Abstract
In plants, phase change from the juvenile stage to maturity involves physiological and anatomical changes, which are initiated and controlled by evolutionary highly conserved microRNAs. This process is of particular significance for the in vitro propagation of woody plant species, as individuals or tissues that have undergone the transition to vegetative maturity are recalcitrant to propagation. Conserved miRNAs differentially expressed between juvenile (including rejuvenated) and mature silver birch tissues were identified using high-throughput sequencing of small RNA libraries. Expression of some miR156 isoforms was high in juvenile tissues and has been previously reported to regulate phase transitions in a range of species. Additional miRNAs, such as miR394 and miR396, that were previously reported to be highly expressed in juvenile woody plant tissues were also differentially expressed in this study. However, expression of miR172, previously reported to be highly expressed in mature tissues, was low in all sample types in this study. The obtained results will provide insight for further investigation of the molecular mechanisms regulating vegetative phase change in silver birch and other perennial woody plant species, by analysing a wider range of genotypes, tissue types and maturation stages. This knowledge can potentially assist in identification of rejuvenated material at an earlier stage than currently possible, increasing the efficiency of silver birch in vitro propagation.