The genome sequence of Sea-Island cotton (Gossypium barbadense) provides insights into the allopolyploidization and development of superior spinnable fibres

Characterization of cotton ARF factors and the role of GhARF2b in fiber development

BACKGROUND

Cotton fiber is a model system for studying plant cell development. At present, the functions of many transcription factors in cotton fiber development have been elucidated, however, the roles of auxin response factor (ARF) genes in cotton fiber development need be further explored.

RESULTS

Here, we identify auxin response factor (ARF) genes in three cotton species: the tetraploid upland cotton G. hirsutum, which has 73 ARF genes, and its putative extent parental diploids G. arboreum and G. raimondii, which have 36 and 35 ARFs, respectively. Ka and Ks analyses revealed that in G. hirsutum ARF genes have undergone asymmetric evolution in the two subgenomes. The cotton ARFs can be classified into four phylogenetic clades and are actively expressed in young tissues. We demonstrate that GhARF2b, a homolog of the Arabidopsis AtARF2, was preferentially expressed in developing ovules and fibers. Overexpression of GhARF2b by a fiber specific promoter inhibited fiber cell elongation but promoted initiation and, conversely, its downregulation by RNAi resulted in fewer but longer fiber. We show that GhARF2b directly interacts with GhHOX3 and represses the transcriptional activity of GhHOX3 on target genes.

CONCLUSION

Our results uncover an important role of the ARF factor in modulating cotton fiber development at the early stage.

The cellulose synthase (CesA) gene family in four Gossypium species: phylogenetics, sequence variation and gene expression in relation to fiber quality in Upland cotton

Cellulose synthases (CesAs) are multi-subunit enzymes found on the plasma membrane of plant cells and play a pivotal role in cellulose production. The cotton fiber is mainly composed of cellulose, and the genetic relationships between CesA genes and cotton fiber yield and quality are not fully understood. Through a phylogenetic analysis, the CesA gene family in diploid Gossypium arboreum and Gossypium raimondii, as well as tetraploid Gossypium hirsutum ('TM-1') and Gossypium barbadense ('Hai-7124' and '3-79'), was divided into 6 groups and 15 sub-groups, with each group containing two to five homologous genes. Most CesA genes in the four species are highly collinear. Among the five cotton genomes, 440 and 1929 single nucleotide polymorphisms (SNPs) in the CesA gene family were identified in exons and introns, respectively, including 174 SNPs resulting in amino acid changes. In total, 484 homeologous SNPs between the A and D genomes were identified in diploids, while 142 SNPs were detected between the two tetraploids, with 32 and 82 SNPs existing within G. hirsutum and G. barbadense, respectively. Additionally, 74 quantitative trait loci near 18 GhCesA genes were associated with fiber quality. One to four GhCesA genes were differentially expressed (DE) in ovules at 0 and 3 days post anthesis (DPA) between two backcross inbred lines having different fiber lengths, but no DE genes were identified between these lines in developing fibers at 10 DPA. Twenty-seven SNPs in above DE CesA genes were detected among seven cotton lines, including one SNP in Ghi_A08G03061 that was detected in four G. hirsutum genotypes. This study provides the first comprehensive characterization of the cotton CesA gene family, which may play important roles in determining cotton fiber quality.

Embryogenic Calli Induction and Salt Stress Response Revealed by RNA-Seq in Diploid Wild Species Gossypium sturtianum and Gossypium raimondii

Wild cotton species can contribute to a valuable gene pool for genetic improvement, such as genes related to salt tolerance. However, reproductive isolation of different species poses an obstacle to produce hybrids through conventional breeding. Protoplast fusion technology for somatic cell hybridization provides an opportunity for genetic manipulation and targeting of agronomic traits. Transcriptome sequencing analysis of callus under salt stress is conducive to study salt tolerance genes. In this study, calli were induced to provide materials for extracting protoplasts and also for screening salt tolerance genes. Calli were successfully induced from leaves of Gossypium sturtianum (C₁ genome) and hypocotyls of G. raimondii (D₅ genome), and embryogenic calli of G. sturtianum and G. raimondii were induced on a differentiation medium with different concentrations of 2, 4-D, KT, and IBA, respectively. In addition, embryogenic calli were also induced successfully from G. raimondii through suspension cultivation. Transcriptome sequencing analysis was performed on the calli of G. raimondii and G. sturtianum, which were treated with 200 mM NaCl at 0, 6, 12, 24, and 48 h, and a total of 12,524 genes were detected with different expression patterns under salt stress. Functional analysis showed that 3,482 genes, which were differentially expressed in calli of G. raimondii and G. sturtianum, were associated with biological processes of nucleic acid binding, plant hormone (such as ABA) biosynthesis, and signal transduction. We demonstrated that DEGs or TFs which related to ABA metabolism were involved in the response to salt stress, including xanthoxin dehydrogenase genes (ABA2), sucrose non-fermenting 1-related protein kinases (SnRK2), NAM, ATAT1/2, and CUC2 transcription factors (NAC), and WRKY class of zinc-finger proteins (WRKY). This research has successfully induced calli from two diploid cotton species and revealed new genes responding to salt stress in callus tissue, which will lay the foundation for protoplast fusion for further understanding of salt stress responses in cotton.

The Cotton Wall-Associated Kinase GhWAK7A Mediates Responses to Fungal Wilt Pathogens by Complexing with the Chitin Sensory Receptors

Plant receptor-like kinases (RLKs) are important players in response to pathogen infections. Verticillium and Fusarium wilts, caused by Verticillium dahliae (Vd) and Fusarium oxysporum f. sp vasinfectum (Fov), respectively, are among the most devastating diseases in cotton (Gossypium spp). To understand the cotton response to these soil-borne fungal pathogens, we performed a genome-wide in silico characterization and functional screen of diverse RLKs for their involvement in cotton wilt diseases. We identified Gossypium hirsutum GhWAK7A, a wall-associated kinase, that positively regulates cotton response to both Vd and Fov infections. Chitin, the major constituent of the fungal cell wall, is perceived by lysin-motif-containing RLKs (LYKs/CERK1), leading to the activation of plant defense against fungal pathogens. A conserved chitin sensing and signaling system is present in cotton, including chitin-induced GhLYK5-GhCERK1 dimerization and phosphorylation, and contributes to cotton defense against Vd and Fov Importantly, GhWAK7A directly interacts with both GhLYK5 and GhCERK1 and promotes chitin-induced GhLYK5-GhCERK1 dimerization. GhWAK7A phosphorylates GhLYK5, which itself does not have kinase activity, but requires phosphorylation for its function. Consequently, GhWAK7A plays a crucial role in chitin-induced responses. Thus, our data reveal GhWAK7A as an important component in cotton response to fungal wilt pathogens by complexing with the chitin receptors.

A Retrotransposon Insertion in GhMML3_D12 Is Likely Responsible for the Lintless Locus li3 of Tetraploid Cotton

Cotton (Gossypium) seed fibers can be divided into lint (long) or fuzz (very short). Using fiberless (fuzzless-lintless) mutants, the lint initiation gene Li₃ was identified by map-based cloning. The gene is an R2R3-MYB transcription factor located on chromosome D12 (GhMML3_D12). Sequence analysis revealed that li₃ is a loss-of-function allele containing a retrotransposon insertion in the second exon that completely blocks the gene’s expression. The genetic loci n₂ and n₃ underlying the recessive fuzzless phenotype in Gossypium hirsutum were also mapped. The genomic location of n₃ overlapped with that of the dominant fuzzless locus N₁, and n₃ appeared to be a loss-of-function allele caused by a single nucleotide polymorphism (SNP) mutation in the coding region of GhMML3_A12. The n₂ allele was found to be co-located with li₃ and originated from G. babardense. n₂ and li₃ are possibly the multiple alleles of the GhMML3_D12 gene. Genetic analysis showed that Li₃ and N₃ are a pair of homologs with additive effects for the initiation of fibers (fuzz or lint). In addition, the presence of another locus was speculated, and it appeared to show an inhibitory effect on the expression of GhMML3. These findings provide new information about the genetic factors affecting the initiation of fibers in cotton.

Targeted Metagenomics of Retting in Flax: The Beginning of the Quest to Harness the Secret Powers of the Microbiota

The mechanical and chemical properties of natural plant fibers are determined by many different factors, both intrinsic and extrinsic to the plant, during growth but also after harvest. A better understanding of how all these factors exert their effect and how they interact is necessary to be able to optimize fiber quality for use in different industries. One important factor is the post-harvest process known as retting, representing the first step in the extraction of bast fibers from the stem of species such as flax and hemp. During this process microorganisms colonize the stem and produce hydrolytic enzymes that target cell wall polymers thereby facilitating the progressive destruction of the stem and fiber bundles. Recent advances in sequencing technology have allowed researchers to implement targeted metagenomics leading to a much better characterization of the microbial communities involved in retting, as well as an improved understanding of microbial dynamics. In this paper we review how our current knowledge of the microbiology of retting has been improved by targeted metagenomics and discuss how related '-omics' approaches might be used to fully characterize the functional capability of the retting microbiome.

Genome-wide analysis of PRR gene family uncovers their roles in circadian rhythmic changes and response to drought stress in Gossypium hirsutum L

BACKGROUND

The circadian clock not only participates in regulating various stages of plant growth, development and metabolism, but confers plant environmental adaptability to stress such as drought. Pseudo-Response Regulators (PRRs) are important component of the central oscillator (the core of circadian clock) and play a significant role in plant photoperiod pathway. However, no systematical study about this gene family has been performed in cotton.

METHODS

PRR genes were identified in diploid and tetraploid cotton using bioinformatics methods to investigate their homology, duplication and evolution relationship. Differential gene expression, KEGG enrichment analysis and qRT-PCR were conducted to analyze PRR gene expression patterns under diurnal changes and their response to drought stress.

