Summary Polycomb group (PcG) proteins are gene repressors that help to maintain cellular identity during development via chromatin remodeling. Fertilization‐independent endosperm (FIE), a member of the PcG complex, operates extensively in plant development, but its role in rice has not been fully investigated to date. We report the isolation and characterization of a PcG member in rice, which was designated OsFIE2 for Oryza sativa Fertilization‐Independent Endosperm 2. OsFIE2 is a single‐copy gene in the rice genome and shows a universal expression pattern. The OsFIE2 RNAi lines displayed pleiotropic phenotypes in vegetative and reproductive organ generation. In unfertilized lines, endosperm formation could be triggered without embryo formation, which indicates that FIE is indeed involved in the suppression of autonomous endosperm development in rice. Furthermore, lateral root generation was promoted early in the roots of OsFIE2 RNAi lines, whereas the primary root was premature and highly differentiated. As the root tip stem cell differentiated, QHB, the gene required for stem cell maintenance in the quiescent center, was down‐regulated. Our data suggest that the OsFIE2–PcG complex is vital for rice reproduction and endosperm formation. Its role in stem cell maintenance suggests that the gene is functionally conserved in plants as well as animals.

BackgroundSince the quality and yield of rice production depends on endosperm development, previous studies have focused on the molecular mechanism that regulates this developmental process. Recently, how this process is epigenetically regulated has become an important topic. However, the gene expression analysis and screening imprinted genes during early endosperm development remain challenging since the isolation of early endosperm has not been possible. Here, we report a procedure for the isolation of endosperm at 24 or 48 HAP (hours after pollination) during the free nuclear stage of endosperm development.ResultsThis technique allows for rapid and convenient collection of pure free nuclear endosperm. Early endosperm RNA can then be extracted from the isolated endosperm cells using dynabeads. Our results showed that the quality of RNA is satisfactory for gene expression analysis and screening the parental-of-origin specific genes in early endosperm.ConclusionsThus, we offer a reliable method to overcome one of the major obstacles in the investigation of the molecular mechanisms of early endosperm development. Our approach can be used for accurate gene expression analysis and screening of imprinted genes, and facilitates the confirmation of endosperm-specific gene expression at the very early stages of endosperm development. This method could also be used in other species to collect early free nuclear endosperm.

Background Parent-of-origin gene expression and its role in seed development have drown a great attention in recent years. Genome-wide analysis has identified hundreds of candidate imprinted genes, a major type of parent-of-origin genes, in rice hybrid endosperms at the stage of 5 days after pollination (dap). However, the expression of these genes in early endosperm have been never confirmed due to technique limitations and the behavior of the imprinted genes in different rice hybridizations are still largely unknown. Results Here, based on our elaborate technique established previously, the expression patterns of PcG genes in the early stages of endosperm development (within 3 dap), were comprehensively analyzed. We revealed that the free nucleus stage of endosperm development is critical for parent-of-origin gene analysis. The expression of the imprinted genes are highly dynamic, likely corresponding to the critical developmental events during this period. Hybridizations between Oryza sativa japonica and indica showed that the expression patterns of the same imprinted gene could be varied by crossing with different parental cultivars, indicative of their parent-dependent character. There are strong alleles that often showed predominant expression over other alleles regardless of the parental origin, which provides a possible explanation for the cultivar-dependent predominant phenotype in crop hybridizations. In addition, we found that the transcripts of the same gene behave differently, with imprinting or non-imprinting patterns, suggesting the existence of not only imprinted and non-imprinted genes but also imprinted or non-imprinted transcripts, which reveals new aspects of the genomic imprinting. Conclusions These findings on the characters of parent-of-origin genes shed light on the understanding the real role of gene imprinting in endosperm development. Electronic supplementary material The online version of this article (10.1186/s12284-019-0306-x) contains supplementary material, which is available to authorized users.

Garden asparagus (Asparagus officinalis L.) is a horticultural crop with high nutritional and medical value, considered an ideal plant for sex determination research among many dioecious plants, whose genomic information can support genetic analysis and breeding programs. In this research, the entire mitochondrial genome of A. officinalis was sequenced, annotated and assembled using a mixed Illumina and PacBio data. The garden asparagus circular mitochondrial genome measures 492,062 bp with a GC value of 45.9%. Thirty-six protein-coding genes, 17 tRNA and 6 rRNA genes were annotated, among which 8 protein-coding genes contained 16 introns. In addition, 254 SSRs with 10 complete tandem repeats and 293 non-tandem repeats were identified. It was found that the codons of edited sites located in the amino acids showed a leucine-formation trend, and RNA editing sites mainly caused the mutual transformation of amino acids with the same properties. Furthermore, 72 sequence fragments accounting for 20,240 bp, presentating 4.11% of the whole mitochondrial genome, were observed to migrate from chloroplast to mitochondrial genome of A. officinalis. The phylogenetic analysis showed that the closest genetic relationship between A. officinalis with onion (Allium cepa) inside the Liliaceae family. Our results demonstrated that high percentage of protein-coding genes had evolutionary conservative properties, with Ka/Ks values less than 1. Therefore, this study provides a high-quality garden asparagus mitochondrial genome, useful to promote better understanding of gene exchange between organelle genomes.

Asparagus setaceus (Kunth) Jessop is a horticultural plant of the genus Asparagus. Herein, the whole chloroplast (cp) genome of A. setaceus was sequenced with PacBio and Illumina sequencing systems. The cp genome shows a characteristic quadripartite structure with 158,076 bp. In total, 135 genes were annotated, containing 89 protein-coding, 38 tRNA, and 8 rRNA genes. Contrast with the previous cp genome of A. setaceus registered in NCBI, we identified 7 single-nucleotide polymorphisms and 15 indels, mostly situated in noncoding areas. Meanwhile, 36 repeat structures and 260 simple sequence repeats were marked out. A bias for A/T-ending codons was shown in this cp genome. Furthermore, we predicted 78 RNA-editing sites in 29 genes, which were all for C-to-U transitions. And it was also proven that positive selection was exerted on the rpoC1 gene of A. setaceus with the K a/K s data. Meanwhile, a conservative gene order and highly similar sequences of protein-coding genes were revealed within Asparagus species. Phylogenetic tree analysis indicated that A. setaceus was a sister to Asparagus cochinchinensis. Taken together, our released genome provided valuable information for the gene composition, genetics comparison, and the phylogeny studies of A. setaceus.