Screening of Zinc Solubilizing Plant Growth Promoting Rhizobacteria (PGPR) as Potential Tool for Biofortification in Rice

Aims: Plant growth-promoting rhizobacteria enhance growth by making plant nutrients available to plants under a variety of growing conditions. The study was designed to screen zinc (Zn) solubilizing rhizobacteria and test their colonization ability in the rice rhizosphere. Place and Duration of Study: The experiments were conducted in the Department of Agricultural Chemistry, Bangladesh Agricultural University, Mymensingh-2202, from January 2019 to July 2019. Methodology: Initially, thirty-two previously isolated rhizobacteria were used for screening Zn solubilizing ability under a solid medium. Selected rhizobacteria from previous screening were used to quantify Zn solubilization in broth culture and evaluate their root colonization in rice using water agar media in a growth chamber. Early seedling growth was monitored for two weeks, and root-shoot lengths were recorded. Eleven of the tested rhizobacteria responded positively to ZnO-containing media. Results: The Zn solubilizing index (ZSI) of the positive isolates ranged between 1.08-2.25 after 5 days of incubation. Isolate MQ1 solubilized the highest Zn both in solid medium (ZSI=2.25) and culture broth (solubilized 220.20 ppm Zn). The colonization of bacteria in the root zone was also investigated via bio-primed rice seeds of Binadhan-20. Eight of 11 isolates (MQ1, MQ2, MQ3, MQ4, OSbr5, OSbr6, EC1, and MQL9) showed substantial colonization in the rhizosphere after two weeks. The germination percentage of bio-primed seeds was not increased over the control, however, in most cases, bio-priming boasted early seedling growth. The OSbr5, OSbr6, and MQ6 isolates were superior performers in case of root and shoot growth. Conclusion: The study identified some Zn solubilizing isolates, revealed their root colonizing ability, and observed early plant growth promotion. These isolates could be used as a potential tool for the Zn biofortification approach in rice.