RESULTS

A total of 44 PRR family members were identified in four Gossypium species, with 16 in G. hirsutum, 10 in G. raimondii, and nine in G. barbadense as well as in G. arboreum. Phylogenetic analysis indicated that PRR proteins were divided into five subfamilies and whole genome duplication or segmental duplication contributed to the expansion of Gossypium PRR gene family. Gene structure analysis revealed that members in the same clade are similar, and multiple cis-elements related to light and drought stress response were enriched in the promoters of GhPRR genes. qRT-PCR results showed that GhPRR genes transcripts presented four expression peaks (6 h, 9 h, 12 h, 15 h) during 24 h and form obvious rhythmic expression trend. Transcriptome data with PEG treatment, along with qRT-PCR verification suggested that members of clade III (GhPRR5a, b, d) and clade V (GhPRR3a and GhPRR3c) may be involved in drought response. This study provides an insight into understanding the function of PRR genes in circadian rhythm and in response to drought stress in cotton.

Sequencing Multiple Cotton Genomes Reveals Complex Structures and Lays Foundation for Breeding

Cotton is a major fiber plant, which provides raw materials for clothing, protecting humans from the harsh environment of cold or hot weathers, enriching the culture and custom of human societies. Due to its importance, the diploid and tetraploid genomes of different cotton plants have been repeatedly sequenced to obtain their complete and fine genome sequences. These valuable genome data sets revealed the evolutionary past of the cotton plants, which were recursively affected by polyploidization, with a decaploidization contributing to the formation of the genus Gossypium, and a neo-tetraploidization contributing to the formation of nowadays widely cultivated cotton plants. Post-polyploidization genome instability resulted in numerous structural changes of the genomes, such as gene loss, DNA inversion and translocation, illegitimate recombination, and accumulation of repetitive sequences, and functional innovation accompanied by elevated evolutionary rates of genes. Many these changes have been asymmetric between subgnomes of the tetraploid cottons, rendering their divergent profiles of biological regulation and function. The availability of whole-genome sequences has now paved the way to identify and clone functional genes, e.g., those relating to fiber development, and to enhance breeding efforts to cultivate cottons to produce high-yield and high-quality fibers, and to resist environmental and biological stress.

Genome-wide characterization of the GRF family and their roles in response to salt stress in Gossypium

BACKGROUND

Cotton (Gossypium spp.) is the most important world-wide fiber crop but salt stress limits cotton production in coastal and other areas. Growth regulation factors (GRFs) play regulatory roles in response to salt stress, but their roles have not been studied in cotton under salt stress.

RESULTS

We identified 19 GRF genes in G. raimondii, 18 in G. arboreum, 34 in G. hirsutum and 45 in G. barbadense, respectively. These GRF genes were phylogenetically analyzed leading to the recognition of seven GRF clades. GRF genes from diploid cottons (G. raimondii and G. arboreum) were largely retained in allopolyploid cotton, with subsequent gene expansion in G. barbadense relative to G. hirsutum. Most G. hirsutum GRF (GhGRF) genes are preferentially expressed in young and growing tissues. To explore their possible role in salt stress, we used qRT-PCR to study expression responses to NaCl treatment, showing that five GhGRF genes were down-regulated in leaves. RNA-seq experiments showed that seven GhGRF genes exhibited decreased expression in leaves under NaCl treatment, three of which (GhGRF3, GhGRF4, and GhGRF16) were identified by both RNA-seq and qRT-PCR. We also identified six and three GRF genes that exhibit decreased expression under salt stress in G. arboreum and G. barbadense, respectively. Consistent with its lack of leaf withering or yellowing under the salt treatment conditions, G. arboreum had better salt tolerance than G. hirsutum and G. barbadense. Our results suggest that GRF genes are involved in salt stress responses in Gossypium.

CONCLUSION

In summary, we identified candidate GRF genes that were involved in salt stress responses in cotton.

Fine-mapping and candidate gene analysis of qFS-Chr. D02, a QTL for fibre strength introgressed from a semi-wild cotton into Gossypium hirsutum

Fibre strength (FS) is an important quality attribute in the modern textile industry, which is genetically controlled by quantitative trait loci (QTLs). Fine-mapping stable QTLs for FS to identify candidate genes would be valuable for uncovering the genetic basis of fibre quality traits in cotton. Here, a single segment introgression line, IL-D2-2, from the cross of (TM-1×TX-1046) reported in our previous studies, was found to have significantly improved FS compared with the recurrent parent TM-1. To fine-map the QTLs of the FS, we further crossed IL-D2-2 with its recurrent parent TM-1 to produce F₂ and F_2:3 populations. QTL analysis and substitution mapping showed qFS-Chr. D02 was anchored into a 550.66 kb-interval between two markers, INTR1027 and JESPR-231. This interval contained 67 genes, among which 27 genes related to cell-wall synthesis were selected to conduct qRT-PCR. The results revealed seven genes were expressed significantly differently during the fibre secondary-wall-thickening stage (10-25 days post-anthesis), three being upregulated and four downregulated in IL-D2-2. Both GH_D02G2269 (UDP-glucosyl transferase 84B1) and GH_D02G2289 (unknown function (DUF869)) with nonsynonymous SNPs in IL-D2-2 had significantly downregulated expression, suggesting they were candidates for qFS-Chr. D02. This research provides information about marker-assisted selection for cotton fibre strength improvement.

The miR319-Targeted GhTCP4 Promotes the Transition from Cell Elongation to Wall Thickening in Cotton Fiber

Plant cell growth involves a complex interplay among cell-wall expansion, biosynthesis, and, in specific tissues, secondary cell wall (SCW) deposition, yet the coordination of these processes remains elusive. Cotton fiber cells are developmentally synchronous, highly elongated, and contain nearly pure cellulose when mature. Here, we report that the transcription factor GhTCP4 plays an important role in balancing cotton fiber cell elongation and wall synthesis. During fiber development the expression of miR319 declines while GhTCP4 transcript levels increase, with high levels of the latter promoting SCW deposition. GhTCP4 interacts with a homeobox-containing factor, GhHOX3, and repressing its transcriptional activity. GhTCP4 and GhHOX3 function antagonistically to regulate cell elongation, thereby establishing temporal control of fiber cell transition to the SCW stage. We found that overexpression of GhTCP4A upregulated and accelerated activation of the SCW biosynthetic pathway in fiber cells, as revealed by transcriptome and promoter activity analyses, resulting in shorter fibers with varied lengths and thicker walls. In contrast, GhTCP4 downregulation led to slightly longer fibers and thinner cell walls. The GhHOX3-GhTCP4 complex may represent a general mechanism of cellular development in plants since both are conserved factors in many species, thus providing us a potential molecular tool for the design of fiber traits.

Combined GWAS and eQTL analysis uncovers a genetic regulatory network orchestrating the initiation of secondary cell wall development in cotton

The cotton fibre serves as a valuable experimental system to study cell wall synthesis in plants, but our understanding of the genetic regulation of this process during fibre development remains limited. We performed a genome-wide association study (GWAS) and identified 28 genetic loci associated with fibre quality in allotetraploid cotton. To investigate the regulatory roles of these loci, we sequenced fibre transcriptomes of 251 cotton accessions and identified 15 330 expression quantitative trait loci (eQTL). Analysis of local eQTL and GWAS data prioritised 13 likely causal genes for differential fibre quality in a transcriptome-wide association study (TWAS). Characterisation of distal eQTL revealed unequal genetic regulation patterns between two subgenomes, highlighted by an eQTL hotspot (Hot216) that established a genome-wide genetic network regulating the expression of 962 genes. The primary regulatory role of Hot216, and specifically the gene encoding a KIP-related protein, was found to be the transcriptional regulation of genes responsible for cell wall synthesis, which contributes to fibre length by modulating the developmental transition from rapid cell elongation to secondary cell wall synthesis. This study uncovered the genetic regulation of fibre-cell development and revealed the molecular basis of the temporal modulation of secondary cell wall synthesis during plant cell elongation.

Gossypium Genomics: Trends, Scope, and Utilization for Cotton Improvement

Cotton (Gossypium spp.) is the most important natural fiber crop worldwide. The diversity of Gossypium species also provides an ideal model for investigating evolution and domestication of polyploids. However, the huge and complex cotton genome hinders genomic research. Technical advances in high-throughput sequencing and bioinformatics analysis have now largely overcome these obstacles, bringing about a new era of cotton genomics. Here, we review recent progress in Gossypium genomics based on whole genome sequencing, resequencing, and comparative genomics, which have provided insights about the genomic basis of fiber biogenesis and the landscape of cotton functional genomics. We address current challenges and present multidisciplinary genomics-enabled breeding strategies covering the breadth of high fiber yield, quality, and environmental resilience for future cotton breeding programs.

Genome-wide identification and expression analysis of PUB genes in cotton

Background

The U-box gene encodes a ubiquitin ligase that contain U-box domain. The plant U-box gene (PUB) plays an important role in the response to stresses, but few reports about PUBs in cotton were available. Therefore research on PUBs is of great importance and a necessity when studying the mechanisms of stress- tolerance in cotton.

