Molecular Regulation of Salt Stress Tolerance in Fruit Crops with Special Emphasis on WRKY Transcription Factors

1. Introduction

Fruit crops contribute significantly to global food security, nutrition, and economic stability, particularly in developing countries. However, fruit production is increasingly threatened by abiotic stresses, among which soil salinity is one of the most damaging. Salinity stress affects nearly 20–30% of irrigated agricultural land globally and is projected to expand further under climate change scenarios. Fruit crops are generally more sensitive to salinity than many annual crops due to their perennial nature, longer growth cycles, and specific physiological requirements. Salt stress adversely affects plant growth and productivity through osmotic stress, ion toxicity, and nutrient imbalance [1]. High concentrations of sodium (Na⁺) and chloride (Cl⁻) ions disrupt cellular homeostasis, impair photosynthesis, reduce water uptake, and cause oxidative damage. In fruit crops, salinity leads to reduced vegetative growth, poor flowering, fruit drop, inferior fruit quality, and ultimately yield losses. Developing salt-tolerant fruit crops is therefore a priority for sustainable horticulture. Plants have evolved sophisticated molecular mechanisms to perceive salt stress and initiate adaptive responses. These responses involve signal perception at the plasma membrane, activation of signaling cascades, transcriptional reprogramming, and metabolic adjustments [2]. Transcription factors act as master regulators by controlling the expression of large sets of stress-responsive genes. Among them, WRKY transcription factors have gained considerable attention due to their significant involvement in abiotic stress tolerance.

WRKY transcription factors are plant-specific proteins characterized by the conserved WRKY domain and zinc finger motifs. Initially identified as regulators of pathogen defense, WRKY proteins are now recognized as crucial components in abiotic stress responses, including salinity, drought, heat, and cold stress. In fruit crops, increasing evidence suggests that WRKY genes play essential roles in modulating salt tolerance through regulation of ion transporters, antioxidant systems, hormone signaling pathways, and stress-responsive genes. With the advent of high-throughput sequencing technologies, genome-wide identification and functional characterization of WRKY gene families have been reported in several fruit crops such as apple, grape, citrus, banana, and tomato. These studies provide valuable insights into the regulatory networks mediated by WRKY transcription factors under salt stress [3]. This review aims to synthesize current knowledge on the molecular regulation of salt stress tolerance in fruit crops, with special emphasis on WRKY transcription factors and their potential applications in crop improvement.

2. Salt Stress and Its Impact on Fruit Crops

Salt stress affects plants through two major phases: osmotic stress and ionic stress. The initial osmotic phase reduces water availability to roots, leading to decreased cell expansion and stomatal closure. The subsequent ionic phase involves accumulation of toxic Na⁺ and Cl⁻ ions in plant tissues, disrupting metabolic processes. In fruit crops, salt stress manifests as reduced leaf area, chlorosis, necrosis, poor root development, delayed flowering, and reduced fruit set. Salinity also affects fruit size, taste, sugar content, and nutritional quality [4]. Sensitive fruit crops such as citrus, grapevine, and apple exhibit significant yield reductions even at moderate salinity levels. Physiological responses to salinity include reduced photosynthetic efficiency, impaired nutrient uptake, and altered hormonal balance. At the cellular level, salt stress induces oxidative stress by generating excessive reactive oxygen species (ROS), leading to membrane damage, protein oxidation, and DNA damage. Therefore, efficient detoxification of ROS and maintenance of ion homeostasis are critical for salt tolerance.

3. Molecular Mechanisms of Salt Stress Tolerance in Plants

Salt tolerance in plants is governed by multiple molecular mechanisms, including ion homeostasis, osmotic adjustment, antioxidant defense, and stress signaling pathways. Key components include ion transporters such as SOS1, NHX, and HKT proteins, which regulate Na⁺ exclusion, compartmentalization, and transport. Signal transduction pathways involving calcium signaling, mitogen-activated protein kinases (MAPKs), and phytohormones such as abscisic acid (ABA) play central roles in stress perception and response. ABA accumulation under salt stress triggers stomatal closure and induces expression of stress-responsive genes [5]. Transcriptional regulation is a crucial step in these adaptive responses. Several transcription factor families, including NAC, MYB, bZIP, DREB, and WRKY, coordinate the expression of downstream target genes involved in stress adaptation. Among these, WRKY transcription factors have emerged as key regulators due to their broad involvement in stress-responsive gene networks.

