Rajesh Kumar Tewari

Micronutrients like iron (Fe), zinc (Zn), copper (Cu), manganese (Mn), boron (B), nickel (Ni), and molybdenum (Mo) perform significant roles in the regulation of plant metabolism, growth, and development. Micronutrients, namely Fe, Zn, Cu, Mn, and Ni, are involved in oxidative stress and antioxidant defense as they are cofactors or activators of various antioxidant enzymes, viz., superoxide dismutase (Fe, Cu/Zn, Mn, and Ni), catalase (Fe), and ascorbate peroxidase (Fe). An effort has been made to incorporate recent advances along with classical work done on the micronutrient deficiency-induced oxidative stress and associated antioxidant responses of plants. Deficiency of a micronutrient produces ROS in the cellular compartments. Enzymatic and non-enzymatic antioxidant defense systems are often modulated by micronutrient deficiency to regulate redox balance and scavenge deleterious ROS for the safety of cellular constituents. ROS can strike cellular constituents such as lipids, proteins, and nucleic acids and can destruct cellular membranes and proteins. ROS might act as a signaling molecule and activate the antioxidant proteins by interacting with signaling partners such as respiratory burst oxidase homolog (RBOH), G-proteins, Ca²⁺, mitogen activated protein kinases (MAPKs), and various transcription factors (TFs). Opinions on probable ROS signaling under micronutrient deficiency have been described in this review. However, further research is required to decipher micronutrient deficiency-induced ROS generation, perception, and associated downstream signaling events, leading to the development of antioxidant responses in plants.

Improving functional Fe status through the application of Fe with synthetic organic ligands is a strategy to overcome the Fe-deficiency problem. The study emphasizes on the potential efficiency of different selected synthetic organic ligands (Fe-EDTA, Fe-EDDHA, Fe-Citrate) and compare their effectiveness in improving functional Fe status by biochemical means and how wheat responds to different Fe ligands at a lower supply of Fe supply, 25 µM ( c.f. 150 µM in maize) under hydroponics. Wheat ( Triticum aestivum ) and maize ( Zea mays ) plants grown under a hydroponic system. Fe was supplied as either FeCl ₃ or in complex with organic ligands, viz. , EDTA, EDDHA, and citrate. Wheat plants exhibited improved functional Fe status, as indicated by high chlorophyll and carotenoids concentrations, lipid peroxidation, and ferric chelate reductase activity in Fe-EDTA and Fe-Citrate-supplied plants. Interveinal chlorosis along with mildly increased superoxide dismutase activity was observed in FeCl ₃ , and Fe-EDDHA-supplied plants. Chlorotic leaves of Fe-EDDHA show a decrease in lipid peroxidation, highlighting the role of Fe in chlorophyll biosynthesis and lipid peroxidation in wheat plants. Fe-EDDHA was found to be an inefficient Fe supplier for wheat too, as indicated by decreased growth with a low shoot Fe concentration ( c.f. maize at 150 µM supply). Both maize and wheat are strategy II plants, but wheat is more efficient than maize plants. Fe-EDTA and Fe-Citrate have been proven to be effective Fe-suppliers in comparison to Fe-EDDHA and FeCl ₃ in wheat plants. Fe-EDDHA is a poor Fe supplier in hydroponic systems as EDDHA forms a strong complex with Fe and its aromatic and hydrophobic nature.

Nitric oxide (NO), once regarded as a poisonous air pollutant, is now understood as a regulatory molecule essential for several biological functions in plants. In this review, we summarize NO generation in different plant organs and cellular compartments, and also discuss the role of NO in iron (Fe) homeostasis, particularly in Fe-deficient plants. Fe is one of the most limiting essential nutrient elements for plants. Plants often exhibit Fe deficiency symptoms despite sufficient tissue Fe concentrations. NO appears to not only up-regulate Fe uptake mechanisms but also makes Fe more bioavailable for metabolic functions. NO forms complexes with Fe, which can then be delivered into target cells/tissues. NO generated in plants can alleviate oxidative stress by regulating antioxidant defense processes, probably by improving functional Fe status and by inducing post-translational modifications in the enzymes/proteins involved in antioxidant defense responses. It is hypothesized that NO acts in cooperation with transcription factors such as bHLHs, FIT, and IRO to regulate the expression of enzymes and proteins essential for Fe homeostasis. However, further investigations are needed to disentangle the interaction of NO with intracellular target molecules that leads to enhanced internal Fe availability in plants.