Results

In this study, we identified 93, 96, 185 and 208 PUBs from four sequenced cotton species G. raimondii (D₅), G. arboreum (A₂), G. hirsutum (AD₁) and G. barbadense (AD2), respectively. Prediction analysis of subcellular localization showed that the PUBs in cotton were widely localized in cells, but primarily in the nucleus. The PUBs in cotton were classified into six subfamilies (A-F) on the basis of phylogenetic analysis, which was testified by the analysis of conserved motifs and exon-intron structures. Chromosomal localization analysis showed that cotton PUBs were unevenly anchored on all chromosomes, varying from 1 to 14 per chromosome. Through multiple sequence alignment analysis, 3 tandem duplications and 28 segmental duplications in cotton genome D₅, 2 tandem duplications and 25 segmental duplications in A₂, and 143 homologous gene pairs in A₂ and D₅ were found; however no tandem duplications in A₂ or D₅ were found. Additionally, 105, 14 and 17 homologous gene pairs were found in the intra-subgenome of At and Dt, At sub-genome and Dt sub-genome of G. hirsutum, respectively. Functional analysis of GhPUB85A and GhPUB45D showed that these genes positively responded to abiotic stresses, but the expression patterns were different. In addition, although the expression levels of these two homologous genes were similar, their contributions were different when responding to stresses, specifically showing different responses to abiotic stresses and functional differences between the two subgenomes of G. hirsutum.

Conclusions

This study reported the genome-wide identification, structure, evolution and expression analysis of PUBs in cotton, and the results showed that the PUBs were highly conserved throughout the evolutionary history of cotton. All PUB genes were involved in the response to abiotic stresses (including salt, drought, hot and cold) to varying degrees.

Salt-tolerance diversity in diploid and polyploid cotton (Gossypium) species

The development of salt-tolerant genotypes is pivotal for the effective utilization of salinized land and to increase global crop productivity. Several cotton species comprise the most important source of textile fibers globally, and these are increasingly grown on marginal or increasingly saline agroecosystems. The allopolyploid cotton species also provide a model system for polyploid research, of relevance here because polyploidy was suggested to be associated with increased adaptation to stress. To evaluate the genetic variation of salt tolerance among cotton species, 17 diverse accessions of allopolyploid (AD-genome) and diploid (A- and D-genome) Gossypium were evaluated for a total of 29 morphological and physiological traits associated with salt tolerance. For most morphological and physiological traits, cotton accessions showed highly variable responses to 2 weeks of exposure to moderate (50 mm NaCl) and high (100 mm NaCl) hydroponic salinity treatments. Our results showed that the most salt-tolerant species were the allopolyploid Gossypium mustelinum from north-east Brazil, the D-genome diploid Gossypium klotzschianum from the Galapagos Islands, followed by the A-genome diploids of Africa and Asia. Generally, A-genome accessions outperformed D-genome cottons under salinity conditions. Allopolyploid accessions from either diploid genomic group did not show significant differences in salt tolerance, but they were more similar to one of the two progenitor lineages. Our findings demonstrate that allopolyploidy in itself need not be associated with increased salinity stress tolerance and provide information for using the secondary Gossypium gene pool to breed for improved salt tolerance.

Open-Bud Duplicate Loci Are Identified as MML10s, Orthologs of MIXTA-Like Genes on Homologous Chromosomes of Allotetraploid Cotton

The open-bud (ob) mutants in cotton display abnormal flower buds with the stigma and upper anthers exposed before blooming. This characteristic is potentially useful for the efficient production of hybrid seeds. The recessive inheritance pattern of the ob phenotype in allotetraploid cotton is determined by duplicated recessive loci (ob1ob1ob2ob2). In this study, ob1, which is a MIXTA-like MYB gene on chromosome D13 (MML10_Dt), was identified by map-based cloning. In Gossypium barbadense (Gb) acc. 3-79, a single nucleotide polymorphism (SNP) (G/A) at the splice site of the first intron and an 8-bp deletion in the third exon of MML10_Dt were found, which are the causative mutations at the ob1 loci. A 1783-bp deletion that leads to the loss of the third exon and accounts for the causal variation at the ob2 loci was found in MML10_At of Gossypium hirsutum (Gh) acc. TM-1. The ob phenotype results from the combination of these two loss-of-function loci. Genotyping assays showed that the ob1 and ob2 loci appeared after the formation of allotetraploid cotton and were specific for Gb and Gh, respectively. All Gb lines and most Gh cultivars carry the single corresponding mutant alleles. Genome-wide transcriptome analysis showed that some of the MYB genes and genes related to cell wall biogenesis, trichome differentiation, cytokinin signal transduction, and cell division were repressed in the ob mutants, which may lead to suppression of petal growth. These findings should be of value for breeding superior ob lines in cotton.

Genome-Wide Characterization and Analysis of CIPK Gene Family in Two Cultivated Allopolyploid Cotton Species: Sequence Variation, Association with Seed Oil Content, and the Role of GhCIPK6

Calcineurin B-like protein-interacting protein kinases (CIPKs), as key regulators, play an important role in plant growth and development and the response to various stresses. In the present study, we identified 80 and 78 CIPK genes in the Gossypium hirsutum and G. barbadense, respectively. The phylogenetic and gene structure analysis divided the cotton CIPK genes into five groups which were classified into an exon-rich clade and an exon-poor clade. A synteny analysis showed that segmental duplication contributed to the expansion of Gossypium CIPK gene family, and purifying selection played a major role in the evolution of the gene family in cotton. Analyses of expression profiles showed that GhCIPK genes had temporal and spatial specificity and could be induced by various abiotic stresses. Fourteen GhCIPK genes were found to contain 17 non-synonymous single nucleotide polymorphisms (SNPs) and co-localized with oil or protein content quantitative trait loci (QTLs). Additionally, five SNPs from four GhCIPKs were found to be significantly associated with oil content in one of the three field tests. Although most GhCIPK genes were not associated with natural variations in cotton oil content, the overexpression of the GhCIPK6 gene reduced the oil content and increased C18:1 and C18:1+C18:1d6 in transgenic cotton as compared to wild-type plants. In addition, we predicted the potential molecular regulatory mechanisms of the GhCIPK genes. In brief, these results enhance our understanding of the roles of CIPK genes in oil synthesis and stress responses.

Identification of a genome-specific repetitive element in the Gossypium D genome

The activity of genome-specific repetitive sequences is the main cause of genome variation between Gossypium A and D genomes. Through comparative analysis of the two genomes, we retrieved a repetitive element termed ICRd motif, which appears frequently in the diploid Gossypium raimondii (D₅) genome but rarely in the diploid Gossypium arboreum (A₂) genome. We further explored the existence of the ICRd motif in chromosomes of G. raimondii, G. arboreum, and two tetraploid (AADD) cotton species, Gossypium hirsutum and Gossypium barbadense, by fluorescence in situ hybridization (FISH), and observed that the ICRd motif exists in the D₅ and D-subgenomes but not in the A₂ and A-subgenomes. The ICRd motif comprises two components, a variable tandem repeat (TR) region and a conservative sequence (CS). The two constituents each have hundreds of repeats that evenly distribute across 13 chromosomes of the D₅genome. The ICRd motif (and its repeats) was revealed as the common conservative region harbored by ancient Long Terminal Repeat Retrotransposons. Identification and investigation of the ICRd motif promotes the study of A and D genome differences, facilitates research on Gossypium genome evolution, and provides assistance to subgenome identification and genome assembling.

High-efficient and precise base editing of C•G to T•A in the allotetraploid cotton (Gossypium hirsutum) genome using a modified CRISPR/Cas9 system

The base-editing technique using CRISPR/nCas9 (Cas9 nickase) or dCas9 (deactivated Cas9) fused with cytidine deaminase is a powerful tool to create point mutations. In this study, a novel G. hirsutum-Base Editor 3 (GhBE3) base-editing system has been developed to create single-base mutations in the allotetraploid genome of cotton (Gossypium hirsutum). A cytidine deaminase sequence (APOBEC) fused with nCas9 and uracil glycosylase inhibitor (UGI) was inserted into our CRISPR/Cas9 plasmid (pRGEB32-GhU6.7). Three target sites were chosen for two target genes, GhCLA and GhPEBP, to test the efficiency and accuracy of GhBE3. The editing efficiency ranged from 26.67 to 57.78% at the three target sites. Targeted deep sequencing revealed that the C→T substitution efficiency within an 'editing window', approximately six-nucleotide windows of -17 to -12 bp from the PAM sequence, was up to 18.63% of the total sequences. The 27 most likely off-target sites predicted by CRISPR-P and Cas-OFFinder tools were analysed by targeted deep sequencing, and it was found that rare C→T substitutions (average < 0.1%) were detected in the editing windows of these sites. Furthermore, whole-genome sequencing analyses on two GhCLA-edited and one wild-type plants with about 100× depth showed that no bona fide off-target mutations were detectable from 1500 predicted potential off-target sites across the genome. In addition, the edited bases were inherited to T1 progeny. These results demonstrate that GhBE3 has high specificity and accuracy for the generation of targeted point mutations in allotetraploid cotton.

Identification of a genome-specific repetitive element in the Gossypium D genome

The activity of genome-specific repetitive sequences is the main cause of genome variation between Gossypium A and D genomes. Through comparative analysis of the two genomes, we retrieved a repetitive element termed ICRd motif, which appears frequently in the diploid Gossypium raimondii (D₅) genome but rarely in the diploid Gossypium arboreum (A₂) genome. We further explored the existence of the ICRd motif in chromosomes of G. raimondii, G. arboreum, and two tetraploid (AADD) cotton species, Gossypium hirsutum and Gossypium barbadense, by fluorescence in situ hybridization (FISH), and observed that the ICRd motif exists in the D₅ and D-subgenomes but not in the A₂ and A-subgenomes. The ICRd motif comprises two components, a variable tandem repeat (TR) region and a conservative sequence (CS). The two constituents each have hundreds of repeats that evenly distribute across 13 chromosomes of the D₅genome. The ICRd motif (and its repeats) was revealed as the common conservative region harbored by ancient Long Terminal Repeat Retrotransposons. Identification and investigation of the ICRd motif promotes the study of A and D genome differences, facilitates research on Gossypium genome evolution, and provides assistance to subgenome identification and genome assembling.

