Improving cotton yield is a major breeding goal for Chinese upland cotton. Lint percentage is an important yield component and a critical economic index for cotton cultivars, and raising the lint percentage has a close relationship to improving cotton lint yield. To investigate the genetic architecture of lint percentage, a diversity panel consisting of 355 upland cotton accessions was grown, and the lint percentage was measured in four different environments. Genotyping was performed with specific-locus amplified fragment sequencing (SLAF-seq). Twelve single-nucleotide polymorphisms (SNPs) associated with lint percentage were detected via a genome-wide association study (GWAS), in which five SNP loci distributed on chromosomes At3 (A02) and At4 (A08) and contained two major-effect QTLs, which were detected in the best linear unbiased predictions (BLUPs) and in more than three environments simultaneously. Furthermore, favorable haplotypes (FHs) of two major-effect QTLs and 47 putative candidate genes in the two linkage disequilibrium (LD) blocks of these associated loci were identified. The expression levels of these putative candidate genes were estimated using RNA-seq data from ten upland cotton tissues. We found that Gh_A02G1268 was very highly expressed during the early fiber development stage, whereas the gene was poorly expressed in the seed. These results implied that Gh_A02G1268 may determine the lint percentage by regulating seed and fiber development. The favorable QTL alleles and candidate genes for lint percentage identified in this study will have high potential for improving lint yield in future Chinese cotton breeding programs.

Although upland cotton (Gossypium hirsutism L.) originated in the tropics, this early maturity cotton can be planted as far north as 46°N in China due to the accumulation of numerous phenotypic and physiological adaptations during domestication. However, how the genome of early maturity cotton has been altered by strong human selection remains largely unknown. Herein, we report a cotton genome variation map generated by the resequencing of 436 cotton accessions. Whole-genome scans for sweep regions identified 357 putative selection sweeps covering 4.94% (112 Mb) of the upland cotton genome, including 5184 genes. These genes were functionally related to flowering time control, hormone catabolism, ageing and defence response adaptations to environmental changes. A genome-wide association study (GWAS) for seven early maturity traits identified 307 significant loci, 22.48% (69) of which overlapped with putative selection sweeps that occurred during the artificial selection of early maturity cotton. Several previously undescribed candidate genes associated with early maturity were identified by GWAS. This study provides insights into the genetic basis of early maturity in upland cotton as well as breeding resources for cotton improvement.

Thirty significant associations between 22 SNPs and five plant architecture component traits in Chinese upland cotton were identified via GWAS. Four peak SNP loci located on chromosome D03 were simultaneously associated with more plant architecture component traits. A candidate gene, Gh_D03G0922, might be responsible for plant height in upland cotton. A compact plant architecture is increasingly required for mechanized harvesting processes in China. Therefore, cotton plant architecture is an important trait, and its components, such as plant height, fruit branch length and fruit branch angle, affect the suitability of a cultivar for mechanized harvesting. To determine the genetic basis of cotton plant architecture, a genome-wide association study (GWAS) was performed using a panel composed of 355 accessions and 93,250 single nucleotide polymorphisms (SNPs) identified using the specific-locus amplified fragment sequencing method. Thirty significant associations between 22 SNPs and five plant architecture component traits were identified via GWAS. Most importantly, four peak SNP loci located on chromosome D03 were simultaneously associated with more plant architecture component traits, and these SNPs were harbored in one linkage disequilibrium block. Furthermore, 21 candidate genes for plant architecture were predicted in a 0.95-Mb region including the four peak SNPs. One of these genes (Gh_D03G0922) was near the significant SNP D03_31584163 (8.40 kb), and its Arabidopsis homologs contain MADS-box domains that might be involved in plant growth and development. qRT-PCR showed that the expression of Gh_D03G0922 was upregulated in the apical buds and young leaves of the short and compact cotton varieties, and virus-induced gene silencing (VIGS) proved that the silenced plants exhibited increased PH. These results indicate that Gh_D03G0922 is likely the candidate gene for PH in cotton. The genetic variations and candidate genes identified in this study lay a foundation for cultivating moderately short and compact varieties in future Chinese cotton-breeding programs.

Due to China's rapidly increasing population, the total arable land area has dramatically decreased; as a consequence, the competition for farming land allocated for grain and cotton production has become fierce. Therefore, to overcome the existing contradiction between cotton grain and fiber production and the limited farming land, development of early-maturing cultivars is necessary. In this research, a high-density linkage map of upland cotton was constructed using genotyping by sequencing (GBS) to discover single nucleotide polymorphism (SNP) markers associated with early maturity in 170 F 2 individuals derived from a cross between LU28 and ZHONG213. The high-density genetic map, which was composed of 3978 SNP markers across the 26 cotton chromosomes, spanned 2480 cM with an average genetic distance of 0.62 cM. Collinearity analysis showed that the genetic map was of high quality and accurate and agreed well with the Gossypium hirsutum reference genome. Based on this high-density linkage map, QTL analysis was performed on cotton early-maturity traits, including FT, FBP, WGP, NFFB, HNFFB and PH. A total 47 QTLs for the six traits were detected; each of these QTLs explained between 2.61% and 32.57% of the observed phenotypic variation. A major region controlling early-maturity traits in Gossypium hirsutum was identified for FT, FBP, WGP, NFFB and HNFFB on chromosome D03. QTL analyses revealed that phenotypic variation explained (PVE) ranged from 10.42% to 32.57%. Two potential candidate genes, Gh_D03G0885 and Gh_D03G0922, were predicted in a stable QTL region and had higher expression levels in the early-maturity variety ZHONG213 than in the late-maturity variety LU28. However, further evidence is required for functional validation. This study could provide useful information for the dissection of earlymaturity traits and guide valuable genetic loci for molecular-assisted selection (MAS) in cotton breeding.

