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miRNAs are essential regulators of cell identity, yet their role in early embryo development in plants remains largely unexplored. To determine the earliest stage at which miRNAs act to promote pattern formation in embryogenesis, we examined a series of mutant alleles in the Arabidopsis thaliana miRNA biogenesis enzymes DICER-LIKE 1 (DCL1), SERRATE (SE), and HYPONASTIC LEAVES 1 (HYL1). Cellular and patterning defects were observed in dcl1, se and hyl1 embryos from the zygote through the globular stage of embryogenesis. To identify miRNAs that are expressed in early embryogenesis, we sequenced mRNAs from globular stage Columbia wild type (wt) and se-1 embryos, and identified transcripts potentially corresponding to 100 miRNA precursors. Considering genome location and transcript increase between wt and se-1, 39 of these MIRNAs are predicted to be bona fide early embryo miRNAs. Among these are conserved miRNAs such as miR156, miR159, miR160, miR161, miR164, miR165, miR166, miR167, miR168, miR171, miR319, miR390 and miR394, as well as miRNAs whose function has never been characterized. Our analysis demonstrates that miRNAs promote pattern formation beginning in the zygote, and provides a comprehensive dataset for functional studies of individual miRNAs in Arabidopsis embryogenesis.

Mediator is a multiprotein complex that regulates transcription at the level of RNA pol II assembly, as well as through regulation of chromatin architecture, RNA processing and recruitment of epigenetic marks. Though its modular structure is conserved in eukaryotes, its subunit composition has diverged during evolution and varies in response to environmental and tissue-specific inputs, suggesting different functions for each subunit and/or Mediator conformation. In animals, Mediator has been implicated in the control of differentiation and morphogenesis through modulation of numerous signaling pathways. In plants, studies have revealed roles for Mediator in regulation of cell division, cell fate and organogenesis, as well as developmental timing and hormone responses. We begin this review with an overview of biochemical mechanisms of yeast and animal Mediator that are likely to be conserved in all eukaryotes, as well as a brief discussion of the role of Mediator in animal development. We then present a comprehensive review of studies of the role of Mediator in plant development. Finally, we point to important questions for future research on the role of Mediator as a master coordinator of development.

In Arabidopsis, leaves produced during the juvenile vegetative phase are simple, while adult leaves are morphologically complex. The juvenile to adult transition is regulated by miR156, a microRNA that promotes juvenility by impeding the function of SPL transcription factors, which specify adult leaf traits. Both leaf derived sugars, as well as the Mediator Cyclin Dependent Kinase 8 (CDK8) module genes CENTER CITY (CCT)/MED12 and GRAND CENTRAL (GCT)/MED13, act upstream of miR156 to promote the juvenile to adult transition. However, it is not known whether sugar, CCT and GCT repress miR156 independently, as part of the same pathway, or in a cooperative manner. Here we show that sugar treatment can repress MIR156 expression in the absence of CCT or GCT. Both cct and the photosynthetic mutant chlorina1 (ch1) (which decreases sugar synthesis) exhibit extended juvenile development and increased MIR156A and MIR156C expression. Compared to ch1 and cct single mutants, the ch1 cct double mutant has a stronger effect on juvenile leaf traits, higher MIR156C levels, and a dramatic increase in MIR156A. Our results show that sugar and the CDK8 module are capable of regulating MIR156 independently, but suggest they normally act together in a synergistic manner.