Sequence Composition of Bacterial Chromosome Clones in a Transgressive Root-Knot Nematode Resistance Chromosome Region in Tetraploid Cotton

Plants evolve innate immunity including resistance genes to defend against pest and pathogen attack. Our previous studies in cotton (Gossypium spp.) revealed that one telomeric segment on chromosome (Chr) 11 in G. hirsutum cv. Acala NemX (rkn1 locus) contributed to transgressive resistance to the plant parasitic nematode Meloidogyne incognita, but the highly homologous segment on homoeologous Chr 21 had no resistance contribution. To better understand the resistance mechanism, a bacterial chromosome (BAC) library of Acala N901 (Acala NemX resistance source) was used to select, sequence, and analyze BAC clones associated with SSR markers in the complex rkn1 resistance region. Sequence alignment with the susceptible G. hirsutum cv. TM-1 genome indicated that 23 BACs mapped to TM-1-Chr11 and 18 BACs mapped to TM-1-Chr 21. Genetic and physical mapping confirmed less BAC sequence (53-84%) mapped with the TM-1 genome in the rkn1 region on Chr 11 than to the homologous region (>89%) on Chr 21. A 3.1-cM genetic distance between the rkn1 flanking markers CIR316 and CIR069 was mapped in a Pima S-7 × Acala NemX RIL population with a physical distance ∼1 Mbp in TM-1. NCBI Blast and Gene annotation indicated that both Chr 11 and Chr 21 harbor resistance gene-rich cluster regions, but more multiple homologous copies of Resistance (R) proteins and of adjacent transposable elements (TE) are present within Chr 11 than within Chr 21. (CC)-NB-LRR type R proteins were found in the rkn1 region close to CIR316, and (TIR)-NB-LRR type R proteins were identified in another resistance rich region 10 cM from CIR 316 (∼3.1 Mbp in the TM-1 genome). The identified unique insertion/deletion in NB-ARC domain, different copies of LRR domain, multiple copies or duplication of R proteins, adjacent protein kinases, or TE in the rkn1 region on Chr 11 might be major factors contributing to complex recombination and transgressive resistance.

Genome-wide quantitative trait loci reveal the genetic basis of cotton fibre quality and yield-related traits in a Gossypium hirsutum recombinant inbred line population

Cotton is widely cultivated globally because it provides natural fibre for the textile industry and human use. To identify quantitative trait loci (QTLs)/genes associated with fibre quality and yield, a recombinant inbred line (RIL) population was developed in upland cotton. A consensus map covering the whole genome was constructed with three types of markers (8295 markers, 5197.17 centimorgans (cM)). Six fibre yield and quality traits were evaluated in 17 environments, and 983 QTLs were identified, 198 of which were stable and mainly distributed on chromosomes 4, 6, 7, 13, 21 and 25. Thirty-seven QTL clusters were identified, in which 92.8% of paired traits with significant medium or high positive correlations had the same QTL additive effect directions, and all of the paired traits with significant medium or high negative correlations had opposite additive effect directions. In total, 1297 genes were discovered in the QTL clusters, 414 of which were expressed in two RNA-Seq data sets. Many genes were discovered, 23 of which were promising candidates. Six important QTL clusters that included both fibre quality and yield traits were identified with opposite additive effect directions, and those on chromosome 13 (qClu-chr13-2) could increase fibre quality but reduce yield; this result was validated in a natural population using three markers. These data could provide information about the genetic basis of cotton fibre quality and yield and help cotton breeders to improve fibre quality and yield simultaneously.

Comparative transcriptome, physiological and biochemical analyses reveal response mechanism mediated by CBF4 and ICE2 in enhancing cold stress tolerance in Gossypium thurberi

Low temperature is one of the key environmental stresses that impair plant growth and significantly restricts the productivity and spatial distribution of crop plants. Gossypium thurberi, a wild diploid cotton species, has adapted to a wide range of temperatures and exhibits a better tolerance to chilling stress. Here, we compared phenotypes and physiochemical changes in G. thurberi under cold stress and found this species indeed showed better cold tolerance. Therefore, to understand the molecular mechanisms of the cold tolerance in G. thurberi, we compared transcription changes in leaves of G. thurberi under cold stress by high-throughput transcriptome sequencing. In total, 35 617 unigenes were identified in the whole-genome transcription profile, and 4226 differentially expressed genes (DEGs) were discovered in the leaves upon cold treatment. Gene Ontology (GO) classification analyses showed that the majority of DEGs belonged to categories of signal transduction, transcription factors (TFs) and carbohydrate transport and metabolism. The expression of several cold-responsive genes such as ICE1, CBF4, RAP2-7 and abscisic acid (ABA) biosynthesis genes involved in different signalling pathways were induced after G. thurberi seedlings were exposed to cold stress. Furthermore, cold sensitivity was increased in CBF4 and ICE2 virus-induced gene silencing (VIGS) plants, and high level of malondialdehyde (MDA) showed that the CBF4 and ICE2 silenced plants were under oxidative stress compared to their wild types, which relatively had higher levels of antioxidant enzyme activity, as evident by high levels of proline and superoxide dismutase (SOD) content. In conclusion, our findings reveal a new regulatory network of cold stress response in G. thurberi and broaden our understanding of the cold tolerance mechanism in cotton, which might accelerate functional genomics studies and genetic improvement for cold stress tolerance in cultivated cotton.

Single-molecule real-time sequencing facilitates the analysis of transcripts and splice isoforms of anthers in Chinese cabbage (Brassica rapa L. ssp. pekinensis)

BACKGROUND

Anther development has been extensively studied at the transcriptional level, but a systematic analysis of full-length transcripts on a genome-wide scale has not yet been published. Here, the Pacific Biosciences (PacBio) Sequel platform and next-generation sequencing (NGS) technology were combined to generate full-length sequences and completed structures of transcripts in anthers of Chinese cabbage.

RESULTS

Using single-molecule real-time sequencing (SMRT), a total of 1,098,119 circular consensus sequences (CCSs) were generated with a mean length of 2664 bp. More than 75% of the CCSs were considered full-length non-chimeric (FLNC) reads. After error correction, 725,731 high-quality FLNC reads were estimated to carry 51,501 isoforms from 19,503 loci, consisting of 38,992 novel isoforms from known genes and 3691 novel isoforms from novel genes. Of the novel isoforms, we identified 407 long non-coding RNAs (lncRNAs) and 37,549 open reading frames (ORFs). Furthermore, a total of 453,270 alternative splicing (AS) events were identified and the majority of AS models in anther were determined to be approximate exon skipping (XSKIP) events. Of the key genes regulated during anther development, AS events were mainly identified in the genes SERK1, CALS5, NEF1, and CESA1/3. Additionally, we identified 104 fusion transcripts and 5806 genes that had alternative polyadenylation (APA).

CONCLUSIONS

Our work demonstrated the transcriptome diversity and complexity of anther development in Chinese cabbage. The findings provide a basis for further genome annotation and transcriptome research in Chinese cabbage.

Intergenomic gene transfer in diploid and allopolyploid Gossypium

BACKGROUND

Intergenomic gene transfer (IGT) between nuclear and organellar genomes is a common phenomenon during plant evolution. Gossypium is a useful model to evaluate the genomic consequences of IGT for both diploid and polyploid species. Here, we explore IGT among nuclear, mitochondrial, and plastid genomes of four cotton species, including two allopolyploids and their model diploid progenitors (genome donors, G. arboreum: A₂ and G. raimondii: D₅).

RESULTS

Extensive IGT events exist for both diploid and allotetraploid cotton (Gossypium) species, with the nuclear genome being the predominant recipient of transferred DNA followed by the mitochondrial genome. The nuclear genome has integrated 100 times more foreign sequences than the mitochondrial genome has in total length. In the nucleus, the integrated length of chloroplast DNA (cpDNA) was between 1.87 times (in diploids) to nearly four times (in allopolyploids) greater than that of mitochondrial DNA (mtDNA). In the mitochondrion, the length of nuclear DNA (nuDNA) was typically three times than that of cpDNA. Gossypium mitochondrial genomes integrated three nuclear retrotransposons and eight chloroplast tRNA genes, and incorporated chloroplast DNA prior to divergence between the diploids and allopolyploid formation. For mitochondrial chloroplast-tRNA genes, there were 2-6 bp conserved microhomologies flanking their insertion sites across distantly related genera, which increased to 10 bp microhomologies for the four cotton species studied. For organellar DNA sequences, there are source hotspots, e.g., the atp6-trnW intergenic region in the mitochondrion and the inverted repeat region in the chloroplast. Organellar DNAs in the nucleus were rarely expressed, and at low levels. Surprisingly, there was asymmetry in the survivorship of ancestral insertions following allopolyploidy, with most numts (nuclear mitochondrial insertions) decaying or being lost whereas most nupts (nuclear plastidial insertions) were retained.

CONCLUSIONS

This study characterized and compared intracellular transfer among nuclear and organellar genomes within two cultivated allopolyploids and their ancestral diploid cotton species. A striking asymmetry in the fate of IGTs in allopolyploid cotton was discovered, with numts being preferentially lost relative to nupts. Our results connect intergenomic gene transfer with allotetraploidy and provide new insight into intracellular genome evolution.

Genome-wide identification and characterization of TALE superfamily genes in cotton reveals their functions in regulating secondary cell wall biosynthesis

BACKGROUND

Cotton fiber length and strength are both key traits of fiber quality, and fiber strength (FS) is tightly correlated with secondary cell wall (SCW) biosynthesis. The three-amino-acid-loop-extension (TALE) superclass homeoproteins are involved in regulating diverse biological processes in plants, and some TALE members has been identified to play a key role in regulating SCW formation. However, little is known about the functions of TALE members in cotton (Gossypium spp.).

