Does salicylic acid regulate antioxidant defense system, cell death, cadmium uptake and partitioning to acquire cadmium tolerance in rice?
Salicylic acid (SA) may accelerate the cell death of cadmium-stressed roots to avoid cadmium (Cd) uptake by plants or may play positive roles in protecting the stressed roots from Cd-induced damage. To test these hypotheses, we performed a series of split-root hydroponic experiments with one-half of rice (Oryza sativa L. cv. Jiahua 1) roots exposed to 50 microM Cd and the other half not exposed. The objectives were to elucidate the effects of SA pretreatment on the time-dependent changes of H(2)O(2) levels in roots, antioxidant defense system in different organs, root cell death and the dynamic distribution of Cd in the plants. In the split-root system, a higher Cd uptake rate was observed in the Cd-stressed portions of roots compared with the treatment with the whole roots exposed to Cd. Furthermore, an appreciable amount of Cd was translocated from the Cd-exposed roots to the unexposed roots and trace amounts of Cd were released into the external solution. The split-root method also caused the two root portions to respond differently to Cd stress. The activities of major antioxidant enzymes (superoxide dismutase, SOD; peroxidase, POD; and catalase, CAT) were significantly suppressed in the Cd-treated roots, hence leading to H(2)O(2) burst, lipid peroxidation, cell death and growth inhibition. By contrast, in the non-Cd-treated roots, the activities of enzymes (SOD, CAT, and POD) and root growth were persistently stimulated during the experimental period. The H(2)O(2) accumulation and lipid peroxidation were also induced in the non-Cd-treated roots, but they were significantly lower than those of the Cd-treated roots. The concentrations of glutathione (GSH) and non-protein thiols (NPT) in the Cd-treated roots were significantly higher than those of the untreated roots. SA pretreatment elevated enzymatic and non-enzymatic antioxidants, and the concentrations of GSH and NPT in roots and shoots, hence leading to alleviation of the oxidative damage as indicated by the lowered H(2)O(2) and MDA levels. Furthermore, SA pretreatment mitigated the Cd-induced growth inhibition in both roots and shoots and increased transpiration compared with non-SA-pretreatment under Cd exposure. It is concluded that Cd can be partly transferred from the Cd-exposed roots to Cd-unexposed roots and that cell death can be accelerated in the Cd-stressed roots in response to Cd stress. The SA-enhanced Cd tolerance in rice can be attributed to SA-elevated enzymatic and non-enzymatic antioxidants and NPT, and to SA-regulated Cd uptake, transport and distribution in plant organs.
Guo
Bin
B
Department of Plant Nutrition, College of Natural Resources and Environmental Sciences, Nanjing Agricultural University, Nanjing, PR China.
Liang
Yongchao
Y
Zhu
Yongguan
Y
D000222:Adaptation, Physiological / Q000187:drug effects*; D000975:Antioxidants / Q000378:metabolism*; D018533:Biomass; D002104:Cadmium / Q000378:metabolism* / Q000633:toxicity; D016923:Cell Death / Q000187:drug effects; D005070:Evans Blue; D005978:Glutathione / Q000378:metabolism; D006861:Hydrogen Peroxide / Q000378:metabolism; D018527:Hydroponics; D008315:Malondialdehyde / Q000378:metabolism; D012275:Oryza / Q000166:cytology* / Q000187:drug effects / Q000276:immunology* / Q000378:metabolism; D018517:Plant Roots / Q000166:cytology / Q000187:drug effects / Q000201:enzymology; D018520:Plant Shoots / Q000187:drug effects / Q000378:metabolism; D018526:Plant Transpiration / Q000187:drug effects; D020156:Salicylic Acid / Q000494:pharmacology*; D012996:Solutions; D013194:Staining and Labeling; D013438:Sulfhydryl Compounds / Q000378:metabolism
D000975:Antioxidants; D012996:Solutions; D013438:Sulfhydryl Compounds; D002104:Cadmium; D005070:Evans Blue; D008315:Malondialdehyde; D006861:Hydrogen Peroxide; D005978:Glutathione; D020156:Salicylic Acid