Asymmetric subgenome selection and cis-regulatory divergence during cotton domestication

Exploration of miRNAs and target genes of cytoplasmic male sterility line in cotton during flower bud development

Cytoplasmic male sterility (CMS) lines provide crucial material to harness heterosis for crop plants, which serves as an important strategy for hybrid seed production. However, the molecular mechanism remains obscure. Although microRNAs (miRNAs) play important roles in vegetative growth and reproductive growth, there are few reports on miRNAs regulating the development of male sterility in Upland cotton. In present study, 12 small RNA libraries were constructed and sequenced for two development stages of flower buds from a CMS line and its maintainer line. Based on the results, 256 novel miRNAs were allocated to 141 new miRNA families, and 77 known miRNAs belonging to 54 conserved miRNA families were identified as well. Comparative analysis revealed that 61 novel and 10 conserved miRNAs were differentially expressed. Further transcriptome analysis identified 232 target genes for these miRNAs, which participated in cellular developmental process, cell death, pollen germination, and sexual reproduction. In addition, expression patterns of typical miRNA and the negatively regulated target genes, such as PPR, ARF, AP2, and AFB, were verified by qRT-PCR in cotton flower buds. These targets were previously reported to be related to reproduction development and male sterility, suggesting that miRNAs might act as regulators of CMS occurrence. Some miRNAs displayed specific expression profiles in special developmental stages of CMS line and its fertile hybrid (F₁). Present study offers new information on miRNAs and their related target genes in exploiting CMS mechanism, and revealing the miRNA regulatory networks in Upland cotton.

Epigenetic perspectives on the evolution and domestication of polyploid plant and crops

Polyploidy or whole genome duplication (WGD) is a prominent feature for genome evolution of some animals and all flowering plants, including many important crops such as wheat, cotton, and canola. In autopolyploids, genome duplication often perturbs dosage regulation on biological networks. In allopolyploids, interspecific hybridization could induce genetic and epigenetic changes, the effects of which could be amplified by genome doubling (ploidy changes). Albeit the importance of genetic changes, some epigenetic changes can be stabilized and transmitted as epialleles into the progeny, which are subject to natural selection, adaptation, and domestication. Here we review recent advances for general and specific roles of epigenetic changes in the evolution of flowering plants and domestication of agricultural crops.

Evolution and Stress Responses of Gossypium hirsutum SWEET Genes

The SWEET (sugars will eventually be exported transporters) proteins are sugar efflux transporters containing the MtN3_saliva domain, which affects plant development as well as responses to biotic and abiotic stresses. These proteins have not been functionally characterized in the tetraploid cotton, Gossypium hirsutum, which is a widely cultivated cotton species. In this study, we comprehensively analyzed the cotton SWEET gene family. A total of 55 putative G. hirsutumSWEET genes were identified. The GhSWEET genes were classified into four clades based on a phylogenetic analysis and on the examination of gene structural features. Moreover, chromosomal localization and an analysis of homologous genes in Gossypium arboreum, Gossypium raimondii, and G. hirsutum suggested that a whole-genome duplication, several tandem duplications, and a polyploidy event contributed to the expansion of the cotton SWEET gene family, especially in Clade III and IV. Analyses of cis-acting regulatory elements in the promoter regions, expression profiles, and artificial selection revealed that the GhSWEET genes were likely involved in cotton developmental processes and responses to diverse stresses. These findings may clarify the evolution of G. hirsutum SWEET gene family and may provide a foundation for future functional studies of SWEET proteins regarding cotton development and responses to abiotic stresses.