RESULTS

In the present study, based on gene homology, 46, 47, 88 and 94 TALE superfamily genes were identified in G. arboreum, G. raimondii, G. barbadense and G. hirsutum, respectively. Phylogenetic and evolutionary analysis showed the evolutionary conservation of two cotton TALE families (including BEL1-like and KNOX families). Gene structure analysis also indicated the conservation of GhTALE members under selection. The analysis of promoter cis-elements and expression patterns suggested potential transcriptional regulation functions in fiber SCW biosynthesis and responses to some phytohormones for GhTALE proteins. Genome-wide analysis of colocalization of TALE transcription factors with SCW-related QTLs revealed that some BEL1-like genes and KNAT7 homologs may participate in the regulation of cotton fiber strength formation. Overexpression of GhKNAT7-A03 and GhBLH6-A13 significantly inhibited the synthesis of lignocellulose in interfascicular fibers of Arabidopsis. Yeast two-hybrid (Y2H) experiments showed extensive heteromeric interactions between GhKNAT7 homologs and some GhBEL1-like proteins. Yeast one-hybrid (Y1H) experiments identified the upstream GhMYB46 binding sites in the promoter region of GhTALE members and defined the downstream genes that can be directly bound and regulated by GhTALE heterodimers.

CONCLUSION

We comprehensively identified TALE superfamily genes in cotton. Some GhTALE members are predominantly expressed during the cotton fiber SCW thicking stage, and may genetically correlated with the formation of FS. Class II KNOX member GhKNAT7 can interact with some GhBEL1-like members to form the heterodimers to regulate the downstream targets, and this regulatory relationship is partially conserved with Arabidopsis. In summary, this study provides important clues for further elucidating the functions of TALE genes in regulating cotton growth and development, especially in the fiber SCW biosynthesis network, and it also contributes genetic resources to the improvement of cotton fiber quality.

Phosphatase GhDsPTP3a interacts with annexin protein GhANN8b to reversely regulate salt tolerance in cotton (Gossypium spp.)

Salinity is among the major factors limiting crop production worldwide. Despite having moderate salt-tolerance, cotton (Gossypium spp.) suffers severe yield losses to salinity stresses, largely due to being grown on saline-alkali and dry lands. To identify genetic determinants conferring salinity tolerance in cotton, we deployed a functional genomic screen using a cotton cDNA library in a virus-induced gene silencing (VIGS) vector. We have revealed that silencing of GhDsPTP3a, which encodes a protein phosphatase, increases cotton tolerance to salt stress. Yeast two-hybrid screens indicated that GhDsPTP3a interacts with GhANN8b, an annexin protein, which plays a positive role in regulating cotton response to salinity stress. Salt stress induces GhANN8b phosphorylation, which is subsequently dephosphorylated by GhDsPTP3a. Ectopic expression of GhDsPTP3a and GhANN8b oppositely regulates plant salt tolerance and calcium influx. In addition, we have revealed that silencing of GhDsPTP3a or GhANN8b exerts opposing roles in regulating GhSOS1 transcript levels, and ectopic expression of GhANN8b elevates Na⁺ efflux in Arabidopsis under salinity stress. Our study demonstrates that a cotton phosphatase GhDsPTP3a and an annexin protein GhANN8b interact and reversely modulate Ca²⁺ and Na⁺ fluxes in cotton salinity responses.

A Comparative Genome-Wide Analysis of the R2R3-MYB Gene Family Among Four Gossypium Species and Their Sequence Variation and Association With Fiber Quality Traits in an Interspecific G. hirsutum × G. barbadense Population

Cotton (Gossypium spp.) is the most important natural fiber crop in the world. The R2R3-MYB gene family is a large gene family involved in many plant functions including cotton fiber development. Although previous studies have reported its phylogenetic relationships, gene structures, and expression patterns in tetraploid G. hirsutum and diploid G. raimondii, little is known about the sequence variation of the members between G. hirsutum and G. barbadense and their involvement in the natural quantitative variation in fiber quality and yield. In this study, a comprehensive genome-wide comparative analysis was performed among the four Gossypium species using whole genome sequences, i.e., tetraploid G. hirsutum (AD1) and G. barbadense (AD2) as well as their likely ancestral diploid extants G. raimondii (D5) and G. arboreum (A2), leading to the identification of 406, 393, 216, and 213 R2R3-MYB genes, respectively. To elucidate whether the R2R3-MYB genes are genetically associated with fiber quality traits, 86 R2R3-MYB genes were co-localized with quantitative trait loci (QTL) hotspots for fiber quality and yield, including 42 genes localized within the fiber length QTL hotspots, in interspecific G. hirsutum × G. barbadense populations. There were 20 interspecific nonsynonymous single-nucleotide polymorphism (SNP) sites between the two tetraploid cultivated species, of which 16 developed from 11 R2R3-MYB genes were significantly correlated with fiber quality and yield in a backcross inbred population (BIL) of G. hirsutum × G. barbadense in at least one of the four field tests. Taken together, these results indicate that the sequence variation in these 11 R2R3-MYB genes is associated with the natural variation (i.e., QTL) in fiber quality and yield. Moreover, the functional SNPs of five R2R3-MYB allele pairs from the AD1 and AD2 genomes were significantly correlated with the gene expression related to fiber quality in fiber development. The results will be useful in further elucidating the role of the R2R3-MYB genes during fiber development.

Fine mapping and molecular characterization of the virescent gene vsp in Upland cotton (Gossypium hirsutum)

The vsp gene was fine mapped to a 353.7-kb region, and a 201-bp deletion that affected chloroplast development and chlorophyll biosynthesis was found in the candidate gene GhPUR4. Virescent mutations can be used as marker traits in heterosis breeding and can also be used to research chloroplast development, chlorophyll biosynthesis and photosynthesis mechanisms. Here, we obtained a light-sensitive virescent mutant, vsp, that has reduced chlorophyll (Chl) content and abnormal chloroplast development. Then, the virescent space (vsp) gene in the vsp mutant was preliminarily mapped to a 38.32-Mb region of chromosome D04 using a high-density SNP genetic map with a total length of 5384.33 cM and 4472 bin markers. Furthermore, the vsp gene was narrowed down to a 353.7-kb region that contains 15 candidate genes using 484 virescent individuals from an F₂ population. Sequence analysis of genes in this region showed that a 201-bp deletion was present in the Gh_D04G1108 (GhPUR4) gene in the vsp mutant. The 201-bp deletion of Gh_D04G1108 caused the deletion of 67 AAs in the GhPUR4 protein. Virus-induced gene silencing (VIGS) of GhPUR4 in normal plants caused reduced GhPUR4 gene expression levels, reduced Chl content, abnormal chloroplast development and virescent true leaves. This study could help us unravel the function of GhPUR4 in chloroplast development and Chl biosynthesis at the early developmental stages of the true leaves in cotton, which could promote the research and application of virescent mutations in cotton heterosis breeding.

A genome-wide analysis of the phospholipid: diacylglycerol acyltransferase gene family in Gossypium

Background

Cotton (Gossypium spp.) is the most important natural fiber crop worldwide, and cottonseed oil is its most important byproduct. Phospholipid: diacylglycerol acyltransferase (PDAT) is important in TAG biosynthesis, as it catalyzes the transfer of a fatty acyl moiety from the sn-2 position of a phospholipid to the sn-3 position of sn-1, 2-diacylglyerol to form triacylglycerol (TAG) and a lysophospholipid. However, little is known about the genes encoding PDATs involved in cottonseed oil biosynthesis.

Results

A comprehensive genome-wide analysis of G. hirsutum, G. barbadense, G. arboreum, and G. raimondii herein identified 12, 11, 6 and 6 PDATs, respectively. These genes were divided into 3 subfamilies, and a PDAT-like subfamily was identified in comparison with dicotyledonous Arabidopsis. All GhPDATs contained one or two LCAT domains at the C-terminus, while most GhPDATs contained a preserved single transmembrane region at the N-terminus. A chromosomal distribution analysis showed that the 12 GhPDAT genes in G. hirsutum were distributed in 10 chromosomes. However, none of the GhPDATs was co-localized with quantitative trait loci (QTL) for cottonseed oil content, suggesting that their sequence variations are not genetically associated with the natural variation in cottonseed oil content. Most GhPDATs were expressed during the cottonseed oil accumulation stage. Ectopic expression of GhPDAT1d increased Arabidopsis seed oil content.

Conclusions

Our comprehensive genome-wide analysis of the cotton PDAT gene family provides a foundation for further studies into the use of PDAT genes to increase cottonseed oil content through biotechnology.

Electronic supplementary material

The online version of this article (10.1186/s12864-019-5728-8) contains supplementary material, which is available to authorized users.

A genome-wide analysis of the phospholipid: diacylglycerol acyltransferase gene family in Gossypium

BACKGROUND

Cotton (Gossypium spp.) is the most important natural fiber crop worldwide, and cottonseed oil is its most important byproduct. Phospholipid: diacylglycerol acyltransferase (PDAT) is important in TAG biosynthesis, as it catalyzes the transfer of a fatty acyl moiety from the sn-2 position of a phospholipid to the sn-3 position of sn-1, 2-diacylglyerol to form triacylglycerol (TAG) and a lysophospholipid. However, little is known about the genes encoding PDATs involved in cottonseed oil biosynthesis.

