Deciphering Defense Mechanisms and Genetic Determinants of Insect Resistance in Brassica Species

Abstract

Brassica crops (genus Brassica) represent globally important vegetables and oilseeds, yet are continuously threatened by insect pests that reduce yield and quality. While classical physiological and chemical defense mechanisms such as the glucosinolate–myrosinase system have been well documented, recent advances in genomics and molecular biology are beginning to unravel the genetic basis of insect resistance in Brassica species. Notably, emerging evidence highlights the central role of jasmonic acid (JA) signaling and the transcription factor MYC2 as a master regulator of inducible defense responses, where stress-induced degradation of JAZ repressors releases MYC2 to activate downstream defense genes and secondary metabolite biosynthesis. This review synthesizes the current understanding of defense mechanisms in Brassica against herbivores, highlights identified resistance genes and their functional roles, and examines the knowledge gaps that hinder progress in molecular breeding. We then explore future molecular approaches including high-throughput omics, gene editing, and resistance gene mining that hold promise for designing durable insect-resistant Brassica cultivars. To our knowledge, major insect resistance loci are relatively scarce compared to pathogen-resistant loci. We argue for integrated strategies combining classical breeding, biotechnology, and ecological management to accelerate the development of resilient Brassica germplasm.

1. Introduction

Brassica crops including Chinese cabbage (Brassica rapa subsp. pekinensis), oilseed rape (Brassica napus), cabbage (Brassica oleracea), mustard (Brassica juncea) and turnip (Brassica rapa subsp. rapa) represent one of the most economically and nutritionally important plant groups worldwide [1,2]. They serve as major sources of edible oil, vegetables, condiments, forage, and industrial raw materials [3]. However, despite their global importance, research specifically focused on insect resistance mechanisms and associated genes in Brassica remains comparatively limited [1,4]. Insect pests such as cabbage aphid (Brevicoryne brassicae L.), green peach aphid (Myzus persicae Sulzer.), diamondback moth (Plutella xylostella L.), and cabbage root fly (Delia radicum L.) continue to cause substantial economic losses, with yield reductions in oilseed Brassicas reaching up to 20% under production conditions [5,6,7,8]. Heavy reliance on chemical pesticides remains the dominant management strategy. Yet, concerns regarding environmental impacts, rising production costs, and rapid evolution of insecticide resistance underscore the need for alternative, durable approaches [9].

Plant resistance to insects has emerged as a sustainable and cost-effective strategy to reduce yield losses and dependence on chemical inputs [10,11]. Brassica crops possess diverse natural defense traits shaped by coevolution with specialist and generalist herbivores. These defenses span morphological barriers such as trichomes and wax layers, biochemical pathways involving secondary metabolites, and inducible molecular responses coordinated by plant hormonal signaling networks [12,13]. dl-β-Aminobutyric acid (BABA) is a non-protein amino acid that can bolster plant defenses against certain diseases [14]. BABA can diminish infestations by phytopathogenic nematodes and has recently been demonstrated to inhibit the growth of aphids that feed on legumes [15]. Among these, the glucosinolate–myrosinase defense system has been the most widely studied and is frequently highlighted as a hallmark of Brassicaceae–insect interactions [16]. However, despite recognition of these mechanisms, the identification and functional validation of resistance genes (R-genes) conferring durable insect resistance in Brassica remains scarce compared to disease resistance research [17]. This discrepancy reflects both biological complexity and historical prioritization of pathogen resistance over herbivore resilience in breeding programs.

Advances in high-throughput sequencing, multi-omics platforms, and comparative genomics have begun to accelerate the discovery of insect resistance loci and regulatory pathways in Brassica crops [18]. Genome-wide association studies (GWAS), transcriptomics, metabolomics, proteomics, epigenomics, and pangenome analyses have revealed new candidate genes, allelic diversity, and novel defense-related gene clusters previously overlooked in single-reference genome studies [19,20]. The emergence of precision molecular tools such as Clustered Regularly Interspaced Short Palindromic Repeats/CRISPR-associated protein (CRISPR/Cas)-based genome editing, RNA interference (RNAi), gene stacking, and synthetic biology provides unprecedented opportunities to dissect insect–plant interactions and engineer enhanced resistance without compromising agronomic performance [21]. Yet, translating this expanding genomic knowledge into breeding pipelines remains challenging and uneven across Brassica species and pest groups.

Given the increasing demand for climate-resilient and pest-resistant crop production systems, a systematic synthesis of current advances and existing research gaps is critically needed. Therefore, this review aims to: (i) summarize established morphological, biochemical, and molecular defense mechanisms in Brassica crops against major insect pests; (ii) consolidate current knowledge on identified insect resistance genes and their associated functional evidence; (iii) highlight key biological, methodological, and breeding-related gaps that limit the effective deployment of insect resistance; and (iv) discuss emerging molecular and genomic strategies to accelerate the development of durable insect-resistant Brassica cultivars.