Recent insights into cotton functional genomics: progress and future perspectives

Functional genomics has transformed from futuristic concept to well-established scientific discipline during the last decade. Cotton functional genomics promise to enhance the understanding of fundamental plant biology to systematically exploit genetic resources for the improvement of cotton fibre quality and yield, as well as utilization of genetic information for germplasm improvement. However, determining the cotton gene functions is a much more challenging task, which has not progressed at a rapid pace. This article presents a comprehensive overview of the recent tools and resources available with the major advances in cotton functional genomics to develop elite cotton genotypes. This effort ultimately helps to filter a subset of genes that can be used to assemble a final list of candidate genes that could be employed in future novel cotton breeding programme. We argue that next stage of cotton functional genomics requires the draft genomes refinement, re-sequencing broad diversity panels with the development of high-throughput functional genomics tools and integrating multidisciplinary approaches in upcoming cotton improvement programmes.

Linkage and association mapping reveals the genetic basis of brown fibre (Gossypium hirsutum)

Brown fibre cotton is an environmental-friendly resource that plays a key role in the textile industry. However, the fibre quality and yield of natural brown cotton are poor, and fundamental research on brown cotton is relatively scarce. To understand the genetic basis of brown fibre cotton, we constructed linkage and association populations to systematically examine brown fibre accessions. We fine-mapped the brown fibre region, Lc1 , and dissected it into 2 loci, qBF-A07-1 and qBF-A07-2. The qBF-A07-1 locus mediates the initiation of brown fibre production, whereas the shade of the brown fibre is affected by the interaction between qBF-A07-1 and qBF-A07-2. Gh_A07G2341 and Gh_A07G0100 were identified as candidate genes for qBF-A07-1 and qBF-A07-2, respectively. Haploid analysis of the signals significantly associated with these two loci showed that most tetraploid modern brown cotton accessions exhibit the introgression signature of Gossypium barbadense. We identified 10 quantitative trait loci (QTLs) for fibre yield and 19 QTLs for fibre quality through a genome-wide association study (GWAS) and found that qBF-A07-2 negatively affects fibre yield and quality through an epistatic interaction with qBF-A07-1. This study sheds light on the genetics of fibre colour and lint-related traits in brown fibre cotton, which will guide the elite cultivars breeding of brown fibre cotton.

Evolutionary dynamics of 3D genome architecture following polyploidization in cotton

The formation of polyploids significantly increases the complexity of transcriptional regulation, which is expected to be reflected in sophisticated higher-order chromatin structures. However, knowledge of three-dimensional (3D) genome structure and its dynamics during polyploidization remains poor. Here, we characterize 3D genome architectures for diploid and tetraploid cotton, and find the existence of A/B compartments and topologically associated domains (TADs). By comparing each subgenome in tetraploids with its extant diploid progenitor, we find that genome allopolyploidization has contributed to the switching of A/B compartments and the reorganization of TADs in both subgenomes. We also show that the formation of TAD boundaries during polyploidization preferentially occurs in open chromatin, coinciding with the deposition of active chromatin modification. Furthermore, analysis of inter-subgenomic chromatin interactions has revealed the spatial proximity of homoeologous genes, possibly associated with their coordinated expression. This study advances our understanding of chromatin organization in plants and sheds new light on the relationship between 3D genome evolution and transcriptional regulation.

Whole genome re-sequencing reveals evolutionary patterns of sacred lotus (Nelumbo nucifera)

Sacred lotus (Nelumbo nucifera or lotus) is an important aquatic plant in horticulture and ecosystems. As a foundation for exploring genomic variation and evolution among different germplasms, we re-sequenced 19 individuals from three cultivated temperate lotus subgroups (rhizome, seed and flower lotus), one wild temperate lotus subgroup (wild lotus), one tropical lotus group (Thai lotus) and an outgroup (Nelumbo lutea). Through genetic diversity and polymorphism analysis by non-missing SNP sites widely distributed in the whole genome, we confirmed that wild and Thai lotus exhibited greater differentiation with a higher genomic diversity compared to cultivated lotus. Rhizome lotus had the lowest genomic diversity and a closer relationship to wild lotus, whereas the genomes of seed and flower lotus were admixed. Genes in energy metabolism process and plant immunity evolved rapidly in lotus, reflecting local adaptation. We established that candidate genes in genomic regions with significant differentiation associated with temperate and tropical lotus divergence always exhibited highly divergent expression pattern. Together, this study comprehensive and credible interpretates important patterns of genetic diversity and relationships, gene evolution, and genomic signature from ecotypic differentiation of sacred lotus.

