Molecular biology experienced a significant shift in thinking in recent years with growing evidence that microRNAs (miRNAs) play a major role in the control of eukaryotic gene expression during development. These tiny RNAs, which have sequence complimentarity to short segments of protein-coding genes, are encoded in regions of the genome distinct from other recognized genes. miRNAs function similarly to small interfering RNAs (siRNAs) which are equivalently sized, tiny RNAs that target viral genomes, transposons, and other foreign or aberrant RNA molecules for destruction. siRNAs have been studied for a number of years in plants since their discovery as the causal agent of RNA silencing, which is believed to represent a natural defense mechanism against viral infection and the activity of transposable elements. siRNAs typically target foreign and potentially pathogenic RNAs for destruction, whereas miRNAs target endogenous messenger RNAs for regulation. miRNAs often show a high degree of evolutionary conservation across species, and there is growing evidence that they represent a major class of regulatory molecule having broad significance in a wide range of developmental processes in plants and animals. miRNAs have been found to play specific roles in plant development, including the regulation of flowering time and floral organ identity, and leaf polarity and morphology.
Researchers William J. Lucas at the University of California, Davis, and Tony J. Lough at AgriGenesis Biosciences, Auckland, New Zealand, and their coauthors, show that small RNA corresponding to authentic siRNAs and miRNAs can enter and move through the phloem of several plant species. Furthermore, these authors identify a novel protein, Cucurbita maxima PHLOEM SMALL RNA-BINDING PROTEIN1 (CmPSRP1), and show that it likely plays a role in trafficking of small RNA through the phloem. The research is significant because small RNAs have not previously been observed in the phloem. In plants, the long-distance transport of protein and RNA through the phloem translocation stream plays a critical role in non-cell-autonomous signaling that contributes significantly to plant development. The systemic spread of RNA silencing via a phloem-transmissible signal is a well-characterized phenomenon that can be readily observed following localized viral infection or with hetero-grafting experiments. Small RNAs have been viewed as likely candidates for the systemic silencing signal, but there has been no direct evidence of their transport through the phloem and the nature of the mobile silencing signal has remained elusive. The extent to which miRNA function requires long-distance transport through the phloem is unknown and has not been explored previously.
In another report, researchers Ramanjulu Sunkar and Jian-Kang Zhu at the University of California, Riverside describe a new library of small RNAs isolated from Arabidopsis seedlings exposed to dehydration, salinity, cold stress, or the plant stress hormone abscisic acid. They identify 15 new miRNA families which include 26 new miRNAs corresponding to 34 loci, and show that some of these miRNAs are expressed in specific tissues and several are up- or down-regulated in response to abiotic stress. This report doubles the number of known miRNA families in Arabidopsis and identifies a number of new potential miRNA target genes, and further suggests that some miRNAs might be associated with regulation of gene expression in response to stress.
Together, these two reports open new avenues of research into small regulatory RNAs, and propel us several steps forward in understanding the biological functions and mechanism of action of this fascinating and important class of regulatory molecule.
Source: Eurekalert & othersLast reviewed: By John M. Grohol, Psy.D. on 21 Feb 2009
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