Cadmium armors the Cd hyperaccumulator Sedum alfredii against aphid attack

Abstract

The cadmium (Cd) hyperaccumulator Sedum alfredii has been identified to have great ability to accumulate >100 ppm (dry weight) of Cd in its aboveground biomass. However, little attention has been paid to the possibility that S. alfredii may benefit from this trait. Here, we investigated the effect of Cd accumulation on the performance of the black bean aphid Aphis fabae in S. alfredii. The results showed that 6 weeks of Cd exposure prevented S. alfredii from being infested by aphids. In another experiment, S. alfredii was pretreated with 100 μmol⋅dm⁻³ CdCl₂ for 7 days. Prolonged Cd exposure significantly reduced the number of aphids in the Cd-pretreated S. alfredii after 7 days of aphid infestation. The Cd concentration in the phloem exudates of S. alfredii was also high. Micro X-ray fluorescence mapping of aphids collected from Cd-treated plants revealed high levels of Cd in the stylets. In summary, Cd protects S. alfredii from A. fabae through toxicity, but not deterrence, which may be related to the abundance of Cd in the phloem.

1 Introduction

Contamination of arable soil with heavy metals has become a severe environmental issue, especially in China (Zhao et al., 2015; He et al., 2017). Cadmium (Cd), the most severe and prevalent heavy metal contaminant in soil, negatively impacts the human health and ecosystems (Wen-Li et al., 2014; Rubina et al., 2020). Phytoremediation is a feasible and cost-efficient strategy for the remediation of soils contaminated with heavy metals (Clemens et al., 2013). Cadmium-hyperaccumulating plants, which can accumulate metals beyond the critical concentration level (>100 ppm, dry weight) in their shoots, are prerequisites for efficient phytoremediation (Kramer, 2010; Verbruggen et al., 2013). In recent decades, considerable interest has been shown in these particular plant species. Findings have revealed that their metal hyperaccumulation ability mainly depends on active root uptake, efficient xylem loading, powerful leaf sequestration, and efficient phloem remobilization (Navariizzo, 2010; Ent et al., 2013; Tian et al., 2016; Tian et al., 2017; Hu et al., 2019b). Although progress has been made concerning research in metal transport and accumulation (Lu et al., 2008; Hu et al., 2019a), few studies have been devoted to understanding the relationship between heavy metals and biotic stress, particularly in insects (Dai et al., 2020; Shimizu-Inatsugi et al., 2021; Liu et al., 2022). Several hypotheses have been proposed regarding the possible ecological functions of metal hyperaccumulation (Tewes et al., 2018). Among these hypotheses, the elemental defense hypothesis is the most popular (Boyd, 2007; Boyd, 2013), which postulates that high internal metal concentrations could protect plants by reducing the performance of pathogens and herbivores. The deterrent and toxic effects of metals on herbivores have been addressed in several studies (Rascio and Navari-Izzo, 2011; Boyd, 2013). Generally, toxic metals are quite effective in defending against tissue-chewing herbivores (Lin et al., 2020), whereas the results of studies on vascular tissue-feeding insects are controversial or negative (Boyd, 2007; Lin et al., 2020). Therefore, enhancing traits related to insect resistance and promoting metal hyperaccumulation in shoots may contribute to optimizing phytoremediation efficiency.

Aphids are typical phloem feeders that tap phloem tissues to obtain sugar and amino acids from the phloem fluid, and they are important pests of agricultural and horticultural crops worldwide (Will et al., 2013). Generally, aphids prefer water-enriched plants to wooden plants (Will et al., 2013). Several studies have shown that metal accumulation may enhance aphid resistance in plants. For example, it has been reported that Zn and Cd in Arabidopsis helleri (Stolpe et al., 2017; Irfan Naikoo et al., 2021b) and Se in Brassica juncea (Hanson et al., 2004; Shi et al., 2020) were effective in reducing aphid performance in corresponding plants. However, contrasting results have also been shown, such as with the aphids that were unaffected by Ni in the hyperaccumulator Streptanthus polygaloides (Boyd and Martens, 1999; Irfan Naikoo et al., 2021a) and by Cd in Brassica juncea (Konopka et al., 2013; Butt et al., 2018). Sedum alfredii is a Cd-hyperaccumulating plant native to China (Yang et al., 2004; Tian et al., 2010). However, aphids are a severe threat to the growth of S. alfredii and heavy metal phytoremediation. Although the ability of S. alfredii to concentrate Cd in its aboveground parts has been widely studied (Lu et al., 2008; Tian et al., 2016; Hu et al., 2019b), there is no consistent conclusion on whether S. alfredii could benefit from Cd accumulation in terms of resisting aphids. Therefore, in this study, we pretreated S. alfredii with 0 or 100 μmol⋅dm⁻³ Cd under aphid infestation to investigate aphid Cd distribution and feeding behavior. Thus, the objectives of this study were to 1) examine the effect of Cd on aphid resistance in S. alfredii and 2) explore the relationship between biotic stress and metal remediation.

