Understanding the genetic basis of the node of the first fruiting branch (NFFB) improves early-maturity cotton breeding. Here we report QTL mapping on 200 F₂ plants and derivative F_2:3 and F_2:4 populations by genotyping by sequencing (GBS). BC₁F₂ population was constructed by backcrossing one F_2:4 line with the maternal parent JF914 and used for BSA-seq for further QTL mapping. A total of 1,305,642 SNPs were developed between the parents by GBS, and 2,907,790 SNPs were detected by BSA-seq. A high-density genetic map was constructed containing 11,488 SNPs and spanning 4,202.12 cM in length. A total of 13 QTL were mapped in the 3 tested populations. JF914 conferred favorable alleles for 11 QTL, and JF173 conferred favorable alleles for the other 2 QTL. Two stable QTL were repeatedly mapped in F_2:3 and F_2:4, including qNFFB-D3-1 and qNFFB-D6-1. Only qNFFB-D3-1 contributed more than 10% of the phenotypic variation. This QTL covered about 24.7 Mb (17,130,008–41,839,226 bp) on chromosome D3. Two regions on D3 (41,779,195–41,836,120 bp, 41,836,768–41,872,287 bp) were found by BSA-seq and covered about 92.4 Kb. This 92.4 Kb region overlapped with the stable QTL qNFFB-D3-1 and contained 8 annotated genes. By qRT-PCR, Ghir_D03G012430 showed a lower expression level from the 1- to 2-leaf stage and a higher expression level from the 3- to 6-leaf stage in the buds of JF173 than that of JF914. Ghir_D03G012390 reached the highest level at the 3- and 5-leaf stages in the buds of JF173 and JF914, respectively. As JF173 has lower NFFB and more early maturity than JF914, these two genes might be important in cell division and differentiation during NFFB formation in the seedling stage. The results of this study will facilitate a better understanding of the genetic basis of NFFB and benefit cotton molecular breeding for improving earliness traits.

Cotton is the most important natural fiber-producing crop globally. High-quality fiber and early maturity are equally important breeding goals in the cotton industry. However, it remains challenging to synchronously improve these traits through conventional breeding techniques. To identify additional genetic information relating to fiber quality and early maturity, 11 phenotypic traits for the F₂ and F₃ generations were tested, and quantitative trait loci (QTL) mapping was performed. Candidate genes were analyzed using published RNA-seq datasets and qRT-PCR assays. All 11 tested traits showed bi-directional transgressive segregation, and most traits followed an approximately normal distribution. Overall, significant positive and significant negative correlations were observed among these traits. During cotton breeding, varieties with strong boll-setting ability can be selected from early-maturing materials that have high-quality fiber. A total of 102 QTLs were mapped, including 4 major and 3 stable QTLs. qFL-D13-1 was mapped in both the F₂ and F₃ generations, achieving a 3.94% to 11.39% contribution rate to the phenotypic variation. Three genes located in the QTL regions were identified based on their high expression levels in the three evaluated RNA-seq datasets. Ghir_A04G014830.1, covered by qHNFFB-A4-1 and qFU-A4-1, encoded ACLA-1. Ghir_D13G015010.1, encoding VTC2, and Ghir_D13G016670.1, encoding GA2OX1, were in the stable QTL qFL-D13-1 region. The qRT-PCR results suggested that these three genes may be involved in regulating seed development, fiber initiation, and fiber elongation. Overall, these findings contribute additional information for the breeding of high-yield, high fiber quality, and early-maturity varieties, as well as serve as a foundation for research on the underlying molecular mechanisms.

Superior fiber quality is one of the most important objectives in cotton breeding. To detect the genetic basis underlying fiber quality, an F2 population containing 413 plants was constructed by crossing Jifeng 914 and Jifeng 173, both of which have superior fiber quality, with Jifeng 173 being better. Five fiber quality traits were investigated in the F2, F2:3, F2:4, and F2:5 populations. Quantitative trait loci (QTL) mapping was conducted based on a high-density genetic map containing 11,488 single nucleotide polymorphisms (SNPs) and spanning 4202.12 cM in length. Transgressive segregation patterns and complex correlations in the five tested traits were observed. A total of 108 QTLs were found, including 13 major effect QTLs that contributed more than 10% toward phenotypic variation (PV) and 9 stable QTLs that could be repeatedly mapped in different generations. Chromosome A7 contained 12 QTL, ranking the first. No QTL was found on chromosomes D1 and D11. Two QTLs could be repeatedly detected in three populations, including qFL-D3-2 in F2, F2:4, and F2:5 with 9.18–21.45% of PV and qFS-A11-1 in F2:3, F2:4, and F2:5 with 6.05–10.41% of PV. Another seven stable QTLs could be detected in two populations, including four major effect QTLs: qFL-A12-3, qFS-D10-2, qMC-D6-2, and qMC-D8-1. Fourteen QTL-overlapping regions were found, which might explain the complex correlations among the five phenotypic traits. Four regions on chromosome A11, D3, D6, and D10 covered by both stable and major effect QTLs are promising for further fine mapping. The genomic regions of the two QTLs detected in three populations and the four major effect QTLs contain 810 genes. Gene functional analysis revealed that the annotated genes are mainly involved in protein binding and metabolic pathways. Fifteen candidate genes in the qFL-D3-2 region are highly expressed in fiber or ovules during fiber initiation, elongation, secondary cell wall thickening, or maturation stages. qRT-PCR revealed that Ghir_D03G005440.1 and Ghir_D03G011310.1 may play a role in promoting fiber initiation, while Ghir_D03G006470.1 may be beneficial for promoting fiber elongation. This study provides more information for revealing the molecular genetic basis underlying cotton fiber quality.

