The plant immune system operates through two interconnected layers that function as an integrated defense network rather than as separate pathways. Pattern-triggered immunity, or PTI, is initiated when pattern-recognition receptors at the cell surface detect pathogen-associated molecular patterns. This recognition activates mitogen-activated protein kinase cascades and stimulates the biosynthesis of salicylic acid, a key immune phytohormone. Salicylic acid accumulation promotes the redox-dependent monomerization of NPR1, allowing it to enter the nucleus, where it partners with TGA transcription factors to activate defense genes such as PR1, PR2 and RBOHD. Effector-triggered immunity, or ETI, is mounted when intracellular nucleotide-binding leucine-rich repeat receptors, known as NLRs, detect specific pathogen effectors. ETI potentiates the reactive oxygen burst through RBOHD, facilitates the assembly of EDS1/PAD4/ADR1 helper complexes and amplifies salicylic acid-dependent transcriptional reprogramming, ultimately triggering localized cell death and systemic acquired resistance. Far from being redundant, the two layers form a continuum: PTI provides the foundation for ETI activation through shared signaling components, while ETI enhances PTI by increasing transcript levels of pattern-recognition receptors and their signaling partners.

Despite this sophistication, engineered resistance faces formidable obstacles. The foremost is the growth-defense trade-off: while timely immune activation is crucial for survival, mistimed or constitutive immune responses can lead to autoimmunity and severe growth penalties. Introducing a new resistance gene therefore requires precise regulation at the DNA, mRNA and protein levels. Immune recognition itself is vulnerable, as pathogens continuously evolve to evade or suppress plant defenses. The rice gene Xa23, for example, encodes an executor resistance protein activated by the transcription activator-like effector AvrXa23 of the bacterial blight pathogen, yet mutant variants of AvrXa23 have already evolved to bypass recognition by Xa23. Genetic variation among plant species further limits the transferability of immune receptors, because recipient plants may lack essential helper receptors or compatible signaling components. These constraints, the review argues, highlight the urgent need for innovative strategies that repurpose immune switches to bypass both evolutionary and physiological limitations.

At the transcriptional level, epigenetic modifications offer reversible and context-dependent control of resistance gene expression. Whole-genome sequencing in common bean has shown that nearly half of all NLR genes are methylated, with roughly 90 methylated NLRs targeted by 24-nucleotide small interfering RNAs and exhibiting reduced expression consistent with RNA-directed DNA methylation. In rice, the allelic inhibitor PigmS antagonizes the resistance protein PigmR through heterodimerization, and RNA-directed DNA methylation controls its tissue-specific expression: the PigmS promoter is hypermethylated and silenced in leaves but hypomethylated in pollen, allowing expression where immunity would compromise yield. Similarly, the resistance gene Xa21G harbors 5-methylcytosine modifications in its promoter that maintain transcriptional silence, and artificial demethylation restores expression and confers resistance to bacterial blight. Methylation can also act positively: methylation of the Pib promoter enhances its inducible expression upon infection by the rice blast fungus. The authors propose a synthetic epigenetic on/off switch that fuses DNA demethylases such as ROS1 with pathogen-inducible promoters, so that bacterial blight infection would relieve methylation-mediated silencing at the Xa21G promoter and induce resistance gene expression precisely when needed.

Pathogen effectors themselves can be turned into tools for transcriptional engineering. During ETI, transcription activator-like effectors from Xanthomonas recognize and bind effector-binding elements in the promoters of executor resistance genes such as Xa23, Xa7, Xa10 and Xa27 in rice, and Bs3 in pepper, thereby activating transcription and inducing cell death. Synthetic pathogen-inducible promoters exploit this specificity by multimerizing minimal effector-responsive elements, typically 12 to 24 base pairs in length, upstream of a core promoter. In one striking example, a combinatorial promoter module comprising ten effector-binding elements was designed to target ten distinct isolates of the bacterial blight and bacterial streak pathogens, and a 220-base-pair DNA cassette was inserted into the TATA-box region of the Xa23 promoter using genome editing, conferring resistance to both diseases. In potato, the synthetic blight-inducible promoter 2×S-4×D-NpCABE combined with the Avr3a allele has generated transgenic lines with significantly enhanced resistance to Phytophthora infestans, the late blight pathogen. The natural salicylic acid-responsive PR-1a promoter has been engineered by inserting jasmonic acid-responsive elements, enabling a single gene to confer resistance to both necrotrophic and biotrophic pathogens. CRISPR activation technology extends this approach further, allowing pathogen-inducible promoters to drive fusion proteins composed of dead Cas9 and transcriptional activators that target the promoters of resistance genes in tomato and common bean.