Identification of SNPs and Candidate Genes Associated With Salt Tolerance at the Seedling Stage in Cotton (Gossypium hirsutum L.)

Salt tolerance in cotton is highly imperative for improvement in the response to decreasing farmland and soil salinization. However, little is known about the genetic basis underlying salt tolerance in cotton, especially the seedling stage. In this study, we evaluated two salt-tolerance-related traits of a natural population comprising 713 upland cotton (Gossypium hirsutum L.) accessions worldwide at the seedling stage and performed a genome-wide association study (GWAS) to identify marker-trait associations under salt stress using the Illumina Infinium CottonSNP63K array. A total of 23 single nucleotide polymorphisms (SNPs) that represented seven genomic regions on chromosomes A01, A10, D02, D08, D09, D10, and D11 were significantly associated with the two salt-tolerance-related traits, relative survival rate (RSR) and salt tolerance level (STL). Of these, the two SNPs i46598Gh and i47388Gh on D09 were simultaneously associated with the two traits. Based on all loci, we screened 280 possible candidate genes showing different expression levels under salt stress. Most of these genes were involved in transcription factors, transporters and enzymes and were previously reported as being involved in plant salt tolerance, such as NAC, MYB, NXH, WD40, CDPK, LEA, and CIPK. We further validated six putative candidate genes by qRT-PCR and found a differential expression level between salt-tolerant and salt-sensitive varieties. Our findings provide valuable information for enhancing the understanding of complicated mechanisms of salt tolerance in G. hirsutum seedlings and cotton salt tolerance breeding by molecular marker-assisted selection.

Genome-Wide Association Studies Reveal Genetic Variation and Candidate Genes of Drought Stress Related Traits in Cotton (Gossypium hirsutum L.)

Cotton is an important industrial crop worldwide and upland cotton (Gossypium hirsutum L.) is most widely cultivated in the world. Due to ever-increasing water deficit, drought stress brings a major threat to cotton production. Thus, it is important to reveal the genetic basis under drought stress and develop drought tolerant cotton cultivars. To address this issue, in present study, 319 upland cotton accessions were genotyped by 55,060 single nucleotide polymorphisms (SNPs) from high-density CottonSNP80K array and phenotyped nine drought tolerance related traits. The two datasets were used to identify quantitative trait nucleotides (QTNs) for the above nine traits using multi-locus random-SNP-effect mixed linear model method. As a result, a total of 20 QTNs distributed on 16 chromosomes were found to be significantly associated with six drought tolerance related traits. Of the 1,326 genes around the 20 QTNs, 205 were induced after drought stress treatment, and 46 were further mapped to Gene ontology (GO) term "response to stress." Taken genome-wide association study (GWAS) analysis, RNA-seq data and qRT-PCR verification, four genes, RD2 encoding a response to desiccation 2 protein, HAT22 encoding a homeobox-leucine zipper protein, PIP2 encoding a plasma membrane intrinsic protein 2, and PP2C encoding a protein phosphatase 2C, were proposed to be potentially important for drought tolerance in cotton. These results will deepen our understanding of the genetic basis of drought stress tolerance in cotton and provide candidate markers to accelerate the development of drought-tolerant cotton cultivars.

A global survey of alternative splicing in allopolyploid cotton: landscape, complexity and regulation

