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.

Heat shock proteins (HSPs) perform a fundamental role in protecting plants against abiotic stresses. Previous studies have made great efforts in the functional analysis of individual family members, but there has not yet been an overall analysis or expression profiling of the HSP70 gene family in soybeans (Glycine max L.). In this study, an investigation of the soybean genome revealed 61 putative HSP70 genes, which were evaluated. These genes were classified into eight sub-families, denoted I–VIII, based on a phylogenetic analysis. In each sub-family, the constituent parts of the gene structure and motif were relatively conserved. These GmHSP70 genes were distributed unequally on 17 of the 20 chromosomes. The analysis of the expression profiles showed that 53 of the 61 GmHSP70 genes were differentially expressed across the 14 tissues. However, most of the GmHSP70s were differentially expressed in a tissue-specific expression pattern. Furthermore, the expression of some of the duplicate genes was partially redundant, while others showed functional diversity. The quantitative real-time PCR (qRT-PCR) analysis of the 61 soybean HSP70 genes confirmed their stress-inducible expression patterns under both drought and heat stress. These findings provide a thorough overview of the evolution and modification of the GmHSP70 gene family, which will help to determine the functional characteristics of the HSP70 genes in soybean growth and development.

Allopolyploidy is an important process in plant speciation, yet newly formed allopolyploid species typically suffer from extreme genetic bottlenecks. One escape from this impasse might be homoeologous meiotic pairing, during which homoeologous exchanges (HEs) generate phenotypically variable progeny. However, the immediate genome-wide patterns and resulting phenotypic diversity generated by HEs remain largely unknown. Here, we analyzed the genome composition of 202 phenotyped euploid segmental allopolyploid individuals from the 4th selfed generation following chromosomal doubling of reciprocal F1 hybrids of crosses between rice subspecies, using whole genome sequencing. We describe rampant occurrence of HEs that, by overcoming incompatibility or conferring superiority of hetero-cytonuclear interactions, generate extensive and individualized genomic mosaicism across the analyzed tetraploids. We show that the resulting homoeolog copy number alteration in tetraploids affects known-function genes and their complex genetic interactions, in the process creating extraordinary phenotypic diversity at the population level following a single initial hybridization. Our results illuminate the immediate genomic landscapes possible in a tetraploid genomic environment, and underscore HE as an important mechanism that fuels rapid phenotypic diversification accompanying the initial stages of allopolyploid evolution.

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.

The allopolyploid speciation process faces the genomic challenge of stoichiometric disruption caused by merging biparental nuclear genomes with only one (usually maternal) of the two sets of progenitor cytoplasmic genomes. The photosynthetic protein 1,5-bisphosphate carboxylase/oxygenase (Rubisco) is composed of nuclear-encoded small subunits (SSUs) and plastome-encoded large subunits (LSUs), making it an ideal enzyme to explore the evolution process of cytonuclear accommodation. We investigated the variation of SSUs and their encoding rbcS genes in synthetic nascent rice (Oryza sativa L.) allotetraploid lineages, formed from the parental subspecies japonica and indica of Asian rice. The evolution of rbcS genes in rice subspecies involves both mutation and concerted homogenization. Within reciprocal rice hybrids and allopolyploids, there was no consistent pattern of biased expression of rbcS alleles or homeologs, nor was there biased gene conversion favoring the maternal gene copies. Instead, we observed an apparently stochastic pattern of rbcS intergenomic gene conversions and biased expression of rbcS homeologs. We conclude that in young rice allopolyploids, cytonuclear coordination either is not selectively favored because of high parental sequence similarity or because there has been insufficient time for subtle selective effects to become observable. P olyploidy, involving the combining of two or more sets of diploid parental nuclear genomes, recursively occurs during plant evolution, generating modern plant lineages that are considered to be paleopolyploid (having • This study characterized the evolutionary features of rbcS genes in diploid rice subspecies and investigated the cytonuclear coevolution of Rubisco genes (biparentally inherited nuclear rbcS and unimaternal plastid rbcL genes encoding small subunits and large subunits, respectively) in synthesized rice hybrids (9N and N9) and allotetraploids (99NN and NN99).