To address these objectives, this review was developed through an extensive, systematic literature search across multiple scientific databases, including Google Scholar, Web of Science, Elsevier, MDPI, and ResearchGate. Literature retrieval was performed using combinations of keywords such as (Brassica OR “Brassica napus” OR “Brassica oleracea” OR “Brassica rapa”) AND (insect resistance OR herbivore resistance OR pest resistance OR anti-insect). However, the number of studies specifically focusing on insect resistance in Brassica species remains relatively limited.

To overcome this limitation and ensure comprehensive coverage of relevant defense mechanisms, additional literature on closely related Brassicaceae model systems, particularly Arabidopsis thaliana, was included. As a widely used model plant, Arabidopsis provides fundamental insights into conserved molecular and hormonal regulatory pathways that are often transferable to Brassica crops [22]. Furthermore, targeted searches addressing hormonal signaling and secondary metabolite regulation were conducted using terms such as Brassica AND (MYC2 OR “jasmonate signaling”) AND (glucosinolate OR “secondary metabolite” OR herbivore OR insect OR aphid OR Plutella). All selected studies were critically evaluated and synthesized to integrate fragmented evidence, identify conserved resistance mechanisms, and highlight key knowledge gaps relevant to insect resistance in Brassica species.

2. Overview of Insect Pests in Brassica Crops

Brassica crops are targeted by a diverse range of insect pests that vary across geographic regions, cropping systems, and climate zones [1,4]. Among the most widespread and damaging pests are diamondback moth, cabbage white butterfly, flea beetles, and various aphid species including green peach aphid and cabbage aphid [5,7]. In temperate agricultural systems, cabbage root fly and pollen beetle (Meligethes aeneus) are also significant threats, particularly for oilseed rape [23]. These insects differ in host specificity, with many Brassica herbivores classified as specialists due to co-evolution with glucosinolate-based chemical defenses [24]. The pest spectrum continues to expand as global trade, climate change, and shifts in agroecosystems facilitate pest migration and the emergence of new invasive species and biotypes [25].

The eating behavior of Brassica insect pests can be classified according to herbivory methods [26]. Herbivorous insects, including P. xylostella, Helicoverpa armigera, and Pieris spp.) and flea beetles, excise substantial sections of plant tissue, frequently resulting in significant foliar damage [27,28]. Conversely, piercing-sucking insects like aphids, whiteflies, and leafhoppers consume phloem sap, hence affecting plant physiology and occasionally serving as vectors for viral infections [29]. Belowground herbivores such as cabbage root fly larvae, wireworms, and nematodes infest roots, hindering nutrient absorption and compromising plant stability [30,31,32].

These functional feeding groups activate distinct and occasionally overlapping defensive mechanisms in Brassica species, thereby affecting the specificity and intensity of resistance responses [33]. Understanding these classes is crucial for correlating feeding mechanisms with defense signaling, plant damage responses, and the function of resistance genes [34].

Table 1. Representative of significant insect pests that infest Brassica crops globally.

Insect	Scientific Name	Type of Oral Appendage	Distribution	Host Plant	References
Cabbage aphid	B. brassicae	Sucking	China, South Asia	Cabbage, oilseed rape	[5,35]
Green peach aphid	M. persicae	Sucking	China and Europe	Chinese cabbage, cabbage, radish	[6]
Turnip aphid	Lipaphis erysimi Kaltenbach.	Sucking	South Asia	Indian mustard	[36]
Diamondback moth	P. xylostella	Chewing	Australia, Asia, Africa	Broccoli, Brussels sprouts, cabbage, cauliflower, kale, mustard, turnip	[37]
Cabbage looper	Trichoplusia ni (Hübner.)	Chewing	North American native found throughout the US, Canada, and Mexico	Broccoli, cabbage, cauliflower, kale, collards, mustard, rutabaga, turnip	[38]
Cabbage butterfly	Pieris brassicae L.	Chewing	North Africa across Europe and Asia to the Himalayas	Kale, cabbage, turnip, black mustard, Ethiopian mustard, swede	[39]
Beet armyworm	Spodoptera exigua Hübner.	Chewing	Southeast Asia, Eastern Asia	mustard	[40]
Cabbage moth	Mamestra brassicae L.	Chewing	Europe, North Africa (Libya, Canary Islands), Japan and sub- tropical Asia, including India	Cabbage, red cabbage, mustard, turnip,	[41]
Leafhoppers	Cicadelliade sp.	Sucking	Asia, Europe	canola	[42]
Flea beetles	Phyllotreta cruciferae	Chewing	Europe, North America	canola	[43]

presents a condensed list of principal insect pests that infest Brassicaceae crops worldwide, highlighting typical species within various feeding categories. This table delineates their host ranges, mouthpart types, and geographic distributions, providing a comparative framework for understanding variations in pest pressure across locations.

