Small RNAs and developmental timing in plants.

MicroRNAs (miRNAs) were originally discovered as regulators of developmental timing in C. elegans. Recent results have revealed that miRNAs also regulate developmental timing in plants, and have provided a long-awaited molecular connection between the juvenile-to-adult transition and flowering. Specifically, the transition from juvenile to adult development in flowering plants is regulated by two temporally expressed miRNAs, miR156 and miR172. These miRNAs target two families of plant-specific transcription factors (respectively, SBP-box and AP2-like factors) that cooperate to regulate phase-specific vegetative traits, as well as genes involved in flowering. Small RNAs have also been shown to play a role in the transition between different stages of gametophyte development in the moss Physcomitrella patens. The use of small RNAs for temporal regulation is therefore quite ancient in plants.