Model for FMB regulation by GUN1-dependent retrograde signal.

Most leaf cells express FIL and have miR165/166 activity just after leaf initiation. However, during the early developmental stages, the FIL-expressing and miR165/166-active cells switch the nuclear gene expression state to that expressing PHB-like genes, thus FMB shifts. When plastid gene expression machinery is functional (A), the gene expression switch in nuclei progress smoothly regardless whether GUN1 is functional or not. The pace of this gene expression switch is important for the full lamina expansion. When the plastid gene expression machinery is impaired (B), the GUN1-dependent retrograde signal affects the nuclei to delay or stop the gene expression switch. This plastid effect contributes to prevent the wide lamina expansion. Possibly, the GUN1-dependent retrograde signal regulates also other nuclear genes to repair the plastid condition. When the plastid gene expression machinery is impaired and the plant is devoid of the GUN1-dependent retrograde signal (C), the switch in nuclear gene expression progress normally and lamina expands despite the absence of photosynthetic activity.