*2.3. Elemental Analysis of Rice Tissues*

Shoot and root samples were freeze-dried in a lyophilizer and then ground into fine powder. Approximately 50 and 150 mg of root or shoot tissue were weighed into digestion tubes containing 3 mL concentrated HNO3. The mixtures were digested at 115 ◦C for 40 min in a heat block and then cooled to ambient temperature. To complete the digestion, 500 μL H2O2 was added to each tube for another 20 min of heating at 115 ◦C. The cooled digests were diluted to 25 mL with deionized water. Inductively coupled plasma optical emission spectrometry (ICP-OES; iCAP 6500, Thermo Fisher Scientific, Waltham, MA, USA) was used to determine the As, Cd, and nutrient element (macronutrients (P, S, Ca, Mg, and K) and micronutrients (Cu, Fe, Mn, and Zn)) contents in the acid digested samples [49]. Yttrium (Y) was used as an internal standard, and a sample of known concentration was measured at every 30 samples.

#### *2.4. Real-Time Quantitative PCR Analysis of As and Cd Transporters in Rice*

Fresh tissues of all five biological replicates in each treatment with 250 mg/L C3N4 were ground into a fine powder in liquid nitrogen. A Sigma-Aldrich Spectrum Plant Total RNA kit (Sigma-Aldrich Corp. St. Louis, MO, USA) was used to isolate total RNA from roots and shoots. The total RNA concentration and quality were determined by a Thermo Scientific NanoDrop Lite Spectrophotometer (NanoDrop 2000, Thermo Fisher Scientific, Waltham, MA, USA). One microgram of the extracted RNA was used as template to synthesize complementary DNA (cDNA) with a Verso cDNA synthesis kit. A complete list of primer sequences for As and Cd transporters is provided in Table S1 [50–52]. The synthesized cDNA was diluted to 50 ng/μL, and was used as the template for the following qPCR analysis. Bio-Rad SsoAdvanced Universal SYBR Green Supermix (Bio-Rad Incorporation, Hercules, CA, USA) was used to run the qPCR, and the working concentration of each primer was 10 μM. The thermal program profile for qPCR amplification was 95 ◦C for 30 s, 95 ◦C for 15 s, and 63 ◦C for 30 s, repeating 40 cycles, melting curve from 65 to 95 ◦C. The total volume of each reaction was 20 μL, and histone H3 was used as a housekeeping gene for normalization. The relative expression of each gene was calculated through the 2−ΔΔCt method [53].

### *2.5. Random Amplified Polymorphic DNA (RAPD) Analysis*

The total DNA of shoots and roots in the treatments with 250 mg/L C3N4 with or without the addition of As or Cd and the treatments with As or Cd alone were extracted using a Qiagen DNeasy Plant Mini Kit. Random (Qiagen Incorporation, Germantown, MD, USA) amplified polymorphic DNA (RAPD) analysis was performed using Taq DNA polymerase with a standard *Taq* buffer. The amplification profile was 92 ◦C for 1 min, 35 ◦C for 1 min, and 72 ◦C for 2 min, and the cycle was repeated 39 times. The RAPD primer used in this assay was OPC20 (ACT TCG CCA C) [54]. PCR products were run in 1% agarose gel, and images were taken under UV light in a gel dock.
