Isolation of endophytic bacteria from cadmium-contaminated rice seeds and their cadmium tolerance and plant growth-promoting traits

Abstract:

[Objective] Endophytes play an important role in the promotion of plant growth and the remediation of heavy metal-contaminated soil.In this study,we isolated the endophytic bacteria from cadmium-contaminated rice seeds and analyzed their cadmium tolerance,plant growth-promoting traits,and effects on rice seed germination and seedling growth under cadmium stress.[Methods] First,we used ethanol and sodium hypochlorite to sterilize the surface of rice seeds and then isolated the endophytic bacteria by standard dilution plating method.Second,we amplified and sequenced the 16S rRNA genes of the strains and identified their taxonomic positions.Third,we analyzed the cadmium tolerance,cadmium removal rate,and plant growth-promoting traits of the endophytic bacteria via microdilution method,inductively coupled plasma mass spectrometry (ICP-MS),and spectrophotometry,respectively.Finally,we infected the rice seeds with bacterial inoculants to observe their effects on seed germination,seedling growth,and cadmium accumulation.[Results] We isolated a total of 133 endophyte strains belonging to 24 species of 12 genera from two rice cultivars.Eighty-three strains were isolated from low-Cd-accumulating caltivar 728B,including Pseudomonas(34.94%),Bacillus(28.92%),and Paenibacillus(10.84%).Fifty strains were isolated from high-Cd-accumulating caltivar BB,including Pantoea(40%),Curtobacterium(22%),and Microbacterium(12%).The minimum inhibitory concentrations of cadmium against 24 representative strains were 80-2 560 μmol/L.Bacillus sp.HNR-4 showed the highest cadmium tolerance.The cadmium removal rate of HNR-4 reached 77.57% after the strain was cultured in the liquid medium containing 5 mmol/L Cd²⁺ for 24 h.Most of the 24 strains were capable of producing plant growth-promoting substances,such as IAA (0.78-40.12 μg/mL),siderophore (1.46-752.74 mg/L),soluble phosphate (0-3.10 mg/L),and ACC deaminase (0-9.22 U/mg).Under cadmium stress,HNR-4 inoculant significantly increased the germination rate and seedling shoot length,while reduced the transfer coefficient of cadmium from root to shoot.[Conclusion] Although the composition of seed endophytic bacteria varied in different rice cultivars,these bacteria all showed different levels of cadmium tolerance and plant growth-promoting activities.Isolation and identification of Cd-resistant endophytic bacteria may help investigate the interaction between endophytic bacteria and host plants exposed to heavy metal contamination,and explore the potential of microbial remediation in farmland.