The economic repercussions of insect pests on Brassica cultivation are significant and are increasingly exacerbated in numerous producing areas [23]. Yield losses in oilseed rape and leafy Brassicas may surpass 20–30% in the absence of adequate intervention, with complete crop failure documented during severe outbreaks [19,44]. Secondary effects such as diminished seed quality, contamination of sellable products, and heightened production expenses owing to pesticide use exacerbate economic losses [45]. Furthermore, the swift development of pesticide resistance, particularly in P. xylostella, aphids, and flea beetles, has reduced the efficacy of traditional chemical control methods [46,47]. Consequently, insect pressure is now acknowledged as a major limitation to sustainable Brassica production. These problems highlight the necessity of cultivating robust host resistance and incorporating it into sustainable pest management systems.

3. Morphological and Physiological Defense Mechanisms

Brassica crops exhibit diverse morphological traits that function as key defenses against insect herbivory [4]. Physical barriers, such as epidermal wax layers, reduce palatability, impede insect adhesion, and alter microsurface cues used by specialist herbivores for host recognition, such as B. napus [48]. Both glandular and non-glandular trichomes may serve as mechanical deterrents by obstructing eating and oviposition or by physically ensnaring small insects [49]. Moreover, leaf structural characteristics, such as increased tissue rigidity, thickened cuticles, and reinforced cell walls, further impede tissue penetration and reduce feeding efficacy, particularly for chewing insects such as flea beetles and lepidopteran larvae [50]. In addition to physical barriers, Brassica species utilize an advanced chemical defense mechanism focused on glucosinolates (GLSs), a class of secondary metabolites unique to the Brassicaceae family [40,51]. Under typical physiological settings, GLSs and the hydrolytic enzyme myrosinase are compartmentalized inside distinct cellular locales [16]. When plant tissue is compromised by insect infestation, myrosinase facilitates the degradation of glucosinolates into bioactive compounds, including isothiocyanates, thiocyanates, and nitriles, which possess poisonous and deterring properties against various chewing and sucking pests [4].

Besides constitutive defenses, Brassica plants activate a range of inducible physiological responses in response to herbivore attacks. Herbivory triggers wound signaling pathways that include mechanical damage signals and chemical elicitors, including insect oral secretions [52]. Hormonal regulators, including as jasmonic acid (JA), salicylic acid (SA), ethylene (ET), and abscisic acid (ABA), are pivotal to these responses, interacting to precisely modulate defensive mechanisms based on the feeding pattern of pests [4]. Insect herbivory generally triggers jasmonic acid (JA)-dependent pathways, resulting in the synthesis of secondary metabolites, protease inhibitors, and structural fortifications, although piercing-sucking insects may initiate crosstalk between JA and salicylic acid (SA), yielding diverse effects [19,53]. These inducible reactions not only diminish insect performance but may also prepare systemic tissues for improved resistance, hence enhancing long-term defensive plasticity.

Numerous case studies in Brassica crops demonstrate the functional significance of these defensive characteristics. In B. oleracea, increased leaf surface wax content correlates with less flea beetle feeding and oviposition [54]. Accessions of B. napus with thick trichomes exhibit reduced infestation rates by aphids and caterpillars, indicating a possible screening trait for resistance [49]. Moreover, experimental herbivory by P. xylostella has demonstrated a fast activation of JA production and subsequent glucosinolate accumulation, validating the combination of morphological defenses with dynamic chemical and hormonal responses [53,55]. These examples illustrate the coordinated operation of Brassica defensive tactics, integrating pre-existing structural features with inducible physiological mechanisms to reduce insect harm.

4. Biochemical Defense Mechanisms: The Glucosinolate–Myrosinase System

The glucosinolate–myrosinase defense mechanism is a prominent biochemical characteristic of Brassica crops and is pivotal in plant–insect interactions [4,56]. Glucosinolates (GLSs) are sulfur-rich secondary metabolites produced via established routes that include amino acid precursors, cytochrome P450 enzymes (such as CYP79 and CYP83 families), and various glycosylation, sulfation, and side-chain modification processes [57]. Following tissue damage, compartmentalized myrosinase enzymes catalyze the hydrolysis of GLSs into physiologically active degradation products, including isothiocyanates, nitriles, thiocyanates, and oxazolidinethiones [58]. The precise product profile is contingent upon the presence of specifier proteins and environmental factors, including pH and metal ions [59]. These chemicals serve as poisons, repellents, or feeding deterrents to numerous generalist herbivores and contribute to the distinctive pungency of Brassica tissues [59,60].

