Abstract
Unlike in animals, the reproductive lineage cells in plants differentiate from within somatic tissues late in development to produce a specific haploid generation of the life cycle—male and female gametophytes. In flowering plants, the male gametophyte develops within the anthers and the female gametophyte—within the ovule. Both gametophytes consist of only a few cells. There are two major stages of gametophyte development—meiotic and post-meiotic. In the first stage, sporocyte mother cells differentiate within the anther (pollen mother cell) and the ovule (megaspore mother cell). These sporocyte mother cells undergo two meiotic divisions to produce four haploid daughter cells—male spores (microspores) and female spores (megaspores). In the second stage, the haploid spore cells undergo few asymmetric haploid mitotic divisions to produce the 3-cell male or 7-cell female gametophyte. Both stages of gametophyte development involve extensive epigenetic reprogramming, including siRNA dependent changes in DNA methylation and chromatin restructuring. This intricate mosaic of epigenetic changes determines, to a great extent, embryo and endosperm development in the future sporophyte generation.
Highlights
Plants have a complex life cycle with alternating diploid and haploid generations (Figure 1) [1]
At the beginning of the “classic” RNA-directed DNA methylation (RdDM) pathway, a plant-specific RNA polymerase IV (Pol IV) is recruited, by SHH1 protein that recognizes unmethylated H3K4 and methylated H3K9 through its Tudor domain, to a subset of its possible target loci to generate short (26–45 nt) single-stranded RNA transcripts. These RNA transcripts are subsequently used as templates by Pol IV-associated RNA-DEPENDENT RNA POLYMERASE 2 (RDR2) to generate double-stranded RNAs, which are cut into double-stranded 24 nt siRNAs by the endoribonuclease DICER-LIKE 3 (DCL3)
Plants have three groups of small RNAs that differ in their biogenesis and function: 20–22 nt microRNAs produced by DCL1, 21–22 nt siRNAs produced by DCL4 and DCL2, and 24 nt siRNAs produced by DCL3 [25]
Summary
Plants have a complex life cycle with alternating diploid (sporophyte) and haploid (gametophyte) generations (Figure 1) [1]. The second and third divisions give rise to four nuclei at each pole Their positioning correlates with future cell specification. Mutation of indeterminate gametophyte results in the supernumerary nuclei in the embryo sac that assume specific cell fates correlated with their position [6]. A high auxin concentration is detectable at the micropylar pole of the embryo sac up to the third mitotic division and eight nuclei positioning before cellularization. A mechanism of lateral inhibition by the EC and CC has been suggested to prevent the excessive gamete cell formation and maintain the identity of accessory cells [8] In flowering plants, both SCs in the pollen are used to fertilize cells of the female gametophyte—a unique process of double fertilization. Its growth is regulated by much different guidance, attraction, and support mechanisms
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