MicroRNAs (miRNAs) are endogenous, single-stranded, small (~21-23nt), non-coding, regulatory RNA molecules that manipulate messenger RNA (mRNA) degradation and translation repression by binding complementary sites in the protein-encoding, or 3’-untranslated regions, of target mRNAs
[1–3]. Contemporary research has revealed numerous miRNAs in diverse plant species, such as Arabidopsis thaliana (mouse-ear cress), Oryza sativa (Asian rice), Chlamydomonas reinhardtii (flagellated green algae), and Populus euphratica (subtropical poplar tree)
[4–7]. An abundance of recent evidence suggests that plant miRNAs are critical to several essential biological processes in plant physiology, such as floral organ identity, leaf morphogenesis, cellular signaling, and stress responses
[8–11]. Because of the diverse and important functions of miRNAs in plants, the dysfunction, mutation, and dysregulation of specific miRNAs has the potential to negatively influence plant growth and productivity.
Initially discovered as regulators of developmental timing in Caenorbabditis elegans (transparent nematode), miRNAs have already been demonstrated to play important regulatory roles, particularly in the complex network of gene regulatory pathways. During abiotic stress to plants, such as drought, salinity, wounding, and high temperature, miRNAs act at the posttranscriptional level in gene regulatory networks associated with stress adaptation and tolerance. Under adverse stress conditions, gene expression can be elaborately regulated to facilitate the action of these tolerance mechanisms
[12–14], providing further evidence of the importance of miRNAs in regulation of plant responses to abiotic stress conditions.
In the past decade, the regulatory mechanisms of miRNAs involved in plant stress responses have garnered increasing attention. For example, miRNAs have been identified to regulate plant response to drought
[15, 16], salinity
[17, 18], low temperature
[19, 20], and nutrient deficiency
[21–23]. More recently, a set of conserved and non-conserved miRNAs from Medicago truncatula (barrel clover)
, Oryza sativa (Asian rice)
, Brassica napus (rapeseed)
, and Arabidopsis thaliana (mouse-ear cress)
 that act to regulate plant response to heavy metal exposure, have been idenfified. These findings indicated that certain stress conditions stimulate specific plant species to produce miRNAs involved in species-specific regulatory processes that vary highly between different species, potentially accounting for differences in plant tolerances to specific stress conditions. Additionally, these results indicate that miRNAs could represent a discriminative evolutionary footprint associated with abiotic stress regulation, relationships that can only be determined by comprehensive organization of miRNA regulation data from many plant species.
To date, more than one thousand miRNAs have been discovered in diverse plant species
. Many of these miRNAs have confirmed involvement in regulatory networks associated with plant stress response. Despite notable recent progress in identifying miRNA regulation during stress conditions, detailed data of miRNA molecular regulation in different plant abiotic stresses, such as miRNA expression pattern and target genes of miRNAs, remains widely distributed in published literature. In the current stage, a cohesive database system is sorely needed for data deposit and further application. Therefore, development of the literature-curated database system “PASmiR” (miRNA molecular regulation in Plant Abiotic Stress) for the storage of regulatory relationships between miRNAs and plant abiotic stresses will be a valuable resource for contemporary researchers. While these immediate, tangible benefits are apparent, the PASmiR database may also serve as a valuable ongoing program for the study of miRNA regulatory mechanisms associated with plant responses to abiotic stress as additional information becomes available in this field.