Isolation and characterization of metal resistant plant growth promoting endophytic bacteria from Cynodon dactylon L....

Heavy metal contamination threatens soil health and ecosystem stability. Endophytic bacteria from metal-tolerant plants can enhance phytoremediation. In this study, twenty-four endophytic bacterial isolates were isolated from leaves and roots of Cynodon dactylon collected from a metal-contaminated industrial site in Islamabad, Pakistan. All isolates were screened for plant growth-promoting (PGP) traits and heavy metal (Zn, Pb, Cu, Cr) tolerance. Quantitative assays revealed that 95.8% of isolates solubilised phosphate (maximum zone 17 mm by CDR11), all produced indole acetic acid (IAA) (3.2-14.7 µg/mL) and ammonia (0.8-6.5 µg/mL). Metal tolerance tests showed minimum inhibitory concentrations (MIC) of 50-100 µg/mL. Isolates CDL3, CDL2 and CDR1 exhibited maximum tolerance to all four metals at 100 µg/mL. Seed germination assays with Linum usitatissimum demonstrated significant (p < 0.05) increases in germination percentage (up to 91.1% by CDR6 vs. 66.6% control) and seedling vigour. 16 S rRNA gene sequencing of six potential isolates identified them as Bacillus thuringiensis CDL2 (99.78%), Bacillus cereus CDL3 (99.90%), Bacillus cereus CDL5 (99.90%), Bacillus sp. CDL9 (98.52%), Bacillus sp. CDR1 (100%) and Bacillus subtilis CDR12 (99.90%). These results demonstrate that metal-resistant Bacillus endophytes from C. dactylon possess multiple PGP traits and significantly improve flax seed germination, highlighting their potential as bio-inoculants for phytoremediation of metal-polluted soils.