2 Materials and methods

2.1 Plant culture

Seeds of Cd-hyperaccumulating S. alfredii were collected from an old Pb/Zn mine in Zhejiang Province, China (Yang et al., 2004). The seeds were grown in non-contaminated soil for several generations, and healthy and uniform shoots were selected and pre-cultured hydroponically, as previously reported (Lu et al., 2008). The hydroponic solution contained 2 mmol⋅dm⁻³ Ca(NO₃)₂, 0.1 mmol⋅dm⁻³ KCl, 0.1 mmol⋅dm⁻³ KH₂PO₄, 0.7 mmol⋅dm⁻³ K₂SO₄, 0.5 mmol⋅dm⁻³ MgSO₄, 10 μmol⋅dm⁻³ H₃BO₃, 0.5 μmol⋅dm⁻³ MnSO₄, 5 μmol⋅dm⁻³ ZnSO₄, 0.2 μmol⋅dm⁻³ CuSO₄, 0.01 μmol⋅dm⁻³ (NH₄)₆Mo₇O₂₄, and 50 μmol⋅dm⁻³ Fe-EDTA and was replaced every 3 days. The pH was adjusted to 5.8 using 0.1 mol⋅dm⁻³ HCl. The seedlings were cultivated in a natural light greenhouse (350 μmol m⁻²s⁻², 16 h light/8 h dark) with a cooling system, maintaining the temperature at 22–25°C and the humidity at 60%–70%.

2.2 Aphid infestation treatment

The aphid used in the present study was the black bean aphid Aphis fabae Scopoli (Hemiptera, Aphididae), which naturally colonizes S. alfredii. In the greenhouse, an aphid attack occurred naturally before the experiment. During the experiment, sick S. alfredii seedlings were continuously infested with A. fabae, making the greenhouse a susceptible environment. A separate experiment was designed for aphid infestation treatment. First, 4-week-old S. alfredii seedlings were pretreated with 0 or 100 μmol⋅dm⁻³ CdCl₂ for 7 days and then exposed to aphids (Figure 1A) for another 7 days of treatment. During this period, plants infested with aphids were continuously treated with 0 or 100 μmol⋅dm⁻³ CdCl₂. Each treatment was replicated four times. Four seedlings were placed in a transparent plastic box with a ventilating window covered with an insect net for each replicate. Two out of four seedlings pretreated with 0 or 100 μmol⋅dm⁻³ CdCl₂ were randomly chosen. Then, 200 adult aphids collected from Cd-free S. alfredii plants (mainly used for aphid culture) were placed at the bottom of the box in the center, where each seedling was equally accessible. One hour later, for each replicate, one host plant with 0 μmol⋅dm⁻³ CdCl₂ and one with 100 μmol⋅dm⁻³ CdCl₂ were randomly chosen for determining the number of aphids on each seedling and the concentration of Cd in aphids and plants. This was used to determine the preference of aphids for choosing a host plant.

FIGURE 1

For another experiment, 4-week-old seedlings of S. alfredii were pretreated with nutrient solutions containing 0 or 100 μmol⋅dm⁻³ CdCl₂ for 7 days, which were reapplied every 4 days. Plants were then placed in a greenhouse (350 μmol m⁻²s⁻², 16 h light/8 h dark, 22–25°C, 60%–70% humidity) with aphids for another 6 weeks of treatment. During this period, the plants infested with aphids were treated with or without 100 μmol⋅dm⁻³ CdCl₂. Each treatment consisted of 12 seedlings (four in one pot per replicate), and the pots were randomized.

