Cross-talk between freezing response and signaling for regulatory transcriptions of MIR475b and its targets by miR475b promoter in Populus suaveolens

MicroRNAs (miRNAs) are small, non-coding RNAs that play important roles in post-transcriptional regulation of their target genes, yet the transcriptional regulation of plant miRNAs by promoter is poorly understood. Here, we firstly clone pri-miR475b cDNA and its native promoter from P. suaveolens, and characterize Psu-MIR475b as class-II gene transcribed by RNA polymerase II. By 5′ deletion analysis of Psu-miR475b promoter in a series of promoter-GUS chimeric vectors, we functionally identify three positive regulatory regions and multiple cis-acting elements responsible for Psu-miR475b promoter activity in response to freezing stress and exogenous hormone treatment. Moreover, the Psu-miR475b promoter activity displays a tissue-specific manner, negatively regulated by freezing stress and positively by MeJA, SA or GA treatment. Importantly, we comparatively analyze the time-course transcriptional profiles of Psu-miR475b and its targets in Psu-miR475b over-expression transgenic plants controlled by Psu-miR475b-specific promoter or CaMV 35S constitutive promoter, and explore the regulatory mechanism of Psu-miR475b promoter controlling transcriptional expressions of Psu-MIR475b and its targets in response to freezing stress and exogenous hormone treatment. Our results reveal that Psu-miR475b promoter-mediated transcriptions of Psu-MIR475b and its targets in response to freezing stress may be involved in a cross-talk between freezing response and stress signaling process.