Physiological and Molecular Studies on Silicon-Induced Cadmium Tolerance in Rice (Oryza sativa L.)
Plant productivity is seriously limited by cadmium (Cd) toxicity. Studies suggest that silicon (Si) plays a significant role in the alleviation of Cd-stress in plants. In this study, gas exchange, chlorophyll fluorescence, growth, tissue-element content, and proteomic analysis were performed on rice plants exposed to varying Si and Cd concentrations/durations towards elucidating the mechanisms involved in Si-induced Cd tolerance in hydroponically-grown rice (Oryza sativa L.) plants. The first part of this study (chapters 1 and 2) involved the investigation of the effects of different Si concentrations (0.0, 0.2, 0.6 mM) and time of Si addition on rice plants exposed to different Cd concentrations (0.0, 2.5 ?M, or 5.0 ?M) and time of Cd treatment. Our results showed that Si-induced Cd tolerance is mediated by a significant inhibition of Cd-uptake, regardless of time and duration of Cd/Si exposure. Additionally, our results suggest that a late addition of 0.6 mM Si is required for the alleviation of low-level (2.5 ?M) long-term Cd-mediated growth inhibition. Results also suggest that Si-induced increase in instantaneous water-use-efficiency is more significant in the alleviation of low-level long-term Cd-stress, while Si-induced increase in photosynthesis rate is more significant in the alleviation of moderate (5.0 ?M) level short-term Cd-stress. Furthermore, we suggest that 0.2 mM Si might be close to an optimum Si-dose requirement for the alleviation of Cd-stress in rice plants. Chlorophyll fluorescence results also provided the first real evidence that Si alleviates Cd toxicity by improving light-use-efficiency.
Studies have suggested that Si-induced Cd tolerance is mediated by a chelation mechanism. Thus, we investigated the effects of a synthetic chelator, EGTA, and Si on Cd-uptake, growth and photosynthesis of rice plants under different Cd levels and exposure durations (chapter 4). Our results suggest that EGTA enhances Si-induced Cd tolerance but the mechanisms employed by EGTA and Si towards systemic Cd-exclusion might differ. Furthermore, we suggest that in the absence of Cd, the combined effect of EGTA and Si inhibits growth and photosynthesis due to a proposed synergistic inhibition of nutrient-uptake. Finally, we combined physiological and proteomic approaches to identify the molecular mechanisms involved in Si induced Cd tolerance in rice plants (chapter 5). Our results showed that in unstressed plants, the addition of Si down-regulated five proteins including glycine dehydrogenase (decarboxylating) but in Cd-stress plants, Si up-regulated five proteins including class III peroxidase and RUBISCO. Furthermore, we observed that Si nutrition inhibited Cd-induced down-regulation of putative ferredoxin-NADP(H) oxidoreductase, a Zn-binding oxidoreductase and cysteine synthase. Taken together, in this dissertation, we proposes that Si-induced Cd tolerance in plants is a holistic process involving physiological and biochemical mechanisms, including Cd-uptake inhibition, enhancement of photosynthetic efficiency and modulation of protein expression.
School Location:USA - Ohio
Source Type:Master's Thesis
Keywords:cadmium stress chlorophyll fluorescence egta photosynthesis proteins silicon stomatal conductance
Date of Publication:01/01/2008