www.czu.cz ; www.raupp.info

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; July 2004.


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.

The research by William Lucas and colleagues was supported by the National
Science Foundation and the Department of Energy, Division of Energy
Biosciences.

The research by Ramanjulu Sunkar and Jian-Kang Zhu was supported by National
Institutes of Health and the National Science Foundation.

You can read the full report News and Reviews article on this research in
The Plant Cell at [www.plantcell.org].

The Plant Cell (http://www.plantcell.org) is published by the American
Society of Plant Biologists. News and Reviews articles are available without
subscription. For more information about ASPB, please visit
[www.aspb.org].

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