Pathogens also deliver transcriptional repressors, and this vulnerability can be repurposed. Fungal and oomycete effectors such as MoHTR1 and MoHTR2 from Magnaporthe oryzae, and CRN108 and Avh23 from Phytophthora sojae, bind cis-regulatory elements in host promoters to suppress defense gene expression. Introducing such elements into the promoters of susceptibility genes using genome editing would downregulate their expression upon pathogen infection, thereby enhancing resistance. The authors propose integrating CRISPR interference, in which pathogen-inducible promoters drive dead Cas9 combined with guide RNAs, to enable tunable repression of susceptibility genes such as the rice SWEET13, whose partial downregulation confers resistance to multiple pathogens without yield penalty. AI-driven direct promoter editing, combining deep learning for promoter sequence analysis with multi-omics approaches, enables the precise deletion of cis-regulatory elements to fine-tune gene expression and balance the growth-defense trade-off. Computational models for de novo design of regulatory DNA are also advancing rapidly: the deep learning model PhytoExpr, trained on 600,000 genes from 17 plant species coupled with over 6,000 matched transcriptomes, can design novel regulatory sequences with experimentally validated biological activity. Future targets could include vascular-specific elements for diseases like bacterial wilt and Huanglongbing, epidermis-specific elements that promote cuticle thickening, and senescence-specific elements activated during fruit ripening to prevent postharvest diseases.

Post-transcriptional mechanisms provide additional layers of control. Small RNA pathways regulate immunity by silencing mRNAs: in Arabidopsis, siRNAs produced at the RPP5 locus mediate cosuppression of SNC1 and neighboring resistance genes, preventing constitutive immune activation, while in rice, miR168 targets AGO1, and overexpression of miR168 increases disease susceptibility whereas expression of a target mimic enhances resistance and improves yield. miR160a confers broad-spectrum resistance to rice blast, bacterial blight and sheath blight by suppressing multiple auxin response factor genes. Beyond intracellular regulation, plants transport specific small RNAs into pathogens such as Botrytis cinerea via exosome-like vesicles to silence virulence genes, a natural cross-kingdom RNA interference adapted into spray-induced, host-induced and microbe-induced gene silencing strategies. Artificial microRNA systems using endogenous miRNA backbones have been engineered to target viral genes, and the authors suggest that hairpin RNA or artificial microRNA constructs directed against susceptibility genes such as OsSWEET14 in rice, TaMlo in wheat or DMR6 in tomato, expressed under pathogen-inducible promoters, could provide broad-spectrum resistance. Alternative splicing offers another switch: the Arabidopsis RPS4 gene undergoes pathogen-induced alternative splicing upon recognition of the AvrRps4 effector, the potato RB gene switches from a non-functional isoform during normal growth to a functional transcript upon infection, and the tobacco N gene fine-tunes antiviral immunity through virus-triggered inclusion of a 70-base-pair mini-exon. CRISPR-mediated knock-in of pathogen-inducible splicing elements could engineer synthetic splicing events that generate highly active immune receptor isoforms upon infection, while alternative polyadenylation, observed in all 72 resistance genes profiled in Arabidopsis, could be redirected by genome editing to shift transcript isoforms toward variants with increased abundance of immunity-related proteins.

