Effects Of Cr～(6+) Stress On Growth And DNA Metabolism In Wheat Seedlings

Abstract As one of five poisons in industry, chromium is a latent and jeopardized environmental pollutant. Due to its powerful oxidizing capabilities and ability to cross biological membranes, Cr⁶⁺ is considered as the most hazardous carcinogens and mutagens to animals and human. At present, the studies on the effects of Cr on plants were mainly focused on uptake and accumulation of Cr, hindrance on plant growth, damages to internal structures, and variety in activities of enzymes, oxidative stress and lesions of photosynthetic system and so on. Some studies indicate that heavy metals (e.g. Ni,Cd,Hg and Cr) are potent inducers of tumors in experimental animals and human cell, and caused a variety of DNA damages such as increasing of the rate of unscheduled DNA synthesis, DNA strand breaks, DNA-protein crosslinks, DNA interstrand crosslinks, as well as expression of genome. But there are a few reports on the DNA damages of plants caused by heavy metals, say nothing of Cr⁶⁺.Therefore, in this study, the investigations were carried out on wheat (Triticum aestivum cv. Xiaoyan No.22) seedlings with different concentrations of Cr⁶⁺ in hydroponics culture. Effects of Cr⁶⁺ on the growth of wheat seedlings, DNA metabolism and PCD in root tip cells were conducted in different stages of wheat seedlings (3-day-old seedlings and 10-day-old seedlings) after 72h of Cr⁶⁺treatment, in order to discuss the mechanisms that Cr⁶⁺ affects the growth and yields of wheat seedlings, that heavy metal causes damage to plants, and that rule of PCD happens under heavy metal stress, so that to establish the theoretic ground for mechanism of recovery and endurance to heavy metal stress in plants. The results were as follows:Effect of Cr⁶⁺ on the growth of wheat seedlings.Studies found that Cr⁶⁺ could clearly affect the growth of 3-day-old seedlings and 10-day-old seedlings. Root length, shoot height, fresh weight and dry weight of roots and shoots of seedlings were all decreased by Cr⁶⁺ tested concentrations, except that root number was increased with 5-20mg·L^-1 Cr⁶⁺ in 3-day-old seedlings and with 5-100mg·L^-1Cr⁶⁺ in 10-day-old seedlings.The growth of roots was reduced by Cr⁶⁺ treatment more than the growth ofshoots. And the growth of 3-day-old seedlings was more sensitive to Cr⁶⁺ stress than that of 10-day-old seedlings."1. Effect of Cr⁶⁺ on DNA damages in wheat seedlings.1. Effect of Cr⁶⁺on DNA content in wheat seedlings. The trial results indicated that DNA contents in roots and shoots of seedlings were all markedly reduced by Cr⁶⁺ tested concentrations. Moreover, the declining extent of DNA content in roots of seedlings was bigger than that in shoots of seedlings, and the declining extent of DNA content in shoots and roots of 3-day-old seedlings was still bigger than that of 10-day-old seedlings.2. Effect of Cr⁶⁺ on DNA fragmentation and DNA interstrand crosslinks in wheat seedlings. The results showed that: (1) The hyperchromicity of DNA in roots of 3-day-old seedlings was bigger than that of control seedlings in the range 5-40mg·L^-1 Cr⁶⁺, also the hyperchromicity of DNA in shoots of 3-day-old seedlings and 10-day-old seedlings and roots of 10-day-old seedlings was bigger than that of control seedlings in the range 5-100mg·L^-1Cr⁶⁺. These indicated that DNA fragmentation happened, and that DNA fragmentation was one of DNA damages effects. (2) The hyperchromicity of DNA in roots of 3-day-old seedlings was smaller than that of control seedlings in the range of 60-100mg·L^-1Cr⁶⁺, which means DNA interstrand crosslinks. And DNA interstrand crosslinks was another of DNA damages effects. So, DNA fragmentation and DNA interstrand crosslinks are possible the mechanisms of inhibition of growth in wheat seedlings.三. Effect of Cr⁶⁺ on DNA methylation in wheat seedlings. The results indicated that: (1) The 5-MeC content in the shoots DNA of 3-day-old seedlings and 10-day-old seedlings with 5-100mg·L^-1Cr⁶⁺ treatment were all higher than that of control seedlings. (2) The 5-MeC content in the roots DNA of 3-day-old seedlings with 5-100mg·L^-1Cr⁶⁺ treatment were all higher than that of control seedlings. But in the roots DNA of 3-day-old seedlings, exception that the 5-MeC content with 100 mg·L^-1Cr⁶⁺ treatment was lower than that of control seedlings, 5-MeC contents with the rest Cr⁶⁺ tested concentrations were still higher than that of control seedlings.å››. Effect of Cr⁶⁺ on the PCD in wheat seedlings root tip cells. DNA Ladder was not examined with the conventional agarose gel electrophoresis in the shoots and roots DNA of wheat seedlings, which was possibly related to the few number of PCD cells. Chromium(VI)-induced PCD in root tip cells was only examined...