Isolation, characterization, and identification of phosphate-solubilizing <i>Burkholderia cepacia</i> from the sweet corn cv. Golden Bantam rhizosphere soil and effect on growth-promoting activities

The average growth and productivity of sweet corn (Zea mays L, cv. Golden Bantam) in India is lower than its potential, likely due to lack of proper phosphorus (P) management. Phosphorus is normally applied to soil as a synthetic P fertilizer. The industrial manufacturer of chemical P fertilizer is a highly energy-intensive process. There can be long-term environmental implications due to eutrophication, soil fertility depletion, and the size of the carbon footprint. Alternate strategies for use of P in sustainable agriculture are being developed. Phosphate-solubilizing bacteria (PSB) can make available insoluble phosphate compounds as an alternate strategy to phosphatic fertilizers. Even though PSB occur in soil, their numbers are usually not high enough in the rhizosphere, their performance in situ is not reliable, and needs to be improved. The project was undertaken to isolate, and characterize, PSB from the rhizosphere of sweet corn (Zea mays L, cv. Golden Bantam) and investigate their potential for the promotion of plant growth. Eight phosphate-solubilizing bacterial colonies were isolated on Pikovskaya’s (PKV) agar medium, containing insoluble tricalcium phosphate. Clear halo zones occurring around colonies indicated they were phosphate solubilizers. The phosphate-solubilization index (PSI) ranged from 1.77 ± 0.92 to 4.88 ± 0.69, broth medium pH ranged from 5.44 ± 0.40 to 3.08 ± 0.08 and phosphate solubilization varied from 125.33 ± 8.78 to 305.49 ± 10 μg∙mL−1. Bacterial isolates, MS1, MS2, and MS5, exhibited high PSI, low pH, and high phosphate solubilization and were further studied. There was a negative correlation (r = −0.72) between soluble P content and medium pH, and a positive correlation (r = 0.90) between soluble P content and PSI, indicating acidification of the medium can aid phosphate solubilization. Pot and field studies indicated isolate MS1 had the greatest efficiency in promoting sweet corn growth and kernel productivity over isolates MS2 and MS5 and the control. Based on the 16S rRNA gene sequence analysis isolate MS1 was closely related to Burkholderia cepacia (99%). Isolates MS2 and MS5 were related to B. contaminans (94%–99%). Isolate B. cepacia was superior over MS2 and MS5 in terms of concentration of soluble P, pH of the broth medium, pot experiment and field experiment, and selected for further study. The HPLC analysis indicated isolate MS1 produced gluconic and formic acids, and an unknown acid, in a culture which may be the principal mechanism for phosphate solubilization. Burkholderia cepacia isolates could be used as a plant growth-promoting bacterium instead of synthetic chemicals and may help to sustain sweet corn productivity.