RESULTS

A comprehensive genome-wide analysis of G. hirsutum, G. barbadense, G. arboreum, and G. raimondii herein identified 12, 11, 6 and 6 PDATs, respectively. These genes were divided into 3 subfamilies, and a PDAT-like subfamily was identified in comparison with dicotyledonous Arabidopsis. All GhPDATs contained one or two LCAT domains at the C-terminus, while most GhPDATs contained a preserved single transmembrane region at the N-terminus. A chromosomal distribution analysis showed that the 12 GhPDAT genes in G. hirsutum were distributed in 10 chromosomes. However, none of the GhPDATs was co-localized with quantitative trait loci (QTL) for cottonseed oil content, suggesting that their sequence variations are not genetically associated with the natural variation in cottonseed oil content. Most GhPDATs were expressed during the cottonseed oil accumulation stage. Ectopic expression of GhPDAT1d increased Arabidopsis seed oil content.

CONCLUSIONS

Our comprehensive genome-wide analysis of the cotton PDAT gene family provides a foundation for further studies into the use of PDAT genes to increase cottonseed oil content through biotechnology.

Metabolism of polysaccharides in dynamic middle lamellae during cotton fibre development

Evidence is presented that cotton fibre adhesion and middle lamella formation are preceded by cutin dilution and accompanied by rhamnogalacturonan-I metabolism. Cotton fibres are single cell structures that early in development adhere to one another via the cotton fibre middle lamella (CFML) to form a tissue-like structure. The CFML is disassembled around the time of initial secondary wall deposition, leading to fibre detachment. Observations of CFML in the light microscope have suggested that the development of the middle lamella is accompanied by substantial cell-wall metabolism, but it has remained an open question as to which processes mediate adherence and which lead to detachment. The mechanism of adherence and detachment were investigated here using glyco-microarrays probed with monoclonal antibodies, transcript profiling, and observations of fibre auto-digestion. The results suggest that adherence is brought about by cutin dilution, while the presence of relevant enzyme activities and the dynamics of rhamnogalacturonan-I side-chain accumulation and disappearance suggest that both attachment and detachment are accompanied by rhamnogalacturonan-I metabolism.

Resequencing of cv CRI-12 family reveals haplotype block inheritance and recombination of agronomically important genes in artificial selection

Although efforts have been taken to exploit diversity for yield and quality improvements, limited progress on using beneficial alleles in domesticated and undomesticated cotton varieties is limited. Given the complexity and limited amount of genomic information since the completion of four cotton genomes, characterizing significant variations and haplotype block inheritance under artificial selection has been challenging. Here we sequenced Gossypium hirsutum L. cv CRI-12 (the cotton variety with the largest acreage in China), its parental cultivars, and progeny cultivars, which were bred by the different institutes in China. In total, 3.3 million SNPs were identified and 118, 126 and 176 genes were remarkably correlated with Verticillium wilt, salinity and drought tolerance in CRI-12, respectively. Transcriptome-wide analyses of gene expression, and functional annotations, have provided support for the identification of genes tied to these tolerances. We totally discovered 58 116 haplotype blocks, among which 23 752 may be inherited and 1029 may be recombined under artificial selection. This survey of genetic diversity identified loci that may have been subject to artificial selection and documented the haplotype block inheritance and recombination, shedding light on the genetic mechanism of artificial selection and guiding breeding efforts for the genetic improvement of cotton.

Identification of cotton MOTHER OF FT AND TFL1 homologs, GhMFT1 and GhMFT2, involved in seed germination

Plant phosphatidylethanolamine-binding protein (PEBP) is comprised of three clades: FLOWERING LOCUS T (FT), TERMINAL FLOWER1 (TFL1) and MOTHER OF FT AND TFL1 (MFT). FT/TFL1-like clades regulate identities of the determinate and indeterminate meristems, and ultimately affect flowering time and plant architecture. MFT is generally considered to be the ancestor of FT/TFL1, but its function is not well understood. Here, two MFT homoeologous gene pairs in Gossypium hirsutum, GhMFT1-A/D and GhMFT2-A/D, were identified by genome-wide identification of MFT-like genes. Detailed expression analysis revealed that GhMFT1 and GhMFT2 homoeologous genes were predominately expressed in ovules, and their expression increased remarkably during ovule development but decreased quickly during seed germination. Expressions of GhMFT1 and GhMFT2 homoeologous genes in germinating seeds were upregulated in response to abscisic acid (ABA), and their expressions also responded to gibberellin (GA). In addition, ectopic overexpression of GhMFT1 and GhMFT2 in Arabidopsis inhibited seed germination at the early stage. Gene transcription analysis showed that ABA metabolism genes ABA-INSENSITIVE3 (ABI3) and ABI5, GA signal transduction pathway genes REPRESSOR OF ga1-3 (RGA) and RGA-LIKE2 (RGL2) were all upregulated in the 35S:GhMFT1 and 35S:GhMFT2 transgenic Arabidopsis seeds. GhMFT1 and GhMFT2 localize in the cytoplasm and nucleus, and both interact with a cotton bZIP transcription factor GhFD, suggesting that both of GhMFT1, 2 have similar intracellular regulation mechanisms. Taken together, the results suggest that GhMFT1 and GhMFT2 may act redundantly and differentially in the regulation of seed germination.

The Cotton High-Affinity K+ Transporter, GhHAK5a, Is Essential for Shoot Regulation of K+ Uptake in Root under Potassium Deficiency

Potassium (K) deficiency is a key limiting factor in cotton (Gossypium hirsutum) production. By grafting two contrasting cotton cultivars, CCRI41 (more susceptible to K+ deficiency) and SCRC22 (more tolerant of K+ deficiency), we established that cotton shoot plays a vital role in the regulation of root K+ uptake. To identify the genetic basis of this finding, we performed RNA sequencing (RNA-seq) of roots of CCRI41 self-grafts (CCRI41/CCRI41, scion/rootstock) and SCRC22/CCRI41 reciprocal-grafts exposed to K+ deficiency. We found that GhHAK5a, an orthologous of Arabidopsis thaliana high-affinity K+ transporter, AtHAK5, was significantly induced in the CCRI41 rootstock by the SCRC22 scion. This gene was mainly expressed in roots and was more highly induced by K+ deficiency in roots of SCRC22 than those of CCRI41. Agrobacterium-mediated virus-induced gene silencing and yeast complementary assay showed that GhHAK5a is a high-affinity K+ uptake transporter. Importantly, silencing of GhHAK5a in the CCRI41 rootstock almost completely inhibited the K+ uptake induced by SCRC22 scion in CCRI41 rootstock. We identified a key high-affinity K+ transporter, GhHAK5a in cotton, which is the essential target for shoot regulation of root K+ uptake under K+ deficiency.

Gossypium barbadense and Gossypium hirsutum genomes provide insights into the origin and evolution of allotetraploid cotton

Allotetraploid cotton is an economically important natural-fiber-producing crop worldwide. After polyploidization, Gossypium hirsutum L. evolved to produce a higher fiber yield and to better survive harsh environments than Gossypium barbadense, which produces superior-quality fibers. The global genetic and molecular bases for these interspecies divergences were unknown. Here we report high-quality de novo-assembled genomes for these two cultivated allotetraploid species with pronounced improvement in repetitive-DNA-enriched centromeric regions. Whole-genome comparative analyses revealed that species-specific alterations in gene expression, structural variations and expanded gene families were responsible for speciation and the evolutionary history of these species. These findings help to elucidate the evolution of cotton genomes and their domestication history. The information generated not only should enable breeders to improve fiber quality and resilience to ever-changing environmental conditions but also can be translated to other crops for better understanding of their domestication history and use in improvement.

Identification and Expression Profiling of Protein Phosphatases (PP2C) Gene Family in Gossypium hirsutum L

The protein phosphatase (PP2C) gene family, known to participate in cellular processes, is one of the momentous and conserved plant-specific gene families that regulate signal transduction in eukaryotic organisms. Recently, PP2Cs were identified in Arabidopsis and various other crop species, but analysis of PP2C in cotton is yet to be reported. In the current research, we found 87 (Gossypiumarboreum), 147 (Gossypiumbarbadense), 181 (Gossypiumhirsutum), and 99 (Gossypiumraimondii) PP2C-encoding genes in total from the cotton genome. Herein, we provide a comprehensive analysis of the PP2C gene family in cotton, such as gene structure organization, gene duplications, expression profiling, chromosomal mapping, protein motif organization, and phylogenetic relationships of each species. Phylogenetic analysis further categorized PP2C genes into 12 subgroups based on conserved domain composition analysis. Moreover, we observed a strong signature of purifying selection among duplicated pairs (i.e., segmental and dispersed) of Gossypiumhirsutum. We also observed the tissue-specific response of GhPP2C genes in organ and fiber development by comparing the RNA-sequence (RNA-seq) data reported on different organs. The qRT-PCR validation of 30 GhPP2C genes suggested their critical role in cotton by exposure to heat, cold, drought, and salt stress treatments. Hence, our findings provide an overview of the PP2C gene family in cotton based on various bioinformatic tools that demonstrated their critical role in organ and fiber development, and abiotic stress tolerance, thereby contributing to the genetic improvement of cotton for the resistant cultivar.

Gossypium barbadense and Gossypium hirsutum genomes provide insights into the origin and evolution of allotetraploid cotton

Allotetraploid cotton is an economically important natural-fiber-producing crop worldwide. After polyploidization, Gossypium hirsutum L. evolved to produce a higher fiber yield and to better survive harsh environments than Gossypium barbadense, which produces superior-quality fibers. The global genetic and molecular bases for these interspecies divergences were unknown. Here we report high-quality de novo-assembled genomes for these two cultivated allotetraploid species with pronounced improvement in repetitive-DNA-enriched centromeric regions. Whole-genome comparative analyses revealed that species-specific alterations in gene expression, structural variations and expanded gene families were responsible for speciation and the evolutionary history of these species. These findings help to elucidate the evolution of cotton genomes and their domestication history. The information generated not only should enable breeders to improve fiber quality and resilience to ever-changing environmental conditions but also can be translated to other crops for better understanding of their domestication history and use in improvement.