Translational regulation exploits the fact that pathogens suppress cap-dependent translation during infection. Purine-rich R-motifs in the 5-prime untranslated regions of many resistance gene transcripts drive cap-independent translation, allowing sustained synthesis of defense proteins when decapping enzymes would otherwise promote mRNA decay. The barley RNA-binding protein La1 specifically recognizes the R-motif and promotes translation of defense-related transcripts, enhancing antiviral immunity. Upstream open reading frames, or uORFs, act as translational brakes: the Arabidopsis TBF1 contains two uORFs that impede ribosomes under normal conditions, but upon pathogen infection ribosomes bypass the uORFs, enabling rapid TBF1 synthesis. The transgenic cassette ProTBF1::uORFTBF1-AtNPR1 confers broad-spectrum disease resistance in rice without compromising yield, and CRISPR-mediated truncation of the uORF in OsTBF1 enables rice to maintain stable yields while achieving broad-spectrum resistance. The CRISPR-aTrE-uORF system constructs different uORF types to fine-tune translation levels precisely, offering heritable control of disease resistance without introducing foreign DNA.

At the post-translational level, the recognition surfaces of immune receptors represent the most direct switches. Structure-guided mutagenesis and domain swapping within the leucine-rich repeat domains of wheat receptors Sr33 and Sr50 have redirected recognition specificity to new pathogen effectors without triggering autoimmunity, and high-throughput mutagenesis of the tomato Tm-2² LRR domain produced variants conferring robust resistance to both tomato brown rugose fruit virus and tobacco mosaic virus. Integrated domains such as the heavy-metal-associated module in NLRs like RGA5 and Pik-1 directly recognize pathogen effectors, and structure-guided mutagenesis of the Pikp HMA domain yielded mutants that acquired recognition of additional effector variants. Nanobody-fused NLRs create artificial immune receptors capable of recognizing defined antigens, while the directed evolution technique GRAPE, which mimics natural evolution in planta, successfully evolved the integrated domain of Pikm-1 to recognize six distinct Avr-Pik variants of the rice blast fungus. Protein cleavage provides a modular cleavage-release switch: replacing the protease cleavage site of the decoy kinase PBS1 with the site recognized by a viral protease conferred resistance to the corresponding virus, and engineered cleavage-activated NLRs carrying cleavage sites for six conserved viral proteases confer resistance to over 100 types of viruses in tobacco, soybean and Arabidopsis.

The review concludes that resistance gene engineering is transitioning from time-consuming cloning from limited genetic resources to a new phase of rational design facilitated by immune switches, high-quality pan-genomes, multi-omics datasets and AI-driven protein design. Single-cell and spatial transcriptomics, ribosome profiling, cross-linking mass spectrometry and cryo-electron microscopy are deepening mechanistic understanding of how switches operate, while AlphaFold-Multimer predictions of effector-NLR interfaces closely match experimental structures. Yet the authors caution that pathogen population dynamics must be monitored continuously, since resistance genes targeting individual effectors often follow a boom-and-bust cycle, as demonstrated when the stem rust race Ug99 rapidly overcame resistance conferred by Sr31 in East African wheat. A novel endogenous sentinel strategy proposes engineering plant endophytes that express effectors in response to generic pathogen-induced signals such as reactive oxygen species, triggering effector-triggered immunity in a generalized manner and decelerating pathogen adaptive evolution. By integrating real-time pathogen population monitoring with engineered immune switches, the authors argue, crop disease resistance can be transformed from a passive to an active strategy, enabling precise and durable disease control aligned with the goals of sustainable agriculture.

Subject of Research: Plant immunity engineering

Article Title: Exploiting plant immune “switches” for resistance engineering

Article References: Wu, B., Xu, K., Bashir, A., Han, X., Sun, Q., Sun, P., Wang, G.-F., Milne, R. J., Zhang, M., Yang, L., Xu, G., & Li, G. (2026). Exploiting plant immune “switches” for resistance engineering. Stress Biology, 6(1), Article 32. https://doi.org/10.1007/s44154-026-00306-4

Image Credits: AI Generated

DOI: 10.1007/s44154-026-00306-4

Keywords: plant immunity, disease resistance, CRISPR, NLR receptors, gene editing, crop protection, synthetic biology, food security, AI protein design, salicylic acid, Exploiting, plant