Isolation of heavy metal-immobilizing and plant growth-promoting bacteria and their potential in reducing Cd and Pb uptake in water spinach.

Heavy metal-immobilizing bacteria are normally capable of stabilizing metals and affecting their absorption by plants. However, few studies have elucidated the mechanisms employed by novel heavy metal-immobilizing and plant growth-promoting bacteria to immobilize Cd and Pb and reduce their uptake by vegetables. In this study, polyamine (PA)-producing strains were isolated and their effects on biomass and metal accumulation in water spinach (Ipomoea aquatica Forssk.) and the underlying mechanisms were investigated. Two PA-producing strains, Enterobacter bugandensis XY1 and Serratia marcescens X43, were isolated. Strains XY1 and X43 reduced the aqueous Cd and Pb levels (49%-52%) under 10 mg L-1 Cd and 20 mg L-1 Pb because of metal ion chelation by bacterially produced PAs and cell adsorption. Further evidence showed that Cd and Pb were bound and precipitated on the bacterial cell surface in the form of Cd(OH)2, CdCO3 and PbO. Compared with strain-free water spinach, greens inoculated with strains XY1 and X43 showed 51%-80% lower Cd and Pb contents. The rhizosphere soil pH and PA contents were significantly higher, and lower contents of the rhizosphere soil acid-soluble fractions of Cd (18%-39%) and Pb (31%-37%) were observed compared to the noninoculated control. Moreover, inoculation with XY1 reduced the diversity of the bacterial community, but the relative abundances of plant growth-promoting and PA-producing bacteria in rhizosphere soil were enriched, which enhanced water spinach resistance to Cd and Pb toxicity. Our findings describe novel heavy metal-immobilizing bacteria that could be used to improve the habitat of vegetables and reduce their uptake of heavy metals.

The complete mitogenome of Chrysopogon zizanioides (L.) Roberty (Poaceae), with its phylogenetic analysis.

Vetiver grass (Chrysopogon zizanioides), is a perennial and tussock C4 grass from the genus Chrysopogon of Poaceae, which has been widely used as a natural and inexpensive resource for multifarious environmental applications. The complete mitogenome of C. zizanioides was 551,622 bp in length, containing 40 protein-coding genes (PCGs), 19 transfer RNA genes (tRNAs), and six ribosomal RNA genes (rRNAs). All PCGs started with ATG and stopped with TNN (TAA, TAG, and TGA). The overall nucleotide composition is: 28.2% A, 28.2% T, 21.7% G, and 21.9% C, with a biased A + T content of 56.4%. Phylogenetic analysis using 14 PCGs of 22 species showed that C. zizanioides display a close relationship with Saccharum officinarum (LC107874) and Sorghum bicolor (DQ984518) in Poaceae.