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Abstract:
MicroRNAs are ubiquitous in plants, as they are in other eukaryotes. Animal
microRNAs, like the canonical let-7 and lin-4 from C. elegans, bind to
imperfect matching sequences in the 3’UTR of target mRNAs rendering in
translation attenuation. However, evidence has accumulated suggesting
that some plant microRNAs can guide cleavage of their targets, in a manner
similar or identical to RNAi.
We identified the JAW locus using an activation tagging screen in Arabidopsis
thaliana (random insertion of viral enhancers). A microarray survey identified
5 TCP transcription factors downregulated in the mutant. These TCPs
share an almost invariant 20 bases motif in their RNA. We showed that JAW
encodes a microRNA (miR-JAW/miR-319a) that targets the TCPs through this
conserved box. In the jaw-D mutant the expression of miR-JAW is increased
around 50 times, being also broader its expression domain. In vitro and in
vivo evidence demonstrate that miR-JAW is able to direct the cleavage of the
target mRNAs and fragmentation products can be detected in plants, being
mapped the cleavage site to the middle of the miRNA matching sequence.
Mutated versions of the TCPs were prepared in which the miRNA target
sequence was modified, but not the encoded amino acids. MicroRNA-guided
cleavage is necessary to prevent aberrant activity of the TCP4 gene
expressed from its native promoter, which unchecked leads to embryo-patterning
defects.
In addition, overexpression of wild-type and microRNA-resistant TCP variants
demonstrate that mRNA cleavage is largely sufficient to restrict TCP function
to its normal domain of activity. JAW and its target sequences are found in
a wide range of species, indicating that microRNA-mediated control of leaf
morphogenesis is conserved in plants with very different leaf forms.
miR-JAW mature sequence has similarity to five other miRNAs from Arabidopsis:
miR-319b, miR-319c, miR-159a, miR-159b and miR159c, that are
predicted to target members of the TCP and Myb families of transcription
factors. Overexpression of miR-159a or miR-159b caused stamen defects
and sterility. An analysis of the system will be presented.