2.3 Parameter determination

2.3.1 Element determination

After the aphid infestation treatment, plant growth and Cd concentrations in plant tissues and aphids for both the control and Cd-treated S. alfredii were determined. Briefly, samples were rinsed with deionized water and dried with a paper towel. The samples were then dried in an oven at 75°C until a constant weight was obtained. After the plant tissues and aphids were digested with HNO₃-H₂O₂ (3:1, v/v) at 180°C, the concentrations were determined by inductively coupled plasma mass spectrometry (Agilent 7500a, United States) (Tian et al., 2011).

2.3.2 Elemental mapping by micro-X ray fluorescence

In each treatment (0 or 100 μmol⋅dm⁻³ CdCl₂), five aphids were randomly selected for micro-X-ray analysis at 1 h and 7 days after the onset of aphid infestation. As previously described (Tian et al., 2011; Tian et al., 2016), the elemental distribution of the aphids was collected by micro-X-ray fluorescence (XRF) in five replicates. In brief, aphids were directly pasted on sulfur-free tape and freeze-dried, and the distribution of Cd, S, and P in the intact aphids was mapped with a vortex silicon drift detector. The energy of the incident X-rays was 13.5 keV.

2.3.3 Phloem exudation

In a separate experiment, 6-week-old S. alfredii seedlings were treated with nutrient solutions containing 0 or 100 μmol⋅dm⁻³ CdCl₂ for 14 days in a growth chamber (LeDian, Shanghai, China) under a 16 h light/8 h dark cycle (25°C/22°C) at 350 μmol m⁻² s⁻² and 65% relative humidity. Petioles were then cut above the stem, surface sterilized in 50% ethanol and 0.0006% bleach for 10 s, rinsed in sterile 1 mmol⋅dm⁻³ EDTA, and submerged in 2 ml of 1 mmol⋅dm⁻³ EDTA and 50 μg ml⁻¹ ampicillin, as described previously (Chanda et al., 2011; Lim et al., 2020). The purity of the phloem exudate was evaluated based on their glucose (Miller, 1959) and sucrose (Zhang et al., 2009) contents. A subsample of phloem exudates (5.0 ml) was mixed with 2 ml of 2% (w/v) HNO₃ to determine Cd, Zn, Ca, and K contents by inductively coupled plasma mass spectrometry.

2.4 Statistical analysis

Data were statistically analyzed using IBM^®SPSS^®Statistics 25 (IBM, United States). The data on aphid numbers and metal concentrations in plants, aphids, and phloem exudates were analyzed by one-way analysis of variance. Statistical significance was set at p < 0.01. All statistical graphs were plotted using Origin version 2021b (OriginLab, Northampton, MA, United States). XRF mappings were processed using the software package SMAK version 0.34, S-4 (http://www.sams-xrays.com/smak).

3 Results

3.1 The influence of aphids on S. alfredii growth

After the 6-week greenhouse experiment, the Cd-treated S. alfredii plants were still healthy, with very few aphids observed, while the Cd-free S. alfredii were heavily infested by A. fabae and showed low biomass, chlorotic leaves, and wilting plants (Figure 2). In the susceptible open environment, A. fabae in control plants was mainly found on young stems and leaves near the growing tip, as indicated in Figure 2A. Moreover, the sticky honeydew produced by the aphids was visible, along with the resulting black sooty mold that grew on the honeydew and the aphid slough abandoned during its growth (Figure 2B). In contrast, the growing tips of the Cd-treated plants were clean (Figure 2C); however, one or two aphids were sometimes present on the plants. Furthermore, the metal concentrations in the shoots of S. alfredii were as high as 3,211 ± 610 μg g⁻¹ dry weight (Table 1).

