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Background: Salinity is one of the most significant environmental factors limiting the productivity of cotton. However, the key genetic components responsible for the reduction in cotton yield in saline-alkali soils are still unclear. Results: Here, we evaluated three main components of lint yield, single boll weight (SBW), lint percentage (LP) and boll number per plant (BNPP), across 316 G. hirsutum accessions under four salt conditions over two years. Phenotypic analysis indicated that LP was unchanged under different salt conditions, however BNPP decreased significantly and SBW increased slightly under high salt conditions. Based on 57,413 high-quality single nucleotide polymorphisms (SNPs) and genome-wide association study (GWAS) analysis, a total of 42, 91 and 25 stable quantitative trait loci (QTLs) were identified for SBW, LP and BNPP, respectively. Phenotypic and QTL analysis suggested that there was little correlation among the three traits. For LP, 8 stable QTLs were detected simultaneously in four different salt conditions, while fewer repeated QTLs for SBW or BNPP were identified. Gene Ontology (GO) analysis indicated that their regulatory mechanisms were also quite different. Via transcriptome profile data, we detected that 10 genes from the 8 stable LP QTLs were predominantly expressed during fiber development. Further, haplotype analyses found that a MYB gene (GhMYB103), with the two SNP variations in cis-regulatory and coding regions, was significantly correlated with lint percentage, implying a crucial role in lint yield. We also identified that 40 candidate genes from BNPP QTLs were salt-inducible. Genes related to carbohydrate metabolism and cell structure maintenance were rich in plants grown in high salt conditions, while genes related to ion transport were active in plants grown in low salt conditions, implying different regulatory mechanisms for BNPP at high and low salt conditions. Conclusions: This study provides a foundation for elucidating cotton salt tolerance mechanisms and contributes gene resources for developing upland cotton varieties with high yields and salt stress tolerance. [ABSTRACT FROM AUTHOR]

• The cotton cytoplasmic subgroup gene GhHSP70-26 is transcribed in response to polyethylene glycol and abscisic acid treatment and salt, cold, and heat stress. • Overexpression of the GhHSP70-26 gene in tobacco and silencing of the GhHSP70-26 gene in cotton showed that the GhHSP70-26 gene can improve plant tolerance to drought stress. • GhbZIP43 and GhHSF8 bind to cis -acting ABREs and HSEs, respectively, to regulate the transcription of GhHSP70-26 , and the GhHSP70-26 protein interacts with the GhAPX2 protein. The 70-kDa heat shock protein (HSP70) is a molecular chaperone that plays an important role in the response of plants to abiotic stress, but the function and molecular mechanism of HSP70 in the cotton drought stress response are unclear. In this study, the HSP70-encoding gene GhHSP70-26 , which belongs to the cytoplasmic HSP70 subgroup, was cloned from upland cotton (Gossypium hirsutum L.). The transcript level of GhHSP70-26 in cotton leaves was higher than that in cotton stems and roots. GhHSP70-26 was found to respond at the transcriptional level to polyethylene glycol (PEG) and abscisic acid (ABA) treatment and to cold, salt, and heat stress, and its transcript level was positively correlated with the drought resistance of different cotton varieties. Heterologous overexpression of the GhHSP70-26 gene improved the drought resistance of transgenic tobacco compared with that of wild-type (WT) tobacco, and the transgenic tobacco plants were characterized by low leaf wilting, high survival, low leaf water loss, increased root length and high chlorophyll content. Moreover, the transgenic tobacco plants overexpressing GhHSP70-26 had higher proline contents, higher superoxide dismutase (SOD) and peroxidase (POD) enzyme activities, and lower malondialdehyde (MDA) and reactive oxygen species (ROS) contents than the WT tobacco plants. The transcript levels of the stress response-related genes NtLEA , NtERD10D , NtPOD , NtSOD , and NtNCED3-1 in transgenic tobacco overexpressing GhHSP70-26 were significantly higher than those in WT tobacco. In contrast, under drought stress, unlike the results obtained with the pTRV2 empty vector (EV)-transformed and WT cotton plants, the use of virus-induced gene silencing (VIGS) technology to silence the GhHSP70-26 gene in cotton resulted in plants with severely wilted leaves, an increased water loss rate, a higher MDA content, and higher relative electrical conductivity (REC). Furthermore, yeast one-hybrid assays showed that GhbZIP43 and GhHSF8 bind to ABA-responsive elements (ABREs) and heat shock elements (HSEs) in the GhHSP70-26 promoter, respectively. Moreover, yeast two-hybrid, bimolecular fluorescence complementation and luciferase complementation assays showed that the GhHSP70-26 protein interacts with the ascorbate peroxidase (APX) GhAPX2 protein. These results indicate that by reducing the degrees of cell membrane damage and cellular damage caused by ROS stress, the GhHSP70-26 protein plays a positive role in the response of plants to drought stress. [ABSTRACT FROM AUTHOR]

