New work from Javier Palatnik and colleagues provides the first direct evidence that microRNAs (miRNAs) control aspects of plant development by guiding the cleavage of mRNAs.

Like small interfering RNAs (siRNAs), which are the effectors of RNA interference, miRNAs can bind to complementary mRNAs and silence the genes that encode them. Despite evidence that they can silence plant genes, previously there was no support for any specific biological function for miRNA silencing.

In an Arabidopsis developmental mutant (jaw-D) that has uneven leaf shape and curvature in adult plants, Palatnik and colleagues identified five TCP genes that were downregulated, which were closely related to a snapdragon gene (CIN) that is involved in leaf morphogenesis. In all five TCP genes, the authors found a motif that was partially complementary to a previously identified plant miRNA and a sequence at the JAW locus.

By hybridizing a probe that was based on the JAW sequence to a low molecular-mass RNA blot, the authors showed that it encoded a 20-nucleotide miRNA (miR-JAW). To try to understand the interaction between miR-JAW and the TCP genes, the authors engineered miRNA-resistantforms of TCP4. Constructs with mutant TCP4 produced stable transcripts that were unaffected by the co-injection of a JAW construct into a heterologous host, unlike wild-type TCP4 constructs, which led to an accumulation ofcleavage products.

The same mutations introduced into TCP4 in situ resulted in a range of developmental defects, most of which led to growth arrest at the seedling stage. This clearly showed that miRNA-guided cleavage of TCP4 is essential for normal plant development.

Neatly completing the circle, the authors then showed that miRNA-resistant TCP2 could rescue the jaw-D phenotype. So, the production of TCP2 transcripts that are resistant to miRNA-mediated degradation might correct aberrant phenotypes that are associated with lower expression levels of the gene.

As miRNA target sequences were found in TCP genes from plants with a range of leaf forms, including lotus and maize, it seems that these micro-masters might have a ubiquitous role in controlling leaf development. Perhaps the time is now ripe for a wider search for such motifs to help identify other developmental pathways that could be influenced by miRNAs.