The roles of non-coding RNAs in male reproductive development and abiotic stress responses during this unique process in flowering plants

Successful male reproductive development is the guarantee for sexual reproduction of flowering plants. Male reproductive development is a complicated and multi-stage process that integrates physiological processes and adaptation and tolerance to a myriad of environmental stresses. This well-coordinated process is governed by genetic and epigenetic machineries. Non-coding RNAs (ncRNAs) play pleiotropic roles in the plant growth and development. The identification, characterization and functional analysis of ncRNAs and their target genes have opened a new avenue for comprehensively revealing the regulatory network of male reproductive development and its response to environmental stresses in plants. This review briefly addresses the types, origin, biogenesis and mechanisms of ncRNAs in plants, highlights important updates on the roles of ncRNAs in regulating male reproductive development and emphasizes the contribution of ncRNAs, especially miRNAs and lncRNAs, in responses to abiotic stresses during this unique process in flowering plants.

Expression patterns of conserved microRNAs in the male gametophyte of loblolly pine (Pinus taeda)

MicroRNAs (miRNAs) are small RNAs that regulate genes involved in various aspects of plant development, but their presence and expression patterns in the male gametophytes of gymnosperms have not yet been established. Therefore, this study identified and compared the expression patterns of conserved miRNAs from two stages of the male gametophyte of loblolly pine (Pinus taeda), which are the mature (ungerminated) and germinated pollen. Microarray was used to identify conserved miRNAs that varied in expression between these two stages of the loblolly pine male gametophyte. Forty-seven conserved miRNAs showed significantly different expression levels between mature and germinated loblolly pine pollen. In particular, miRNAs representing 14 and 8 families were up- and down-regulated in germinated loblolly pine pollen, respectively. qRT-PCR was used to validate their expression patterns using representative miRNAs. Target genes and proteins were identified using psRNATarget program. Predicted targets of the 22 miRNA families belong mostly to classes of genes involved in defense/stress response, metabolism, regulation, and signaling. qRT-PCR was also used to validate the expression patterns of representative target genes. This study shows that conserved miRNAs are expressed in mature and germinated loblolly pine pollen. Many of these miRNAs are differentially expressed, which indicates that the two stages of the male gametophyte examined are regulated at the miRNA level. This study also expands our knowledge of the male gametophytes of seed plants by providing insights on some similarities and differences in the types and expression patterns of conserved miRNAs between loblolly pine with those of rice and Arabidopsis.

Emerging Roles for Non-Coding RNAs in Male Reproductive Development in Flowering Plants

Knowledge of sexual reproduction systems in flowering plants is essential to humankind, with crop fertility vitally important for food security. Here, we review rapidly emerging new evidence for the key importance of non-coding RNAs in male reproductive development in flowering plants. From the commitment of somatic cells to initiating reproductive development through to meiosis and the development of pollen—containing the male gametes (sperm cells)—in the anther, there is now overwhelming data for a diversity of non-coding RNAs and emerging evidence for crucial roles for them in regulating cellular events at these developmental stages. A particularly exciting development has been the association of one example of cytoplasmic male sterility, which has become an unparalleled breeding tool for producing new crop hybrids, with a non-coding RNA locus.

Epigenetic control of cell specification during female gametogenesis

In flowering plants, the formation of gametes depends on the differentiation of cellular precursors that divide meiotically before giving rise to a multicellular gametophyte. The establishment of this gametophytic phase presents an opportunity for natural selection to act on the haploid plant genome by means of epigenetic mechanisms that ensure a tight regulation of plant reproductive development. Despite this early acting selective pressure, there are numerous examples of naturally occurring developmental alternatives that suggest a flexible regulatory control of cell specification and subsequent gamete formation in flowering plants. In this review, we discuss recent findings indicating that epigenetic mechanisms related to the activity of small RNA pathways prevailing during ovule formation play an essential role in cell specification and genome integrity. We also compare these findings to small RNA pathways acting during gametogenesis in animals and discuss their implications for the understanding of the mechanisms that control the establishment of the female gametophytic lineage during both sexual reproduction and apomixis.

Small RNA activity and function in angiosperm gametophytes

Small non-coding RNAs are key post-transcriptional and transcriptional regulators of plant gene expression in angiosperm sporophytes. In recent years, gametophytic small RNAs have also been investigated, predominantly in Arabidopsis male gametophytes, revealing features in common with the sporophyte as well as some surprising differences. Transcriptomic and deep-sequencing studies confirm that multiple small RNA pathways operate in male gametophytes, with over 100 miRNAs detected throughout development. Trans-acting siRNA pathways that are associated with novel phased transcripts in pollen, and the nat-siRNA pathway have important roles in pollen maturation and gamete function. Moreover, a role for siRNA-triggered silencing of transposable elements in male and female germ cells has been established, a feature in common with the role of piRNAs in animal germlines. Current evidence supports an integral role for small RNAs in angiosperm gametophyte development and it can be anticipated that novel small RNAs with significant roles in germline development and genome integrity await discovery.

Male gametogenesis and germline specification in flowering plants

During angiosperm male gametophyte development, the male germline is segregated by an asymmetric cell division of the haploid microspore. This review encompasses recent advances in understanding the genetic and molecular mechanisms involved in generating the male germline from this pluripotent germline initial and in specifying the production of the twin sperm cells required for double fertilization. Genetic studies and access to the transcriptome of isolated gametes have enabled remarkable progress in understanding some of the key regulators that control and integrate germ cell cycle progression with germline specification, and an emerging regulatory model is presented. Rapid advances have also been made in understanding epigenetic regulation and small RNA pathways in the male gametophyte and germline that impact on genome integrity and gamete development, traits that are shared with animal germlines. The review concludes with a perspective of the outstanding issues and directions of future research that will further our understanding of germline specification and the gametophytic control of pollen development.

Small RNAs in angiosperm gametophytes: from epigenetics to gamete development

The established role of various small RNA pathways in the epigenetic regulation of gene expression in the dipolid sporophytic generation of flowering plants contrasts sharply with the lack of knowledge of their role in haploid gametophyte generation. Several recent studies now uncover the operation of multiple small RNA pathways in male and female gametophytes and their essential roles in genome integrity, cell specification, and, most recently, sperm cell function, as described in the May 15, 2010, issue of Genes & Development by Ron and colleagues (pp. 1010-1021).