FIGURE 2.

Mechanisms driving heterochrony in plants. (A) Cellular heterochrony. The sequence of rosette leaves produced during vegetative development is shown. The phenotype of Arabidopsis Columbia wild type plants is taken as a hypothetical ancestral state; a paedomorphic scenario (neoteny) and a peramorphic scenario (acceleration) are also shown. Arrows represent the direction of the wave of the cell division arrest in a basipetal gradient, where cells below the dotted line are still actively dividing whereas the cells above are expanding and/or differentiating. (B) Transcriptional and metabolic heterochrony. A graphic representation of the predicted abundance of the microRNA miR156/7 and its targets, the SPL genes, as well as sugar abundance along the time of vegetative development, corresponding to the phenotypes in (A). Dotted lines represent the threshold of miR156/7 and SPL abundance which leads to juvenility (above the threshold) or adulthood (below the threshold). In this case, a delay in the repression of miR156 and activation of SPL expression and sugar production results in paedomorphosis, whereas a precocious decay of miR156 and early activation of SPL expression and sugar production results in peramorphosis. These drawings are simplified representations, and do not reflect the exact abundances in nature. The actual pattern of miR156/SPL abundance is closer to that depicted for “peramorphosis” state.