Myrosinases are essential for the hydrolysis of GLSs and belong to the glycoside hydrolase family 1 (GH1) [16]. The conventional myrosinases (TGG1–TGG6) in A. thaliana are encoded by six β-glucosidase genes from the GH1 family (BGLU34–BGLU39) [56]. Silencing TGG1 and TGG2, which are mostly expressed in leaves and floral structures, significantly reduces resistance to the generalist Mamestra brassicae larvae and the specialist herbivore P. xylostella [61]. Conversely, TGG3 and TGG6 are non-functional pseudogenes, whilst TGG4 and TGG5 are only expressed in roots [56,62]. The evolutionary links between myrosinase genes in Chinese cabbage, cabbage, radish, and rapeseed are shown in Figure 1.

To investigate the evolutionary relationships of myrosinase genes across Brassicaceae species, comparative sequence analyses were conducted. The BoTGG1 protein sequence from broccoli (Brassica oleracea var. italica) was used as a query for BLAST (v2.16.0+) searches against the Chiifu V4.1protein database of Chinese cabbage (B. rapa ssp. pekinensis) available in the Brassicaceae Database (BRAD), resulting in the identification of the homologous gene BraA02g043080.4.1Cwith 99.1% sequence identity (Figure 1A). In addition, TGG1 gene sequences from rapeseed, radish, broccoli, and Chinese cabbage were retrieved and subjected to multiple sequence alignment using MAFFT (v7.526), followed by pairwise sequence similarity analysis (Figure 1B). For phylogenetic inference, protein sequences from representative Brassicaceae species, including Chinese cabbage, cabbage, radish, and rapeseed, were obtained from Brassicaceae Database (BRAD) and the National Genomics Data Center. The A. thaliana AtTGG1 protein was used as a query sequence for reciprocal BLAST analyses. Multiple sequence alignment was performed using MAFFT, and a phylogenetic tree was constructed using IQ-TREE (v2.4.0). The resulting evolutionary relationships among myrosinase genes across Brassicaceae species are summarized in Figure 1C.

In the glucosinolate–myrosinase system has a role in insect resistance in Brassica, generalist insects generally exhibit diminished survival, prolonged development, or feeding aversion when subjected to elevated concentrations of GLS hydrolysis products, whereas specialist herbivores like P. xylostella and Pieris spp. have developed detoxification strategies and occasionally utilize GLS profiles for host selection [23]. Numerous studies indicate that herbivory triggers alterations in GLS biosynthesis genes, transporter proteins, and myrosinase activity, demonstrating a meticulously regulated defense mechanism that responds to the kind of insect feeding, developmental stage, and genotype [4,56,59]. Transcriptomic investigations of B. napus and B. juncea subjected to aphid and caterpillar assault have demonstrated significant activation of the jasmonate-related GLSbiosynthesis pathway, signifying hormonal integration of biochemical defense mechanisms [63,64].

The interplay between constitutive and induced GLS levels is a crucial factor influencing Brassica resistance outcomes. Constitutive GLS concentrations differ among species, cultivars, and tissues, establishing a fundamental chemical defense that may affect initial herbivore behavior, feeding initiation, and oviposition selection [65]. Conversely, induced reactions are generally triggered post-feeding and may entail localized or systemic accumulation of particular GLS classes, such as aliphatic or indolic compounds, contingent upon the identity of the pest [66]. The fitness trade-offs linked to sustaining elevated constitutive GLS levels, such as adverse impacts on plant growth, nutritional quality, and beneficial organisms, highlight the necessity of comprehending regulating mechanisms [67]. Clarifying the interaction between constitutive and inducible components will be crucial for optimizing breeding techniques to improve effective and enduring biochemical resistance in Brassica crops.

5. Identified Resistance Genes and Molecular Genetic Evidence

Investigations into mapped or cloned resistance genes (R-genes) in Brassica have predominantly concentrated on disease resistance, rather than insect herbivory, mirroring past research interests [68]. Thoroughly documented R-genes in B. napus, B. rapa, and B. oleracea encompass loci that confer resistance to Leptosphaeria maculans, Hyaloperonospora brassicae, and various other significant pathogens, many of which produce nucleotide-binding leucine-rich repeat (NLR) proteins or receptor-like kinases [69,70]. Although these pathogen-related R-gene resources indicate the existence of gene families with potential defensive functions, verified insect-specific R-genes in Brassica are still limited [1,4]. Emerging research indicates that several genes associated with early immunological awareness, secondary metabolism, and structural reinforcement may serve dual functions in resistance to pathogens and insects, revealing functional overlap between biotic stress pathways and underscoring latent genetic potential [4,11,71].

