The THO Complex Non-Cell-Autonomously Represses Female Germline Specification through the TAS3-ARF3 Module

Functional dissection of the ARGONAUTE7 promoter

ARGONAUTES are the central effector proteins of RNA silencing which bind target transcripts in a small RNA-guided manner. Arabidopsis thaliana has 10 ARGONAUTE (AGO) genes, with specialized roles in RNA-directed DNA methylation, post-transcriptional gene silencing, and antiviral defense. To better understand specialization among AGO genes at the level of transcriptional regulation we tested a library of 1497 transcription factors for binding to the promoters of AGO1,AGO10, and AGO7 using yeast 1-hybrid assays. A ranked list of candidate DNA-binding TFs revealed binding of the AGO7 promoter by a number of proteins in two families: the miR156-regulated SPL family and the miR319-regulated TCP family, both of which have roles in developmental timing and leaf morphology. Possible functions for SPL and TCP binding are unclear: we showed that these binding sites are not required for the polar expression pattern of AGO7, nor for the function of AGO7 in leaf shape. Normal AGO7 transcription levels and function appear to depend instead on an adjacent 124-bp region. Progress in understanding the structure of this promoter may aid efforts to understand how the conserved AGO7-triggered TAS3 pathway functions in timing and polarity.

Sex Determination in Ceratopteris richardii Is Accompanied by Transcriptome Changes That Drive Epigenetic Reprogramming of the Young Gametophyte

The fern Ceratopteris richardii is an important model for studies of sex determination and gamete differentiation in homosporous plants. Here we use RNA-seq to de novo assemble a transcriptome and identify genes differentially expressed in young gametophytes as their sex is determined by the presence or absence of the male-inducing pheromone called antheridiogen. Of the 1,163 consensus differentially expressed genes identified, the vast majority (1,030) are up-regulated in gametophytes treated with antheridiogen. GO term enrichment analyses of these DEGs reveals that a large number of genes involved in epigenetic reprogramming of the gametophyte genome are up-regulated by the pheromone. Additional hormone response and development genes are also up-regulated by the pheromone. This C. richardii gametophyte transcriptome and gene expression dataset will prove useful for studies focusing on sex determination and differentiation in plants.

Be my baby: patterning toward plant germ cells

In flowering plants, germ cells are formed via tightly coordinated patterning processes that facilitate specification of spore mother cells and meiosis during sporogenesis, as well as functional differentiation of germ cells in gametogenesis. Studies using the conventional Arabidopsis system and the newly emerged bryophyte system have revealed novel interactions between regulatory factors that restrict the number of spore mother cells, and evolutionarily conserved factors that promote germ cell differentiation. This short review summarizes recent advances in our understanding of the cellular events that lead to the formation of germ cells in plants, and highlights questions that remain to be addressed in the field.

KLU suppresses megasporocyte cell fate through SWR1-mediated activation of WRKY28 expression in Arabidopsis

Germ-line specification is essential for sexual reproduction. In the ovules of most flowering plants, only a single hypodermal cell enlarges and differentiates into a megaspore mother cell (MMC), the founder cell of the female germ-line lineage. The molecular mechanisms restricting MMC specification to a single cell remain elusive. We show that the Arabidopsis transcription factor WRKY28 is exclusively expressed in hypodermal somatic cells surrounding the MMC and is required to repress these cells from acquiring MMC-like cell identity. In this process, the SWR1 chromatin remodeling complex mediates the incorporation of the histone variant H2A.Z at the WRKY28 locus. Moreover, the cytochrome P450 gene KLU, expressed in inner integument primordia, non-cell-autonomously promotes WRKY28 expression through H2A.Z deposition at WRKY28. Taken together, our findings show how somatic cells in ovule primordia cooperatively use chromatin remodeling to restrict germ-line cell specification to a single cell.

A reciprocal inhibition between ARID1 and MET1 in male and female gametes in Arabidopsis

Both female and male gametophytes harbor companion cells and gametes. MET1, a DNA methyltransferase, is down-regulated in companion cells. However, how MET1 is differentially regulated in gametophytes remains unexplored. ARID1, a transcription factor that is specifically depleted in sperm cells, is occupied by MET1-dependent CG methylation. Here, we show that MET1 confines ARID1 to the vegetative cell of male gametes, but ARID1 conversely represses MET1 in the central cell of female gametes. Compared to the vegetative cell-localization in wild type pollen, ARID1 expands to sperm cells in the met1 mutant. To understand whether MET1-dependent ARID1 inhibition exists during female gametogenesis, we first show that ARID1 is expressed in the megaspore mother cell (MMC), ARID1 but not MET1 is detectable in the central cell at maturity. Interestingly, compared to the absence of MET1 in the central cell and the egg cell of wild type ovules, MET1 significantly accumulates in these two cells in arid1 ovules. Lastly, we show that both ARID1 and MET1 are required for the cell specification of MMC. Collectively, our results uncover a reciprocal dependence between ARID1 and MET1, and provide a clue to further understand how the specification of MMC is likely regulated by DNA methylation.