Abstract
BackgroundApomixis is considered an evolutionary deviation of the sexual reproductive pathway leading to the generation of clonal maternal progenies by seeds. Recent evidence from model and non-model species suggested that this trait could be modulated by epigenetic mechanisms involving small RNAs (sRNAs). Here we profiled floral sRNAs originated from apomictic and sexual Paspalum notatum genotypes in order to identify molecular pathways under epigenetic control that might be involved in the transition from sexuality to agamospermy.ResultsThe mining of genes participating in sRNA-directed pathways from floral Paspalum transcriptomic resources showed these routes are functional during reproductive development, with several members differentially expressed in apomictic and sexual plants. Triplicate floral sRNA libraries derived from apomictic and a sexual genotypes were characterized by using high-throughput sequencing technology. EdgeR was apply to compare the number of sRNA reads between sexual and apomictic libraries that map over all Paspalum floral transcripts. A total of 1525 transcripts showed differential sRNA representation, including genes related to meiosis, plant hormone signaling, biomolecules transport, transcription control and cell cycle. Survey for miRNA precursors on transcriptome and genome references allowed the discovery of 124 entities, including 40 conserved and 8 novel ones. Fifty-six clusters were differentially represented in apomictic and sexual plants. All differentially expressed miRNAs were up-regulated in apomictic libraries but miR2275, which showed different family members with opposed representation. Examination of predicted miRNAs targets detected 374 potential candidates. Considering sRNA, miRNAs and target surveys together, 14 genes previously described as related with auxin metabolism, transport and signaling were detected, including AMINO ACID/AUXIN PERMEASE 15, IAA-AMIDO SYNTHETASE GH3–8, IAA30, miR160, miR167, miR164, miR319, ARF2, ARF8, ARF10, ARF12, AFB2, PROLIFERATING CELL FACTOR 6 and NITRATE TRANSPORTER 1.1.ConclusionsThis work provides a comprehensive survey of the sRNA differential representation in flowers of sexual and apomictic Paspalum notatum plants. An integration of the small RNA profiling data presented here and previous transcriptomic information suggests that sRNA-mediated regulation of auxin pathways is pivotal in promoting apomixis. These results will underlie future functional characterization of the molecular components mediating the switch from sexuality to apomixis.
Highlights
Apomixis is considered an evolutionary deviation of the sexual reproductive pathway leading to the generation of clonal maternal progenies by seeds
Identification of Paspalum genes involved in key small RNA pathways Small-RNA directed pathways all rely on the activities of DICER-LIKE (DCL) enzymes that generate 21–24-nt small RNAs by processing: 1) hairpins of single-stranded RNAs synthesized from RNA POLYMERASE II (Pol II); 2) long double-stranded RNAs, whose biogenesis requires transcription by Pol II or RNA POLYMERASE IV (Pol IV) followed by Double-stranded RNA (dsRNA) synthesis by RNADEPENDENT RNA POLYMERASEs (RDRs); and 3) long dsRNAs arising from the hybridization of sense and antisense transcripts, from the fold-back of an invertedrepeat sequences, or from the hybridization of unrelated RNA molecules with sequence complementarity [1]
Small RNAs are loaded onto ARGONAUTE proteins (AGO), which mediate targeting of complementary RNAs or DNAs, resulting in posttranscriptional gene silencing (PTGS; degradation through cleavage or translational repression) or RNAdependent DNA methylation (RdDM) through the recruitment of DNA methylation factors [1, 2]
Summary
Apomixis is considered an evolutionary deviation of the sexual reproductive pathway leading to the generation of clonal maternal progenies by seeds. Small RNAs perform critical functions in development, stress responses, and transgenerational inheritance [2]. They contribute to numerous aspects of sexual reproduction [3, 4]. Adventitious embryony relates to the spontaneous generation of somatic embryos from the maternal tissues involved in seed formation, whereas gametophytic apomixis encompasses the formation of functional, unreduced (2n) embryo sacs (2n-ES) from either diploid megaspore mother cells (diplospory) or nucellar companion cells (apospory) [8]. Since seeds are formed in absence of meiosis and egg cell fertilization, they contain clonal embryos genetically identical to the mother plant
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