Quantitative trait locus (QTL) mapping initiatives have begun to identify genomic regions associated with insect resistance in Brassica, although results remain fragmented and species-specific [72]. QTLs linked to resistance against P. xylostella, B. brassicae, and flea beetles have been identified in crucifer crops, including B. napus [73,74], with certain loci co-localizing with genes involved in glucosinolate biosynthesis or defense signaling. Introgression from wild relatives, such as B. villosa and other Brassicaceae species, has broadened the reservoir of resistance genes, particularly for traits associated with leaf trichomes, glucosinolate variation, and physical robustness [73]. Notwithstanding these advancements, the majority of insect-related QTLs remain extensive and inadequately delineated for direct breeding applications, highlighting the necessity for enhanced genetic mapping and functional validation [74].

High-throughput transcriptomics, metabolomics, and reverse-genetic platforms are currently expediting the identification of insect-responsive genes and regulatory networks in Brassica crops [1,75]. RNA-seq investigations of herbivory by aphids, flea beetles, and lepidopteran larvae have demonstrated the activation of jasmonate-regulated defense mechanisms, cytochrome P450 genes associated with glucosinolate modification, and transcription factors from the MYB, WRKY, and TIFY families that function as key regulators of inducible responses [76,77]. Key genes in the jasmonic acid (JA) signaling pathway play a central role in coordinating insect resistance, as JA and its bioactive derivative JA-Ile rapidly accumulate following herbivore attack and subsequently trigger defense cascades, including the production of lignin, lectins, chitinases, toxic metabolites, protease inhibitors, and volatile organic compounds (VOCs) that deter herbivores or attract their natural enemies [78,79]. As shown in Figure 2, JA-mediated signaling relies on the dynamic balance between MYC2-regulated transcriptional activation and repression by JAZ proteins, where stress-induced COI1–JAZ degradation releases MYC2 to initiate downstream defense responses [78,80].

MYC2/3/4 are basic helix–loop–helix (bHLH) transcription factors that act as central regulators of jasmonate (JA)-responsive gene networks controlling plant development and secondary metabolite biosynthesis broadly, particularly glucosinolates that are critical for insect defense [81]. However, its direct roles in Brassica insect resistance remain underexplored [4]. In B. rapa ssp. pekinensis, multiple MYC paralogs (BrMYC2, BrMYC3, and BrMYC4) have been identified and shown to be closely related to their A. thaliana orthologs, which are well established as key regulators of JA-mediated insect resistance [82]. Functional studies demonstrate that BrMYC2/3/4 from B. rapa localize to the nucleus, and that their ectopic expression in Arabidopsis substantially alters growth, development, and secondary metabolite accumulation [83]. Notably, BrMYC2 overexpression leads to pronounced increases in glucosinolate levels, thereby directly supporting its role in anti-insect defense. Although enhanced resistance was also reported against Sclerotinia sclerotiorum, a fungal pathogen, this phenotype should be interpreted as ancillary evidence of MYC2 involvement in JA-dependent defense regulation rather than a primary indicator of insect resistance. Recent findings further reveal that MYC2 activity is strengthened by transcriptional cofactors such as MED25 and MED16, while additional regulators including CPH, the wound-induced peptide REF1 (SPR9), and terpene synthases such as CsELE expand the JA-centered regulatory network [84], positioning MYC-centered modules as promising targets for improving insect resistance in crop breeding programs.

Evidence from multiple Brassica species further highlights MYC2 as a key integrator of insect-induced JA signaling. In B. oleracea, MYC2 expression is rapidly induced within 15 min of Delia radicum feeding when plants are pre-infested by P. xylostella, whereas plants exposed to D. radicum alone exhibit delayed induction, indicating transient MYC2-dependent defense priming [85]. Consistently, in Brussels sprouts (B. oleracea var. gemmifera), both MYC2 and the JA-responsive transcription factor ORA59 show elevated expression in Plutella-induced roots relative to controls [86]. In B. napus, herbivory by Mamestra brassicae triggers coordinated upregulation of JA biosynthesis and signaling genes, including MYC2 and multiple JAZ repressors, reinforcing the central role of MYC2 in insect-responsive JA pathways [87]. Similarly, in B. juncea, insect feeding activates JA signaling dominated by MYC2 and its functionally redundant partners MYC3 and MYC4, with MYC2 showing sustained upregulation at later feeding stages and concomitant induction of downstream target genes involved in glucosinolate biosynthesis via the COI1/JAZ/MYC2 pathway [40]. Together, these studies underscore MYC2 as a conserved and dominant regulator orchestrating insect-induced defense responses across Brassica species.

