Iron is an essential nutrient for most of the life forms present in earth crust. Iron may cause physiological disorders in plants, contribute to microbial aggressiveness and exert beneficial effect on plant growth and defense (Ward 2025). Macronutrients nitrogen, phosphorus and potassium significantly influence plant defense mechanisms and are essential for synthesis of defense compounds, immune signaling and physiological responses like stomatal regulation (Dutta et al., 2024). Whereas the micronutrients like zinc, copper and iron are essential for balancing reactive oxygen species (ROS) and other reactive compound involved in the plant immune response (Marreiro et al., 2017; Dutta et al., 2024). Insufficient iron uptake causes retarded growth, interveinal chlorosis and reduced fitness (Shmidt et al., 2020). Plant and associated microbes often compete for the acquisition of iron from soil. Iron can exist in both ferrous (Fe2+) and ferric (Fe3+) forms, allowing them to function as a catalytic component of enzymes that mediate redox reaction in many cellular processes such as photosynthesis, respiration and deoxyribonucleic acid (DNA) replication (Verbon et al., 2017; Pandey 2023). Host plant may use iron to increase local oxidative stress in defense response against the pathogen (Aznar et al., 2015).
Plant defense against pathogen and plant response to iron deficiency share many features such as secretion of phenolic compounds and use of common hormone signaling pathways (Aznar et al., 2015). The problems caused due to iron deficiency; plants use two strategies for iron absorption from the rhizosphere. First strategy is based on rhizosphere acidification and iron reduction, while second strategy is based on iron chelation (Montejano-Ramirez and Valencia-Cantero 2023). Pathogenic defense and iron uptake is not an isolated phenomenon: the antimicrobial phenols are produced by the plant during plant defense, chelation and solubilization of iron, hence the production and secretion of these molecules increase in response to iron deficiency (Montejano-Ramirez and Valencia-Cantero 2023). Iron deficiency also induces the expression of defense-related genes (Montejano-Ramirez and Valencia-Cantero 2023).
Excess iron can produce highly active ROS which are toxic to the cells of plant and pathogens (Liu et al., 2021). Iron acts as a catalyst with hydrogen peroxide through the Fenton reaction, producing ROS, including highly reactive hydroxide ion (Herlihy et al., 2020). Fenton reaction is the reaction between iron and hydrogen peroxide, generating hydroxyl radical, which is highly reactive and highly toxic to the living cells (Abe et al., 2022). The formation of hydroxyl radicals oxidizes lipids, proteins and DNA, that threatens the survival of cells (Pierre and Fontecave 1999). Therefore, plant and microbes must modulate their iron homeostasis to remain fit.
Siderophore-mediated iron uptake pathway is a common strategy used by bacteria and fungi (Liu et al., 2021). Plant use iron-withholding strategies to reduce pathogen virulence or to locally increase iron level to activate a toxic oxidative burst (Verbon et al., 2017). Few plant pathogen counteract such defenses by secreting iron-scavenging siderophores that promote iron uptake and alleviate iron-regulated host immune response (Verbon et al., 2017). Nobori et al.(2018) found that bacterial iron acquisition pathway is commonly suppressed by multiple plant immune-signaling pathway. Siderophores are biosynthetically produced and secreted by several bacteria, yeast, fungi and plants, to scavenge for Fe3+ ion (Chu et al., 2010). Siderophores are classified into three main families depending on the characteristic functional group i.e. hydroxamates catecholates and carboxylates (Ahmed and Holmstrom 2014). Iron is also required by pathogen for survival. The invaded plant is the only source of iron for the pathogen. To successfully infect host plant and grow, pathogen employ multiple iron uptake systems (Liu et al., 2021). Iron has a role in pathogen virulence and most plants have evolved iron-withholding strategies to reduce iron availability to the pathogen (Ye et al., 2014). Fungal infection progressed rapidly via biotrophic to necrotrophic growth in iron-deficient leaves, while an adequate iron nutritional status of maize suppressed the formation of infection structures of the fungal pathogen Colletotrichum graminicola during the early biotrophic growth phase (Ye et al., 2014).
In wheat-Blumeria graminis f. sp. tritici and rice-Magnaporthe oryzae pathosystems, ferric iron is accumulated at the site of the pathogen attack (Liu et al., 2021). The accumulated iron suppresses the growth of pathogen through a ROS burst produced by Fenton reaction (Verbon et al., 2017; Liu et al., 2021). To scavenge host iron for survival, fungal pathogens have evolved at least two iron acquisition systems. 1) System depends on secretion and subsequent uptake of Fe3+ iron-specific siderophores and 2) the other system uses cell wall iron reductases to free bound or insoluble Fe3+ iron by reducing it to Fe2+ iron for uptake (Greenshields et al., 2007). Bacteria produce all three families of siderophore. Most bacteria produce siderophore catecholates (i.e. enterobactin), few produce carboxylates (i.e. rhizobactin) and hydroxamates (i.e. ferrioxamine B) (Ahmed and Holmstrom 2014). Certain type of bacterial siderophore contain a mix of main functional group (i.e. pyoverdines) (Cornelis 2010; Ahmed and Holmstrom 2014). The most common fungal siderophores is hydroxamates (Ahmed and Holmstrom 2014; Liu et al., 2021).
Dickeya dadantii a plant pathogenic enterobacterium causes soft rot disease in a wide range of hosts including Arabidopsis thaliana (Aznar et al., 2015). During symptom progression, two distinct zone are visible: a macerated zone displaying weak iron content and a healthy zone displaying strong iron content. Immunolabelling of cell wall methylated pectin shows that pectin degradation is correlated with iron release from cell wall, indicating a strong relationship between cell wall integrity and iron in plant tissues (Aznar et al., 2015).
Fungi can take up iron when present in any wide variety of forms, which includes free iron ion, low affinity iron chelates, siderophore-iron chelates, transferrin, heme and hemoglobin (Philpott 2006). All known plant fungal pathogens that require siderophore production for virulence are necrotrophs. (Greenshields et al., 2007). Plant nutritional iron status can control infection by acting on pathogen virulence and plant defense.
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