At the Max Planck Institute for Developmental Biology in Tubingen, developmental biologists have discovered that the regulatory mechanism that switches Arabidopsis to flower depends on a group of related regulators, the SPL proteins. Its production on the other hand is triggered with the decrease in concentration of microRNA in the shoot apex of flowering plants. This activation process occurs even in the absence of other regulators that measure daylength or external temperature, which may allow a sufficiently old plant to flower even in an unfavorable environment.

MicroRNAs are very short RNA sequences that function as essential regulators in the proper functioning of genes in plants and animals. By binding to the complementary motifs in a messenger RNA (coding for amino acids), they inhibit its translation to protein and the consequent inactivity of the corresponding gene. As the microRNA concentration is lowered, plants can synthesize the corresponding proteins needed for flowering. 

See the report at http://www.mpg.de/english/illustrationsDocumentation/documentation/pressReleases/2009/
pressRelease20090817/index.html.