FIGURE 2

3.2 The effect of Cd on S. alfredii aphid defense

The results showed that S. alfredii pretreated with Cd remained healthy despite being grown in the same closed environment full of plants attacked by aphids (Figure 1). As shown in Figures 1B, D, upon immediate infestation, there was no apparent difference in the number of aphids between S. alfredii with and without Cd treatment. However, the number of aphids in Cd-treated S. alfredii severely decreased to 10 after 7 days of infestation. Furthermore, concomitant with the high level of Cd in the phloem exudate of S. alfredii (Figure 1F), the concentration of Cd in the aphids significantly increased 3-fold (Figures 1C–E). Meanwhile, no glucose (data not shown) but high levels of sucrose were detected in the phloem exudates (Figure 3A). Interestingly, there was no significant difference in the distribution of other elements, such as Ca, K, and Zn, in the phloem exudates of S. alfredii with and without Cd treatment (Figure 3).

FIGURE 3

3.3 The elemental distribution patterns in the aphid body

Figure 4 shows the micro XRF images with the distribution patterns of Cd in the aphids collected from S. alfredii leaves treated with 100 μmol⋅dm⁻³ Cd. It is noteworthy that although Cd, S, and P were preferentially distributed in the aphids, the distribution pattern for each element was notably different (Figure 4). It is clear that the concentration of Cd in the stylets, the piercing–sucking mouthpart of aphids, was substantially higher than that in other parts of the aphid body. In contrast, S and P were uniformly distributed throughout the aphid body (Figure 4).

FIGURE 4

4 Discussion

4.1 Cd accumulation plays an important role in S. alfredii aphid defense

The protective role of metal hyperaccumulation in plants could be due to the toxicity of metals and their deterrent effects on insect feeding (Morkunas et al., 2018). In this study, we found that the number of aphids that colonized the Cd-pretreated S. alfredii was significantly lower than that in the control plants after 7 days of exposure to 200 aphids (Figure 1D). This is also consistent with the evidence provided in previous studies (Morkunas et al., 2018; Ali et al., 2019; Shi et al., 2020). Interestingly, we still found a few aphids in Cd-treated S. alfredii (Figure 1D), indicating that some insects were unable to immediately adjust their feeding behaviors in response to the food metal composition (Stolpe et al., 2017). This also indicates the bidirectional movement of aphids in detecting Cd (Jhee et al., 2005; Stolpe and Müller, 2016). These results align with the elemental defense hypothesis (Boyd, 2007; Rascio and Navari-Izzo, 2011) since elemental defense is inexpensive and is likely a key mechanism for plant invasion into metal-polluted sites. Heavy metals may change the chemical composition of plants and affect the performance of herbivores (Stolpe et al., 2017). Taking these results together, we conclude that the role of Cd in defending aphids in the hyperaccumulator S. alfredii may not be due to a deterrent effect on aphids.

4.2 Efficient phloem transport significantly enhanced Cd defense ability

In a closed environment experiment, we still found a considerably high Cd concentration in aphids collected from Cd-treated S. alfredii plants (Figure 1E) and many dead aphids below the seedlings (data not shown). We deduced that Cd directly poisoned the aphids and, therefore, prevented their colonization. The connection between the phloem metal concentration and aphid resistance has been proposed for decades (Boyd and Marrten, 1999; Hanson et al., 2004; Stolpe et al., 2017). As Stolpe et al. (2017) showed, Cd in the phloem of A. helleri plays an important role in aphid resistance. However, most heavy metals are stored in the cell vacuoles (Kupper et al., 2000; Rascio and Navari-Izzo, 2011). Previously, we detected a high concentration of Cd in the phloem of S. alfredii by micro XRF (Tian et al., 2016), and Cd was remobilized from mature to developing organs via the phloem (Hu et al., 2019a). In the present study, abundant Cd was also detected in the phloem exudates of S. alfredii (Figure 1F). In contrast to Cd, the concentrations of Ca, K, and Zn were not significantly changed in the phloem. This may partly be due to the characteristics of S. alfredii, which mainly stores a large amount of metals in its cell vacuoles (Tian et al., 2017). In contrast, the non-accumulator species Zea mays (Poaceae) shows reduced K concentration when exposed to high Pb concentrations (Małkowski et al., 2002). Therefore, the high Cd levels found in aphids (Figure 1E) provide strong evidence that high Cd levels in the phloem (or other factors triggered by high Cd) kill the aphids that feed on Cd-treated S. alfredii (Stolpe et al., 2017).

