Abstract
Germ line specification is a crucial step in the life cycle of all organisms. For sexual plant reproduction, the megaspore mother cell (MMC) is of crucial importance: it marks the first cell of the plant “germline” lineage that gets committed to undergo meiosis. One of the meiotic products, the functional megaspore, subsequently gives rise to the haploid, multicellular female gametophyte that harbours the female gametes. The MMC is formed by selection and differentiation of a single somatic, sub-epidermal cell in the ovule. The transcriptional network underlying MMC specification and differentiation is largely unknown. We provide the first transcriptome analysis of an MMC using the model plant Arabidopsis thaliana with a combination of laser-assisted microdissection and microarray hybridizations. Statistical analyses identified an over-representation of translational regulation control pathways and a significant enrichment of DEAD/DEAH-box helicases in the MMC transcriptome, paralleling important features of the animal germline. Analysis of two independent T-DNA insertion lines suggests an important role of an enriched helicase, MNEME (MEM), in MMC differentiation and the restriction of the germline fate to only one cell per ovule primordium. In heterozygous mem mutants, additional enlarged MMC-like cells, which sometimes initiate female gametophyte development, were observed at higher frequencies than in the wild type. This closely resembles the phenotype of mutants affected in the small RNA and DNA-methylation pathways important for epigenetic regulation. Importantly, the mem phenotype shows features of apospory, as female gametophytes initiate from two non-sister cells in these mutants. Moreover, in mem gametophytic nuclei, both higher order chromatin structure and the distribution of LIKE HETEROCHROMATIN PROTEIN1 were affected, indicating epigenetic perturbations. In summary, the MMC transcriptome sets the stage for future functional characterization as illustrated by the identification of MEM, a novel gene involved in the restriction of germline fate.
Highlights
The life cycle of flowering plants alternates between a diploid sporophytic and a haploid gametophytic phase
The functional megaspore (FMS) develops into the haploid embryo sac through three rounds of mitosis followed by cellularization, typically forming a seven-celled embryo sac, including two gametes, two synergids, and three antipodals [2,5,6]
Statistical data analysis comparing these results with the transcriptomes of 71 other types of cells and tissues revealed the importance of translational control pathways and RNA helicases for plant germline development, a feature reminiscent of the animal germline
Summary
The life cycle of flowering plants alternates between a diploid sporophytic and a haploid gametophytic phase. In contrast to animals, which form gametes directly by meitotic division from a diploid germline, plants form female and male spores by meiotic division during megasporogenesis and microsporogenesis, respectively. During mega- and microgametogenesis, the spores develop by mitotic division and cell differentiation into the female and male gametophytes, respectively. The two morphologically distinct gametophytes develop within specialized reproductive structures in the female and male organs of the flower, the ovules and the anthers. In the model plant Arabidopsis thaliana, the archespore differentiates directly into the megaspore mother cell (MMC), which is committed to undergo meiosis and gives rise to a tetrad of haploid megaspores. Double fertilization of the female gametes by one sperm cell each initiates
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