Alternative splicing (AS) is a crucial regulatory mechanism in eukaryotes, which acts by greatly increasing transcriptome diversity. The extent and complexity of AS has been revealed in model plants using high-throughput next-generation sequencing. However, this technique is less effective in accurately identifying transcript isoforms in polyploid species because of the high sequence similarity between coexisting subgenomes. Here we characterize AS in the polyploid species cotton. Using Pacific Biosciences single-molecule long-read isoform sequencing (Iso-Seq), we developed an integrated pipeline for Iso-Seq transcriptome data analysis (https://github.com/Nextomics/pipeline-for-isoseq). We identified 176 849 full-length transcript isoforms from 44 968 gene models and updated gene annotation. These data led us to identify 15 102 fibre-specific AS events and estimate that c. 51.4% of homoeologous genes produce divergent isoforms in each subgenome. We reveal that AS allows differential regulation of the same gene by miRNAs at the isoform level. We also show that nucleosome occupancy and DNA methylation play a role in defining exons at the chromatin level. This study provides new insights into the complexity and regulation of AS, and will enhance our understanding of AS in polyploid species. Our methodology for Iso-Seq data analysis will be a useful reference for the study of AS in other species.

The JASMONATE ZIM-Domain Gene Family Mediates JA Signaling and Stress Response in Cotton

JASMONATE ZIM-domain (JAZ) family proteins are involved in regulating diverse biological processes in plants. However, their functions have not been well characterized in cotton (Gossypium spp.). In the present study, 13, 15, 25 and 30 JAZ genes were identified in Gossypium arboretum, Gossypium raimondii, Gossypium barbadense and Gossypium hirsutum, respectively, based on gene homology. Selection and variation analyses showed that the single nucleotide polymorphism (SNP) density of GhJAZ genes in wild species was much higher than that in domesticated species. Expression pattern analysis showed that all the GhJAZ genes are expressed in at least one tissue and respond to one or more stress factors, as well as being induced by some phytohormones. The co-expression network indicated that GhJAZ genes might mediate multiple stress response pathways. Yeast two-hybrid (Y2H) experiments showed extensive interactions among GhJAZ proteins, including homo- and heteromeric interactions. Overexpressing one member of the JAZ gene family, GhJAZ2 (Gh_D06G0810), significantly enhanced sensitivity to salt stress in transgenic cotton. Transcriptome analysis indicated that GhJAZ2 regulates stress responses possibly by participating in α-linolenic acid metabolism and jasmonate signaling, and is involved in the repression of GhMYC2 regulated by GhJAZ2. Our data provide important clues for further elucidating the functions of JAZ genes in cotton.

Cotton functional genomics reveals global insight into genome evolution and fiber development

Due to the economic value of natural textile fiber, cotton has attracted much research attention, which has led to the publication of two diploid genomes and two tetraploid genomes. These big data facilitate functional genomic study in cotton, and allow researchers to investigate cotton genome structure, gene expression, and protein function on the global scale using high-throughput methods. In this review, we summarized recent studies of cotton genomes. Population genomic analyses revealed the domestication history of cultivated upland cotton and the roles of transposable elements in cotton genome evolution. Alternative splicing of cotton transcriptomes was evaluated genome-widely. Several important gene families like MYC, NAC, Sus and GhPLDα1 were systematically identified and classified based on genetic structure and biological function. High-throughput proteomics also unraveled the key functional proteins correlated with fiber development. Functional genomic studies have provided unprecedented insights into global-scale methods for cotton research.

Isolation of an indigenous Chlorella vulgaris from swine wastewater and characterization of its nutrient removal ability in undiluted sewage

Bio-treatment of wastewater mediated by microalgae is considered as a promising solution. This work aimed to isolate an indigenous microalgal strain (named MBFJNU-1) from swine wastewater effluent and identify as Chlorella vulgaris. After 12days, the removal efficiencies of total nitrogen (TN) and total phosphorus (TP) in undiluted swine slurry were 90.51% and 91.54%, respectively. Stress tolerance in response to wastewater was verified by cultivating in artificial wastewater containing different levels of chemical oxygen demand (COD), TN and TP. MBFJNU-1 could grow well in undiluted swine slurry and artificial wastewater containing 30,000mg/L COD or 2000mg/L TN. Furthermore, global nuclear DNA methylation (5-mC) of MBFJNU-1 was employed to explore the possible mechanism in response to wastewater stress. The results showed that the level of 5-mC was inversely proportional to the growth of MBFJNU-1 in different diluted swine slurry, helping to understand 5-mC variation in response to stress environment.