Cumulative and different genetic effects contributed to yield heterosis using maternal and paternal backcross populations in Upland cotton

Heterosis has been utilized in commercial production, but the heterosis mechanism has remained vague. Hybrid cotton is suitable to dissect the heterosis mechanism. In order to explore the genetic basis of heterosis in Upland cotton, we generated paternal and maternal backcross (BC/P and BC/M) populations. Data for yield and yield-component traits were collected over 2 years in three replicated BC/P field trials and four replicated BC/M field trials. At single-locus level, 26 and 27 QTLs were identified in BC/P and BC/M populations, respectively. Six QTLs shared in both BC populations. A total of 27 heterotic loci were detected. Partial dominant and over-dominant QTLs mainly determined yield heterosis in the BC/P and BC/M populations. QTLs for different traits displayed varied genetic effects in two BC populations. Eleven heterotic loci overlapped with QTLs but no common heterotic locus was detected in both BC populations. We resolved the 333 kb (48 genes) and 516 kb (25 genes) physical intervals based on 16 QTL clusters and 35 common QTLs, respectively, in more than one environment or population. We also identified 189 epistatic QTLs and a number of QTL × environment interactions in two BC populations and the corresponding MPH datasets. The results indicated that cumulative effects contributed to yield heterosis in Upland cotton, including epistasis, QTL × environment interaction, additive, partial dominance and over-dominance.

Evolution of PEPC gene family in Gossypium reveals functional diversification and GhPEPC genes responding to abiotic stresses

Phosphoenolpyruvate carboxylase (PEPC) family genes play important roles in regulating plant growth and abiotic stress response. Based on the sequenced Gossypium genomes, we performed comprehensive analysis of PEPC homolog genes in cotton, which six, six, eleven and ten PEPC genes were identified in Gossypium arboreum (A₂), G. raimondii (D₅), G. hirsutum (AD₁) and G. barbadense (AD₂), respectively. These genes were divided into six subgroups: PEPC-i, PEPC-ii, PEPC-iii, PEPC-iv, PEPC-v and PEPC-vi; PEPC genes in each subgroup displayed conserved gene structure and motifs. Segmental duplication and whole genome duplication (WGD) events yielded the expansion of PEPC genes. Expression assays showed that the duplicated PEPC genes displayed diverse expression patterns, indicating that they experienced functional divergence. Of which, genes in PEPC-iv subgroup played crucial role for substrate distribution in cottonseed. Cis-elements, putative miRNAs and expression analyses showed that GhPEPC homologs might respond to abiotic stresses, expression levels of GhPEPC1 and GhPEPC2/GhPEPC2D genes were larger induced than other GhPEPC genes under cold, heat, salt, and drought stresses, indicating the crucial roles in abiotic stresses response. Present study serves new information to decipher the evolution and function of PEPC genes in Gossypium.

The susceptibility of sea-island cotton recombinant inbred lines to Fusarium oxysporum f. sp. vasinfectum infection is characterized by altered expression of long noncoding RNAs

Disease resistance is one of the most complicated yet important plant traits. The potential functions of long noncoding RNAs (lncRNAs) in response to pathogenic fungi remain unclear. In this study, we sequenced the transcriptomes of four different sea-island cotton (Gossypium barbadense) recombinant inbred lines (RILs) with susceptible, highly susceptible, highly resistant, or super highly resistant phenotypes and compared their responses to Fusarium oxysporum f. sp. vasinfectum (Fov) infection with those of their susceptible and resistant parents. Infection-induced protein coding genes were highly enriched in similar disease resistance-related pathways regardless of fungal susceptibility. In contrast, we found that the expression of a large number of Fov infection-induced lncRNAs was positively correlated with plant susceptibility. Bioinformatics analysis of potential target mRNAs of lncRNAs with both trans-acting and cis-acting mechanisms showed that mRNAs co-expressed or co-located with Fov-regulated lncRNAs were highly enriched in disease resistance-related pathways, including glutathione metabolism, glycolysis, plant hormone signal transduction, anthocyanin biosynthesis, and butanoate metabolism. Together these results suggest that lncRNAs could play a significant role in the response to pathogenic fungal infection and the establishment of disease resistance. The transcriptional regulation of these infection-susceptible lncRNAs could be coordinated with infection-susceptible mRNAs and integrated into a regulatory network to modulate plant-pathogen interactions and disease resistance. Fov-susceptible lncRNAs represent a novel class of molecular markers for breeding of Fov-resistant cotton cultivars.

Gene duplication and evolution in recurring polyploidization-diploidization cycles in plants

BACKGROUND

The sharp increase of plant genome and transcriptome data provide valuable resources to investigate evolutionary consequences of gene duplication in a range of taxa, and unravel common principles underlying duplicate gene retention.

RESULTS

We survey 141 sequenced plant genomes to elucidate consequences of gene and genome duplication, processes central to the evolution of biodiversity. We develop a pipeline named DupGen_finder to identify different modes of gene duplication in plants. Genes derived from whole-genome, tandem, proximal, transposed, or dispersed duplication differ in abundance, selection pressure, expression divergence, and gene conversion rate among genomes. The number of WGD-derived duplicate genes decreases exponentially with increasing age of duplication events-transposed duplication- and dispersed duplication-derived genes declined in parallel. In contrast, the frequency of tandem and proximal duplications showed no significant decrease over time, providing a continuous supply of variants available for adaptation to continuously changing environments. Moreover, tandem and proximal duplicates experienced stronger selective pressure than genes formed by other modes and evolved toward biased functional roles involved in plant self-defense. The rate of gene conversion among WGD-derived gene pairs declined over time, peaking shortly after polyploidization. To provide a platform for accessing duplicated gene pairs in different plants, we constructed the Plant Duplicate Gene Database.

CONCLUSIONS

We identify a comprehensive landscape of different modes of gene duplication across the plant kingdom by comparing 141 genomes, which provides a solid foundation for further investigation of the dynamic evolution of duplicate genes.

Genome-Wide Identification, Sequence Variation, and Expression of the Glycerol-3-Phosphate Acyltransferase (GPAT) Gene Family in Gossypium

Cotton is an economically important crop grown for natural fiber and seed oil production. Cottonseed oil ranks third after soybean oil and colza oil in terms of edible oilseed tonnage worldwide. Glycerol-3-phosphate acyltransferase (GPAT) genes encode enzymes involved in triacylglycerol biosynthesis in plants. In the present study, 85 predicted GPAT genes were identified from the published genome data in Gossypium. Among them, 14, 16, 28, and 27 GPAT homologs were identified in G. raimondii, G. arboreum, G. hirsutum, and G. barbadense, respectively. Phylogenetic analysis revealed that a total of 108 GPAT genes from cotton, Arabidopsis and cacao could be classified into three groups. Furthermore, through comparison, the gene structure analyses indicated that GPAT genes from the same group were highly conserved between Arabidopsis and cotton. Segmental duplication could be the major driver for GPAT gene family expansion in the four cotton species above. Expression patterns of GhGPAT genes were diverse in different tissues. Most GhGPAT genes were induced or suppressed after salt or cold stress in Upland cotton. Eight GhGPAT genes were co-localized with oil and protein quantitative trait locus (QTL) regions. Thirty-two single nucleotide polymorphisms (SNPs) were detected from 12 GhGPAT genes, sixteen of which in nine GhGPAT genes were classified as synonymous, and sixteen SNPs in ten GhGPAT genes non-synonymous. Two SNP markers of the GhGPAT16 and GhGPAT26 genes were significantly correlated with cotton oil content in one of the three field tests. This study shed lights on the molecular evolutionary properties of GPAT genes in cotton, and provided reference for improvement of cotton response to abiotic stress and the genetic improvement of cotton oil content.

The Cotton Apoplastic Protein CRR1 Stabilizes Chitinase 28 to Facilitate Defense against the Fungal Pathogen Verticillium dahliae

The apoplast serves as the first battlefield between the plant hosts and invading microbes; therefore, work on plant-pathogen interactions has increasingly focused on apoplastic immunity. In this study, we identified three proteins in the apoplast of cotton (Gossypium sp) root cells during interaction of the plant with the fungal pathogen Verticillium dahliae Among these proteins, cotton host cells secrete chitinase 28 (Chi28) and the Cys-rich repeat protein 1 (CRR1), while the pathogen releases the protease VdSSEP1. Biochemical analysis demonstrated that VdSSEP1 hydrolyzed Chi28, but CRR1 protected Chi28 from cleavage by Verticillium dahliae secretory Ser protease 1 (VdSSEP1). In accordance with the in vitro results, CRR1 interacted with Chi28 in yeast and plant cells and attenuated the observed decrease in Chi28 level that occurred in the apoplast of plant cells upon pathogen attack. Knockdown of CRR1 or Chi28 in cotton plants resulted in higher susceptibility to V. dahliae infection, and overexpression of CRR1 increased plant resistance to V dahliae, the fungus Botrytis cinerea, and the oomycete Phytophthora parasitica var nicotianae By contrast, knockout of VdSSEP1 in V. dahliae destroyed the pathogenicity of this fungus. Together, our results provide compelling evidence for a multilayered interplay of factors in cotton apoplastic immunity.

Transcriptomic and biochemical analysis of upland cotton (Gossypium hirsutum) and a chromosome segment substitution line from G. hirsutum × G. barbadense in response to Verticillium dahliae infection

BACKGROUND

Verticillium wilt (VW), also known as "cotton cancer," is one of the most destructive diseases in global cotton production that seriously impacts fiber yield and quality. Despite numerous attempts, little significant progress has been made in improving the VW resistance of upland cotton. The development of chromosome segment substitution lines (CSSLs) from Gossypium hirsutum × G. barbadense has emerged as a means of simultaneously developing new cotton varieties with high-yield, superior fiber, and resistance to VW.

