Cadmium telluride quantum dots (CdTe-QDs) and enhanced ultraviolet-B (UV-B) radiation trigger antioxidant enzyme metabolism and programmed cell death in wheat seedlings

Abstract

Nanoparticles (NPs) are becoming increasingly widespread in the environment. Free cadmium ions released from commonly used NPs under ultraviolet-B (UV-B) radiation are potentially toxic to living organisms. With increasing levels of UV-B radiation at the Earth's surface due to the depletion of the ozone layer, the potential additive effect of NPs and UV-B radiation on plants is of concern. In this study, we investigated the synergistic effect of CdTe quantum dots (CdTe-QDs), a common form of NP, and UV-B radiation on wheat seedlings. Graded doses of CdTe-QDs and UV-B radiation were tested, either alone or in combination, based on physical characteristics of 5-day-old seedlings. Treatments of wheat seedlings with either CdTe-QDs (200 mg/L) or UV-B radiation (10 KJ/m(2)/d) induced the activation of wheat antioxidant enzymes. CdTe-QDs accumulation in plant root cells resulted in programmed cell death as detected by DNA laddering. CdTe-QDs and UV-B radiation inhibited root and shoot growth, respectively. Additive inhibitory effects were observed in the combined treatment group. This research described the effects of UV-B and CdTe-QDs on plant growth. Furthermore, the finding that CdTe-QDs accumulate during the life cycle of plants highlights the need for sustained assessments of these interactions.

Figure 1. TEM image of synthesized CdTe-QDs.

Figure 2. X-ray diffraction spectrum of CdTe-QDs.

Figure 3. Emission spectra of QDs in H₂O over 7 days.

Figure 4. Effects of different CdTe-QDs concentrations on 5-day-old wheat seedlings.

CK, control group; C, treatment groups with different concentrations of CdTe-QDs (C1, 25 mg/L; C2, 50 mg/L; C3, 100 mg/L; C4, 200 mg/L; C5, 400 mg/L). Data are means±SD (n = 3). Means with the same letter are not significantly different at Tukey’s test (p≤0.05).

Figure 5. Effects of different enhanced UV-B radiation doses on 5-day-old wheat seedlings.

CK, control group; B, treatment groups with graded doses of UV-B radiation (B1, 2.5 KJ/m²/d; B2, 5 KJ/m²/d; B3, 7.5 KJ/m²/d; B4, 10 KJ/m²/d; B5, 12.5 KJ/m²/d). Data are means±SD (n = 3). Means with the same letter are not significantly different at Tukey’s test (p≤0.05).

Figure 6. Two-day old seedlings visualized with UV light.

Control, CK; UV-B treatment alone, B; CdTe-QDs treatment alone, C; combined CdTe-QDs and UV-B treatment, C+B.

Figure 7. Cd concentration in tissues of 5-day-old wheat seedlings under different treatments.

Control, CK; UV-B treatment alone, B; CdTe-QDs treatment alone, C; combined CdTe-QDs and UV-B treatment, C+B.

Figure 8. Schematic showing Cd²⁺ release from QDs as a result of UV-B oxidation of CdTe-QDs surfaces , .

Figure 9. Contents of H₂O₂ and O₂ ⁻ in the root and shoot of 5 day old seedlings following different treatments. Data are means±SD (n = 3).

Means with the same letter are not significantly different at Tukey’s test (p≤0.05).

Figure 10. Schematic of the antioxidative defense system and Cd degradation mechanism in plants.

Figure 11. Antioxidant enzyme activities in shoots and roots of 5-day-old wheat seedlings.

Data are means±SD (n = 3). Means with the same letter are not significantly different at Tukey’s test (p≤0.05).

Figure 12. DNA laddering in wheat tissues.

Electrophoresis gels of DNA extracted from wheat with different treatments. CK: control group; B: enhanced UV-B radiation; C: CdTe-QDs treatment; B+C: combined UV-B and CdTe-QDs treatment.

Figure 13. Confocal imaging of CdTe-QDs distribution in wheat root cells.

CdTe-QDs are indicated by fluorescence imaging (g, h, i, m, n, o, s), DIC images demonstrate cell integrity (a, b, c, d, e, f, j, k, l, p, q, r, t). Regions highlighted in red in panel e are shown in panels s, t, and u. Pictures of the bottom right corner in figure j, k, l, p, q, r is the merge figures of g and j, h and k, i and l, m and p, n and q, o and r separately. Scale bars = 20 µm.

Figure 14. Schematic of a putative mechanism through which CdTe-QDs are processed by plant cells.

Figure 15. Average fluorescence intensity of QDs in 50 cells after different treatment durations.

C, CdTe-QDs treatment alone; B+C, combined enhanced UV-B and CdTe-QDs treatment. Values are means±SD (n = 3). Means with the same letter are not significantly different at Tukey’s test (p≤0.05).

Figure 16. Cells undergoing programmed cell death.

DAPI-stained DNA is shown in blue. Apoptotic bodies are indicated with white arrows. Shoot cells (a) and root cells (b) are shown. Scale bars = 10 µm.