Study On The Alleviating Effect And Mechanism Of Polyamines On Cadmium Stress In Aquatic Plants

Polyamines (PAs) are important antioxidants which have close relationship with the tolerance of plant under stress. In this paper, three aquatic plants with different ecotypes (Potamogeton crispus, Potamogeton malaianus, Hydrocharis dubia and Sagittarai trifolia) were chosen as experimental materials, cadmium (Cd²⁺) was selected as the stress factor. Four aquatic plants were cultivated in solution containing different concentrations of Cd+ under lab condition; Hydrocharis dubia was also treated with spermidine (Spd); Potamogeton malaianus was treated with Spd and spermine (Spm). The relationship between endogenous PAs levels and heavy metal stress as well as the mechanisms for the protection of aquatic plants from the toxic effects of heavy metals were systemically studied. The results are as follow:(1) The toxic effect of various concentrations of Cd²⁺(0,30,50,70,90?M) was studied on the contents of chlorophyll, malondiadehyde (MDA), hydrogen peroxide (H₂O₂), free proline (Pro), mineral nutrient, ascorbate (AsA), reduced glutathione (GSH) and PAs, the activities of protective enzyme, polyamine oxidase (PAO) and diamine oxidase (DAO), as well as the generation of O2·- and ultrastructure changes in leaves of Potamogeton crispus. The results showed that the content of chlorophyll and the ratio of Chl (a+b)/Car decreased, with increasing the concentration of Cd²⁺. AsA, GSH and Pro contents and superoxide dismutase (SOD), peroxidase (POD), PAO and DAO activities first increased and then decreased ascorbate peroxidase (APX) and glutathione reductase (GR) activities and O2·-, H₂O₂, MDA and soluble sugar contents always increased, but the data of O2·- was a little down at Cd²⁺ 90?M concentration. The results also showed that:the mineral nutrient absorption was affected by Cd²⁺ stress, it increased the absorption of Ca²⁺?Na+?Mn²⁺?Fe3+ and Cu²⁺, and reduced that of P and K+, with increasing the concentration of Cd²⁺. In addition, Cd treatment increased the putrescine (Put) content, while decreasing Spd and Spm contents, which reduced the ratio of free (Spd+Spm)/Put in the leaves. Total and PS-conjugated PAs changed in the same pattern as free PAs, while PIS-bound PAs was different. PIS-bound Put content enhanced with the increase of Cd²⁺concentration up to 50?M and then decreased, and PIS-bound Spd and Spm contents decreased to a lesser extent. Cd²⁺ exposure resulted in significant damage in chloroplasts and mitochondria, suggesting that Cd²⁺ hastened the senescence of the tested plants.(2) The toxic effects of different gradient concentrations of Cd²⁺ (0,18,36,54,72 ?M) was sdudied on Chlorophyll content, antioxidase system, MDA content, soluble sugar content, soluble protein content and ultrastructure of the cells of Potamogeton malaianus were studied. With Cd increasing, the results showed that Chlorophyll content, soluble protein content and the activity of SOD decreased drastically. MDA content and soluble sugar content increased and a little decreased at 72?M. The activities of catalase (CAT) and POD increased first at 18?M and decreased afterwards. GSH and AsA contents increased first and decreased after 36?M. SDS-PAGE revealed that Cd²⁺ decreased the amount of four polypeptides with apparent molecular weights of 52.1kDa,42.9kDa, 27.4kDa and 14.1kDa, while it promoted the expression of one polypeptide with apparent molecular weight of 18.1kDa. Ultrastructural observation showed that the extent of ultrastructural damage was much more serious with higher pollutant concentration.(3) In order to investigate the effects of Spd and Spm on Cd²⁺ stress, it was checked that the content of chlorophyll, H₂O₂, MDA, soluble protein and Pro, the rate of O2·-generation and the antioxidant enzyme activities of SOD, POD, APX and GR in Potamogeton malaianus. Exogenous application of Spd or Spm Significantly enhanced the level of proline, retarded the loss of chlorophyll, enhanced photosynthesis, decreased the rate of O2·- generation and H₂O₂ content, and prevented Cd-induced lipid peroxidation. Spd and Spm also effectively maintained the balance of antioxidant enzymes activities under Cd²⁺ stress; however, GR activity was found to increase slightly in response to PAs. The antioxidant system which were modified by PAs, are able to moderate the radical scavenging system and to lessen in this way the oxidative stress.(4) The toxic effect of various concentrations of Cd²⁺(0,20,40,60,80?M) was studied on the contents of chlorophyll, MDA, H₂O₂, mineral nutrient, AsA and GSH, the activities of protective enzymes, as well as the ultrastructure changes in leaves of Hydrocharis dubia. The results showed that the content of chlorophyll and the activity of SOD decreased, with increasing the concentration of Cd²⁺. POD and CAT activities first increased and then decreased, and reached the peak value at 40?M and 20?M Cd²⁺, respectively. AsA, GSH, H₂O₂ and MDA contents always increased, but the data of MDA was a little down at Cd²⁺ 80?M concentration. The results also showed that:the mineral nutrient absorption was affected by Cd²⁺ stress, it increased the absorption of K+?Fe3+ and Mg²⁺, and reduced that of P?Ca²⁺ and Cu²⁺, with increasing the concentration of Cd²⁺. Cd²⁺ exposure resulted in swelling of chloroplast, disruption and disappearance of chloroplast membrane and disintegration of chloroplasts, dilatation of cristae and vacuolization of mitochondria, which suggested that Cd hastened the senescence of the tested plants. (5) The effect of exogenous Spd on arginine decarboxylase (ADC), ornithine decarboxylase (ODC), PAO and DAO activities, the content of free Pro and PAs contents in Hydrocharis dubia leaves under Cd²⁺ toxicity was studied after 6 days treatment. The results showed that Cd stress increased Put level and lowered Spd and Spm levels. PS-conjugated and PS-bound PAs changed in the same pattern as free PAs. In addition, the activities of ADC, DAO and PAO were increased, while that of ODC was decreased. Exogenous application of Spd markedly reversed these Cd-induced effects, the activities of ADC, DAO and PAO were significantly decreased, however, ODC activity was found to increase slightly in response to Spd. It also significantly retarded the loss of Pro and prevented lipid peroxidation. These results suggest that exogenous Spd can enhance the tolerance of H.dubia to Cd²⁺.(6) The toxic effect of various concentrations of Cd²⁺(0,5,25,50,100?M) was studied on the contents of MDA, soluble protein, mineral nutrient and PAs, the activities of protective enzyme as well as the generation of O2·- in leaves of Sagittarai trifolia. The results showed that the content of soluble protein and the activities of SOD, POD and CAT decreased, with increasing the concentration of Cd²⁺. The generation of O2·- first increased and then decreased, but MDA content always increased. The results also showed that:the mineral nutrient absorption was affected by Cd²⁺ stress, it reduced the absorption of K+?P?Na+?Mg²⁺?Cu²⁺ and Zn²⁺, with increasing the concentration of Cd²⁺. In addition, Cd²⁺ treatment decreased free Put content, while increasing free Spd and Spm contents. PS-conjugated and PIS-bound PAs also changed in diffferent patterns. PS-conjugated Spd first increased and then decreased, and reached the peak value at 5?M Cd²⁺ concentration, and PS-conjugated Put and Spm contents decreased to a lesser extent; PIS-bound Put and Spm contents enhanced with the increase of Cd²⁺, and PIS-bound Spd content decreased to a lesser extent. The significant increased of PIS-bound Put was important for plant metal tolerance in S. trifolia.