4.3 The unique distribution of Cd in aphids

As phloem-feeding insects, aphids use stylets to obtain nutrition from plant phloem tissues (Will et al., 2013). The stylets are the most direct channel for the plant phloem sap to enter the aphid’s body (Will et al., 2013). There is a highly competitive interaction between Cd and Ca in S. alfredii (Lu et al., 2008; Tian et al., 2011). In our previous research (Tian et al., 2016; Hu et al., 2019a), we found that the S. alfredii phloem has a strong ability to remobilize Cd. The level of Cd in the phloem exudate of S. alfredii leaves was 2- to 50-fold higher than those of other elements (such as P and S), and Ca deficiency triggers phloem remobilization of Cd (Tian et al., 2016). These changes may be strengthened further by aphid saliva (Will et al., 2013; Stolpe et al., 2017). Before ingestion, stylets secrete watery saliva containing proteins that can bind to Ca²⁺ and regulate the element influx to prevent sieve tube plugging (Will et al., 2007; Carolan et al., 2009; Will et al., 2013). This is strongly supported by our results, as shown in Figure 4, which indicate a large amount of Cd accumulated in the stylets of aphids.

In conclusion, this study reported the effects of Cd accumulation on aphid performance in the Cd hyperaccumulator S. alfredii. The results revealed that Cd or other factors affected by Cd significantly protected S. alfredii plants from aphid infestation, probably by directly poisoning the insects rather than by deterring them. It is likely that the high Cd level in the phloem sap of S. alfredii helps fight against the black bean aphid. Consequently, the current study significantly improves our understanding of the “heavy metal defense” hypothesis to develop strategies for insect resistance and promote heavy metal accumulation in the shoots of these hyperaccumulators, which may contribute to the optimized phytoremediation efficiency.

References

• 1

  AliS.UllahM. I.SaeedM. F.KhalidS.SaqibM.ArshadM.et al (2019). Heavy metal exposure through artificial diet reduces growth and survival of Spodoptera litura (Lepidoptera: Noctuidae). Environ. Sci. Pollut. Res.26 (14), 14426–14434. 10.1007/s11356-019-04792-0

  • CrossRef
  • Google Scholar

• 2

  BoydR. S. (2013). Exploring tradeoffs in hyperaccumulator ecology and evolution. New Phytol.199 (4), 871–872. 10.1111/nph.12398

  • CrossRef
  • Google Scholar

• 3

  BoydR. S.MartensS. N. (1999). Aphids are unaffected by the elemental defence of the nickel hyperaccumulator Streptanthus polygaloides (Brassicaceae). Chemoecology9, 1–7.

  • Google Scholar

• 4

  BoydR. S. (2007). The defense hypothesis of elemental hyperaccumulation: Status, challenges and new directions. Plant Soil293 (1-2), 153–176. 10.1007/s11104-007-9240-6

  • CrossRef
  • Google Scholar

• 5

  ButtA.Qurat ulA.RehmanK.KhanM. X.HesselbergT. (2018). Bioaccumulation of cadmium, lead, and zinc in agriculture-based insect food chains. Environ. Monit. Assess.190 (12), 698. 10.1007/s10661-018-7051-2

  • CrossRef
  • Google Scholar

• 6

  CarolanJ. C.FitzroyC. I. J.AshtonP. D.DouglasA. E.WilkinsonT. L. (2009). The secreted salivary proteome of the pea aphid Acyrthosiphon pisum characterised by mass spectrometry. Proteomics9, 2457–2467. 10.1002/pmic.200800692

  • CrossRef
  • Google Scholar

• 7

  ChandaB.XiaY.MandalM. K.YuK.SekineK-T.GaoQ-m.et al (2011). Glycerol-3-phosphate is a critical mobile inducer of systemic immunity in plants. Nat. Genet.43 (5), 421–427. 10.1038/ng.798

  • CrossRef
  • Google Scholar

• 8

  ClemensS.AartsM. G. M.ThomineS.VerbruggenN. (2013). Plant science: The key to preventing slow cadmium poisoning. Trends Plant Sci.18 (2), 92–99. 10.1016/j.tplants.2012.08.003

