Iminodisuccinic acid (IDS), as a "green" chelating agent with low toxicity and fast degradation, has great potential for stabilizing metal ions (Dorota et al., 2009; Jing and Wang, 2003), promoting plant growth and decreasing Cd content in plants (Ren et al., 2013). However, only a few study focused on the regulation effects of IDS on Cd resistance in plants, physiological characteristics, Cd subcellular distribution, and chelation of Cd with different CW components in plants. The present study analyzed plant biomass, Cd distribution at organ, subcellular, and cell wall levels, and different CW components. The results indicated that 0.3% IDS application to rapeseed cultured in Cd-contaminated soil effectively promoted plant dry mass accumulation, decreased root-shoot Cd transportation, relieved ROS oxidative damage to leaf cells, and increased Cd distribution in leaf cell walls by enhancing Cd chelation with CSP.
4.1. IDS inhibited Cd transportation from roots to shoots in rapeseed
In plants, water, mineral nutrients and heavy metals such as Cd absorbed from soil are transported from roots to shoots (De Boer and Volkov, 2003; Luo and Zhang, 2019). In this study, IDS supplication did not significantly affect the mobility of Cd in soil (Table S1) and Cd accumulation in rapeseed roots (Table 1). However, our study showed lower Cd contents and Cd distribution ratio in leaves treated with IDS (Table 1), indicating that IDS reduced Cd distribution to leaves of rapeseed. In the meantime, there was a significantly decreased Cd transfer coefficient from roots to shoots (Table 1), suggesting that IDS inhibited the root-shoot transportation of Cd and ultimately alleviated Cd stress in the edible part of rapeseed (Wu et al. 2020a).
4.2 IDS decreased ROS accumulation and activated antioxidant enzymes in leaves
Excessive Cd in plants usually increases the MDA content, which may raise membrane lipid peroxidation degree and indirectly aggravate the oxidative stress in cells (Qiu et al., 2009; Wu et al., 2020b; Wu et al., 2021a). ROS accumulation also causes oxidative damage to plants (Shahid et al., 2017). Higher antioxidant enzyme activity has been widely perceived to reduce ROS accumulation in plants under different stress (Charton et al., 2019; Rasafi et al., 2020; Riaz et al., 2018). Many studies have proposed that the activated antioxidant enzyme system plays an essential role in improving plant Cd resistance and acts as ROS scavengers (Chen et al., 2019; Wu et al., 2020b). In the present study, activated activities of CAT and SOD by IDS application contributed to decreased MDA content and reduced H2O2 accumulation under Cd toxicity in leaves (Fig. 2). However, considering that the Cd content was significantly decreased in leaves with IDS supply (Table 1), the lower Cd content may result in higher antioxidant enzyme activity and lower MDA and ROS accumulation.
4.3 IDS enhanced Cd fixation in leaf CWs by increasing the Cd chelation with CSP
It has been reported that the Cd subcellular reallocation determines Cd resistance in plants (Hu et al., 2021; Shahid et al., 2017; Zhang et al., 2019). CW fixation and vacuole compartmentalization both play pivotal roles in enhancing plant Cd resistance (Huang et al., 2021; Peng et al., 2017; Sharma et al., 2016). Our results found 0.3% IDS had no remarkable effect on Cd sequestration into vacuoles, but promoted Cd fixation in CWs, thereby decreasing Cd stress in organelles (Fig. 3). The CW is the first barrier preventing Cd from entering cells, and CW fixation of Cd depends on the chelation of Cd in different CW components (Gutsch et al., 2018; Peng et al., 2017). To further investigate which CW component was mainly involved in improving Cd chelation with leaf CWs by IDS treatment, the content of different CW component and their Cd contents were further analyzed. The results indicated that IDS did not affect the content of ISP, cellulose, CSP, and hemicellulose, while the Cd contents in CSP were increased by IDS (Fig. 4A-B).
Among the several CW components, pectin is the main component in Cd adsorption to CWs, and the pectin is demethylated by PME and releases COO- that could form complexes with Cd2+ (Gutsch et al., 2018; Kartel et al., 1999). Our study found that IDS did not increase CSP content (Fig. 4B), but significantly improved the degree of demethylation of CSP by raising PME activity (Fig. 4C-D), producing more available groups for Cd binding to CWs, and, thereby, improving Cd detoxification (Wu et al., 2020a; Zhu et al., 2018). The results indicated that the primary cause of improving Cd chelation with pectin was due to enhanced PME activity of CSP by IDS supply.