It is becoming increasingly clear that microRNA are important regulators of gene expression within the animal kingdom. However, microRNA are also found in plants, behaving more like small inhibiting RNA (siRNA) during target gene knockdown. A recent review published in Genome Biology aims to discuss the differences between animal and plant microRNA and highlights the important role of each within the two kingdoms. Axtell et al. serves to showcase the important similarities and differences between microRNA in separate kingdoms and uses the model plant organism Arabidopsis thaliana as an example of classic plant microRNA function. In plants, microRNA are transcribed by RNA polymerase II as in animals, but the entire process of microRNA biogenesis is undertaken within the plant nucleus. The mature microRNA are exported out of the nucleus by Hasty, an exportin 5-like protein found in plants. A major difference between plant and animal microRNA falls within target recognition. Axtell et al. reviews the target recognition process between plants and animals; notably the direct mRNA cleavage of a microRNA target in plants due to near-perfect base complementation between the microRNA and its target. This differs vastly in animals where protein repression is believed to occur by translation inhibition as well as mRNA degradation. Hybridization of microRNA to target in animals is less stringent near the 3’ end of the microRNA strand and relies on the canonical 7-8 nucleotide “seed sequence” to drive microRNA target recognition.
After highlighting the similarities and differences between plants and animals, the review dives into some evolutionally perspectives and driving factors of microRNA evolution in plants and animals. Interestingly, Axtell et al. discusses events that lead to the emergence of new microRNA genes in plants and animals. Briefly, it is more common in plants for microRNA genes to emergence via mechanisms of inverted duplication events, where as in animals it is more common for microRNA hairpins to evolve from mutational events in “unstructured” sequences of the genome. These evolutionary driving factors and mechanisms for newly acquired microRNA genes can perhaps help researchers identify novel microRNA targets within gene loci of interest. Even though most research in microRNA regulation of target genes is primarily focused on animal gene regulation, and specifically within human disease states, acknowledging the breadth and scope of microRNA regulation across kingdoms may provide useful insights into microRNA research.
Axtell, MJ., et al.
Vive la difference: biogenesis and evolution of microRNAs in plants and animals.
Genome Biology. 12(2011): p. 221-234.