Cd2+ resources are widely used in the industrialization in modern society, Cd2+ has a cumulative effect and is gradually enriched in water, soil and various organisms (Afzal et al., 2017). Cd2+ can bind to the hydroxyl, amino, and sulfhydryl groups of proteins, thereby inhibiting the biological activity of enzymes, affecting the expression of genes related to cell apoptosis and proliferation, and bringing serious harm to physical and mental health (Cuypers et al., 2010). Consequently, efforts have been made to effectively remove Cd2+ from contaminated environments. For example, Wang et al. and Yang et al. designed and obtained a variety of metal-free nanomaterials which could effectively remove and reduce various organic pollutants and heavy mental Cr(VI) (Yang et al, 2020, Wang et al,2020). Currently, many strategies have been proposed to remove Cd2+ from the environment, including chemical precipitation (Agwaramgbo et al., 2013), electrolysis (Champault et al., 2014), ion exchange (Hosseini et al., 2020), membrane separation technology (Mohammed and Sahu, 2019), activated carbon adsorption (Vajihe et al., 2019), etc. However, these strategies are generally characterized by high operation cost, complicated operation, high energy consumption and possible secondary pollution (Kavita and Keharia, 2012; Simonescu and Ferdes, 2012). Microbial biodegradation is considered to be a promising strategy in dealing with heavy metal and recovering contaminated environments due to its low operation cost, easy operation, low energy consumption and lack of secondary pollution (Xu et al., 2020).
The microorganism absorption modes of fungi for heavy metals include the metabolism-independent binding the metals bind to the cell walls, and metabolism-dependent intracellular accumulation (Kirillova et al., 2017; Xu et al., 2014). In the metabolic independent mode, heavy metals were adsorbed by microorganisms through surface complexation, coordination, chelation, extracellular precipitation, and ion exchange (Paknikar et al., 2003). The metabolism-dependent mode mainly includes the cell membrane efflux, vacuolar compartment detention, heavy metal chelation and oxidoreductase detoxification (Delalande et al., 2010). Generally, heavy metals are first chelated to the microbial cell wall through surface bonds, and then enters the cell through endocytosis and are captured by the vacuole or interact with glutathione, metallothionein, citrate and phytochelatin (Jacquart et al., 2017). As one kind of heavy metals, Cd2+ is eventually deposited inside the cells, which may affect the growth and metabolism of microorganisms. Although the absorption process of Cd2+ by microorganisms has been reported (Feng et al., 2018), the genetic response of microbes to Cd2+ is still unclear. Studying the microbial gene response to Cd2+ stress is crucial for clarifying the tolerance mechanism of microorganisms to Cd2+ and providing a genetic basis for heavy metal bioremediation.
Beauveria bassiana is a classical entomogenous fungus used in the microbial control of pests, widely applied in the field of agriculture and forestry (Luo, 2016). Beauveria bassiana has attracted extensive attention due to its super stress resistance under the dual stress of external environmental factors, such as high temperature, sunlight, ultraviolet radiation, chemical pesticides, and the toxin in the host caused by the pests. It has become one of the model strains for studying the interaction between filamentous fungi and host as well as between fungi and environment (Zhang, 2016). Previous studies have shown that Beauveria bassiana has the potential to tolerate heavy metals. The maximum tolerance concentrations of Pb2+ and Cd2+ by Beauveria bassiana jb15 were 1200 mg/L and 200 mg/L respectively. Under the optimal absorption conditions, the absorption rates of Pb2+ and Cd2+ were 52.27% and 62.38% (Xie et al.,2020). In this study, a highly Cd2+-tolerant fungus named Beauveria bassiana Z1 was isolated, and the absorption capacity and absorption characteristics of strain Z1 to Cd2+ were studied. Moreover, the transcriptome database of strain Z1 under Cd2+ stress was constructed and the gene response to Cd2+ stress was investigated. This research is of great significance for understanding the genetic response of microorganisms to Cd2+ and elucidating the mechanism of microorganisms against Cd2+ stress.