4. WRKY Transcription Factors: Structure and Classification

WRKY transcription factors are defined by the conserved WRKY domain containing the amino acid sequence WRKYGQK and a zinc finger motif. Based on the number of WRKY domains and zinc finger types, WRKY proteins are classified into three major groups: Group I, Group II, and Group III. Group I WRKYs possess two WRKY domains, while Group II and III contain a single WRKY domain. Group II is further subdivided into several subgroups based on phylogenetic analysis. WRKY proteins bind to W-box elements (TTGACC/T) in the promoter regions of target genes to regulate transcription. Genome-wide analyses have identified large WRKY gene families in fruit crops, indicating their evolutionary expansion and functional diversification [6]. Differential expression patterns of WRKY genes under salt stress suggest their involvement in stress-specific regulatory networks.

5. Role of WRKY Transcription Factors in Salt Stress Tolerance

WRKY transcription factors regulate salt stress tolerance through multiple mechanisms. One key role is the regulation of ion homeostasis by controlling genes involved in Na⁺ transport and compartmentalization. Certain WRKY proteins modulate the expression of SOS pathway genes, thereby enhancing Na⁺ exclusion and reducing cytosolic toxicity [7]. WRKY factors also play a critical role in antioxidant defense by regulating genes encoding enzymes such as superoxide dismutase, catalase, and peroxidases. Enhanced antioxidant capacity helps mitigate oxidative damage under salinity stress.

Hormonal signaling pathways, particularly ABA signaling, are closely associated with WRKY-mediated stress responses. Several WRKY proteins act as positive or negative regulators of ABA-responsive genes, influencing stomatal behavior and stress adaptation. In fruit crops, WRKY genes have been shown to confer salt tolerance when overexpressed. For example, MdWRKY9 in apple has been reported to enhance salt tolerance by improving antioxidant activity and maintaining ion balance. Similar findings have been reported in grapevine, citrus, and tomato, highlighting the conserved role of WRKY transcription factors across species.

6. WRKY Transcription Factors in Fruit Crops under Salt Stress

Functional studies in fruit crops have demonstrated that WRKY transcription factors are differentially expressed in response to salt stress. Transcriptome analyses reveal that several WRKY genes are rapidly induced upon salt exposure, indicating their role in early stress signaling. In apple, WRKY genes regulate salt tolerance by modulating ROS scavenging systems and stress-responsive gene expression. In grapevine, specific WRKY members are associated with enhanced salt and drought tolerance. Citrus WRKY genes have been implicated in ion transport and stress signaling, while tomato WRKYs contribute to improved growth and yield under saline conditions [8]. These findings suggest that WRKY transcription factors act as central nodes in stress regulatory networks and represent promising targets for genetic improvement of fruit crops.

7. Biotechnological and Breeding Approaches Targeting WRKY Genes

Advances in molecular biology and biotechnology offer new opportunities to exploit WRKY transcription factors for improving salt tolerance in fruit crops. Genetic engineering approaches, including overexpression and gene editing using CRISPR/Cas systems, enable precise manipulation of WRKY genes. Marker-assisted selection and genomic selection strategies can also be employed to identify and incorporate favorable WRKY alleles into breeding programs [9-11]. Integration of omics approaches such as transcriptomics, proteomics, and metabolomics provides a comprehensive understanding of WRKY-mediated stress responses.

8. Future Perspectives

The significant progress, challenges remain in translating laboratory findings into field applications. Functional redundancy among WRKY genes and their complex regulatory interactions require further investigation. Future research should focus on understanding WRKY networks at the systems level and their interaction with other transcription factors [12]. The conventional breeding with molecular and genomic tools will accelerate the development of salt-tolerant fruit crops. Harnessing WRKY transcription factors holds great promise for enhancing resilience of horticultural systems under increasing salinity stress.

9. Conclusion

Salt stress poses a major threat to fruit crop productivity worldwide. WRKY transcription factors play a crucial role in regulating molecular and physiological responses to salinity stress in fruit crops. Through modulation of ion homeostasis, antioxidant defense, and hormone signaling, WRKY proteins enhance plant tolerance to saline conditions. Advances in genomics and biotechnology provide valuable tools to exploit WRKY genes for crop improvement. A deeper understanding of WRKY-mediated regulatory networks will facilitate the development of salt-tolerant fruit crops, contributing to sustainable horticulture and food security under changing climatic conditions.

References

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