The glycosyltransferase (GT) 47 family is involved in the biosynthesis of xylose, pectin and xyloglucan and plays a significant role in maintaining the normal morphology of the plant cell wall. However, the functions of GT47s are less well known in cotton. In the present study, a total of 53, 53, 105 and 109 GT47 genes were detected by genome-wide identification in Gossypium arboreum, G. raimondii, G. hirsutum and G. barbadense, respectively. All the GT47s were classified into six major groups via phylogenetic analysis. The exon/intron structure and protein motifs indicated that each branch of the GT47 genes was highly conserved. Collinearity analysis showed that GT47 gene family expansion occurred in Gossypium spp. mainly through whole-genome duplication and that segmental duplication mainly promoted GT47 gene expansion within the A and D subgenomes. The Ka/Ks values suggested that the GT47 gene family has undergone purifying selection during the long-term evolutionary process. Transcriptomic data and qRT-PCR showed that GhGT47 genes exhibited different expression patterns in each tissue and during fiber development. Our results suggest that some genes in the GhGT47 family might be associated with fiber development and the abiotic stress response, which could promote further research involving functional analysis of GT47 genes in cotton.

WRKY transcription factors play important roles in plant defense, stress response, leaf senescence, and plant growth and development. Previous studies have revealed the important roles of the group IIa GhWRKY genes in cotton. To comprehensively analyze the group IIa GhWRKY genes in upland cotton, we identified 15 candidate group IIa GhWRKY genes in the Gossypium hirsutum genome. The phylogenetic tree, intron-exon structure, motif prediction and Ka/Ks analyses indicated that most group IIa GhWRKY genes shared high similarity and conservation and underwent purifying selection during evolution. In addition, we detected the expression patterns of several group IIa GhWRKY genes in individual tissues as well as during leaf senescence using public RNA sequencing data and real-time quantitative PCR. To better understand the functions of group IIa GhWRKYs in cotton, GhWRKY17 (KF669857) was isolated from upland cotton, and its sequence alignment, promoter cis-acting elements and subcellular localization were characterized. Moreover, the over-expression of GhWRKY17 in Arabidopsis up-regulated the senescence-associated genes AtWRKY53, AtSAG12 and AtSAG13, enhancing the plant’s susceptibility to leaf senescence. These findings lay the foundation for further analysis and study of the functions of WRKY genes in cotton.

Background Premature senescence can reduce the yield and quality of crops. WRKY transcription factors (TFs) play important roles during leaf senescence, but little is known about their ageing mechanisms in cotton. Results In this study, a group III WRKY TF, GhWRKY27 , was isolated and characterized. The expression of GhWRKY27 was induced by leaf senescence and was higher in an early-ageing cotton variety than in a non-early-ageing cotton variety. Overexpression of GhWRKY27 in Arabidopsis promoted leaf senescence, as determined by reduced chlorophyll content and elevated expression of senescence-associated genes (SAGs). Yeast two-hybrid (Y2H) and bimolecular fluorescence complementation (BiFC) assays showed that GhWRKY27 interacted with an MYB TF, GhTT2 . Putative target genes of GhWRKY27 were identified via chromatin immunoprecipitation followed by sequencing (ChIP-seq). Yeast one-hybrid (Y1H) assay and electrophoretic mobility shift assay (EMSA) revealed that GhWRKY27 binds directly to the promoters of cytochrome P450 94C1 ( GhCYP94C1 ) and ripening-related protein 2 ( GhRipen2–2 ). In addition, the expression patterns of GhTT2 , GhCYP94C1 and GhRipen2–2 were identified during leaf senescence. Transient dual-luciferase reporter assay indicated that GhWRKY27 could activate the expression of GhCYP94C1 and GhRipen2–2 . Conclusions Our work lays the foundation for further study of the functional roles of WRKY genes during leaf senescence in cotton. In addition, our data provide new insights into the senescence-associated mechanisms of WRKY genes in cotton. Electronic supplementary material The online version of this article (10.1186/s12870-019-1688-z) contains supplementary material, which is available to authorized users.

BackgroundWRKY transcription factors (TFs) participate in various physiological processes of plants. Although WRKY genes have been well studied in model plants, knowledge of the functional roles of these genes is still extremely limited in cotton.ResultsIn this study, a group IId WRKY gene from cotton, GhWRKY42, was isolated and characterized. Our data showed that GhWRKY42 localized to the nucleus. A transactivation assay in yeast demonstrated that GhWRKY42 was not a transcriptional activator. A β-glucuronidase (GUS) activity assay revealed that the promoter of GhWRKY42 showed fragment deletion activity in Nicotiana tabacum and was mainly expressed in the roots, stems and leaves of ProGhWRKY42::GUS transgenic Arabidopsis plants. Quantitative real-time PCR (qRT-PCR) analysis indicated that GhWRKY42 was up-regulated during leaf senescence and was induced after exposure to abiotic stresses. Constitutive expression of GhWRKY42 in Arabidopsis led to a premature aging phenotype, which was correlated with an increased number of senescent leaves, reduced chlorophyll content and elevated expression of senescence-associated genes (SAGs). In addition, virus-induced gene silencing (VIGS) was used to silence the endogenous GhWRKY42 gene in cotton, and this silencing reduced plant height.ConclusionsOur findings indicate that GhWRKY42 is involved in abiotic stress responses, premature leaf senescence and stem development. This work establishes a solid foundation for further functional analysis of the GhWRKY42 gene in cotton.Electronic supplementary materialThe online version of this article (10.1186/s12863-018-0653-4) contains supplementary material, which is available to authorized users.