), Article 62. https://doi.org/10.1007/s44154-026-00336-y

Image Credits: AI Generated

DOI: 10.1007/s44154-026-00336-y

Keywords: aphids, plant immunity, transcription factors, salicylic acid, jasmonic acid, salivary effectors, PTI, ETI, systemic acquired resistance, secondary metabolites, crop resistance, genome editing