Two class I family teosinte branched1/cycloidea/proliferating cell factor1 (TCP) proteins from allotetraploid cotton are involved in cotton fiber cell differentiation and elongation and root hair development. However, the biological function of most class II TCP proteins is unclear. This study sought to reveal the characteristics and functions of the sea-island cotton class II TCP gene GbTCP4 by biochemical, genetic, and molecular biology methods. GbTCP4 protein localizes to nuclei, binding two types of TCP-binding cis-acting elements, including the one in its promoter. Expression pattern analysis revealed that GbTCP4 is widely expressed in tissues, with the highest level in flowers. GbTCP4 is expressed at different fiber development stages and has high transcription in fibers beginning at 5 days post anthesis (DPA). GbTCP4 overexpression increases primary root hair length and density and leaf and stem trichomes in transgenic Arabidopsis relative to wild-type plants (WT). GbTCP4 binds directly to the CAPRICE (CPC) promoter, increasing CPC transcript levels in roots and reducing them in leaves. Compared with WT plants, lignin content in the stems of transgenic Arabidopsis overexpressing GbTCP4 increased, and AtCAD5 gene transcript levels increased. These results suggest that GbTCP4 regulates trichome formation and root hair development in Arabidopsis and may be a candidate gene for regulating cotton fiber elongation. [ABSTRACT FROM AUTHOR]

Plant-specific heat shock protein 70 s (HSP70s) play crucial roles in response to various environmental stimuli. GhHSP70-26, an HSP70 member, is involved in the response to multiple abiotic stresses in cotton (Gossypium hirsutum). This study explored the relationship between the genetic diversity of GhHSP70–26 and drought resistance in cotton to aid crop breeding. Indel sites at −650 bp upstream of the GhHSP70–26 promoter were discovered in cotton and designated an M-650 marker. The M-650 marker was significantly associated with the effective boll numbers (M-650-In360 > M-650-Del) in 110 types of cotton under drought stress conditions for three years. The relative expression levels of GhHSP70–26 was linearly correlated with comprehensive drought resistance in cotton seedlings. The gene expression levels were enhanced in BC 2 F 2 (pGhHSP70–26 In360 ) of cotton and the transgenic lines pGhHSP70–26 In360 :GhHSP70–26 of Arabidopsis , and drought resistance was also enhanced. After PEG, ABA and SA treatments, the transgenic lines of M-650-In360 promoter showed higher GUS expression activity than the M-650-del promoter in Arabidopsis. The GhFLZ, GhABF3, and GhRVE8 transcription factors could regulate to In360 bp of pGhHSP70–26 to activate high levels of gene expression. These results provide important genetic information to study the natural variation of drought tolerance in cotton, and the loci or exceptional promoters identified can be used as direct targets for cotton trait improvement and genetic engineering. [ABSTRACT FROM AUTHOR]