Abstract
Thailand is an agricultural country. However, agricultural productivity relies on the heavy use of herbicides, especially paraquat. Paraquat accumulation is emerging as a problem in an ever-growing portion of agricultural land. Paraquat residues are toxic to plants, animals, and aquatic organisms in the environment. Biological remediation is a process that can mitigate agricultural chemical contaminants. One of the interesting bioremediators is bacteria. Not only do certain soil bacteria remediate paraquat, but some of them also possess plant growth-promoting properties, which provide advantages in field application. Thus, this study aimed to screen soil bacteria that could degrade paraquat and, at the same time, promote plant growth. Bacteria were isolated from paraquat-treated agricultural soil in Mueang Kaen Pattana municipality, Chiang Mai province, Thailand. On the basis of morphological and 16S rDNA sequence analyses, the selected bacterium was identified as Bacillus aryabhattai strain MoB09. It is capable of growing in nitrogen-free media. B. aryabhattai growth and paraquat degradation were found to be optimum at pH 7 and 30°C. This selected strain also possessed plant growth-promoting abilities, including indole production, siderophore production, phosphate solubilization, and 1-aminocyclopropane-1-carboxylic acid deaminase activity. Paraquat degradation was also evaluated in pot experiments of cowpea (Vigna unguiculata). It was found that this strain could remediate the paraquat residue in both sterilized and non-sterilized soils. The cowpea plants grown in paraquat-contaminated soil with B. aryabhattai showed longer root and shoot lengths than those grown in soil without bacterial inoculation. In addition, B. aryabhattai also promoted the growth of cowpea under induced drought stress. These results suggested that B. aryabhattai could be applied to mitigate paraquat residue in soil and also to promote plant productivity for the organic crop production.
Highlights
Vegetables have long been consumed worldwide, and the trend for vegetable consumption as clean and healthy food is currently increasing
Paraquat-degrading bacteria were isolated from two methods: molybdenum-reducing ability using low phosphate molybdate agar (LPMA) and paraquat utilization capacity using mineral salt medium (MSM) containing paraquat
It was found that 12 morphologically different isolates were selected from LPMA and assigned as MoB strains
Summary
Vegetables have long been consumed worldwide, and the trend for vegetable consumption as clean and healthy food is currently increasing. Paraquat (1,1-dimethyl-4,4-bipyridyl dichloride) is a herbicide that has been widely used in agriculture for more than 60 years, and but at the present, it has been banned or disallowed in many countries (Bromilow, 2004). It is a non-selective herbicide that diverges electron flow from the photosystem, resulting in the production of lethally reactive free radicals, superoxide radicals, which damage plant tissues (Reczek et al, 2017). The concentrations of paraquat contamination in some areas are still below the tolerance limit, long-term exposure to those paraquat residues might result in harmful effects on humans and mammals and the food chain (Huang et al, 2019)
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