Soluble silicon modulates expression of Arabidopsis thaliana genes involved in copper stress

Since soluble silicon (Si) has been shown to alleviate copper (Cu) toxicity in Arabidopsis thaliana, the expression of genes involved in responses to Cu toxicity was examined by quantitative reverse transcription-polymerase chain reaction. Expression levels of three metallothionein (MT) genes were increased under Cu stress conditions whereas Cu-stressed plants treated with Si either maintained high levels or contained even higher levels of MT RNA. Cu/zinc superoxide dismutase (SOD) enzyme activity was induced by Cu toxicity. However, SOD activity was increased even more if plants were provided with extra Si and toxic levels of Cu. Previously, plants treated with elevated Cu showed increased phenylalanine ammonia lyase (PAL) activity that was reduced when the plants were also provided with extra Si. Since the Arabidopsis genome encodes 4 PAL genes (PAL1-4), we examined which ones were responsive to Cu and Si. PAL 1, PAL 2, and PAL 3 all showed similar patterns of gene expression that matched previous enzymatic data while PAL4 was elevated by the presence of high Cu whether Si was present or not. Taken together, these data suggested that Si permitted plants to respond to Cu toxicity more effectively and that these changes occurred at the gene expression level.