Over-expression of microRNA171 affects phase transitions and floral meristem determinancy in barley

To investigate the roles of mir171 and its target genes in a monocot, the Hvu pri-miR171a was over-expressed in barley (Hordeum vulgare L. cv. Golden promise) leading to reduced expression of at least one HvSCL gene. The resulting transgenic plants displayed a pleiotropic phenotype which included branching defects, an increased number of short vegetative phytomers and late flowering. These phenotypes appear to be the consequence of changes in the organisation of the shoot meristem. In addition, the data show that miR171 over-expression alters the vegetative to reproductive phase transition by activating the miR156 pathway and repressing the expression of the TRD (THIRD OUTER GLUME) and HvPLA1 (Plastochron1) genes.Our data suggest that some of the roles of miR171 and its target genes that have been determined in Arabidopsis are conserved in barley and that they have additional functions in barley including activation of the miR156 pathway.During their life cycle, flowering plants undergo three developmental phases, the juvenile and adult vegetative stages and a reproductive phase. During the last decade it has become clear that microRNAs (miRNAs) are important regulators of transitions between these phases. miRNAs are small regulatory RNA molecules which trigger the post-transcriptional repression of target genes through a base-pairing mechanism [1]. There are some 20 miRNA families that are highly conserved in flowering plants. Many of these conserved miRNA families control crucial developmental processes through the down-regulation of conserved transcription factor-encoding genes.The miRNAs with the best-defined roles in regulating phase changes in the shoot meristem are miR156 and miR172 which form a regulatory module that is widely conserved in plants. In Arabidopsis, rice and maize, miR156 regulates shoot branching, leaf initiation and juvenile-to-adult phase transition through the down-regulation of several SPL genes [2-9]. In grass plants, over-expression of m