  • CrossRef
  • Google Scholar

• 9

  DaiZ. C.CaiH. H.QiS. S.LiJ.ZhaiD. L.WanJ. S. H.et al (2020). Cadmium hyperaccumulation as an inexpensive metal armor against disease in Crofton weed. Environ. Pollut.267, 115649. 10.1016/j.envpol.2020.115649

  • CrossRef
  • Google Scholar

• 10

  EntA. V. D.BakerA. J. M.ReevesR. D.PollardA. J.SchatH. (2013). Hyperaccumulators of metal and metalloid trace elements: Facts and fiction. Plant Soil362, 319–334. 10.1007/s11104-012-1287-3

  • CrossRef
  • Google Scholar

• 11

  HansonB.LindblomS. D.LoefflerM. L.EahP. S. (2004). Selenium protects plants from phloem-feeding aphids due to both deterrence and toxicity. New Phytol.162, 655–662. 10.1111/j.1469-8137.2004.01067.x

  • CrossRef
  • Google Scholar

• 12

  HeS.YangX.HeZ.BaligarV. C. (2017). Morphological and physiological responses of plants to cadmium toxicity: A review. Pedosphere27 (3), 421–438. 10.1016/s1002-0160(17)60339-4

  • CrossRef
  • Google Scholar

• 13

  HuY.TianS.FoyerC. H.HouD.WangH.ZhouW.et al (2019a). Efficient phloem transport significantly remobilizes cadmium from old to young organs in a hyperaccumulator Sedum alfredii. J. Hazard. Mater.365, 421–429. 10.1016/j.jhazmat.2018.11.034

  • CrossRef
  • Google Scholar

• 14

  HuY.XuL.TianS.LuL.LinX. (2019b). Site-specific regulation of transcriptional responses to cadmium stress in the hyperaccumulator, Sedum alfredii: Based on stem parenchymal and vascular cells. Plant Mol. Biol.99 (4-5), 347–362. 10.1007/s11103-019-00821-1

  • CrossRef
  • Google Scholar

• 15

  Irfan NaikooM.Ahmad KhanF.AhmedN.RinklebeJ.SonneC.NishantaR.et al (2021a). Biotransfer, bioaccumulation and detoxification of nickel along the soil - faba bean - aphid - ladybird food chain. Sci. Total Environ.785, 147226. 10.1016/j.scitotenv.2021.147226

  • CrossRef
  • Google Scholar

• 16

  Irfan NaikooM.FarihaR.Irfan DarM.Ahmad KhanF.KamelH.ParvaizA. (2021b). Uptake, accumulation and elimination of cadmium in a soil - faba bean (Vicia faba) - aphid (Aphis fabae) - ladybird (Coccinella transversalis) food chain. Chemosphere279, 130522. 10.1016/j.chemosphere.2021.130522

  • CrossRef
  • Google Scholar

• 17

  JheeE. M.BoydR. S.EubanksM. D. (2005). Nickel hyperaccumulation as an elemental defense of Streptanthus polygaloides (brassicaceae): Influence of herbivore feeding mode. New Phytol.168 (2), 331–344. 10.1111/j.1469-8137.2005.01504.x

  • CrossRef
  • Google Scholar

• 18

  KonopkaJ. K.HanyuK.MacfieS. M.McneilJ. N. (2013). Does the response of insect herbivores to cadmium depend on their feeding strategy?J. Chem. Ecol.39, 546–554. 10.1007/s10886-013-0273-4

  • CrossRef
  • Google Scholar

• 19

  KraemerU. (2010). “Metal hyperaccumulation in plants,” in Annual review of plant biology. Editors MerchantS.BriggsW. R.OrtD., 61, 517–534.