Functional investigations employing RNA interference (RNAi), CRISPR/Cas gene editing, and mutant screening have confirmed the involvement of potential resistance genes associated with cell wall remodeling, hormone signaling, and secondary metabolism [53]. Although in their preliminary phases, these integrated molecular methodologies are bridging the gap between QTL identification, gene function elucidation, and breeding application, providing a pathway to the creation of genetically informed pest-resistant Brassica cultivars. Until recently, the genetic basis of diamondback moth (DBM) resistance in Brassica species had not been comprehensively investigated, and robust QTL or gene-mapping studies for this trait were largely lacking [88]. Subsequent quantitative trait loci (QTL) mapping using a segregating B. oleracea population evaluated across four consecutive years identified eight QTLs distributed over five linkage groups, with four loci consistently detected and a major QTL (qDbm6) demonstrating stability across multiple years. The development of tightly linked molecular markers for these loci highlights their potential application in marker-assisted breeding to achievedurable insect resistance [89]. Although still at relatively early stages, such integrated molecular methodologies effectively bridge QTL discovery, functional inference, and breeding application, thereby providing a foundation for the development of genetically informed pest-resistant Brassica cultivars.

Table 2. Genes identified in Brassica species associated with insect resistance.

Insect	Scientific Name	Gene Name	Application	Function	Model Plant	References
cabbage looper and cabbage butterfly	Trichoplusia ni and Pieris rapae	Cry1C	NA	Increase insect resistance	B. oleracea ssp. italica	[90]
C. suppressalis	C. suppressalis	CpTI	traditional transgenic transformation (Agrobacterium-mediated gene transfer)	Increase insect resistance	Brassica oleracea var. capitata cultivars Yingchun and Jingfeng	[91]
mustard aphids	Lipaphis erysimi	CAC, TUA and DUF179	microarray	Increase aphid resistance	B. juncea	[92]
diamondback moth	P. xylostella	BtCry1Ac	transgenic (genetically modified) approach	Increase insect resistance	B. napus and B. rapa	[93]
flea beetles	Phyllotreta cruciferae and P. striolata	AtGL3	classical transgenic insertion (T-DNA) and modified expression via transgenic constructs	Increase leaf trichome coverage	B. napus	[94,95]
flea beetles	Phyllotreta cruciferae and P. striolata	BnTTG1	classical transgenic insertion (T-DNA) and modified expression via transgenic constructs	Increase leaf trichome coverage	B. napus	[94,95]
cabbage butterfly	P. brassicae	LecRK-I.1	classical genetic mapping/QTL mapping	Increase insect resistance	B. rapa	[96]
diamondback moth	P. xylostella	Bt cry1C	NA	Increase insect resistance	collard and Indian mustard	[68]
diamondback moth	P. xylostella	Chitinase (chi)	Agrobacterium-mediated transformation	Increase insect resistance	B. napus	[97]
diamondback moth	P. xylostella	BmkIT(Bmk)	Agrobacterium-mediated transformation	Increase insect resistance	B. napus	[97]
cabbage root fly	D. radicum	MYC2	CRISPR/Cas9	Increase insect resistance	B. oleracea	[85]
diamondback moth and cabbage root fly	P. xylostella and D. radicum	MYC2 and ORA59	NA	Increase insect resistance	B. oleracea	[86]
cabbage moth	M. brassicae	LOX2, AOS, AOC2, OPCL1, OPR1, ACX1, KAT1, MYC2	NA	Increase insect resistance	B. napus	[87]
omnivorous beet armyworm	Spodoptera exigua	MYC2	NA	Increase insect resistance	B. juncea	[40]
diamondback moth	P. xylostella	PEN1	NA	Increase insect resistance	B. napus	[98]
diamondback moth	P. xylostella	COMT1	Agrobacterium-mediated transformation	Increase insect resistance	B. napus	[99]
cabbage butterfly	P. brassicae	Pbc1 and Pbc2	QTLs	Increase insect resistance	B. rapa	[96]
diamondback moth	P. xylostella	qDbm 1, qDbm 6, and qDbm 8	QTLs	Increase insect resistance	B. oleracea	[89]
cabbage whitefly	Aleyrodes proletella	Wf2, Wf9	QTLs	Increase insect resistance	B. oleracea	[100]
Beet armyworm	Spodoptera exigua	CYP79B2 and CYP83B1	Agrobacterium-mediated transformation	Increase insect resistance	B. rapa ssp. chinensis var. utilis Tsen et Lee	[101]
diamondback moth	P. xylostella	cryIAa	Agrobacterium-mediated transformation	Increase insect resistance	B. oleracea	[102]
Tobacco cutworm	Spodoptera litura	GTR1 and GTR2	Agrobacterium-mediated transform	Increase insect resistance	B. juncea	[103]

provides an updated summary of genes in Brassica that have been targeted or functionally characterized using RNAi and CRISPR-based genome editing technologies This table highlights recent advances in applying gene silencing and gene editing to enhance insect resistance in Brassica crops.

