Mechanisms Of Salicylic Acid-mediated Alleviation Of Cadmium Toxicity In Rice (Oryza Sativa)

A series of hydroponic experiments were conducted to study the effect of pretreatmentwith salicylic acid (SA) on the changes in germination, growth, ions uptake, photosynthesis,antioxidant system, Cd distribution in root cells of rice (Oryza sativa cv. Jiahua 1) undercadmium (Cd) stress. The effects of SA were also explored on the response of rice to Cdexposure in split-root system. The results obtained are presented as follows.1. Effect of presoaking SA on germination and growth of rice under Cd stressPretreatment of seeds with 0.5 mM SA or treatment with 25μM Cd alone significantlyincreased germination rate and germination tendency of rice at initial stages, whilepretreatment with 1.0 mM SA or treatment with 50μM Cd alone significantly decreasedthe germination rate. Pretreatment with 0.5 mM SA alleviated the growth inhibition in riceseedlings subject to 50μM Cd stress, while pretreatment with either 0.5 mM or 1.0 mM SAdid not affect the rice growth under different Cd stresses in the tillering stage.2. Effect of pretreatment SA on ions uptake by rice under Cd stressPretreatment with 10μM SA mitigated the growth inhibitions in rice shoot under 50μM Cd stress, while pretreatment with 100μM SA further inhibited the root growth. Cdtreatment significantly decreased both K and P concentrations in rice root, but did not affectCa and S concentrations both in roots and in shoots. Under Cd stress, 10μM SA increasedP and S concentrations in roots, but decreased K concentration in roots. Pretreatment with10μM SA significantly decreased the concentration of K in shoots and Ca in roots. SAtreatment at two levels failed to reduce the Cd concentration both in roots and in shoots,suggestingt that the alleviating effect on Cd-induced growth inhibition can be contributed toSA-enhanced tolerance of rice to Cd toxicity.3. Effect of pretreatment SA Cd distribution in cells of rice rootsCd treatment decreased root biomass by 40% compared with the control andpretreatment with SA significantly mitigated the Cd-induced inhibition of root growth.There was no significant difference in root cell wall composition or lignification betweenthe treatment with Cd alone and the treatment with Cd with SA. No effects were observed of SA pretreatment on the activities of phenylalanine ammonia-lyase (PAL), peroxidase(POD) or polyphenol oxidase (PPO) either compared with Cd treatment alone. Furthermore,soluble Cd concentration in root cells was significantly higher in the treatment with Cd withSA than in the treatment with Cd alone. However, H₂O₂ and MDA concentrations in riceroots were significantly lower but non-protein thiols (NPT) levels were higher in thetreatment with Cd with SA than in the treatment with Cd alone, indicating that SAalleviated Cd-induced oxidative damage. It seems to suggest that SA-mediatedenhancement of Cd tolerance was not due to enhanced Cd retention in cell wall but toenhanced Cd bindings with SH group.4. Role of SA in alleviating oxidative damage in rice roots subjected to Cd stressExposure to 50μM Cd resulted in a significant decrease in root growth, activities ofsuperoxide dismutase (SOD), catalase (CAT) and peroxidase (POD), and increase inconcentrations of H₂O₂, malondialdehyde (MDA), ascorbic acid (AsA), glutathione (GSH)and NPT in rice roots during the 6-day experimental period, indicating Cd-inducedoxidative stress. However, pretreatment with 10μM SA for 24 h alleviated the inhibition ofroot growth by Cd stress and significantly enhanced the activities of the antioxidantenzymes (POD on Day 2; SOD and CAT throughout the experimental time) and theconcentrations of AsA, GSH, and NPT, but lowered the concentrations of H₂O₂ and MDAin Cd-stressed rice roots compared with the Cd treatments without SA pretreatment.5. Effect of SA on the photosynthesis of rice under Cd stressCd exposure to rice decreased the net photosynthetic rate (Pn), stomatal conductance(Gs), transpiration rate (Tr), chlorophyll content and chl a/b compared with the control.Intercellular CO₂ concentration (Ci) was decreased on Day 1 under Cd stress, but thereverse was observed as the experiment continued. Pretreatment with SA mitigated thedecreases induced by Cd stress in net photosynthetic rate (Pn), stomatal conductance (Gs),transpiration rate (Tr), chlorophyll content and chl a/b.6. Effect of SA on the response of rice to Cd stress in a split-root systemExposure of half of the root system to Cd led to higher levels of H₂O₂, more cell deathand growth inhibition than in the treatment in which the whole root system was exposed toCd. By contrast, the growth of the non-stressed root part was stimulated. Exposure of thehalf root system to Cd increased Cd uptake with higher transpiration compared with thewhole-root exposure treatment. Cd was translocated from the exposed root part to theunexposed root part. Exposure to Cd resulted in an inhibition of the activities of antioxidant enzymes (SOD, POD and CAT) in the treated root part, while the enzyme activities werestimulated in the untreated root part. The concentrations of GSH and NPT in roots andshoots were also increased by Cd exposure. Pretreatment with SA alleviated the growthinhibition by Cd both in roots and in shoots, increased transpiration compared with thenon-SA pretreatment under Cd exposure. The alleviative effect by SA appeared to be due toan enhanced antioxidative capacity to cope with Cd-induced oxidative stress, and toincreased NPT, which would enhance Cd detoxification by chelation.