Zinc solubilizing bacteria and their potential as bioinoculant for growth promotion of green soybean ( L. Merr.).

Zinc-solubilizing rhizobacteria can convert insoluble zinc to an accessible form and increase Zn bioavailability in soil, which help mitigate Zn deficiency in crops. In this work, 121 bacterial isolates were isolated from the rhizosphere soils of peanuts, sweet potatoes, and cassava, and their capability to solubilize Zn was evaluated using Bunt and Rovira's agar containing 0.1% ZnO and ZnCO. Among these isolates, six showed high Zn solubilization efficiencies ranging from 1.32 to 2.84 and 1.93 to 2.27 on the medium supplemented with 0.1% ZnO and ZnCO, respectively. In a quantitative analysis of soluble Zn in liquid medium supplemented with 0.1% ZnO, the isolate KAH109 showed the maximum soluble zinc concentration of 62.89 mg L. Among the six isolates, the isolate KAH109 also produced the most indole-3-acetic acid (IAA) at 33.44 mg L, whereas the isolate KEX505 also produced IAA at 17.24 mg L along with showing zinc and potassium solubilization activity. These strains were identified as KAH109 and KEX505 based on 16S rDNA sequence analysis. In a greenhouse experiment conducted in Nakhon Pathom, Thailand the ability of KAH109 and KEX505 to stimulate the growth and production of green soybeans was examined. The results revealed that inoculation with KAH109 and KEX505 considerably increased plant dry weight by 26.96% and 8.79%, respectively, and the number of grains per plant by 48.97% and 35.29% when compared to those of the uninoculated control. According to these results, both strains can be considered as a potential zinc solubilizing bioinoculant to promote the growth and production yield of green soybeans.