Identification of Imprinted Genes Based on Homology: An Example of Fragaria vesca

Identification and characterization of miRNAs and PHAS loci related to the early development of the embryo and endosperm in Fragaria × ananassa

BACKGROUND

The strawberry fleshy fruit is actually enlarged receptacle tissue, and the successful development of the embryo and endosperm is essential for receptacle fruit set. MicroRNAs (miRNAs) and phased small interfering RNAs (phasiRNAs) play indispensable regulatory roles in plant growth and development. However, miRNAs and phasiRNAs participating in the regulation of strawberry embryo and endosperm development have yet to be explored.

RESULTS

Here, we performed genome-wide identification of miRNA and phasiRNA-producing loci (PHAS) in strawberry seeds with a focus on those involved in the development of the early embryo and endosperm. We found that embryos and endosperm have different levels of small RNAs. After bioinformatics analysis, the results showed that a total of 404 miRNAs (352 known and 52 novel) and 156 PHAS genes (81 21-nt and 75 24-nt genes) could be found in strawberry seed-related tissues, of which four and nine conserved miRNA families displayed conserved expression in the endosperm and embryo, respectively. Based on refined putative annotation of PHAS loci, some auxin signal-related genes, such as CM3, TAR2, AFB2, ASA1, NAC and TAS3, were found, which demonstrates that IAA biosynthesis is important for endosperm and embryo development during early fruit growth. Additionally, some auxin signal-related conserved (miR390-TAS3) and novel (miR156-ASA1) trigger-PHAS pairs were identified.

CONCLUSIONS

Taken together, these results expand our understanding of sRNAs in strawberry embryo and endosperm development and provide a genomic resource for early-stage fruit development.

Exploring Breakthroughs in Three Traits Belonging to Seed Life

Based on prior knowledge and with the support of new methodology, solid progress in the understanding of seed life has taken place over the few last years. This update reflects recent advances in three key traits of seed life (i.e., preharvest sprouting, genomic imprinting, and stored-mRNA). The first breakthrough refers to cloning of the mitogen-activated protein kinase-kinase 3 (MKK3) gene in barley and wheat. MKK3, in cooperation with ABA signaling, controls seed dormancy. This advance has been determinant in producing improved varieties that are resistant to preharvest sprouting. The second advance concerns to uniparental gene expression (i.e., imprinting). Genomic imprinting primarily occurs in the endosperm. Although great advances have taken place in the last decade, there is still a long way to go to complete the puzzle regarding the role of genomic imprinting in seed development. This trait is probably one of the most important epigenetic facets of developing endosperm. An example of imprinting regulation is polycomb repressive complex 2 (PRC2). The mechanism of PRC2 recruitment to target endosperm with specific genes is, at present, robustly studied. Further progress in the knowledge of recruitment of PRC2 epigenetic machinery is considered in this review. The third breakthrough referred to in this update involves stored mRNA. The role of the population of this mRNA in germination is far from known. Its relations to seed aging, processing bodies (P bodies), and RNA binding proteins (RBPs), and how the stored mRNA is targeted to monosomes, are aspects considered here. Perhaps this third trait is the one that will require greater experimental dedication in the future. In order to make progress, herein are included some questions that are needed to be answered.

The evolution of imprinting in plants: beyond the seed

Genomic imprinting results in the biased expression of alleles depending on if the allele was inherited from the mother or the father. Despite the prevalence of sexual reproduction across eukaryotes, imprinting is only found in placental mammals, flowering plants, and some insects, suggesting independent evolutionary origins. Numerous hypotheses have been proposed to explain the selective pressures that favour the innovation of imprinted gene expression and each differs in their experimental support and predictions. Due to the lack of investigation of imprinting in land plants, other than angiosperms with triploid endosperm, we do not know whether imprinting occurs in species lacking endosperm and with embryos developing on maternal plants. Here, we discuss the potential for uncovering additional examples of imprinting in land plants and how these observations may provide additional support for one or more existing imprinting hypotheses.