Suppression of Chlorella vulgaris growth by cadmium, lead, and copper stress and its restoration by endogenous brassinolide

Abstract

Brassinosteroids play a significant role in the amelioration of various abiotic and biotic stresses. In order to elaborate their roles in plants subjected to heavy metals stress, Chlorella vulgaris cultures treated with 10(-8) M brassinolide (BL) were exposed to 10(-6)-10(-4) M heavy metals (cadmium, lead and copper) application. Under heavy metals stress, the growth and chemical composition (chlorophyll, monosaccharides, and protein content) have been decreased during the first 48 h of cultivation. The inhibitory effect of heavy metals on C. vulgaris cultures was arranged in the following order: copper > lead > cadmium. C. vulgaris cultures treated with BL in the absence or presence of heavy metals showed no differences in the endogenous level of BL. On the other hand, treatment with heavy metals results in BL level very similar to that of control cell cultures. These results suggest that the activation of brassinosteroids biosynthesis, via an increase of endogenous BL, is not essential for the growth and development of C. vulgaris cells in response to heavy metals stress. Simultaneously, BL enhanced the content of indole-3-acetic acid, zeatin, and abscisic acid in cultures treated with heavy metals. Levels per cell of chlorophylls, protein, and monosaccharides are all increased by BL treatment when compared to nontreated control cells. Application of BL to C. vulgaris cultures reduced the accumulation of heavy metals stress on growth, prevented chlorophyll, monosaccharides, and protein loss, and increased phytochelatins content. The arrested growth of C. vulgaris cells treated with heavy metals was restored by the coapplication of BL. It suggested that BL overcame the inhibitory effect of heavy metals. From these results, it can be concluded that BL plays the positive role in the alleviation of heavy metals stress.

Fig. 1

Effect of heavy metals (Cd, Pb, and Cu) in the absence or presence of BL on the growth of C. vulgaris expressed as a number of cells. Data are means ± SE (n = 15). Before treatment of C. vulgaris cultures, the number of cells was established at 5 × 10⁵ cells/ml. Treatments with the same letter are not significantly different according to Duncan’s test

Fig. 2

Effect of heavy metals (Cd, Pb, and Cu) in the absence or presence of BL on the endogenous content of phytohormones in C. vulgaris cultures for the 48-hr treatment. Data are means ± SE (n = 15). Treatments with the same letter are not significantly different according to Duncan’s test

Fig. 3

Effect of heavy metals (Cd, Pb, and Cu) in the absence or presence of BL on the total chlorophyll content in C. vulgaris cultures. Data are means ± SE (n = 15). Before treatment of C. vulgaris cultures, the chlorophyll content was established at 3.5 fg/cell. Treatments with the same letter are not significantly different according to Duncan’s test

Fig. 4

Effect of heavy metals (Cd, Pb, and Cu) in the absence or presence of BL on the monosaccharides content in C. vulgaris cultures. Data are means ± SE (n = 15). Before treatment of C. vulgaris cultures, the sugar content was established at 19 fg/cell. Treatments with the same letter are not significantly different according to Duncan’s test

Fig. 5

Effect of heavy metals (Cd, Pb, and Cu) in the absence or presence of BL on the protein content in C. vulgaris cultures. Data are means ± SE (n = 15). Before treatment of C. vulgaris cultures, the protein content was established at 25 fg/cell. Treatments with the same letter are not significantly different according to Duncan’s test

Fig. 6

Effect of heavy metals (Cd, Pb, and Cu) in the absence or presence of BL on the total PCs content in C. vulgaris cultures. Data are means ± SE (n = 15). Before treatment of C. vulgaris cultures, the total PCs content was established at zero. Treatments with the same letter are not significantly different according to Duncan’s test