Thidiazuron (TDZ) is the main chemical agent for cotton defoliation, which plays a key role in realizing cotton mechanical harvesting. At present, there are few studies on the molecular regulation mechanism of TDZ induced cotton leaf abscission in the field environment. In this study, TDZ sensitive variety (Jifeng 914) and non-sensitive line (04-1685) were used as the experimental materials. Ethylene, cytokinin, and abscisic acid contents in leaves were determined. RNA-seq was used to analyze the changes of gene expression in leaves at 0, 1, 3, and 5 d after TDZ treatment. WGCNA (Weighted gene coexpression network analysis), GO and KEGG were used to analyze and screen gene coexpression modules and gene functions, respectively. The results showed that the ethylene, cytokinin, and abscisic acid contents of Jifeng 914 reached the highest level at 1 d after treatment, and the hormone contents of 04-1685 reached the highest level at 3 d after treatment. A total of 33,283 differentially expressed genes (DEGs) were screened by RNA-seq. Through WGCNA analysis, 2 and 3 modules highly related to hormone and samples were screened respectively. The genes in the modules mainly participate in abiotic stress response, circadian rhythm, and material metabolism. By analyzing the relative expression of plant hormone, stress response, and circadian rhythm related genes, the expression of hormone related genes in sensitive variety Jifeng 914 was higher, among which ethylene signal response genes were kept at a higher expression level within 5 days after treatment, abscisic acid signal response genes were kept at a higher expression level within 1 day after treatment, and cytokinin metabolism related genes expressed very low within 1 day after treatment and increased sharply at 3 d and 5 d after treatment. At 1 d and 3 d after treatment, heat shock protein genes (HSP70, HSP90, and ATJ3), calcium homeostasis regulating genes (CRT3, NCL), and circadian rhythm genes (LNK2, APRR9, CCA1) were significantly down regulated, while circadian rhythm genes (ARR8) were up regulated. Spraying TDZ in the field environment, while causing changes in hormone content, may damage the plant's circadian rhythm and reduce the stress ability to abiotic stress, and thus accelerating leaf senescence and abscission. This study provides more ideas for enriching the molecular regulation mechanism of TDZ induced leaf abscission.

Superior fiber quality is one of the most important objectives in cotton breeding. To detect the genetic basis underlying fiber quality, an F2 population containing 413 plants was constructed by crossing Jifeng 914 and Jifeng 173, both of which have superior fiber quality, with Jifeng 173 being better. Five fiber quality traits were investigated in the F2, F2:3, F2:4, and F2:5 populations. Quantitative trait loci (QTL) mapping was conducted based on a high-density genetic map containing 11,488 single nucleotide polymorphisms (SNPs) and spanning 4,202.12 cM in length. Transgressive segregation patterns and complex correlations in the five tested traits were observed. A total of 108 QTLs were found, including 27 for fiber length (FL), 16 for fiber strength (FS), 24 for micronaire (MC), 17 for fiber uniformity (FU), and 24 for fiber elongation rate (FE). Chromosome A7 contained 12 QTL, ranking the first. No QTL was found on chromosome D1 and D11. Each QTL contributed 1.98%–21.45% to the phenotypic variation (PV), including 13 major effect QTLs that contributed more than 10% toward PV. Two QTLs could be repeatedly detected in three populations, including qFL-D3-2 in F2, F2:4, and F2:5 with 9.18%–21.45% of PV, qFS-A11-1 in F2:3, F2:4 and F2:5 with 6.05%–10.41% of PV. Another seven stable QTLs could be detected in two populations, including four major effect QTLs: qFL-A12-3, qFS-D10-2, qMC-D6-2, and qMC-D8-1. Fourteen QTL-overlapping regions were found, which might explain the complex correlations among the five phenotypic traits. Four regions on chromosome A11, D3, D6, and D10 covered by both stable and major effect QTLs are promising for further fine mapping. The genomic regions of the two QTLs detected in three populations and the four major effect QTLs contain 810 genes. Gene functional analysis revealed that the annotated genes are mainly involved in protein binding and metabolic pathways. Fifteen candidate genes in the qFL-D3-2 region are highly expressed in fiber or ovules during fiber initiation, elongation, secondary cell wall thickening, or maturation stages. qRT-PCR revealed that Ghir_D03G005440.1 and Ghir_D03G011310.1 may play a role in promoting fiber initiation, Ghir_D03G006470.1 may be beneficial for promoting fiber elongation. This study provides more information for revealing the molecular genetic basis underlying cotton fiber quality.