Mechanisms of Metalloid-Induced Biotic Stress Tolerance in Plants (By PB Kale, PV Jadhav)

 Mechanisms of Metalloid-Induced Biotic Stress Tolerance in Plants

PB Kale, PV Jadhav

The role of metalloids such as Silicon (Si), Boron (B), and Selenium (Se) in enhancing plant resilience against biotic stresses is complex and multi-dimensional. These metalloids contribute to plant defense through structural reinforcement, biochemical responses, molecular signaling, and priming mechanisms, all of which reduce the impact of pathogens, pests, and other biotic stressors. Here, we outline these mechanisms to illustrate the potential of metalloids in sustainable biotic stress management in plants.

1. Cell Wall Reinforcement

One of the primary roles of Silicon in plant defense is the reinforcement of cell walls, acting as a physical barrier to biotic stressors.

1.     Silicon Deposition: Silicon is absorbed and deposited as amorphous silica in cell walls, particularly in the epidermis. This deposition strengthens the cell wall, making it harder for pathogens to penetrate plant tissues.

2.     Resistance to Herbivory: The increased rigidity of Silicon-enhanced cell walls deters herbivorous insects and reduces damage from chewing and piercing-sucking activities, indirectly lowering pathogen entry points.

3.     Protection against Fungal Pathogens: In legumes, Silicon deposits have been shown to reduce infection by fungal pathogens such as Fusarium and Rhizoctonia, providing a structural line of defense.

2. Enhanced Antioxidant Defense

Metalloids help manage reactive oxygen species (ROS), which accumulate as a natural response to biotic stress but can damage cells when unregulated.

1.     Activation of Antioxidant Enzymes: Metalloids stimulate the production of antioxidant enzymes, such as superoxide dismutase (SOD), catalase (CAT), and peroxidase (POD), which neutralize ROS and prevent oxidative stress-related damage.

2.     Reduction in Oxidative Damage: By controlling ROS levels, metalloids mitigate cellular damage, allowing plants to maintain a balanced response to pathogens without excessive oxidative harm.

3.     Enhanced Resilience: This antioxidant activity has shown positive effects in legume crops under attack by pathogens such as Phytophthora and Colletotrichum, reducing disease severity and improving survival.

3. Gene Expression and Signal Transduction

Metalloids like Boron and Selenium play a significant role in regulating gene expression and signaling pathways associated with stress responses.

1.     Defense Gene Activation: Boron induces genes responsible for producing pathogenesis-related (PR) proteins, which act as antimicrobial agents, while Selenium upregulates genes that regulate defense signaling pathways.

2.     Secondary Metabolite Synthesis: Boron and Selenium activate pathways for the synthesis of secondary metabolites, such as phytoalexins and phenolic compounds, which inhibit pathogen growth and spread.

3.     Systemic Acquired Resistance (SAR): Metalloids contribute to SAR, an immune response that confers long-term resistance across the plant. SAR involves the accumulation of PR proteins, preparing the plant for future biotic challenges.

4. Induction of Secondary Metabolite Production

Secondary metabolites, such as phytoalexins, phenolics, terpenoids, and lignin, play a critical role in plant defense, acting as antimicrobials and insect deterrents.

1.     Phytoalexins and Phenolics: Metalloids stimulate the production of phytoalexins (e.g., glyceollins in soybean, pisatin in peas) and phenolics, which accumulate at infection sites and hinder pathogen spread.

2.     Terpenoids and Alkaloids: Silicon and Boron increase terpenoid levels, which act as antifungal and insect-repellent compounds. Selenium also promotes alkaloid production, deterring herbivores and pathogens.

3.     Lignin Synthesis: Silicon and Boron facilitate lignin production, enhancing cell wall rigidity and limiting pathogen entry to vascular tissues, ultimately reducing disease spread.

5. Modulation of Hormonal Pathways and Priming for Defense

Metalloids influence key hormonal pathways, including salicylic acid (SA), jasmonic acid (JA), and ethylene (ET), which regulate plant immune responses and create a "primed" state for rapid response to stress.

1.     Salicylic Acid (SA) Pathway: SA is critical for defense against biotrophic pathogens. Metalloids like Selenium and Boron enhance SA production, activating SAR and preparing uninfected tissues for possible pathogen invasion.

2.     Jasmonic Acid (JA) and Ethylene (ET) Pathways: JA and ET play vital roles in defending against necrotrophic pathogens and herbivores. Silicon and Boron activate JA and ET pathways, boosting genes related to cell wall reinforcement, antimicrobial protein synthesis, and secondary metabolite production.

3.     Priming for Defense: Metalloids induce a primed state in legumes, allowing for faster and more robust responses to subsequent biotic stress. This preparedness enhances resource efficiency by ensuring that defense mechanisms are only fully activated upon stress detection, contributing to sustainable crop protection.

Summary

Metalloids such as Silicon, Boron, and Selenium enhance biotic stress tolerance in plants through multiple mechanisms, including structural cell wall reinforcement, ROS management, activation of defense genes, secondary metabolite production, and hormonal regulation. These mechanisms collectively strengthen plants’ resilience, offering a sustainable alternative to chemical treatments. With further research, metalloid-based approaches could be fine-tuned for specific crops, paving the way for eco-friendly agricultural practices that improve crop productivity and health.