6. Omics and Future Molecular Tools for Insect Resistance

Rapid advances in genomics and pangenome resources are transforming the discovery and utilization of insect resistance genes in Brassica crops [104]. The development of high-quality reference genomes for major cultivated species, along with comprehensive pangenomes capturing structural variation, copy-number dynamics, and presence–absence variation, has increased the resolution of genetic mapping for defense traits [105]. The GWASs applied to diverse Brassica germplasm and wild relatives are increasingly identifying loci linked to trichome development, glucosinolate diversity, hormonal regulation, and herbivore performance traits [13]. These genomic resources enable resistance gene mining beyond classical single-reference approaches, revealing novel alleles and gene families potentially associated with insect resistance that were previously undetectable [106]. As genomic databases expand, integration with comparative Brassicaceae datasets further supports the identification of conserved defense pathways and lineage-specific adaptations to herbivory [95].

Multi-omics approaches including transcriptomics, proteomics, and metabolomics have emerged as key tools for elucidating dynamic plant responses to insect herbivory in Brassica species [64,107]. Transcriptomic analyses have provided insight into transcriptional reprogramming under insect attack, revealing complex hormonal crosstalk and activation of biosynthetic pathways related to glucosinolates, phenolics, and cell wall modifications [64]. Proteomic studies complement these findings by identifying post-translational modifications and protein–protein interaction networks that modulate defense responses [108]. Proteomic investigations of Brassica–insect interactions are scarce, underscoring the necessity for extensive protein-level analyses to elucidate defense mechanisms against herbivorous pests [109]. Progress has been evidenced in disease-related research, exemplified by a proteomic study on clubroot-resistant and -susceptible B. napus lines, which identified 6626 differentially abundant proteins and highlighted critical defense-associated proteins and genes beneficial for the development of clubroot-resistant cultivars [110].

Meanwhile, research on metabolomics in Brassica–insect interactions remains constrained, despite its significance for elucidating defense-related metabolic pathways and volatile mediated reactions that affect herbivore and natural enemy behavior. Advancements have been evidenced in disease-centric investigations, notably in studies concerning Turnip mosaic virus (TuMV) within resistant and susceptible B. rapa lines [111]. This study unveiled substantial alterations in volatile organic compounds and transcriptomic responses, highlighting pronounced downregulation of differentially expressed metabolites and shifts in pathways related to auxin, zeatin, brassinosteroid, and α-linolenic acid metabolism, thereby offering insights that bolster future resistance breeding efforts [111]. Together, these omics platforms provide a systems-level understanding of herbivore defense, enabling prioritization of candidate genes, regulatory nodes, and biochemical pathways for functional validation and breeding integration.

Looking ahead, gene editing and synthetic biology present powerful opportunities to engineer insect resistance in Brassica with unprecedented precision. Technologies such as CRISPR/Cas enable targeted modification of defense regulators, fine-tuning of glucosinolate biosynthesis and hydrolysis pathways, and manipulation of hormone signaling without undesirable pleiotropic effects [112]. Emerging strategies include engineering transporter proteins to modulate glucosinolate spatial distribution, modifying specifier protein activity to alter hydrolysis outcomes, and designing synthetic promoter elements to strengthen inducible resistance while minimizing growth penalties [4]. Synthetic biology platforms may also enable stacking of multi-layered defense modules combining volatile signaling, structural defense traits, and biochemical deterrents to develop durable resistance architectures less vulnerable to pest adaptation [113]. These advancing technologies, coupled with expanding multi-omics knowledge, position Brassica crops at the forefront of next-generation molecular breeding for sustainable insect resistance.

7. Knowledge Gaps and Challenges

Despite significant advances in research, substantial information gaps persist in understanding and improving insect resistance in Brassica crops. In contrast to the comprehensive catalog of disease-resistant loci, verified main genes that give insect resistance are infrequent, and much of the existing evidence is based on quantitative or polygenic features exhibiting intricate inheritance patterns [33,93]. This scarcity illustrates both biological and methodological obstacles, including the co-evolution of Brassica crops with specialist herbivores that have developed mechanisms to detoxify or exploit host defenses [85,86]. Moreover, the absence of distinct gene-for-gene interactions hampers the implementation of conventional resistance-breeding tactics that have been effectively utilized against diseases [23]. Consequently, the majority of pest management in Brassica relies on chemical control or integrated pest management instead of genetic resistance, underscoring a significant research and breeding impediment.

