Due to the improper application of pesticides and chemical fertilizers in recent years, heavy metal pollutants have been increasing in the soil of farm fields and facility agriculture fields, which greatly threats the safety of agriculture production (Li et al. 2020; Yang et al. 2019; Zhang et al. 2018). Cadmium (Cd), categorized as one of the class І cancerogens by the World Health Organization (WHO), is a primary element of heavy metal pollutants in soil. Cd mostly exists in the ionic state; most cadmium compounds are easily dissolved in water and absorbed by vegetables and crops, severely increasing the risk of human health through the food chain (Yang et al. 2016).
In plants, some materials and ion transporters that can transport Cd2+ into the vacuole and out of the cell membrane also function as key Cd detoxification pathways, such as ATP binding cassette (ABC) transporters, heavy metal ATPase (HMA). For example, (heavy metal transporter) HMT1 and HMA3 can isolate Cd into the vacuole in yeast and plants (Huang et al. 2012; Miyadate et al. 2011), and AtPDR8 excludes Cd2+ outside of the root cell membrane in Arabidopsis (Kim et al. 2007). In Arabidopsis, Nramp1 (natural resistance-associated macrophage protein 1) was identified to mediate the absorbance of Cd and Mn (Cailliatte et al. 2010), and AtHMA2 and AtHMA4 are involved in transporting Cd into xylem (Mills et al. 2005; Wong and Cobbett 2009). In rice, OsNramp1 localizes in plasma membrane and participates in cellular Cd2+ transport (Takahashi et al. 2011). OsNramp5 plays an imperative role in Cd and Mn transport in the root of rice, and knockout of OsNramp5 led to the decrease of Cd and Mn accumulation in rice (Sasaki et al. 2012; Yang et al. 2014). OsHMA2 were referred to the loading of Cd into the xylem (Yamaji et al. 2013).
In addition to Cd transporters, plants respond to Cd stress by chelating Cd2+ with Cd chelating metabolites, thereby reducing the concentration of Cd2+ and relieving the toxicity of Cd. Reduced glutathione (GSH) is an imperative active material to chelate free radical, which widely exists in plants and animals (Griffith et al. 1978). When plants were under Cd stress, the content of GSH increased rapidly and chelated Cd2+, thus relieving the damage of Cd stress to the metabolic activities in the cytosol (Marrs, 1996). Under Cd stress, phytochelatin synthase (PCs) can transfer c-Glu-Cys to GSH, and then generate phytochelatin (PC) (Thangavel et al. 2007; Zhao et al. 2010). PC can not only combine Cd2+ directly, but also reduce the damage of oxidative stress caused by heavy metals (Schutzendubel et al. 2001; Schutzendubel and Polle 2002). Moreover, GSH and PC facilitate most of ABC transporters in the transport of Cd (Bovet et al. 2005; Rauser 1995; Yamaguchi et al. 2020).
In addition, in plants, metallothionein and organic acid can also bind with Cd2+, thereby alleviating the toxicity of Cd to plants (Choppala et al. 2014; Gallego et al. 2012). In recent years, studies have shown that, under Cd stress, plants generate low molecular weight organic acids, such as oxalic acid and malic acid. These organic acids can be exported from the root, and combine with Cd2+, thereby blocking Cd2+ from the plants, and reducing Cd2+ absorbance in the root (Montiel-Rozas et al. 2016). Under Cd stress, the root of tomato can secret oxalic acid, thereby keeping Cd2+ outside, and enhance the Cd tolerance of the tomato (Zhu et al. 2011). Application of citric acid and malic acid to rice roots is able to alleviate the Cd content of the leaves significantly (Sebastian and Prasad 2018). Besides chelating Cd2+ directly, the metabolites produced by the plants are used to deal with other stresses caused by the heavy metal stresses, such as osmotic and oxidative stress (Arbona et al. 2013; Hernández et al. 2015).
Chinese cabbage (Brassica rapa) is the representative and most popular leafy vegetable in East Asia. Leafy vegetables in the Brassicaceae family have a high ability of Cd accumulation in the leaf parts (Kuboi et al. 1986). With the aggravation of heavy metal pollution in vegetable fields, Cd accumulation in Chinese cabbage is increasingly severe. However, the mechanism associated with Cd accumulation and tolerance is still poorly understood.
Here, we carried out the metabolic and transcript profiling of Chinese cabbage under Cd stresses with different levels of Cd (low (5 µM), medium (25 µM) and high (100 µM)), and investigated the potential mechanism of the tolerance to different Cd levels by examining the levels of specific differentially expressed genes (DEGs) and metabolites (DEMs) and pathways. We found that the DEGs and their enrichment pathways were significantly different from the DEMs and their pathways. In the metabolic level, the GSH metabolism pathway was induced under both the medium and high Cd stresses, and the GSH metabolism pathway was more significantly activated under the high Cd stress. In the following test, we found that GSH reduced Cd accumulation in the Chinese cabbage under the medium and high Cd stresses, however, GSH elevated the Cd accumulation under the low Cd stress. It suggested that GSH could play different roles in Cd accumulation of Chinese cabbage under the different Cd concentration. Cd accumulation measurement in the mild Cd contaminated vegetable field was also conducted, and the result showed that GSH tended to accumulate more Cd in the tested cultivar Beijinxin3, which is one of the most popular Chinese cabbage cultivar in the market. This result is different from previous reports about the function of GSH in Cd exclusion in kinds of crops (Cai et al. 2010; Cao et al. 2015; Huang et al. 2019). Our result indicated that GSH could be utilized as Cd scavenging agent by Chinese cabbages, and cannot be used for preventing Cd accumulation of Chinese cabbage in the most vegetable field.