Chromium phytoextraction and physiological responses of the hyperaccumulator Leersia hexandra Swartz to plant growth-promoting rhizobacterium inoculation

Highlights

● Improved Cr phytoextration efficiency was achieved by B. cereus inoculation.

● B. cereus could produce plant-beneficial PGPR factors at diverse Cr stresses.

● Enhanced resistance of inoculated L. hexandra towards elevated Cr stress.

● The majority of Cr existed in the stable forms in the tissues of L. hexandra.

Abstract

Phytoextraction is a promising option for purifying hexavalent chromium (Cr(VI))-laden wastewater, but the long remediation period incurred by poor growth rate of Cr hyperaccumulators remains a primary hindrance to its large-scale application. In this study, we performed a hydroponic experiment to evaluate the feasibility of promoting the growth and phytoextraction efficiency of Cr hyperaccumulator Leersia hexandra Swartz (L. hexandra) by inoculating plant growth-promoting rhizobacteria (PGPR) Bacillus cereus (B. cereus). In batch tests, the Cr(VI) removal rates of L. hexandra and B. cereus co-culture were greater than the sum of their respective monocultures. This was likely due to the microbial reduction of Cr(VI) to Cr(III), which is amiable to plant uptake. Besides, the PGPR factors of B. cereus, including indoleacetic acid (IAA) production, 1-aminocyclopropane-1-carboxylic acid deamination (ACCd) activity, phosphate solubilization capacity, and siderophore production, were quantified. These PGPR factors helped explain the biomass augmentation, root elongation and enhanced Cr enrichment of the inoculated L. hexandra in pot experiments. Despite the increased Cr uptake, no aggravated oxidative damage to the cell membrane was observed in the inoculated L. hexandra. This was attributed to its capacity to confront the increased intracellular Cr stress by upregulating both the activities of antioxidative enzymes and expression of metal-binding proteins/peptides. Moreover, L. hexandra could always conserve the majority of Cr in the residual and oxalic integrated forms with low mobility and phytotoxicity, irrespective of the B. cereus inoculation. These results highlight the constructed Cr hyperaccumulator-rhizobacteria consortia as an effective candidate for decontaminating Cr(VI)-laden wastewater.

Graphical abstract

Keywords

Hexavalent chromium / Hyperaccumulator / Rhizobacteria / Leersia hexandra Swartz / Bacillus cereus / Consortia

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Acknowledgements

This work was supported by the National Natural Science Foundation of China (Nos. 52000046, 52100034, 52170154, and 52070051), the Special Project of Guangxi Science and Technology Base and Talent (Nos. GuiKe AD20297009 and GuiKe AD20297007), the Middle-aged and Young Teachers’ Basic Ability Promotion Project of Guangxi (Nos. 2020KY05039 and 2021KY0221).

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