A genome-wide survey with different rapeseed ecotypes uncovers footprints of domestication and breeding

Rapeseed (Brassica napus L.) is an important oilseed crop. Despite a short period of domestication and breeding, rapeseed has formed three diverse ecotype groups, namely spring, winter, and semi-winter. However, the genetic changes among the three ecotype groups have remained largely unknown. To detect selective signals, a set of 327 accessions from a worldwide collection were genotyped using a Brassica array, producing 33 186 high-quality single nucleotide polymorphisms (SNPs). Linkage disequilibrium (LD) was unevenly distributed across the genome. A total of 705 (78.2%) weak LD regions were found in the A subgenome, whereas 445 (72.6%) strong LD regions were in the C subgenome. By calculating the nucleotide diversity and population differentiation indices, a total of 198 selective sweeps were identified across ecotype groups, spanning 5.91% (37.9 Mb) of the genome. Within these genome regions, a few known functional genes or loci were found to be in association with environmental adaptability and yield-related traits. In particular, all 12 SNPs detected in significant association with flowering time among accessions were in the selection regions between ecotype groups. These findings provide new insights into the structure of the B. napus genome and uncover the footprints of domestication and breeding.

Activation of ABA Receptors Gene GhPYL9-11A Is Positively Correlated with Cotton Drought Tolerance in Transgenic Arabidopsis

The sensitivity to abscisic acid (ABA) by its receptors, pyrabactin resistance-like proteins (PYLs), is considered a most important factor in activating the ABA signal pathway in response to abiotic stress. However, it is still unknown which PYL is the crucial ABA receptor mediating response to drought stress in cotton (Gossypium hirsutum L.). Here, we reported the identification and characterization of highly induced ABA receptor GhPYL9-11A in response to drought in cotton. It is observed that GhPYL9-11A was highly induced by ABA treatment. GhPYL9-11A binds to protein phosphatase 2Cs (PP2Cs) in an ABA-independent manner. Moreover, the GhPYL-11A-PP2C interactions are partially disrupted by mutations, proline (P84) and histidine (H111), in the gate-latch region. Transgenic Arabidopsis overexpressing GhPYL9-11A plants were hypersensitive to ABA during seed germination and early seedling stage. Further, the increased in root growth and up regulation of drought stress-related genes in transgenic Arabidopsis as compared to wild type confirmed the potential role of GhPYL9-11A in abiotic stress tolerance. Consistently, the expression level of GhPYL9-11A is on average higher in drought-tolerant cotton cultivars than in drought-sensitive cottons under drought treatment. In conclusion, the manipulation of GhPYL9-11A expression could be a useful strategy for developing drought-tolerant cotton cultivars.

QTLs Analysis and Validation for Fiber Quality Traits Using Maternal Backcross Population in Upland Cotton

Cotton fiber is renewable natural fiber source for textile. Improving fiber quality is an essential goal for cotton breeding project. In present study, F₁₄ recombinant inbred line (RIL) population was backcrossed by the maternal parent to obtain a backcross (BC) population, derived from one Upland cotton hybrid. Three repetitive field trials were performed by randomized complete block design with two replicates in three locations in 2015, together with the BC population, common male parent and the RIL population. Totally, 26 QTLs in BC population explained 5.00-14.17% of phenotype variation (PV) and 37 quantitative trait loci (QTL) were detected in RIL population explaining 5.13-34.00% of PV. Seven common QTLs detected simultaneously in two populations explained PV from 7.69 to 23.05%. A total of 20 QTLs in present study verified the previous results across three environments in 2012. Particularly, qFL-Chr5-2 controlling fiber length on chromosome 5 explained 34.00% of PV, while qFL-Chr5-3 only within a 0.8 cM interval explained 13.93% of PV on average in multiple environments. These stable QTLs explaining great variation offered essential information for marker-assisted selection (MAS) to improve fiber quality traits. Lots of epistasis being detected in both populations acted as one of important genetic compositions of fiber quality traits.