microRNAs; master regulators of gene activity and development.

School of Biology

It was once thought that all the important regulatory functions of higher organisms were performed by proteins, and that the DNA between protein-coding genes was nothing other than junk DNA. As it turns out there are many important genes in this so-called "junk", where they code for RNA structures (see Figure A) that are processed into small 21 nucleotide molecules (Fig B) known as microRNAs (miRNAs).

With our CSIRO colleagues, we have isolated a mutant in which the model plant Arabidopsis fails to produce one type of miRNA known as miR159. This has dramatic consequences for the plant's development, where the miRNA mutant develops curled leaves, smaller fruits and smaller seeds. These traits are a consequence of the de-regulation of a family of genes that code for transcription factors known as GAMYBs, where each family member has a sequence to which miR159 can bind to and then destroy the gene's transcript (mRNA). This regulation is shown with a reporter gene (gives a blue colour) that has been joined to a GAMYB gene; in the miRNA mutant there is strong activity (indicating loss of regulation), whereas in the normal wild-type plant no activity can be detected, indicating the gene is being naturally "silenced". Correlating this gene activity with the mutant traits demonstrates that this natural gene silencing mechanism is critical for proper plant development. Understanding this recently discovered form of gene regulation may provide insights on how to manipulate leaf shape, fruit and seed size, all extremely important agronomic traits.

Allen, RS, Li J, Stahle MI, Dubroué A, Gubler F, Millar AA (2007) Genetic analysis demonstrates functional redundancy and the major target genes of the Arabidopsis miR159 family. Proceedings of the National Academy of Sciences, USA (in press).