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MicroRNAs are tiny ribonucleic acid (RNA) molecules (~22 nucleotides long)
that recently have been found to play important roles in regulating gene
expression in eukaryotic organisms, including plants and animals. Research
conducted by three independent groups and reported this month in The Plant
Cell shows that fundamental developmental processes controlled by the plant
hormone auxin are regulated by microRNAs in the model plant Arabidopsis
thaliana. This work provides an important contribution to our understanding
of plant development, May 2005.


The plant hormone indole-3-acetic acid (IAA), commonly referred to as
auxin, plays a major role in regulating plant growth and development. Auxin
influences development by affecting the expression of numerous genes that
control the processes of cell division and cell expansion in specific plant
tissues at specific stages during the plant life cycle - e.g. for leaves,
roots, and floral organs to develop in the correct patterns and correct time
sequence. Research reported this month in The Plant Cell shows that
microRNAs control the accumulation of transcription factor proteins that
regulate the expression of genes in the auxin response pathway.

Messenger RNA (mRNA) molecules are encoded by genes and are themselves
templates for the proteins that carry the main metabolic functions in a
cell. The mRNA levels in a cell are fine tuned by different mechanisms, one
of which is driven by microRNA molecules. MicroRNAs are ~22 nucleotide long
RNA molecules that provide substrate specificity to a protein complex known
as the RNA-induced silencing complex. Within the complex, microRNAs are
thought to bind to mRNA molecules containing a complementary stretch of RNA
sequence. The complex then cleaves the mRNA into smaller pieces, thereby
preventing translation of the protein it encodes, and thus inhibiting or
"silencing" gene expression. mRNAs corresponding to several regulatory genes
that mediate auxin responses contain short stretches of sequence that are
complementary to microRNAs, and therefore have been considered potential
targets of microRNA-mediated regulation. One of these targets is the
transcription factor AUXIN RESPONSE FACTOR17 (ARF17), which is thought to
repress the expression of a number of other genes involved in auxin
responses.

Dr. Bonnie Bartel at Rice University in Houston, TX together with Drs. David
Bartel and Allison Mallory at the Massachusetts Institute of Technology and
the Whitehead Institute for Biomedical Research in Cambridge, MA report
experiments using transgenic Arabidopsis plants that produce a version of
ARF17 mRNA that resists microRNA-mediated cleavage. The plants showed
increased accumulation of ARF17 mRNA and altered levels of mRNAs
corresponding to several genes that may be regulated by ARF17. These changes
were correlated with dramatic development defects in leaves, roots, and
flowers, showing that microRNA-mediated regulation of ARF17 is essential for
normal plant development.

Bonnie Bartel notes that "we have known for several years now that microRNAs
regulate genes implicated in auxin responses; with these three reports, we
are beginning to understand the consequences of this regulation for the
development of the plant." David Bartel adds, "We were particularly struck
by the unusual quadrilaterally symmetric seedlings we uncovered in our
study. This result implies that the Arabidopsis embryo relies on microRNA
restriction of ARF17 activity to achieve normal bilateral symmetry."

A second report focuses on the function of the protein ARGONAUTE1 (AGO1), a
major component of the RNA-induced silencing complex in Arabidopsis. There
are many AGO1-like proteins in animals and other eukaryotes as well,
indicating that the RNA-induced silencing complex is of ancient evolutionary
origin, and that microRNA-mediated regulation of gene expression is shared
among many eukaryotes. Arabidopsis ago1 mutants lacking the AGO1 protein
have numerous severe developmental defects, supporting the notion that
regulation by microRNAs is critical for normal plant growth. Dr. Catherine
Bellini at The Swedish University of Agricultural Sciences in Ume?, Sweden
and colleagues at several other institutions noticed that the ago1 mutant
failed to form adventitious roots - a type of root that develops from aerial
parts of the plant and is important for propagation through cuttings. Auxin
is known to be a major regulator of adventitious root formation and normal
Arabidopsis plants form multiple adventitious roots on the hypocotyl (stem
just above the root) when treated with auxin but the ago1 mutants do not.
Dr. Bellini and her colleagues found that the mutant plants over-accumulate
ARF17 mRNA within the hypocotyl, pointing to ARF17 as a major regulator of
adventitious rooting and microRNA-mediated regulation as a major regulator
of ARF17.

In a third report, Dr. Nam-Hai Chua of Rockefeller University in New York
and scientists at the Temasek Life Science Laboratories, Singapore, and the
Chinese Academy of Sciences in Beijing show that microRNA is important in
the regulation of a transcription factor that is induced by auxin, called
NAC1. NAC1 functions in other aspects of the auxin response, such as the
formation of lateral roots (roots that grow off the main tap root below
ground, as opposed to the adventitious roots that grow off of the stem or
other above-ground plant parts). Like ARF17, NAC1 mRNA contains a stretch of
sequence that is complementary to microRNAs, and thus shows potential for
microRNA-mediated regulation. In experiments analogous to those of Bartel's
group, Chua and colleagues created transgenic Arabidopsis plants that
produce a version of NAC1 that is resistant to microRNA-mediated cleavage.
Compared to wild-type plants, the transgenic plants overaccumulated NAC1
mRNA and produced more lateral roots. It was also found that accumulation of
the microRNA that targets NAC1 mRNA is induced by auxin. The pattern of
induction suggested a model wherein auxin induction of NAC1 mRNA is later
followed by induction of the microRNA responsible for NAC1 mRNA cleavage.
MicroRNA-mediated regulation of gene expression therefore appears to be an
important mechanism for fine-tuning auxin signaling during plant
development.

These reports provide significant new information on microRNA-mediated
regulation of plant development and show that microRNAs play important roles
in regulating the auxin response pathway.

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

The research papers cited in this report are available at the following
links:
[www.aspb.org]
[www.aspb.org]
[www.aspb.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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