A secondary difficulty is the existence of trade-offs linked to defense expression, which may limit the implementation of resistance traits in breeding programs. Increased glucosinolate levels or improved structural defenses may confer resistance advantages, although they can concurrently diminish palatability, nutritional quality, or agricultural efficacy [6]. In rapeseed, specific glucosinolate degradation products induce anti-nutritional or toxic effects in cattle feed, necessitating meticulous optimization of defense chemistry [114]. While MYC2 is well established as a central regulator of jasmonate-mediated insect resistance in Arabidopsis, functional validation of MYC2 and related JA signaling components in Brassica species, including B. rapa remains limited [115]. This represents an important knowledge gap, highlighting the need for targeted studies to confirm whether MYC2-mediated mechanisms operate similarly in Brassica crops and can be leveraged in breeding programs for durable insect resistance. Moreover, inducible defense mechanisms may impose metabolic constraints that diminish growth rates or seed output, especially when resistance traits are continuously produced [115]. Achieving a balance between resistance durability and satisfactory agronomic and nutritional results is a critical unresolved challenge, requiring a more profound comprehension of regulatory networks to provide precise fine-tuning instead of general overexpression.

Translational obstacles persist in obstructing the transition of laboratory findings into commercial cultivars [7]. Despite wild relatives and landraces possessing beneficial genes for insect resistance, many are under characterized or inaccessible due to restricted germplasm exchange, pre-breeding bottlenecks, or inadequate agronomic compatibility [77]. Despite the identification of potential loci or candidate genes through omics and genetic research, validation in realistic field circumstances frequently proves to be difficult, resource-intensive, and hampered by environmental variability and changing pest pressures due to climate change [116]. The disparity between gene discovery and practical breeding is exacerbated by regulatory, technological, and economic obstacles linked to the implementation of gene-edited or synthetic biology solutions [117]. To tackle these problems, it is essential to implement coordinated breeding pipelines, engage in multidisciplinary research, and commit to long-term investment in resistance phenotyping networks to guarantee that future Brassica cultivars provide effective, stable, and environmentally sustainable insect resistance.

8. Future Molecular Approaches and Breeding Strategies

Future breeding strategies for insect-resistant Brassica crops will combine advanced molecular tools with traditional methods to enhance trait deployment and durability. Key techniques like marker-assisted selection, genomic selection, and resistance gene pyramiding are crucial for developing pest-resistant cultivars. Leveraging genetic diversity from wild relatives and underutilized species through hybridization and speed breeding will further enrich resistance traits. Furthermore, modern cytogenetic tools and expanding pangenomic resources will facilitate the precise selection and deployment of resistance alleles. The integration of engineered resistance lines via CRISPR/Cas and careful regulatory considerations will be essential, alongside field testing to ensure effectiveness within ecosystems [118,119]. Ultimately, successful breeding will harmonize host resistance with biological control and ecological practices to achieve resilient and sustainable Brassica cultivars. This integrated approach supports the long-term effectiveness of resistance traits by reducing pest pressure through natural enemies, crop diversification, and habitat management, thereby minimizing the risk of resistance breakdown [120,121]. Incorporating such agroecological strategies into breeding programs aligns the development molecular resistance with integrated pest management (IPM) frameworks, thereby enhancing both environmental sustainability and field-level durability of insect resistance.

9. Conclusions

Brassica crops continue to face significant pressure from insect pests, yet the molecular understanding of insect resistance in these species remains limited compared to disease resistance. Although physical defenses and the glucosinolate–myrosinase system are well characterized, functionally validated insect resistance genes are still scarce, highlighting a critical knowledge gap. Recent advances reveal that jasmonic acid (JA) signaling and the transcription factor MYC2 play pivotal roles in orchestrating inducible defense responses, acting as central hubs that integrate damage perception with downstream activation of defense gene expression and secondary metabolite pathways. However, MYC2-mediated resistance mechanisms remain underexplored in Brassica, emphasizing the need for deeper functional characterization and translation into crop improvement.

Advances in multi-omics, gene editing, and synthetic biology now provide new opportunities to accelerate the discovery and deployment of resistance mechanisms. Moving forward, research should prioritize exploration of diverse germplasm, robust validation of candidate genes under realistic field conditions, and strategies to incorporate resistance traits without reducing yield or compromising food quality. Ultimately, combining molecular innovations, including targeted investigation of MYC2 and its regulatory components, with conventional breeding and ecologically based pest management will be essential to develop durable, sustainable insect-resistant Brassica cultivars.