UVB-Pretreatment-Enhanced Cadmium Absorption and Enrichment in Poplar Plants

Abstract

The phenomenon of cross adaptation refers to the ability of plants to improve their resistance to other stress after experiencing one type of stress. However, there are limited reports on how ultraviolet radiation B (UVB) pretreatment affects the enrichment, transport, and tolerance of cadmium (Cd) in plants. Since an appropriate UVB pretreatment has been reported to change plant tolerance to stress, we hypothesized that this application could alter plant uptake and tolerance to heavy metals. In this study, a woody plant species, 84K poplar (Populus alba × Populus glandulosa), was pretreated with UVB and then subjected to Cd treatment. The RT-qPCR results indicated that the UVB-treated plants could affect the expression of Cd uptake, transport, and detoxification-related genes in plants, and that the UVB-Pretreatment induced the ability of Cd absorption in plants, which significantly enriched Cd accumulation in several plant organs, especially in the leaves and roots. The above results showed that the UVB-Pretreatment further increased the toxicity of Cd to plants in UVB-Cd group, which was shown as increased leaf malonaldehyde (MDA) and hydrogen peroxide (H₂O₂) content, as well as downregulated activities of antioxidant enzymes such as Superoxide Dismutase (SOD), Catalase (CAT), and Ascorbate peroxidase (APX). Therefore, poplar plants in the UVB-Cd group presented a decreased photosynthesis and leaf chlorosis. In summary, the UVB treatment improved the Cd accumulation ability of poplar plants, which could provide some guidance for the potential application of forest trees in the phytoremediation of heavy metals in the future.

Keywords: UVB; cadmium enrichment; gene expression; phytoremediation; poplar.

Figure 1

Phenotypic differences of the poplar before and after treatment. (a–d) T of the whole plant. (e) Root and (f) leaf phenotypes of poplar after treatment. (g) Plant height. (h) Net plant height growth (n = 9 plants for each treatment). Different letters represent significant differences among four treatment groups (p < 0.05).

Figure 2

The photosynthetic gas exchange parameters of poplar under different treatments. (a) Net photosynthetic rate (Pn). (b) Transpiration rate (Tr). (c) Intercellular CO₂ concentration (Ci). (d) Stomatal conductance (Gs). Different letters represent significant differences among four treatment groups (p < 0.05, n = 9 plants for each treatment).

Figure 3

Indicators related to the antioxidant capacity of 84K poplar leaves in different treatment groups. The content of MDA (a) and H₂O₂ (b), APX (c), CAT (d), and SOD (e) of poplar leaves in different treatment groups (n = 9 plants for each treatment). Different letters represent significant differences among four treatment groups (p < 0.05).

Figure 4

Cd localization and accumulation in roots (a1–a4), stems (b1–b4) and leaves (c1–c4) of poplar under different treatments. The red arrow indicates the precipitation of Cd-dithizone. Red bar scale = 100 μm, black bar scale = 500 μm. ph, phloem; xyl, xylem; lv, leaf vein; mc, mesophyll cells. The experiment was repeated at least three times, each time with similar results. A group of photographs was selected for display.

Figure 5

The total accumulation of Cd in different tissues (a) and the whole plant (b), and the Cd content in different tissues (c) and the whole plant (d) under different treatments (n = 9 plants for each treatment). Asterisks represent significant differences between two treatment groups: * represents p < 0.05; ** represents p < 0.01.

Figure 6

Relative expression of genes related to Cd uptake, transport, and detoxification in poplar leaf under different treatments. The expression levels of ABCC1 (a), ABCC2 (b), F3H (c), MYB12 (d), GSH (e), PCS1 (f), ZIP2 (g), ZIP4 (h), ZIP6 (i), CAX1 (j), CAX2 (k), CAX3 (l), CAX4 (m), CAX5 (n), CAX6 (o), CAX7 (p), NRAMP1 (q), YSL1 (r), and MTP1 (s) in poplar leaf (n = 5 plants for each treatment). Different letters represent significant differences among four treatment groups (p < 0.05).

Figure 7

Relative expression of genes related to Cd uptake, transport, and detoxification in poplar root under different treatments. The expression levels of ABCC1 (a), ABCC2 (b), F3H (c), MYB12 (d), GSH (e), PCS1 (f), ZIP2 (g), ZIP4 (h), ZIP6 (i), CAX1 (j), CAX2 (k), CAX3 (l), CAX4 (m), CAX5 (n), CAX6 (o), CAX7 (p), NRAMP1 (q), YSL1 (r), and MTP1 (s) in poplar root (n = 5 plants for each treatment). Different letters represent significant differences among four treatment groups (p < 0.05).

Figure 8

Physiological and molecular mechanisms of UVB Pretreatment enhanced Cd absorption and enrichment in poplar.