Nicotine is widely found in Solanaceae plants and is abundant in tobacco, accounting for 2–8% of its dry weight. The chemical molecules of nicotine are mainly a heterocyclic compounds composed of pyridine and pyrrole rings. Nicotine is not only toxic and carcinogenic to humans, but also poses a great threat to the environment, resulting in a serious damage to the ecological balance in soil and groundwater (McGrath-Morrow et al., 2020; Chaffee et al., 2021; Wittenberg et al., 2020; Herman and Tarran, 2020).
Nicotine is the main toxic pollutant in the waste from tobacco cultivation, and improper handling can leave high levels of it in the environment (Cheng et al., 2021). Nicotine is considered as a toxic discharge; for example, European Union Regulations classify tobacco waste as "toxic and harmful" when the nicotine content exceeds 0.05% (w/w) (Novotny and Zhao, 1999). Therefore, the degradation of nicotine in tobacco and its waste is an essential environmental concern to reduce the risk of ecosystem toxicity.
Tobacco is an important economic crop in many countries (Zou et al., 2021). Of the six trillion cigarettes consumed globally each year, four and a half trillion are disposed of somewhere in the environment (Araújo and Costa, 2019). Tons of tobacco waste from plant to product need to be correctly treated (Novotny and Zhao, 1999; Lam et al., 2022). At present, the main methods of treating tobacco waste are (1) direct incineration causing air pollution; (2) extraction and utilization of useful substances, causing problems concerning the disposal of residual waste; (3) resource utilization and harmless treatment technologies, such as composting or reconstituted tobacco leaf technology (Di et al., 2022; Cai et al., 2016). Some physical and chemical methods are used for tobacco waste treatment but these have negative effects on the subsequent utilization of tobacco resources. In contrast, the biological treatments such as nicotine degradation mediated by microbes or enzymes are eco-friendly and low-cost methods (Zhang et al., 2022). Many nicotine-degrading microorganisms have been used for nicotine degradation in tobacco waste. At present, most of them are bacteria, including Pseudomonas, Arthrobacter, Paleobacter, Agrobacterium and so on (Liu et al., 2015; Zhong et al., 2010; Zhang et al., 2022; Guo et al.,2019). Nicotine-degrading bacteria (NDB) are mostly used in the composting of tobacco waste (Ye et al., 2017; Wang et al.,2013), the treatment of tobacco factory wastewater (Sabzali, Nikaeen, and Bina, 2012), and as a bacterial agent to remediate nicotine-contaminated soil (Shang et al., 2017). For example, a nicotine-degrading bacterium, Arthrobacter histidinolovorans EA-17, was added to the compost where tobacco straw as raw material was decomposed into organic fertilizer and the nicotine degradation rate was increased by 74.5% in comparison with the control (Wang et al., 2015).
Previous studies have demonstrated three main pathways of bacterial nicotine degradation. In the pyridine degradation pathway represented by the Gram-positive bacteria Arthrobacter, the main degradation enzymes include nicotine dehydrogenase, 6-hydroxynicotine oxidase, 6-hydroxypseudooxynicotine dehydrogenase, 2,6-dihydroxypseudooxynicotine hydrolase, 2,6-dihydroxypyridine 3-monooxygenase and nicotine blue oxidoreductase (Sachelaru et al., 2005; Mihasan et al., 2007; Schenk et al., 1998; Deay et al.,2020). In the pyrrolidine pathway represented by the Gram-negative bacteria Pseudomonas, the most important enzymes are nicotine oxidoreductase, and 3-succinoylpyridine monooxygenase (Hu et al., 2019; Xia et al., 2018). A variant of the pyridine and pyrrolidine pathways, named the VPP pathway (Huang et al., 2020), is found in the bacteria Agrobacterium sp. S33 (Shang et al., 2021), Stenotrophomonas geniculata N1 (Liu et al., 2014) and so on. Nonetheless, the degradation mechanisms of many NDB have not been elucidated so far, such as in Rhizobium spp. and Bacillus spp. (Mu et al., 2020; Liu et al., 2015; Zhang et al., 2022). Isolation and application of high-efficiency NDB are still important for nicotine degradation in environmental and human health; for example, the colonization of nicotine-degrading bacterium Bacteroides xylanisolvens in the gut reduced tobacco smoking-exacerbated non-alcoholic fatty liver disease (Chen et al., 2022).
This paper aims to investigate NDB diversity and the relationship between the abundance of nicotine-degrading genes and nicotine content in tobacco cultivation soils. Various NDB were isolated from tobacco rhizosphere soil and two efficient NDB were successfully applied in the composting of tobacco waste. In addition, nicotine-degrading genes and their function in soil were analyzed by using meta-genomic methods.