MicroRNAs (miRNAs) are a class of 22 nucleotide non-coding regulatory RNAs which are universal in eukaryotes. They act as negative regulators of gene expression either by targeting mRNAs for cleavage or by translational repression serving as guides in silencing complexes. They participate in regulation of gene expression associated with plant developmental processes like organ morphogenesis and signal transduction pathway. There are more than 872 miRNAs, belonging to 42 families, reported from 71 plant species identified by genetic screening, direct cloning, computational strategy and EST analysis. The paper reviews the present status of miRNA-mediated regulation in plant developmental processes.

MicroRNAs (miRNAs) are small (approximately 21-24 nucleotides), single-stranded RNAs that act as negative regulators of target gene expression at the post-transcriptional level (Bartel, 2004). They play a critical role in a wide range of developmental processes, including plant development, hormone signaling (Palatnik et al., 2003), maintenance of homeostasis, and responses to environmental and nutritional signals (Jones-Rhoades et al., 2006).

The discovery of specific cis-acting modifications and transacting proteins in miRNA pathway which affects miRNA half-life plays a vital role in developmental regulation. Maturation steps specific to individual miRNAs are yet to be discovered which reveal an abundance of regulatory options that affects miRNA processing efficiency. Although their mode of action has gained much importance, the principles governing their expression and activity are only beginning to be addressed.

Research on miRNA target genes has progressed at a very rapid rate in plants. miRNAs participate in diverse functions, including transcription, catalysis and binding and transporter activity (Archak and Nagaraju, 2007). There are more than 872 miRNAs, belonging to 42 families, reported from 71 plant species by genetic screening, direct cloning after isolation of small RNAs, computational strategy and Expressed Sequence Tag (EST) analysis and the number is constantly increasing (Zhang et al., 2006). Research on the mechanism and consequences of miRNA action have been reported in recent years. With many whole genome sequences available in plants, a plant miRNA database (PMRD) was developed which integrated available plant miRNA data deposited in public databases, collected from the recent literature and data generated in-house. This database contained sequence information, secondary structure, target genes, expression profiles and a genome browser for each miRNA (http://bioinformatics.cau.edu.cn/PMRD). In total, there are 8,433 miRNAs collected from 121 plant species in PMRD, including model plants and major crops such as rice, wheat, soybean, maize, sorghum, barley, capsicum etc. (Zhang et al., 2010).

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