Pattern formation involves temporally and spatially regulated changes in gene expression but the temporal component of pattern formation is poorly understood. On p. , Scott Poethig and colleagues begin to fill this knowledge gap by reporting that two newly identified genes — GRAND CENTRAL (GCT) and CENTER CITY (CCT) — are involved in the temporal regulation of radial polarity in Arabidopsis embryos. They show that mutations in GCT and CCT delay the specification of the central and peripheral domains of Arabidopsis embryos without affecting their initial growth rate or eventual pattern. They identify GCT and CCT as the Arabidopsis orthologues of MED13 and MED12, respectively; these conserved proteins repress transcription by inhibiting core Mediator, a complex that allows transcription factors to activate RNA polymerase II. The researchers suggest, therefore, that GCT/MED13 and CCT/MED12 regulate pattern formation during Arabidopsis embryogenesis by transiently repressing the transcription of genes that would otherwise interfere with radial pattern formation, thus revealing a novel temporal regulatory mechanism in plant embryogenesis.