  • Google Scholar

• 20

  KupperH.LombiE.ZhaoF. J.McGrathS. P. (2000). Cellular compartmentation of cadmium and zinc in relation to other elements in the hyperaccumulator Arabidopsis halleri. Planta212 (1), 75–84. 10.1007/s004250000366

  • CrossRef
  • Google Scholar

• 21

  LimG-H.LiuH.YuK.LiuR.ShineM. B.FernandezJ.et al (2020). The plant cuticle regulates apoplastic transport of salicylic acid during systemic acquired resistance. Sci. Adv.6 (19), eaaz0478. 10.1126/sciadv.aaz0478

  • CrossRef
  • Google Scholar

• 22

  LinT. T.ChenJ. P.ZhouS. S.YuW. H.ChenG.ChenL. H.et al (2020). Testing the elemental defense hypothesis with a woody plant species: Cadmium accumulation protects Populus yunnanensis from leaf herbivory and pathogen infection. Chemosphere247, 125851. 10.1016/j.chemosphere.2020.125851

  • CrossRef
  • Google Scholar

• 23

  LiuZ.SunZ. Z.ZengC. Z.DongX. J.LiM.LiuZ. X.et al (2022). The elemental defense effect of cadmium on Alternaria brassicicola in Brassica juncea. BMC Plant Biol.22 (1), 17. 10.1186/s12870-021-03398-4

  • CrossRef
  • Google Scholar

• 24

  LuL. L.TianS. K.YangX. E.WangX. C.BrownP.LiT. Q.et al (2008). Enhanced root-to-shoot translocation of cadmium in the hyperaccumulating ecotype of Sedum alfredii. J. Exp. Bot.59, 3203–3213. 10.1093/jxb/ern174

  • CrossRef
  • Google Scholar

• 25

  MałkowskiE.KitaA.GalasW.KarczW.KuperbergJ. M. (2002). Lead distribution in corn seedlings (Zea mays L.) and its effect on growth and the concentrations of potassium and calcium. Plant Growth Regul.37, 69–76. 10.1023/a:1020305400324

  • CrossRef
  • Google Scholar

• 26

  MillerG. L. (1959). Use of dinitrosalicylic acid reagent for determination of reducing sugar. Anal. Chem.31, 426–428. 10.1021/ac60147a030

  • CrossRef
  • Google Scholar

• 27

  MorkunasI.WozniakA.Van ChungM.Rucinska-SobkowiakR.JeandetP. (2018). The role of heavy metals in plant response to biotic stress. Molecules23 (9), 2320. 10.3390/molecules23092320

  • CrossRef
  • Google Scholar

• 28

  NavariizzoF. (2010). Heavy metal hyperaccumulating plants: How and why do they do it? And what makes them so interesting?Plant Sci.180, 169–181. 10.1016/j.plantsci.2010.08.016

  • CrossRef
  • Google Scholar

• 29

  RascioN.Navari-IzzoF. (2011). Heavy metal hyperaccumulating plants: How and why do they do it? And what makes them so interesting?Plant Sci.180 (2), 169–181. 10.1016/j.plantsci.2010.08.016

  • CrossRef
  • Google Scholar

• 30

  RubinaK.AnjaniK.NayakA. K.ShahidMdRahulT.VijayakumarS.et al (2020). Metal(loid)s (as, Hg, Se, Pb and Cd) in paddy soil: Bioavailability and potential risk to human health. Sci. Total Environ.699, 134330. 10.1016/j.scitotenv.2019.134330

  • CrossRef
  • Google Scholar

• 31

  ShiZ.WangS.PanB.LiuY.LiY.WangS.et al (2020). Effects of zinc acquired through the plant-aphid-ladybug food chain on the growth, development and fertility of Harmonia axyridis. Chemosphere259, 127497. 10.1016/j.chemosphere.2020.127497

  • CrossRef
  • Google Scholar

• 32

  Shimizu-InatsugiR.MilosavljevicS.ShimizuK. K.Schaepman-StrubG.TanoiK.SatoY. (2021). Metal accumulation and its effect on leaf herbivory in an allopolyploid species Arabidopsis kamchatica inherited from a diploid hyperaccumulator A. halleri. Plant Species Biol.36 (2), 208–217. 10.1111/1442-1984.12304

  • CrossRef
  • Google Scholar

• 33

  StolpeC.KrämerU.MüllerC. (2017). Heavy metal (hyper)accumulation in leaves of Arabidopsis halleri is accompanied by a reduced performance of herbivores and shifts in leaf glucosinolate and element concentrations. Environ. Exp. Bot.133, 78–86. 10.1016/j.envexpbot.2016.10.003