RESULTS

In this study, VW-resistant investigations were first conducted in an artificial greenhouse, a natural field, and diseased nursery conditions, resulting in the identification of one stably VW-resistant CSSL, MBI8255, and one VW-susceptible G. hirsutum, CCRI36, which were subsequently subjected to biochemical tests and transcriptome sequencing during V991 infection (0, 1, and 2 days after inoculation). Eighteen root samples with three replications were collected to perform multiple comparisons of enzyme activity and biochemical substance contents. The findings indicated that VW resistance was positively correlated with peroxidase and polyphenol oxidase activity, but negatively correlated with malondialdehyde content. Additionally, RNA sequencing was used for the same root samples, resulting in a total of 77,412 genes, of which 23,180 differentially expressed genes were identified from multiple comparisons between samples. After Gene Ontology and Kyoto Encyclopedia of Genes and Genomes enrichment analysis on the expression profiles identified using Short Time-series Expression Miner, we found that the metabolic process in the biological process, as well as the pathways of phenylpropanoid biosynthesis and plant hormone signal transduction, participated significantly in the response to VW. Gene functional annotation and expression quantity analysis indicated the important roles of the phenylpropanoid metabolic pathway and oxidation-reduction process in response to VW, which also provided plenty of candidate genes related to plant resistance.

CONCLUSIONS

This study concentrates on the preliminary response to V991 infection by comparing the VW-resistant CSSL and its VW-susceptible recurrent parent. Not only do our findings facilitate the culturing of new resistant varieties with high yield and superior performance, but they also broaden our understanding of the mechanisms of cotton resistance to VW.

Transcriptomic and biochemical analysis of upland cotton (Gossypium hirsutum) and a chromosome segment substitution line from G. hirsutum × G. barbadense in response to Verticillium dahliae infection

BACKGROUND

Verticillium wilt (VW), also known as "cotton cancer," is one of the most destructive diseases in global cotton production that seriously impacts fiber yield and quality. Despite numerous attempts, little significant progress has been made in improving the VW resistance of upland cotton. The development of chromosome segment substitution lines (CSSLs) from Gossypium hirsutum × G. barbadense has emerged as a means of simultaneously developing new cotton varieties with high-yield, superior fiber, and resistance to VW.

RESULTS

In this study, VW-resistant investigations were first conducted in an artificial greenhouse, a natural field, and diseased nursery conditions, resulting in the identification of one stably VW-resistant CSSL, MBI8255, and one VW-susceptible G. hirsutum, CCRI36, which were subsequently subjected to biochemical tests and transcriptome sequencing during V991 infection (0, 1, and 2 days after inoculation). Eighteen root samples with three replications were collected to perform multiple comparisons of enzyme activity and biochemical substance contents. The findings indicated that VW resistance was positively correlated with peroxidase and polyphenol oxidase activity, but negatively correlated with malondialdehyde content. Additionally, RNA sequencing was used for the same root samples, resulting in a total of 77,412 genes, of which 23,180 differentially expressed genes were identified from multiple comparisons between samples. After Gene Ontology and Kyoto Encyclopedia of Genes and Genomes enrichment analysis on the expression profiles identified using Short Time-series Expression Miner, we found that the metabolic process in the biological process, as well as the pathways of phenylpropanoid biosynthesis and plant hormone signal transduction, participated significantly in the response to VW. Gene functional annotation and expression quantity analysis indicated the important roles of the phenylpropanoid metabolic pathway and oxidation-reduction process in response to VW, which also provided plenty of candidate genes related to plant resistance.

CONCLUSIONS

This study concentrates on the preliminary response to V991 infection by comparing the VW-resistant CSSL and its VW-susceptible recurrent parent. Not only do our findings facilitate the culturing of new resistant varieties with high yield and superior performance, but they also broaden our understanding of the mechanisms of cotton resistance to VW.

Fiber Quality Improvement in Upland Cotton (Gossypium hirsutum L.): Quantitative Trait Loci Mapping and Marker Assisted Selection Application

Genetic improvement in fiber quality is one of the main challenges for cotton breeders. Fiber quality traits are controlled by multiple genes and are classified as complex quantitative traits, with a negative relationship with yield potential, so the genetic gain is low in traditional genetic improvement by phenotypic selection. The availability of Gossypium genomic sequences facilitates the development of high-throughput molecular markers, quantitative trait loci (QTL) fine mapping and gene identification, which helps us to validate candidate genes and to use marker assisted selection (MAS) on fiber quality in breeding programs. Based on developments of high density linkage maps, QTLs fine mapping, marker selection and omics, we have performed trait dissection on fiber quality traits in diverse populations of upland cotton. QTL mapping combined with multi-omics approaches such as, RNA sequencing datasets to identify differentially expressed genes have benefited the improvement of fiber quality. In this review, we discuss the application of molecular markers, QTL mapping and MAS for fiber quality improvement in upland cotton.

GbSOBIR1 confers Verticillium wilt resistance by phosphorylating the transcriptional factor GbbHLH171 in Gossypium barbadense

Receptor-like kinases (RLKs) are important components of plant innate immunity. Although recent studies have revealed that the RLK suppressor of BIR1-1 (SOBIR1) can interact with multiple receptor-like proteins and is required for resistance against fungal pathogens, how the signal is transduced and triggers immune responses remains enigmatic. In this study, we identified a defence-related RLK from Gossypium barbadense (designated GbSOBIR1) and investigated its functional mechanism. Expression of the GbSOBIR1 gene is ubiquitous in cotton plants and is induced by Verticillium dahliae inoculation. Knock-down of GbSOBIR1 by virus-induced gene silencing resulted in attenuated resistance of cotton plants to V. dahliae, while heterologous overexpression of GbSOBIR1 in Arabidopsis improves resistance. We also found that the kinase region of GbSOBIR1 interacts with a basic helix-loop-helix (bHLH) transcription factor identified as GbbHLH171 in a yeast-two-hybrid screen. GbbHLH171 could interact with and be phosphorylated by GbSOBIR1 in vitro and in vivo and contributes positively to the resistance of cotton against V. dahliae. Furthermore, we found that this phosphorylation is essential to the transcriptional activity and functional role of GbbHLH171. We also show by spectrometric analysis and site-directed mutagenesis that Ser413 is the GbSOBIR1-mediated phosphorylation site of GbbHLH171. These results demonstrate that GbSOBIR1 interacts with GbbHLH171 and plays a critical role in cotton resistance to V. dahliae.

Insights into the Evolution of the New World Diploid Cottons (Gossypium, Subgenus Houzingenia) Based on Genome Sequencing

We employed phylogenomic methods to study molecular evolutionary processes and phylogeny in the geographically widely dispersed New World diploid cottons (Gossypium, subg. Houzingenia). Whole genome resequencing data (average of 33× genomic coverage) were generated to reassess the phylogenetic history of the subgenus and provide a temporal framework for its diversification. Phylogenetic analyses indicate that the subgenus likely originated following transoceanic dispersal from Africa about 6.6 Ma, but that nearly all of the biodiversity evolved following rapid diversification in the mid-Pleistocene (0.5-2.0 Ma), with multiple long-distance dispersals required to account for range expansion to Arizona, the Galapagos Islands, and Peru. Comparative analyses of cpDNAversus nuclear data indicate that this history was accompanied by several clear cases of interspecific introgression. Repetitive DNAs contribute roughly half of the total 880 Mb genome, but most transposable element families are relatively old and stable among species. In the genic fraction, pairwise synonymous mutation rates average 1% per Myr, with nonsynonymous changes being about seven times less frequent. Over 1.1 million indels were detected and phylogenetically polarized, revealing a 2-fold bias toward deletions over small insertions. We suggest that this genome down-sizing bias counteracts genome size growth by TE amplification and insertions, and helps explain the relatively small genomes that are restricted to this subgenus. Compared with the rate of nucleotide substitution, the rate of indel occurrence is much lower averaging about 17 nucleotide substitutions per indel event.

Nulliplex-branch, a TERMINAL FLOWER 1 ortholog, controls plant growth habit in cotton

Nulliplex-branch (nb) mutants in cotton display a specific architecture. The gene responsible for the nb phenotype was identified, and its modulation mode was further studied. Plant architecture is an important agronomic factor influencing various traits such as yield and variety adaptability in crop plants. Cotton (Gossypium) simultaneously displays monopodial and sympodial growth. Nulliplex-branch (nb) mutants showing determinate sympodial shoots have been reported in both G. hirsutum (Ghnb) and G. barbadense (Gbnb). In this study, the gene responsible for the nb phenotype was identified. GhNB and GbNB were found to be allelic loci and are TERMINAL FLOWER 1 orthologs on the Dt subgenome, though the At copies remain native. Sequencing and association analyses identified four (Gh-nb1-Gh-nb4) and one (Gb-nb1) type of point mutation in the coding sequences of Ghnb and Gbnb, respectively. The NB gene was mainly expressed in the root and shoot apex, and expression rhythms were also observed in these tissues, suggesting that the expression of the NB gene could be regulated by photoperiod. Constitutive overexpression of GhNB suppresses the differentiation of the reproductive shoots. Knockout of both copies of GhNB caused the main and lateral shoots to terminate in flowers, which is a more determinate architecture than that of the nb mutants and implies that its function might be dosage dependent. A protein lipid overlay assay indicated that the amino acid substitutions in Gh-nb1 and Gb-nb1 weaken the ligand-binding activity of the NB protein in vitro. These findings suggest that the NB gene plays crucial roles in regulating the determinacy of shoots, and the modulation of this gene should constitute an effective crop improvement approach through adjusting the growth habit of cotton.