Discovery and functional association of microR-NAs (miRNAs) have led to a large new research area in the previously unsuspected world of non-coding RNAs (Lee et al. 1993; Reinhart et al. 2002). The miRNAs are endogenous, small 21-24 nucleotide, single stranded, non-protein coding RNAs that have recently emerged as important regulators of gene expression (Bartel 2004).
These regulate target gene expression by catalyzing posttranscriptional gene silencing (Palatnik et al. 2003) or translation repression (Chen 2004) . Targets of miRNA comprises transcription factors or other regulatory proteins that function in plant development or signal transduction. Recently, research on micro-RNAs (miRNAs) have suggested an association between miRNAs and plant stress responses (Patade and Suprasanna 2010). However, the relationship between micro-RNAs and stress response is just beginning to be explored. Several miRNAs are either up- or down-regulated by abiotic stresses, suggesting to be involved in stress-responsive gene expression and stress adaptation affecting a variety of cellular and physiological processes (Sunkar and Zhu 2004; Shukla et al. 2008).
Sunkar and Zhu ( 2004) identified novel and abiotic stress-regulated miRNAs and reported differential expression of some of the identified miR-NAs in Arabidopsis seedlings exposed to dehydration, salinity, or cold stress. In order to unravel function of microRNA, Zhao et al. (2007) studied transcript expression profiles of miRNAs in rice (O. sativa) under drought stress. The drought-induced expression of miR-169g and miR393 was validated by microarray expression profiling and confirmed greater expression of miR-169g in roots rather than shoots. Sequence analysis revealed occurrence of two proximate DREs (dehydration-responsive element) in the upstream of the MiR-169g, suggesting transcript expression regulation of miR-169g by CBF/DREBs.
Sunkar et al. (2006) provided evidence on involvement of miRNA in oxidative stress responses by targeting cytosolic and chloroplas-tic superoxide dismutases that detoxify superoxide radicals. Transcript expression of miR398 in response to oxidative stress was down-regulated, leading to posttranscriptional accumulation of the SOD mRNA and thus oxidative stress tolerance. Moreover, transgenic Arabidopsis plants overexpressing a miR398-resistant form of SOD accumulated more mRNA than plants overex-pressing a regular form and were consequently much more tolerant to high light, heavy metals and other oxidative stresses. Arabidopsis have been shown to trigger the accumulation of miR159 in response to ABA, drought stress, and gibberellic acid (GA) treatment and the miRNA was predicted to target four MYB transcription factors (Reyes and Chua 2007) . Recently, Patade and Suprasanna )2010) characterized transcript expression of mature miR159 in response to short- and long-term salt and PEG-induced osmotic stress in sugarcane. A change in mature transcript levels of miR159 was not detected in response to long-term (15 days) NaCl or iso-osmotic (-0.7 MPa) PEG stress. However, short-term (up to 24 h) salt or PEG stresses increased transcript level of the mature miRNA as compared to the control. The early induction of the gene under the short treatments supports its involvement in the regulation of genes involved in stress perception and/or signalling.
Zhou et al. (2008) developed a computational transcriptome-based approach to annotate stress-inducible miRNAs in plants. Interestingly, the promoter analysis of the miRNA genes revealed the presence of many known stress-responsive cis-regulatory elements. Continued efforts are needed to identify the complete set of miRNAs and other small RNAs that are fundamental to the stress regulation pathways. The identification and functional validation of stress-regulated small RNAs including miRNAs will help in designing new strategies for improving stress tolerance (Sunkar et al. 2006; Katiyar-Agarwal et al. 2007).
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