  • CrossRef
  • Google Scholar

• 34

  StolpeC.MüllerC. (2016). Effects of single and combined heavy metals and their chelators on aphid performance and preferences. Environ. Toxicol. Chem.35, 3023–3030. 10.1002/etc.3489

  • CrossRef
  • Google Scholar

• 35

  TewesL. J.StolpeC.KerimA.KraemerU.MuellerC. (2018). Metal hyperaccumulation in the Brassicaceae species Arabidopsis halleri reduces camalexin induction after fungal pathogen attack. Environ. Exp. Bot.153, 120–126. 10.1016/j.envexpbot.2018.05.015

  • CrossRef
  • Google Scholar

• 36

  TianS. K.LuL. L.YangX. O.WebbS. M.DuY. H.BrownP. H. (2010). Spatial imaging and speciation of lead in the accumulator plant Sedum alfredii by microscopically focused synchrotron X-ray investigation. Environ. Sci. Technol.44 (15), 5920–5926. 10.1021/es903921t

  • CrossRef
  • Google Scholar

• 37

  TianS. K.LuL. L.LabavitchJ.YangX. E.HeZ. L.HuH. N.et al (2011). Cellular sequestration of cadmium in the hyperaccumulator plant species Sedum alfredii. Plant Physiol.157, 1914–1925. 10.1104/pp.111.183947

  • CrossRef
  • Google Scholar

• 38

  TianS. K.XieR. H.WangH. X.HuY.GeJ.LiaoX. C.et al (2016). Calcium deficiency triggers phloem remobilization of cadmium in a hyper-accumulating species. Plant Physiol.172, 2300–2313. 10.1104/pp.16.01348

  • CrossRef
  • Google Scholar

• 39

  TianS. K.XieR. H.WangH. X.HuY.HouD. D.LiaoX. C.et al (2017). Uptake, sequestration and tolerance of cadmium at cellular levels in the hyperaccumulator plant species Sedum alfredii. J. Exp. Bot.68, 2387–2398. 10.1093/jxb/erx112

  • CrossRef
  • Google Scholar

• 40

  VerbruggenN.JuraniecM.BaliardiniC.MeyerC-L. (2013). Tolerance to cadmium in plants: The special case of hyperaccumulators. Biometals26 (4), 633–638. 10.1007/s10534-013-9659-6

  • CrossRef
  • Google Scholar

• 41

  Wen-LiZ.YuD.Miao-MiaoZ.QiS. (2014). Cadmium exposure and its health effects: A 19-year follow-up study of a polluted area in China. Sci. Total Environ.470-471, 224–228. 10.1016/j.scitotenv.2013.09.070

  • CrossRef
  • Google Scholar

• 42

  WillT.TjallingiiW. F.ThonnessenA.van BelA. J. E. (2007). Molecular sabotage of plant defense by aphid saliva. Proc. Natl. Acad. Sci. U. S. A.104 (25), 10536–10541. 10.1073/pnas.0703535104

  • CrossRef
  • Google Scholar

• 43

  WillT.FurchA. C. U.ZimmermannM. R. (2013). How phloem-feeding insects face the challenge of phloem-located defenses. Front. Plant Sci.4, 336. 10.3389/fpls.2013.00336

  • CrossRef
  • Google Scholar

• 44

  YangX. E.LongX. X.YeH. B.HeZ. L.CalvertD. V.StoffellaP. J. (2004). Cadmium tolerance and hyperaccumulation in a new Zn-hyperaccumulating plant species (Sedum alfredii Hance). Plant Soil259, 181–189. 10.1023/b:plso.0000020956.24027.f2

  • CrossRef
  • Google Scholar

• 45

  ZhangZ. L.QuW. J.LiX. F. (2009). Plant physiology experimental guide. 4th edition. Beijing, China: China Higher Education Press, 106.

  • Google Scholar

• 46

  ZhaoF. J.MaY. B.ZhuY. G.TangZ.McGrathS. P. (2015). Soil contamination in China: Current status and mitigation strategies. Environ. Sci. Technol.49 (2), 750–759. 10.1021/es5047099

  • CrossRef
  • Google Scholar