The non-random spatial packing of chromosomes in the nucleus plays a critical role in orchestrating gene expression and genome function. Here, we present a Hi-C analysis of the chromatin interaction patterns in rice (Oryza sativa L.) at hierarchical architectural levels. We confirm that rice chromosomes occupy their own territories with certain preferential inter-chromosomal associations. Moderate compartment delimitation and extensive TADs (Topologically Associated Domains) were determined to be associated with heterogeneous genomic compositions and epigenetic marks in the rice genome. We found subtle features including chromatin loops, gene loops, and off-/near-diagonal intensive interaction regions. Gene chromatin loops associated with H3K27me3 could be positively involved in gene expression. In addition to insulated enhancing effects for neighbor gene expression, the identified rice gene loops could bi-directionally (+/-) affect the expression of looped genes themselves. Finally, web-interleaved off-diagonal IHIs/KEEs (Interactive Heterochromatic Islands or KNOT ENGAGED ELEMENTs) could trap transposable elements (TEs) via the enrichment of silencing epigenetic marks. In parallel, the near-diagonal FIREs (Frequently Interacting Regions) could positively affect the expression of involved genes. Our results suggest that the chromatin packing pattern in rice is generally similar to that in Arabidopsis thaliana but with clear differences at specific structural levels. We conclude that genomic composition, epigenetic modification, and transcriptional activity could act in combination to shape global and local chromatin packing in rice. Our results confirm recent observations in rice and A. thaliana but also provide additional insights into the patterns and features of chromatin organization in higher plants.

Summary Allopolyploidization, which entails interspecific hybridization and whole genome duplication (WGD), is associated with emergent genetic and epigenetic instabilities that are thought to contribute to adaptation and evolution. One frequent genomic consequence of nascent allopolyploidization is homoeologous exchange (HE), which arises from compromised meiotic fidelity and generates genetically and phenotypically variable progenies. Here, we used a genetically tractable synthetic rice segmental allotetraploid system to interrogate genome‐wide DNA methylation and gene expression responses and outcomes to the separate and combined effects of hybridization, WGD and HEs. Progenies of the tetraploid rice were genomically diverse due to genome‐wide HEs that affected all chromosomes, yet they exhibited overall methylome stability. Nonetheless, regional variation of cytosine methylation states was widespread in the tetraploids. Transcriptome profiling revealed genome‐wide alteration of gene expression, which at least in part associates with changes in DNA methylation. Intriguingly, changes of DNA methylation and gene expression could be decoupled from hybridity and sustained and amplified by HEs. Our results suggest that HEs, a prominent genetic consequence of nascent allopolyploidy, can exacerbate, diversify and perpetuate the effects of allopolyploidization on epigenetic and gene expression variation, and hence may contribute to allopolyploid evolution.

Trithorax-group proteins (TrxGs) play essential regulatory roles in chromatin modification to activate transcription. Although TrxGs have been shown to be extensively involved in the activation of developmental genes, how the specific TrxGs function in the dehydration and abscisic acid (ABA)-mediated modulation of downstream gene expression remains unknown. Here, we report that two evolutionarily conserved Arabidopsis thaliana TrxGs, ARABIDOPSIS TRITHORAX4 (ATX4) and ATX5, play essential roles in the drought stress response. atx4 and atx5 single loss-of-function mutants showed drought stress-tolerant and ABA-hypersensitive phenotypes during seed germination and seedling development, while the atx4 atx5 double mutant displayed further exacerbation of the phenotypes. Genome-wide RNA-sequencing analyses showed that ATX4 and ATX5 regulate the expression of genes functioning in dehydration stress. Intriguingly, ABA-HYPERSENSITIVE GERMINATION 3 (AHG3), an essential negative regulator of ABA signaling, acts genetically downstream of ATX4 and ATX5 in response to ABA. ATX4 and ATX5 directly bind to the AHG3 locus and trimethylate histone H3 of Lys 4 (H3K4). Moreover, ATX4 and ATX5 occupancies at AHG3 are dramatically increased under ABA treatment, and are also essential for RNA polymerase II (RNAPII) occupancies. Our findings reveal novel molecular functions of A. thaliana TrxGs in dehydration stress and ABA responses.

Aneuploidy, a condition of unbalanced chromosome content, represents a large-effect mutation that bears significant relevance to human health and microbe adaptation. As such, extensive studies of aneuploidy have been conducted in unicellular model organisms and cancer cells. Aneuploidy also frequently is associated with plant polyploidization, but its impact on gene expression and its relevance to polyploid genome evolution/functional innovation remain largely unknown. Here, we used a panel of diverse types of whole-chromosome aneuploidy of hexaploid wheat (Triticum aestivum), all under the common genetic background of cv Chinese Spring, to systemically investigate the impact of aneuploidy on genome-, subgenome-, and chromosome-wide gene expression. Compared with prior findings in haploid or diploid aneuploid systems, we unravel additional and novel features of alteration in global gene expression resulting from the two major impacts of aneuploidy, cisand trans-regulation, as well as dosage compensation. We show that the expression-altered genes map evenly along each chromosome, with no evidence for coregulating aggregated expression domains. However, chromosomes and subgenomes in hexaploid wheat are unequal in their responses to aneuploidy with respect to the number of genes being dysregulated. Strikingly, homeologous chromosomes do not differ from nonhomologous chromosomes in terms of aneuploidy-induced trans-acting effects, suggesting that the three constituent subgenomes of hexaploid wheat are largely uncoupled at the transcriptional level of gene regulation. Together, our findings shed new insights into the functional interplay between homeologous chromosomes and interactions between subgenomes in hexaploid wheat, which bear implications to further our understanding of allopolyploid genome evolution and efforts in breeding new allopolyploid crops.

Allopolyploidy often triggers phenotypic novelty and gene expression remolding in the resulting polyploids. In this study, we employed multiple phenotypic and genetic approaches to investigate the nature and consequences of allotetraploidization between A- and S-subgenome of tetraploid wheat. Results showed that karyotype of the nascent allopolyploid plants (AT2) is stable but they showed clear novelty in multiple morphological traits which might have positively contributed to the initial establishment of the tetraploids. Further microarray-based transcriptome profiling and gene-specific cDNA-pyrosequencing have documented that transcriptome shock was exceptionally strong in AT2, but a substantial proportion of the induced expression changes was rapidly stabilized in early generations. Meanwhile, both additive and nonadditive expression genes showed extensive homeolog expression remodeling and which have led to the subgenome expression dominance in leaf and young inflorescence of AT2. Through comparing the homeolog-expressing patterns between synthetic and natural tetraploid wheats, it appears that the shock-induced expression changes at both the total expression level and subgenome homeolog partitioning are evolutionarily persistent. Together, our study shed new light on how gene expression changes have rapidly occurred at the initial stage following allotetraploidization, as well as their evolutionary relevance, which may have implications for wheat improvements.