Finger Millet (Eleusine coracana) Plant–Endophyte Dynamics: Plant Growth, Nutrient Uptake, and Zinc Biofortification

{"Background": "Zinc deficiency is a widespread micronutrient inadequacy in crop plants globally, leading to reduced yields and contributing to human zinc deficiency in populations relying on staple crops. Finger millet (Eleusine coracana) is a nutritionally dense cereal grown in semi-arid regions of Asia and Africa, but its production is limited by low zinc availability in soils\u2014up to 47% of Indian soils may be zinc-deficient. While chemical zinc fertilizers exist, they are often not cost-effective or sustainable. Endophytic microorganisms capable of solubilizing insoluble zinc compounds into plant-available forms represent a promising alternative for biofortification. This study aimed to isolate and characterize zinc-solubilizing endophytes from finger millet and evaluate their potential to enhance zinc uptake, NPK content, and plant growth.", "Methods": "Seeds of three finger millet cultivars (PRM-1, VL-348, VL-352) were sown in research fields in Uttarakhand, India. Endophytes were isolated from surface-sterilized root, stem, and leaf tissues. A total of 70 fungal and 112 bacterial endophytes were obtained. Qualitative zinc solubilization screening was performed on Tris minimal medium agar plates supplemented with 0.1% zinc oxide, zinc carbonate, or zinc phosphate. Four potent isolates\u2014two fungi (EC1F-15, EC1F-23) and two bacteria (EC3B-22, EC3B-23)\u2014were selected based on solubilization indices. These were characterized for plant growth-promoting traits including IAA production (Salkowski reagent method), HCN production (qualitative and quantitative picrate assay), ammonia production (Nessler's reagent), siderophore production (CAS agar method, quantitative psu), phosphate solubilization (NBRI-BPB agar), organic acid production (bromocresol green plate assay), and extracellular enzyme activities (amylase, lipase, urease, tannase, gelatinase, protease, phytase). Stress tolerance was assessed across pH 4\u201310, temperatures 5\u201350\u00b0C, and NaCl 0.5\u201312%. Molecular identification was performed via ITS sequencing (fungi) and 16S rRNA sequencing (bacteria). A pot experiment was conducted in 4 L pots with sterile soil, using zinc carbonate as the zinc source. Treatments included: control (uninoculated), zinc carbonate alone (ZC), ZC+EC3B-22, ZC+EC3B-23, ZC+EC1F-15, and ZC+EC1F-23. Plants were harvested after 90 days. Growth parameters (shoot/root length, dry biomass) were measured. Grain zinc content was analyzed by atomic absorption spectrophotometry after acid digestion. NPK content was determined using standard methods (Kjeldahl for N, molybdovanadate for P, flame photometry for K). Statistical analysis used ANOVA with Duncan's multiple range test at p<0.05.", "Key Results": "EC1F-15 (Aspergillus terreus) showed maximum zinc solubilization index of 3.0 mm for zinc carbonate, 2.9 mm for zinc oxide, and 2.6 mm for zinc phosphate. EC1F-23 (Lecanicillium sp.) showed indices of 3.3 mm, 3.6 mm, and 3.1 mm respectively. EC3B-22 (Pseudomonas bijieensis) showed 4.3 mm, 3.4 mm, and 2.1 mm. EC3B-23 (Priestia megaterium) showed 4.8 mm, 5.6 mm, and 2.3 mm. IAA production ranged from 35.4 \u00b5g/mL (EC1F-15) to 117.2 \u00b5g/mL (EC3B-22). Siderophore production ranged from 23.74 psu (EC3B-22) to 71.8 psu (EC3B-23). HCN production was highest in EC3B-22 (OD 0.043). Only EC3B-23 produced ammonia. All isolates produced organic acids, reducing pH to 4.5\u20135.2. In the pot experiment, the highest plant height (68.3 cm) was observed in VL-352 with EC3B-23, versus 53.1 cm in control. Maximum root length (22.1 cm) was also in VL-352 with EC3B-23. The highest plant dry weight (21.2 g) was in PRM-1 with EC3B-23, versus 15.9 g in control. Grain zinc content increased by up to 18.8%: in PRM-1, EC3B-23 gave 2.73 mg zinc (18.8% over control); in VL-348, EC3B-23 gave 18.8% increase; in VL-352, EC3B-22 gave 2.75 mg (18.02% increase). NPK concentrations were significantly higher in endophyte-treated plants across varieties, with bacterial treatments generally showing the greatest improvements.", "Clinical Implications": "This study demonstrates that native endophytic microbes from finger millet can effectively solubilize insoluble zinc compounds and enhance zinc biofortification of grains by up to 18.8%, alongside improving NPK uptake and plant growth. For populations where finger millet is a dietary staple, particularly in zinc-deficient regions of India and Africa, deploying such endophytes as bioinoculants could improve dietary zinc intake without the cost and environmental drawbacks of chemical fertilizers. The identified strains\u2014Aspergillus terreus, Lecanicillium sp., Pseudomonas bijieensis, and Priestia megaterium\u2014also produce phytohormones, siderophores, and HCN, suggesting additional benefits for plant health and biocontrol. However, the authors note that field trials are required to confirm commercial viability before these endophytes can be recommended as biofertilizers for widespread finger millet cultivation."}