Differential and dynamic regulation of miR398 in response to ABA and salt stress in Populus tremula and Arabidopsis thaliana.

MicroRNAs (miRNAs) are endogenous small RNAs of ~22 nucleotides (nt) that play a key role in down regulation of gene expression at the post-transcriptional level in plants and animals. Various studies have identified numerous miRNAs that were either up regulated or down regulated upon stress treatment. Here, we sought to understand the temporal regulation of miRNAs in different plant species under abscisic acid (ABA) and salt (NaCl) stress. Our results showed that the regulation of miR398 in response to ABA and salt stress was more dynamic in plants than previously reported. In poplars, miR398 was first induced upon 3-4 h of ABA or salt stress. However, this induction declined after 48 h and finally accumulated again over a prolonged stress (72 h). We referred to this kind of regulation as dynamic regulation. In contrast, such dynamic regulation of miR398 under salt stress was completely absent in Arabidopsis, in which miR398 was steadily and unidirectionally suppressed. Interestingly, ABA treatment caused a deviate dynamic regulation of miR398 in Arabidopsis, showing an opposite response as compared to that in poplars. We referred to the difference in regulation between Arabidopsis and poplars as differential regulation. Furthermore, the expression of the miR398 target, copper superoxide dismutase1 (CSD1), was in reverse correlation with the miR398 level, suggesting a control of this specific target expression predominantly by miR398 under abiotic stress. Together, these data consistently show a correlated regulation between miR398 and its representative target, CSD1, by ABA and salt stresses, and raise the possibility that regulation of miRNAs in plants is twofold: a dynamic regulation within a plant species and a differential regulation between different plant species.