*2.1. Plant Material*

*A. thaliana* accession Landsberg *erecta* (L. Heyn) mutants *atnp01* and *atnp02* were previously isolated by screening for resistance to CdS QDs 378 transposon insertional lines obtained from the Nottingham Arabidopsis Stock Centre (uNASC; http://arabidopsis. info/ (accessed on 2 February 2021)). Marmiroli et al., [14] reported the conditions of isolation and characterization of the two mutants when treated with CdS QDs and CdSO4. All reagents and standards were purchased from Sigma-Aldrich (St. Louis, MO, USA) unless otherwise stated.

### *2.2. Synthesis and Characterization of the CdS QDs*

The synthesis and characterization of CdS QDs are reported in "Supplementary Materials S.1" (Figures S1 and S2). X-ray diffraction (XRD) (XRD Empirean alpha1, Malvern Panalytical, UK) and high-resolution transmission electron microscopy (HR-TEM) (Hitachi HT7700, Hitachi High Technologies America, Pleasanton, CA, US) demonstrated that the average static diameter of the CdS QD NPs (nanoparticles) was 5 nm; the crystal structure was hexagonal wurtzite (ZnS) with about 78% Cd. The average particle size of the aggregates measured by dynamic light scattering, and zeta potential (ζ) estimated in ddH2O, were 178.7 nm and + 15.0 mV, respectively [14].

#### *2.3. Seed Germination, Growth, and Treatments*

Twenty-five seeds of *A. thaliana* wild type (wt) and two mutants, *atnp01* and *atnp02*, were sown on Petri dishes containing Murashige and Skoog (MS) nutrient medium (Duchefa Biochemie, Haarlem, Netherlands) containing 1% *w*/*v* sucrose and 0.8% *w*/*v* agar, then placed in the dark, under controlled conditions in a growth chamber. After germination, seedlings were grown at 24 ◦C, with a relative humidity of 30%, and under a 16-h photoperiod (light intensity 120 μM m−<sup>2</sup> s−<sup>1</sup> photosynthetic photon flux) in the MS medium in the absence of treatment for 14 days. After that time span, seedlings were transferred to a fresh MS medium containing 80 mg L−<sup>1</sup> CdS QDs (treatment) or without CdS QDs (control) and grown for a further 21 days in the same conditions as above. Because 80 mg L−<sup>1</sup> CdS QDs is a sub-inhibitory concentration, the stocks of seeds of both mutants and wt plants were checked for their capacity to grow in the presence of CdS QDs up to 250 mg L<sup>−</sup>1, which was totally inhibiting for wt. All six samples were collected after 21 days by removing them carefully from the medium; they were gently washed with distilled H2O, frozen in liquid nitrogen and then stored at −80 ◦C until use for protein extraction.
