Beryllium Stress-Induced Modifications in Antioxidant Machinery and Plant Ultrastructure in the Seedlings of Black and Yellow Seeded Oilseed Rape

Abstract

Beryllium (Be) could be a threatening heavy metal pollutant in the agroecosystem that may severely affect the performance of crops. The present study was conducted to evaluate the toxic effects of Be (0, 100, 200, and 400 μM) on physiological, ultrastructure, and biochemical attributes in hydroponically grown six-day-old seedlings of two cultivars of Brassica napus L., one tolerant (ZS 758, black seeded) and one sensitive (Zheda 622, yellow seeded). Higher Be concentrations reduced the plant growth, biomass production, chlorophyll contents, and the total soluble protein contents. A significant accumulation of ROS (H₂O₂, OH^-) and MDA contents was observed in a dose-dependent manner. Antioxidant enzymatic activities including SOD, POD, GR, APX, and GSH (except CAT) were enhanced with the increase in Be concentrations in both cultivars. Relative transcript gene expression of above-mentioned antioxidant enzymes further confirmed the alterations induced by Be as depicted from higher involvement in the least susceptible cultivar ZS 758 as compared to Zheda 622. The electron microscopic study showed that higher level of Be (400 μM) greatly damaged the leaf mesophyll and root tip cells. More damage was observed in cultivar Zheda 622 as compared to ZS 758. The damage in leaf mesophyll cells was highlighted as the disruption in cell wall, immature nucleus, damaged mitochondria, and chloroplast structures. In root tip cells, disruption in Golgi bodies and damage in cell wall were clearly noticed. As a whole, the present study confirmed that more inhibitory effects were recorded in yellow seeded Zheda 622 as compared to black seeded ZS 758 cultivar, which is regarded as more sensitive cultivar.

Figure 1

Effects of different concentrations of beryllium (Be) (0, 100, 200, and 400 μM) on the activities of (a, b) superoxide dismutase (SOD), (c, d) guaiacol peroxidase (POD), (e, f) catalase (CAT), (g, h) ascorbate peroxidase (APX), and (i, j) glutathione reductase (GR), respectively, in the leaves and roots of 6-day-old seedlings of two Brassica napus cultivars (ZS 758, black seeded; Zheda 622, yellow seeded). Vertical bars represent standard deviation from three independent replicates. Means followed by the same letters are not significantly different by Duncan's multiple range test (P < 0.05).

Figure 2

Effects of different concentrations of beryllium (Be) (0, 100, 200, and 400 μM) on the transcript level of (a, b) superoxide dismutase (SOD), (c, d) guaiacol peroxidase (POD), (e, f) catalase (CAT), (g, h) ascorbate peroxidase (APX), and (i, j) glutathione reductase (GR) related gene expression, respectively, in the leaves and roots of 6-day-old seedlings of two Brassica napus cultivars (ZS 758, black seeded; Zheda 622, yellow seeded). Vertical bars represent standard deviation from three independent replicates. Means followed by the same letters are not significantly different by Duncan's multiple range test (P < 0.05).

Figure 3

Electron micrographs of leaf mesophyll of 6-day-old seedlings of two cultivars of Brassica napus (ZS 758, black seeded; Zheda 622, yellow seeded) grown under control and 400 μM Be. (a-b) leaf mesophyll cells of ZS 758 and Zheda 622 at control level, respectively, show well-developed cell wall (CW), chloroplasts (Chl), plastoglobuli (P), starch grain (SG), and mitochondria (M). (c) leaf mesophyll cell of ZS 758 at 400 μM Be shows an unmatured nucleus (N) with nucleolus (Nue), disturbed nuclear membrane (NM), deshaped thylakoids (Thy), and small-sized plastoglobuli (P). (d) leaf mesophyll cell of Zheda 622 at 400 μM Be shows damaged thylakoid membranes (Thy), disturbed nuclear membrane (NM), very small plastoglobuli (P), ruptured mitochondria (M), and chloroplast structures.

Figure 4

Electron micrographs of root cells of 6-day-old seedlings of two cultivars of Brassica napus (ZS 758, black seeded; Zheda 622, yellow seeded) grown at control and with 400 μM Be. (a-b) root cells of ZS 758 and Zheda 622 at control level, respectively, show well-developed nucleus (N) with nucleolus (Nue), vacuole (Vac), nuclear membrane (NM), cell wall (CW), and well-developed mitochondria (M) as well as Golgi bodies. (c) root cell of ZS 758 at 400 μM Be shows a damaged cell wall (CW), deshaped nucleus (N) with nucleolus (Nue), small-sized mitochondria (M), disturbed large vacuole (Vac), and disrupted nuclear membrane (NM). (d) root cell of Zheda 622 at 400 μM Be shows a broken cell wall (CW), disturbed nucleus (N) with nucleolus (Nue), damaged cell membrane (CM), and nuclear membrane (NM).