Plant miRNAs differ from animal miRNAs in that many plant miRNAs have perfect homology to their target mRNAs, and they act through the RNAi pathway to cause mRNA degradation (Rhoades 2002).
It is likely, however, that some plant miRNAs base-pair imperfectly with their miRNA target sites and act via a pathway similar to animal miRNAs. In plants and yeast there is also evidence that miRNAs are involved in repression of transcription by guiding chromatin methylation.
A key observation made by two laboratories is that mRNAs containing multiple, non-overlapping miRNA binding sites are more responsive to miRNA-induced translational repression than those containing a single miRNA binding site (Doench 2003, Zeng 2003). Furthermore, comparisons of repression by miRNAs bound to 2, 4, and 6 binding sites on a reporter construct indicate that translation decreases with each additional site (Zeng 2003).
This suggests that the expression of miRNA target genes can be fine-tuned in animals (and potentially plants) by altering the concentrations or identities of miRNAs within cells. This observation coupled with the predictions that many mRNAs have target sites for many different miRNAs suggests that gene expression in various tissues and cells can be greatly influenced by the miRNA populations in those cells.
This could also explain why at least some miRNAs have such broad functionality, and conversely why translational control of some genes is so complex. If miRNAs indeed regulate the translation of, but not the stability of target mRNAs, this might at least partially explain why gene expression profiles based on mRNA analysis do not always correlate with protein expression data (Kern 2003).
Getting started with MicroRNA research
Isolate and quantitate miRNA from plants
miRNA research guide